# Optically-thick structure in early B type supergiant stellar winds at low
metallicities

Timothy N. Parsons1, Raman K. Prinja1, Matheus Bernini-Peron2, Alex W.
Fullerton3, Derck L. Massa4, Lidia M. Oskinova5, Daniel Pauli5, Matthew J.
Rickard1 and Andreas A.C. Sander2
1Department of Physics and Astronomy, University College London, Gower Street,
London WC1E 6BT, UK
2 Zentrum für Astronomie der Universität Heidelberg, Astronomiches Rechen-
Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
3Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, USA
4Space Science Institute (SSI), 4750 Walnut Street, Suite 205, Boulder, CO
80301, USA
5Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str.
24/25, 14476 Potsdam, Germany E-mail<EMAIL_ADDRESS>(TNP)
0000-0002-7714-7823 0000-0002-7714-7823 0000-0003-1113-0727
0000-0002-9139-2964 0000-0003-0708-4414 0000-0002-5453-2788
0000-0002-5255-8116 0000-0002-2090-9751

(Accepted 2023 December 21. Received 2023 December 20; in original form 2023
October 28)

###### Abstract

Accurate determination of mass-loss rates from massive stars is important to
understanding stellar and galactic evolution and enrichment of the
interstellar medium. Large-scale structure and variability in stellar winds
have significant effects on mass-loss rates. Time-series observations provide
direct quantification of such variability. Observations of this nature are
available for some Galactic early supergiant stars but not yet for stars in
lower metallicity environments such as the Magellanic Clouds. We utilise
ultraviolet spectra from the Hubble Space Telescope ULLYSES program to
demonstrate that the presence of structure in stellar winds of supergiant
stars at low metallicities may be discerned from single-epoch spectra. We find
evidence that, for given stellar luminosities and mean stellar wind optical
depths, structure is more prevalent at higher metallicities. We confirm, at
Large Magellanic Cloud (0.5 $Z_{\odot}$), Small Magellanic Cloud (0.2
$Z_{\odot}$) and lower (0.14 – 0.1 $Z_{\odot}$) metallicities, earlier
Galactic results that there does not appear to be correlation between the
degree of structure in stellar winds of massive stars and stellar effective
temperature. Similar lack of correlation is found with regard to terminal
velocity of stellar winds. Additional and revised values for radial velocities
of stars and terminal velocities of stellar winds are presented. Direct
evidence of temporal variability, on timescales of several days, in stellar
wind at low metallicity is found. We illustrate that narrow absorption
components in wind-formed profiles of Galactic OB stellar spectra remain
common in early B supergiant spectra at low metallicities, providing means for
better constraining hot, massive star mass-loss rates.

###### keywords:

stars: early-type – Magellanic Clouds – stars: winds, outflows – stars: mass-
loss – ultraviolet: stars

††pubyear: 2023††pagerange: Optically-thick structure in early B type
supergiant stellar winds at low metallicities–C

##  1 Introduction

Their relative rarity notwithstanding, massive stars are the key drivers of
galactic evolution and of enrichment of the interstellar medium (ISM). Stars
with zero-age main sequence masses greater than approximately $8\,M_{\odot}$
evolve rapidly and undergo significant mass-loss, during both main sequence
and post-main sequence phases. These stars, with some exceptions in the very
high mass regime, end their lives as core-collapse supernovae, thereby
returning significant amounts of highly-processed material to the surrounding
ISM on relatively short timescales.

Unlike lower mass stars, massive stars exhibit strong stellar winds. These
winds are accelerated, not simply by continuum-derived radiation pressure, but
very significantly by “line-driving”, whereby resonance line absorption of
ultraviolet (UV) photons can impart significant momentum, and substantial
velocities, to stellar wind material (see, for example, discussion of this
process in Mihalas, 1978; Puls et al., 2000; Hubeny & Mihalas, 2015).

In this work, we aim primarily to identify the presence of large-scale
structure in the stellar winds of massive stars at low metallicities,
specifically early B-type supergiants in the Large and Small Magellanic Clouds
(LMC, SMC) and some very low-metallicity Local Group dwarf galaxies (IC 1613
and NGC 3109), from single-epoch UV spectra. We seek to demonstrate that the
presence of optically-thick structure in these winds may be reliably
identified even in the absence of extensive time-series observations. The
presence of such structure is likely to have significant consequence for
accurate estimation of mass-loss rates from these stars.

As demonstrated by Oskinova et al. (2007), among others, neglecting the
effects of macro-scale optically thick clumping in stellar winds can lead to a
significant underestimation of the rates of mass-loss from hot, massive stars:
a key factor driving stellar and galactic evolution. Fullerton et al. (2006)
and Prinja & Massa (2013) show that this discrepancy may be at an order-of-
magnitude scale. The presence and nature of stellar wind structure has been
identified and analysed in considerable detail, both temporally and spatially,
in respect of some Galactic B supergiant stars using time series data obtained
with the International Ultraviolet Explorer (IUE) (see, for example, Massa et
al., 1995; Fullerton et al., 1997; Prinja et al., 2002). To date however, no
similar data are available for similar stars at lower metallicities. In
addition, we identify examples where evidence of temporal variability may be
identified in our data, indicating the value that future time-series
observations would be able to provide.

In order to undertake the wind structure analysis described, we make use of
stellar physical parameters for our target sample of stars largely from
existing literature. These are referenced in detail in Table 1 below. Some of
these values are revised or newly-derived, as described below. The derivation
and more precise quantification of many such physical parameters represents a
substantial, and ongoing, programme and reference is made in particular to the
XShootU collaboration in this regard (see Vink et al., 2023).

We further attempt to identify whether any correlations are observed between
effective temperatures, stellar luminosity, metallicity and rotational
velocity on the one hand and the extent of wind structure which may be
discerned from the method here discussed and currently available data.

Use of the “Sobolev with exact integration” (SEI) method described in Section
3 and the conclusions drawn from its application to existing single-epoch data
are motivated by the need to provide a more reliable starting point for the
determination of mass-loss over the full lifetime of massive stars in low-
metallicity conditions. In this way, we aim to provide better constraints to
models of galactic evolution in the early Universe.

Finally, we use the results presented in Section 4 to identify some suitable
targets for obtaining time-series UV observations of massive stars in low-
metallicity environments for the purpose of obtaining direct empirical
evidence to test the conclusions reached in this paper. Such targeted
observations, together with the results presented here, would provide powerful
constraints upon the true extent of variability in mass-loss rates from
massive stars at low metallicities and, by extension, in early phases of
galactic evolution.

##  2 Observational data, target selection and data reduction

###  2.1 Target stars and ULLYSES data

We use UV spectra obtained with the Cosmic Origins Spectrograph (COS) and
Space Telescope Imaging Spectrograph (STIS) instruments on the Hubble Space
Telescope (HST). The primary data come from the HST Ultraviolet Legacy Library
of Young Stars as Essential Standards (ULLYSES) observing program (Roman-Duval
et al., 2020), up to and including Data Release 5 (June 2022). We concentrate
on early B supergiant stars in the LMC and SMC. The target stars examined are
all early B-type supergiant stars (spectral types B0 - B5). B supergiants are
the most numerous hot, luminous stars (Prinja et al., 2005). Stars of these
spectral types generally provide well-developed wind profiles in their UV
spectra which are, in many cases, not saturated, particularly in the case of
stars in the SMC where the lower metallicity environment can generally be
expected to produce weaker stellar winds (Kudritzki & Puls, 2000; Trundle et
al., 2004; Bouret et al., 2015; Rickard et al., 2022).

In addition to the foregoing, we have included a small number of stars of
similar spectral types from ULLYSES Data Release 6 (March 2023), being early B
supergiant stars observed as part of the “low Z” programme of observing such
stars in certain very low metallicity Local Group dwarf galaxies (IC 1613, NGC
3109 and Sextans A). These are among suitable targets identified in previous
studies of these galaxies, such as Bresolin et al. (2007) and Lorenzo et al.
(2022).

Table 1: Tabulation of LMC (first block) and SMC (second block) B0 - B5
supergiants in the ULLYSES DR5 sample and Low Z early B supergiants (third
block) in the ULLYSES DR6 sample which are suitable for analysis, spectral
types and values for principal stellar physical parameters. Targets marked (*)
are not susceptible to separate line profile fitting for each doublet element
and a combined fit with floating inferred oscillator strength ratio is used,
as described in the text. New and varied values of stellar radial velocity and
terminal wind velocity derived for this work are indicated in the table.

Star | Spectral type | Ref | $T_{\rm eff}$ | log $g$ | log $L$ | Ref | $\varv\textsubscript{rad}$ | Ref | $\varv\textsubscript{$\infty$}$ | Ref |  $\varv$ sin $i$ | Ref
---|---|---|---|---|---|---|---|---|---|---|---|---
|  |  | (K) | (cgs) | $(L/L_{\odot})$ |  | (km s-1) |  | (km s-1) |  | (km s-1) |
Sk -69 43 * | B0.5 Ia | 4 | 22,845 | 2.62 | 5.50 | 24 | 242 | $x$ | 900 | $xx$ | 102 | 19
Sk -68 140 * | B0.7 Ib-Iab Nwk | 22 | 23,500 | 2.75 | 5.64 | 22 | 266 | 21 | 1,150 | 30 | 53 | 22
Sk -66 35 | BC1 Ia | 5 | 22,000 | 2.6 | 5.73 | 24 | 255 | $x$ | 810 | 30 | 80 | -
Sk -68 129 * | B1 I | 30 | 22,200 | 2.5 | 5.25 | 30 | 275 | $x$ | 1,380 | 30 | 96 | 19
Sk -67 14 * | B1.5 Ia | 8 | 22,890 | 2.68 | 5.74 | 24 | 280 | $x$ | 900 | 30 | 88 | 19
Sk -68 26 | BC2 Ia | 5 | 18,160 | 2.19 | 5.71 | 24 | 290 | $x$ | 390 | 30 | 80 | -
Sk -67 78 | B3 Ia | 5 | 16,150 | 2.03 | 5.2 | 24 | 315 | $x$ | 470 | $xx$ | 50 | -
Sk -70 50 | B3 Ia | 5 | 15,000 | 2.0 | 5.2 | - | 240 | $x$ | 360 | $xx$ | 50 | 29
Sk -69 140 * | B4 I | 1 | 15,000 | 2.25 | 4.73 | 30 | 275 | $xx$ | 1,100 | $x$ | 50 | -
Sk -70 16 | B4 I | 1 | 15,000 | 2.25 | 4.62 | 30 | 258 | $xx$ | 910 | 30 | 50 | -
Sk -68 8 | B5 Ia+ | 4 | 14,200 | 2.2 | 5.54 | 30 | 290 | 28 | 300 | 30 | 50 | 30
NGC 2004 ELS 3 | B5 Ia | 14 | 14,450 | 2.1 | 5.1 | 17 | 309 | 14 | 440 | 30 | 42 | 17
AzV 215 * | B0 (Ib)/BN0 Ia | 9,6 | 27,000 | 2.9 | 5.63 | 12 | 145 | $x$ | 1,540 | 30 | 92 | 19
AzV 235 * | B0 Iaw | 2 | 27,500 | 2.9 | 5.72 | 11 | 170 | 18 | 1,480 | $x$ | 90 | 19
AzV 317 * | B0 Iw | 3 | 27,000 | 3.00 | 5.40 | 25 | 118 | 23 | 1,240 | $x$ | 80 | -
AzV 104 | B0.5 Ia | 6 | 27,500 | 3.1 | 5.31 | 12 | 190 | $x$ | 600 | 30 | 56 | 19
AzV 488 * | B0.5 Iaw | 2 | 27,500 | 2.9 | 5.74 | 11 | 186 | $x$ | 1,200 | 30 | 87 | 19
AzV 242 | B0.7 Iaw | 2 | 25,000 | 2.85 | 5.67 | 13 | 187 | $x$ | 950 | 11 | 83 | 11,19
AzV 264 | B1 Ia | 6 | 22,500 | 2.55 | 5.44 | 13 | 131 | 23 | 720 | $x$ | 116 | 19
AzV 266 | B1 I | 23 | 22,000 | 2.50 | 5.09 | 25 | 149 | 23 | 990 | $x$ | 61 | 19,23
AzV 96 | B1 I/B1.5 Ia | 23,6 | 20,000 | 2.55 | 5.39 | 13 | 166 | 23 | 800 | $x$ | 70 | 19
AzV 78 | B1.5 Ia+ | 6 | 21,500 | 2.40 | 5.92 | 12 | 170 | $x$ | 590 | 30 | 46 | 13
Sk 191 | B1.5 Ia | 6 | 22,500 | 2.55 | 5.77 | 12 | 120 | $x$ | 460 | $x$ | 118 | 19
AzV 210 | B1.5 Ia | 6 | 20,500 | 2.4 | 5.41 | 12 | 178 | $x$ | 750 | 11 | 51 | 27
AzV 18 | B2 Ia | 6 | 19,000 | 2.3 | 5.44 | 12 | 150 | $x$ | 400 | $x$ | 49 | 12
NGC 330 ELS 4 | B2.5 Ib | 14 | 17,000 | 2.3 | 4.77 | 17 | 158 | 18 | 460 | $x$ | 36 | 17
AzV 187 | B3 Ia | 6 | 14,600 | 2.0 | 4.91 | - | 135 | $x$ | 470 | $xx$ | 45 | -
AzV 234 | B3 Iab | 27 | 15,700 | 2.15 | 4.91 | 27 | 165 | $xx$ | 280 | 30 | 43 | 27
NGC 330 ELS 2 | B3 Ib | 14 | 14,590 | 2.15 | 4.73 | 17 | 154 | 14 | 460 | $x$ | 14 | 17
AzV 324 | B4 Iab | 7 | 14,600 | 2.00 | 4.89 | 30 | 153 | $xx$ | 270 | $x$ | 50 | -
AzV 22 | B5 Ia | 6 | 14,500 | 1.9 | 5.04 | 12 | 144 | 26 | 300 | 30 | 46 | 12
AzV 445 | B5 (Iab) | 6 | 14,500 | 2.00 | 5.0 | - | 175 | $x$ | 350 | $xx$ | 50 | -
NGC 3109 EBU07 * | B0-1 Ia | 16 | 27,000 | 2.9 | 5.82 | 16 | 382 | 16 | 1,350 | $xx$ | 80 | -
IC 1613 BUG2007 B3 | B0 Ia | 15 | 24,500 | 2.65 | 5.53 | 15 | -250 | 15 | 1,100 | $x$ | 100 | 20
IC 1613 BUG2007 A10 | B0.5 Ia/B1 Ia | 20,15 | 25,000 | 2.7 | 5.71 | 15 | -255 | 15 | 1,075 | 20 | 50 | 20
IC 1613 BUG2007 B4 | B1.5 Ia | 15 | 22,500 | 2.6 | 5.20 | 15 | -243 | 15 | 875 | 20 | 50 | 20

References: 1 Rousseau et al. (1978), 2 Walborn (1983), 3 Garmany et al.
(1987), 4 Fitzpatrick (1988), 5 Fitzpatrick (1991), 6 Lennon (1997), 7 Smith
Neubig & Bruhweiler (1997), 8 Walborn et al. (2002), 9 Evans et al. (2004b),
10 Evans et al. (2004c), 11 Evans et al. (2004d), 12 Trundle et al. (2004), 13
Trundle & Lennon (2005), 14 Evans et al. (2006), 15 Bresolin et al. (2007), 16
Evans et al. (2007), 17 Trundle et al. (2007), 18 Evans & Howarth (2008), 19
Penny & Gies (2009), 20 Garcia et al. (2014), 21 Evans et al. (2015), 22
McEvoy et al. (2015), 23 Lamb et al. (2016), 24 Urbaneja et al. (2017), 25
Castro et al. (2018), 26 Neugent et al. (2018), 27 Dufton et al. (2019), 28
Jönsson et al. (2020) 29 derived from the method in Bestenlehner et al.
(2023), 30 Hawcroft et al. (2023).

($x$) = Derived for this work (and varying earlier published results), ($xx$)
= New result derived for this work, (-) = No data available and result
estimated from other parameters or comparable stars.

Note: source references listed after luminosity data apply also to temperature
and surface gravity data, except: Sk -69 43 log $L$ figure is estimated; Sk
-69 140, Sk -70 16, AzV 266 and AzV 324log $g$ figures are estimated, AzV 96
$T_{\rm eff}$ figure is derived from UV spectral fitting; AzV 266 $T_{\rm
eff}$ figure is from Ref 13.

Table 1 sets out the list of early B supergiant LMC and SMC stars from ULLYSES
Data Release 5 which are suitable for analysis using the technique presented
here, as well as some very low metallicity stars from other Local Group dwarf
galaxies from ULLYSES Data Release 6. Stellar identifications are from
Azzopardi et al. (1975) and Azzopardi & Vigneau (1982) (AzV), Sanduleak (1968,
1970) (Sk), Evans et al. (2006) (ELS), Evans et al. (2007) (EBU) and Bresolin
et al. (2007) (BUG). A number of early B supergiants in the ULLYSES data are
excluded from these lists for one or more of the following reasons:

  1. 1.

saturation of absorption profiles;

  2. 2.

terminal wind velocity being too high for separate line-fitting to be reliable
over a reasonable velocity range;

  3. 3.

the presence of prominent emission line characteristics or other peculiar
spectral features within the resonance line profile;

  4. 4.

prominent interstellar absorption lines affecting critical parts of the
resonance line absorption profile (particularly prevalent in the case of LMC
stars with low terminal wind velocities, comparable in magnitude to the radial
velocity of the star);

  5. 5.

lack of well-developed P Cygni-type resonance line profiles.

###  2.2 Data reduction method

The high level science products from ULLYSES Data Releases 5 and 6 comprise
.fits files containing wavelength and flux data for a given star from either,
or both, of the COS and STIS instruments described above (as well as shorter
wavelength FUSE data in some cases). These may be made from more than one
observation in each case. The ULLYSES data include a metatable of principal
stellar parameters for target stars extracted from earlier literature
(although the metatable is not complete for all targets). Primary sources for
stellar data are referenced in Table 1.

Radial velocities for each target star were required as part of the data
reduction process undertaken for this work. In preference to using “bulk”
radial velocities for each galaxy, radial velocity data for individual target
stars were each checked (without initial reference to existing literature
values) using the DIPSO spectral analysis code, part of the STARLINK software
package (Currie et al., 2014), by overplotting of the limited number of
available photospheric absorption lines in the ULLYSES UV data: in each case,
using at least two of Si iii 1312.59Å, C ii 1323.95Å and Si iii 1417.24Å.
These values were then compared to existing literature and, in some cases
where there are significant differences identified and the results obtained
via this method are sufficiently unambiguous, have been used in preference to
literature figures. Where values were not given by existing literature, this
method has been used to discern a suitable value. This process has provided
radial velocity measurements to a precision of order 5 km s-1, which is
sufficient for the fitting techniques applied here.

Reliable terminal wind velocities are an essential aspect of line profile
fitting using the SEI method. For those target stars contained within the
ULLYSES Data Release 3 sample (August 2021), results from the comprehensive
analysis contained in Hawcroft et al. (2023) have generally been used. In a
small number of those cases, a different value has been used, generally
arising from the use in this work of individual stellar radial velocity
values, derived as described in the preceding paragraph, rather than the
applicable bulk galactic values adopted in that work. For target stars
contained within more recent ULLYSES data releases, we have applied similar
techniques to those described in Hawcroft et al. (2023) to measure terminal
velocities or to revisit and/or confirm other existing literature results. As
noted in that work, applying the fitting process used there and in this work
to unsaturated profiles will provide only a lower bound on terminal wind
velocity, rather than the more precise determination possible from using
saturated profiles. Where possible, we have therefore further constrained
terminal velocity results by applying the technique of measuring the velocity
displacement of narrow absorption component (NAC) features in unsaturated Si
iv 1393.76, 1402.77Å profiles, as described by Prinja et al. (1990) or by
direct measurement from the width of saturated C iv 1548.20, 1550.77Å
profiles, as also described by Prinja et al. (1990) and by Hawcroft et al.
(2023).

Checks have also been made against other results in the literature where the
given value of a parameter appears uncertain, or is absent. Use has also been
made of results produced by application of the spectroscopic analysis pipeline
recently introduced by Bestenlehner et al. (2023). Further investigations have
been undertaken where those results suggest stellar parameters significantly
differing from the existing literature.

For each target star, a suitable photospheric spectrum is selected from the
TLUSTY model grids (see Hubeny, 1988; Lanz & Hubeny, 2007, and later guides),
based upon the effective temperature and surface gravity for the star, as
derived from the process discussed above. Metallicity is taken to be 0.5
$Z_{\odot}$ for LMC stars and 0.2 $Z_{\odot}$ for SMC stars. In each case, a
further visual check of the selected TLUSTY spectrum is made against the
observed spectrum to identify any clearly discrepant spectra. In a small
number of cases, the definite presence, or absence, of photospheric lines from
particular ionized states in one spectrum compared to the other has required
the selection of an alternative TLUSTY model spectrum to that suggested by the
effective temperature provided in the literature. For the “low Z” target
stars, metallicity is taken to be 0.1 $Z_{\odot}$ when selecting TLUSTY grid
models. Garcia et al. (2014) utilised SMC metallicity (0.2 $Z_{\odot}$) to
select photospheric models for stars in IC 1613, however this difference does
not significantly affect the results derived here.

Once a suitable photospheric spectrum is selected, that spectrum is “spun up”
using a short FORTRAN procedure, ROTIN3, which forms part of the wider TLUSTY
software package. This provides appropriate rotational broadening of the
spectral lines, based upon the $\varv\sin i$ rotational velocity parameter for
the star. The resulting spectrum is also normalised through division by a
suitable continuum spectrum from the TLUSTY model grids.

The observed spectrum for each star is normalised manually using the DIPSO
spectral analysis code previously referred to. A suitable continuum is adopted
and the observed spectrum divided by this. The widely-acknowledged difficulty
in normalising UV spectra (due to extensive line-blanketing making the true
continuum difficult to locate) means that this process does involve some
trial-and-error in order to produce a reasonably well-normalised spectrum. The
resulting spectrum is then smoothed for clarity using the qsm Gaussian
smoothing function available within DIPSO.

The two normalised spectra are then combined, with the observed spectrum being
suitably blue-shifted (or in the case of stars in IC 1613, red-shifted) to
account for the stellar radial velocity previously derived. This is carried
out using an IDL procedure, auto_windlines2, which uses the Sobolev with exact
integration method described in the following section together with weighted
non-linear least-squares minimisation, using the Marquardt-Levenburg method,
to produce a suitable model fit to the observed wind profile for the ion or
ions selected (see Lamers et al., 1987; Massa et al., 2003; Prinja & Massa,
2010).

This procedure provides identification of the photospheric contribution to the
spectrum in the region(s) examined and seeks to provide a best fit model
spectrum for a specified terminal wind velocity ($\varv_{\infty}$) with
principal outputs being the radial optical depth of the wind across the
normalised velocity range of the wind ($\tau_{\rm rad}(w)$, where
$w=\varv/\varv_{\infty}$), the wind acceleration parameter ($\beta$) and a
wind turbulence factor ($w_{\rm turb}$) expressed as a fraction of normalised
terminal wind velocity ($\varv/\varv_{\infty}$ = 1). A single $\beta$ law to
describe the wind velocity at any radius is assumed (Castor et al., 1975),
such that the wind velocity at a given radius is expressed as:

$\varv(r)=\varv_{\infty}\left(1-\frac{b}{r}\right)^{\beta}.$ (1)

The value of the constant, b, sets the wind velocity at or near the stellar
photosphere. A fixed value of b = 1 was used for each fit, representing a wind
launched from zero velocity at the photosphere. Adopting this value permits
the wind velocity equation to be written as:

$\varv(r)=\varv_{\infty}\left(1-\frac{R_{*}}{r}\right)^{\beta},$ (2)

where $R_{*}$ is the photospheric radius of the star.

It can readily be seen that higher values of $\beta$ describe winds with a
lower rate of radial acceleration, and vice-versa for lower values of $\beta$.
Empirically-derived typical $\beta$ values for OB stars range between 0.7-1.5,
with supergiants tending to the upper end of this range. Note that $\beta$
values may fall outside this range and can be significantly larger for later
type stars (Kudritzki & Puls, 2000). Higher values of $\beta$ for blue
supergiants have also been derived in earlier literature, although in some
cases these are derived assuming a smooth, non-clumped wind (see, for example,
Trundle & Lennon, 2005). Bernini-Peron et al. (2023) adopted $\beta=2$ when
including both clumping in stellar winds and X-ray effects in their
description of winds of cool B supergiants (spectral types B2 - B5), based on
values previously obtained by Crowther et al. (2006) and the fact that Haucke
et al. (2018) identified a trend of $\beta\geq 2$ for cooler B supergiants
(spectral types B3 and later).

The weighted least-squares line fitting process used as part of the SEI method
permits the values for the acceleration parameter and the wind turbulence
factor to float as part of the process of finding the best fit model profile.
The terminal wind velocity needs however to be specified to be an appropriate
value in order for the fitting process to operate effectively, as described
above.

(a)  (b)

Figure 1: SEI-derived model fits for (a) blue and (b) red components of the Si
iv 1393.76, 1402.77 Å resonance line doublet feature in the UV spectrum of SMC
star AzV 210 (spectral type B1.5 Ia). As described in the introduction to
Section 3, the lower panels of each plot respectively show the radial optical
depths of the stellar wind at differing normalised velocities which produce
the best-fit model line (lower left panels) and the photospheric contribution
to the spectrum as derived from the model photospheric spectrum selected based
upon the physical parameters of the star (lower right panels). A detailed
explanation of each of the parameters derived and displayed with the plots is
set out in Subsection 2.2. Rest wavelengths of the blue and red elements of
the doublet are highlighted in those colours. Note also that the decoupled
treatment of each element means that the model fit for each element “ignores”
the presence of the other element, as can be seen in the upper panel of plot
(a). This does not affect the model fit within the relevant doublet element.

##  3 The SEI method for line profile fitting

Use is made of a modified form of the SEI method introduced by Hamann (1981)
and developed, particularly, by Lamers et al. (1987). This employs a single
application of the Sobolev approximation to determine the source function and
combines this with an exact solution to the formal integral for the radiative
transfer equation. Massa et al. (2003) utilised this method to derive wind
profiles and parameters for a large sample of O type stars in the LMC, which
those authors then used to derive mass-loss rates for those stars using the
formulae and methods introduced by Vink et al. (2000) and Vink et al. (2001).
The technique has recently also been used to estimate the terminal wind
velocities of a large sample of the HST ULLYSES (Roman-Duval et al., 2020)
observing program’s LMC and SMC target stars, through line fitting to the C iv
1548.20, 1550.77Å feature (Hawcroft et al., 2023).

Wind-driven features visible in the UV spectra of early supergiant stars
frequently exhibit characteristic P Cygni-type line profiles indicative of
significant mass-loss from these stars and of the presence of a dense,
radially accelerating envelope of surrounding material. We observe that
certain resonance line (i.e. ground state transition) doublet features present
well-developed and separated, but unsaturated, profiles in the case of many
early B supergiants, particularly at lower than solar metallicities. The Si iv
1393.76, 1402.77Å feature in particular often provides considerable diagnostic
data capable of being derived from the shape, depth and width of the
absorption parts of that feature. We concentrate therefore on this feature in
the following analysis. In some cases, use is made of the Al iii 1854.72,
1862.79Å feature.

The data reduction process required to use the SEI technique to produce a
best-fit model of the observed P Cygni profiles is described in detail in
Subsection 2.2 above. To illustrate our approach, we show in Figure 1 an
example fitted spectrum for the SMC star AzV 210 (spectral type B1.5 Ia)
produced using this technique. The plots show the observed spectrum in
velocity space centred on the rest wavelength of the blue element of the Si iv
resonance line doublet at 1393.76Å in the left upper plot and the same for the
red element at 1402.77Å in the right upper plot. The best-fit model line for
each feature is shown in the appropriate colour overplotted onto the
applicable observed spectrum. The contributions to the model line from
transmitted and scattered photons are also plotted. These plots further show
the fundamental parameters of the stellar wind which are either derived from
the fitting process or are adopted as fixed inputs. Each of these parameters
is described in Subsection 2.2 above. The lower panels show, on the left in
each case, the radial optical depths of the stellar wind at differing
normalised velocities which produce the best-fit model line. This process is
described in Subsection 3.1 below. The lower panel on the right in each group
of plots shows the photospheric contribution to the spectrum as derived from
the model photospheric spectrum selected based upon the physical parameters of
the star.

###  3.1 Radial optical depths

When carrying out the line fitting of the above-referenced Si iv or Al iii
doublets, we also obtain and plot the radial optical depths ($\tau_{\rm rad}$)
of the stellar wind which give rise to the best-fit line profile. These
optical depths are recorded in 21 “bins” at normalised velocities
($w=\varv(r)/\varv_{\infty}$) ranging from zero at the stellar photosphere up
to and including the terminal wind velocity ($\varv_{\infty}$, i.e. $w=1$).
Toward the higher velocity region of the stellar wind, these bins are narrower
in width, and more concentrated in number.

Examining the measured ratios between the optical depths of the components of
a doublet provides a strong indicator of clumping in the stellar wind. If a
source is obscured by a uniform distribution of gas which absorbs only in the
lines of the doublet being examined, the ratio of the optical depths inferred
from the observed absorption lines is simply the ratio of the oscillator
strengths (f) of the doublet elements. Fundamental atomic theory provides a
fixed ratio between the relative strengths of each of these transitions: for
this Si iv doublet, 2.013:1 (Morton, 1991) (or 2.015:1 from Mas-Ribas &
Mauland, 2019) and, for this Al iii doublet, 2.010:1 (Morton, 1991) (or 2.012
from Mas-Ribas & Mauland, 2019).

If however the source is partially obscured due to the presence of highly
optically thick clumps or other features, the ratio of the optical depths will
depend only upon the fraction of the source which remains uncovered. Thus, we
can expect the apparent ratio of the radial optical depths required to produce
a fit to the observed spectrum to diverge from the canonical figure for the
f-ratio and to approach unity as those opaque clumps cover an increasing
proportion of the stellar disk (Ganguly et al. 1999 (in the context of active
galactic nuclei); Massa et al. 2008; Prinja & Massa 2010). The numerical
relationship between covering factor and observed optical depth ratios is
described in detail in Massa et al. (2008).

###  3.2 Coupled and decoupled doublet elements

A development of the SEI technique, using a weighted least-squares approach to
fit model line profiles separately to individual elements of the Si iv
1393.76, 1402.77Å resonance line doublet was introduced by Prinja & Massa
(2010). Provided the terminal velocity ($\varv_{\infty}$) of the stellar wind
being considered does not exceed approximately half of the separation of the
doublet’s elements, those authors demonstrated that it is possible to treat
each element as being radiatively decoupled and, therefore, to derive separate
model line fits for each. In the case of Si iv, this means we can reliably
produce separate fits for stars with $\varv_{\infty}\lesssim$ 970 km s-1. This
technique is employed here, where possible, to derive radial optical depths
for each element of the Si iv doublet. In a small number of cases where that
doublet feature is saturated, we have been able to make use of the unsaturated
Al iii 1854.72, 1862.79Å resonance line doublet, which can provide similar
diagnostic capabilities for stars with $\varv_{\infty}\lesssim$ 650 km s-1.

In order to increase the size of the data set, in cases where the terminal
wind velocity significantly exceeds the figure permitting reliable separate
fitting of the elements of the doublet, a single fit is performed across the
whole doublet. In these cases however, the ratio of the oscillator strengths
is allowed to be a floating parameter in the fitting process in order to mimic
the separate fitting process described above, adopting the approach of Prinja
& Massa (2010). Targets where this technique is adopted are identified in the
tables of stellar data and in subsequent plots. Although this single-fitting
method does not permit individual fitting of each element of the doublet, it
nevertheless provides evidence of wind structure surrounding additional stars
once we have established the appropriateness of the separate fitting process
for establishing the presence of such structure, as described in this work.

##  4 Results

We set out below a discussion of the principal conclusions derived from the
foregoing analysis and, in some cases, limitations on the usable data.
Individual stars are ordered, as in Table 1, by host galaxy (LMC, SMC and
others) and by spectral type, earliest first, within each group.

Figure 2 contains plots of the fitted model optical depths of the red element
of the relevant doublet against the fitted model optical depth of the blue
element of that doublet for each normalised radial velocity “bin” for those
LMC target stars where separate fitting of each element has been possible. The
optical depths plotted are the optical depths of the stellar wind at varying
radial velocities which best reproduce the observed absorption profile in the
blue or red, as the case may be, element of the observed resonance-line
doublet feature in the UV stellar spectrum. The aim here is to demonstrate, in
a more quantitative fashion, that the wind structure which may be
qualitatively observed by comparing the similarities between the shapes of the
blue and red element SEI-derived plots of optical depths for a given star’s
stellar wind is indeed present and not an artifact of the SEI-fitting process.
“Structure” is taken here to be evident from the different optical depths able
to be inferred at different radial velocities within the stellar wind (and
therefore distances from the stellar photosphere). The individual plots are
ordered by spectral type, from earliest to latest.

Figure 3 shows the same plots for all of the SMC stars examined for which
separate fitting of doublet elements has been possible, once again ordered by
spectral type, from earliest to latest. Figure 4 sets out the same comparison
for the “low Z” sample.

In each of the plots of optical depths of red against blue doublet elements,
there are varying numbers of data points between stars. This results from the
necessary exclusion of data points with very small or very large optical
depths. Consistently with the approach taken by Prinja & Massa (2010), optical
depths in the range 0.3(blue)/0.2(red) - 6.0(blue)/5.0(red) are considered, in
order to avoid poor quality and overly-saturated situations. The plotted
uncertainties for individual optical depth data points have been broadly
estimated by manually adjusted optical depths in selected radial velocity bins
for some stars to identify the extent of variation required to produce a
clearly discernible change in the model fitted line profile. This generally
varies between 10 and 20 per cent of the radial optical depth value. In order
to avoid using unjustified levels of precision in the plotted results, the
larger uncertainty figure has been adopted throughout.

Figure 2: Plots of Blue and red radial optical depths for LMC stars in the
sample for which separate SEI fits for doublet elements are possible. In this
and the equivalent plots for SMC (Fig. 3) and Low Z (Fig. 4) stars, the
optical depth ratios at different radial velocities derived from SEI model
fitting are shown. The diagonal lines show the extremes of optical depth
ratios to be expected from minimal optically-thick coverage of the stellar
disk (green) to extensive coverage (magenta). Figure 3: Plots of Blue and red
radial optical depths for SMC stars in the sample for which separate SEI fits
for doublet elements are possible. The optical depth ratios at different
radial velocities derived from SEI model fitting are shown. The diagonal lines
show the extremes of optical depth ratios to be expected from minimal
optically-thick coverage of the stellar disk (green) to extensive coverage
(magenta). Figure 4: Plots of Blue and red radial optical depths for Low Z
stars in the sample for which separate SEI fits for doublet elements are
possible. The optical depth ratios at different radial velocities derived from
SEI model fitting are shown. The diagonal lines show the extremes of optical
depth ratios to be expected from minimal optically-thick coverage of the
stellar disk (green) to extensive coverage (magenta).

###  4.1 Numerical results

We set out in Table 2 a summary of key stellar parameters, repeated from
earlier tables for ease of reference, together with our results for the mean
radial optical depth ratios derived from the stellar data and procedures
described above and the mean optical depths of the blue element of the
applicable resonance line doublet used for each star. In each case, the mean
ratio has been calculated using the optical depth ratios for normalised radial
velocities between 0.35 and 0.75, both inclusive, of the respective terminal
wind velocity for each target star. From these results, we consider whether
there is a correlation of stellar effective temperature, luminosity and other
stellar parameters with the prevalence of structure in the stellar wind, as
signified by the mean optical depth ratio. The uncertainties for mean optical
depth ratios are obtained in one of two ways. For the stars where a single fit
of the entire doublet has been carried out, the (asymmetric) uncertainty is
taken directly from the uncertainty in the oscillator strength ratio obtained
by allowing this parameter to float in the fitting process, as described in
Subsection 3.2 above. For stars where each doublet element has been separately
fitted, the uncertainty is the standard error of the mean optical depth ratio
for the star.

In most cases, utilising only this central “continuum” range of wind
velocities excludes those parts of the wind absorption profile for which
either the SEI method breaks down (lowest velocities) or the profile is
significantly affected by ISM absorption lines. It should be noted that in
some instances, particularly of LMC stars with low terminal wind velocities,
there is nevertheless some contamination of this continuum region by Milky Way
interstellar lines when the stellar spectrum is shifted into the rest frame of
the star in question.

Table 2: Tabulation of LMC (first block), SMC (second block) and Low Z (third block) target star parameters: spectral type, effective temperature and luminosity; with results for mean optical depth ratios and mean blue doublet component optical depths (for the range $w$ = 0.35-0.75) derived from the SEI fitting techniques described in this work for the relevant line profiles. “O.D.R” refers to the ratio of the optical depths of blue and red components of the relevant line profile, being that for Si iv, unless otherwise noted. Star | Spectral type | $T_{\rm eff}$ | log $L$ | Mean O.D.R. | Unc. | Mean Opt. Depth |
---|---|---|---|---|---|---|---
|  | (K) | $(L/L_{\odot})$ |  |  | (Blue) |
Sk -69 43 | B0.5 Ia | 22,845 | 5.50 | 1.61 | ${}^{+\,2.13}_{-0.58}$ | (Al iii) 1.59 |
Sk -68 140 | B0.7 Ib-Iab Nwk | 25,100 | 5.64 | 1.38 | ${}^{+\,0.22}_{-0.17}$ | 2.79 |
Sk -66 35 | BC1 Ia | 22,000 | 5.73 | 1.10 |  $\pm$0.18 | 2.34 |
Sk -68 129 | B1 I | 22,000 | 5.25 | 1.58 | ${}^{+\,0.66}_{-0.36}$ | 2.08 |
Sk -67 14 | B1.5 Ia | 22,890 | 5.74 | 1.65 | ${}^{+\,1.20}_{-0.49}$ | (Al iii)1.74 |
Sk -68 26 | BC2 Ia | 18,160 | 5.71 | 1.01 |  $\pm$0.18 | 3.73 |
Sk -67 78 | B3 Ia | 16,150 | 5.50 | 2.47 |  $\pm$0.49 | 2.53 |
Sk -70 50 | B3 Ia | 15,000 | 5.54 | 0.96 |  $\pm$0.27 | 2.11 |
Sk -69 140 | B4 I | 15,000 | 4.73 | 1.56 | ${}^{+\,0.37}_{-0.25}$ | 2.63 |
Sk -70 16 | B4 I | 15,000 | 4.62 | 1.62 |  $\pm$0.42 | 2.61 |
Sk -68 8 | B5 Ia+ | 14,200 | 5.54 | 2.54 |  $\pm$0.31 | 2.91 |
NGC 2004 ELS 3 | B5 Ia | 14,450 | 5.10 | 2.11 |  $\pm$0.43 | 2.46 |
AzV 215 | B0 (Ib)/BN0 Ia | 27,000 | 5.63 | 1.75 | ${}^{+\,0.37}_{-0.26}$ | 3.99 |
AzV 235 | B0 Iaw | 27,500 | 5.72 | 1.65 | ${}^{+\,0.53}_{-0.32}$ | 1.42 |
AzV 317 | B0 Iw | 27,000 | 5.40 | 1.79 | ${}^{+\,0.67}_{-0.38}$ | 3.86 |
AzV 104 | B0.5 Ia | 27,500 | 5.31 | 0.64 | - | 0.81 |
AzV 488 | B0.5 Iaw | 27,500 | 5.74 | 1.24 | ${}^{+\,0.39}_{-0.24}$ | 3.41 |
AzV 242 | B0.7 Iaw | 25,000 | 5.67 | 1.59 |  $\pm$0.24 | 2.00 |
AzV 264 | B1 Ia | 22,500 | 5.44 | 2.04 |  $\pm$0.32 | 2.14 |
AzV 266 | B1 I | 22,000 | 5.09 | 1.69 |  $\pm$0.24 | 1.36 |
AzV 96 | B1 I/B1.5 Ia | 20,000 | 5.39 | 1.64 |  $\pm$0.24 | 1.47 |
AzV 78 | B1.5 Ia+ | 21,500 | 5.92 | 1.98 |  $\pm$0.39 | (Al iii) 3.03 |
Sk 191 | B1.5 Ia | 22,500 | 5.77 | 2.14 |  $\pm$0.49 | (Al iii) 2.77 |
AzV 210 | B1.5 Ia | 20,500 | 5.41 | 1.37 |  $\pm$0.17 | 3.14 |
AzV 18 | B2 Ia | 19,000 | 5.44 | 1.32 |  $\pm$0.22 | 3.91 |
NGC 330 ELS 4 | B2.5 Ib | 17,000 | 4.77 | 1.60 |  $\pm$0.30 | 2.31 |
AzV 187 | B3 Ia | 14,600 | 4.91 | 1.26 |  $\pm$0.22 | 2.35 |
AzV 234 | B3 Iab | 15,700 | 4.91 | 1.09 |  $\pm$0.24 | 1.59 |
NGC 330 ELS 2 | B3 Ib | 14,590 | 4.73 | 1.80 |  $\pm$0.38 | 1.83 |
AzV 324 | B4 Iab | 14,600 | 4.89 | 2.19 |  $\pm$0.51 | 1.73 |
AzV 22 | B5 Ia | 14,500 | 5.04 | 1.28 |  $\pm$0.21 | 1.02 |
AzV 445 | B5 (Iab) | 14,500 | 5.00 | 1.39 |  $\pm$0.23 | 2.76 |
IC 1613 BUG2007 B3 | B0 Ia | 24,500 | 5.53 | 1.59 |  $\pm$0.33 | 1.08 |
IC 1613 BUG2007 A10 | B1 Ia | 25,000 | 5.71 | 1.74 |  $\pm$0.28 | 3.68 |
NGC 3109 EBU07 | B0-1 Ia | 25,000 | 5.60 | 1.36 | ${}^{+\,0.96}_{-0.40}$ | 1.39 |
IC 1613 BUG2007 B4 | B1.5 Ia | 22,500 | 5.20 | 1.71 |  $\pm$0.46 | 0.98 |

###  4.2 General observations

For those target stars where separate fitting of individual doublet elements
is possible, we observe that in almost all cases there is a broad visual
correlation between the shapes of the radial optical depth plots for the best
fit model profile for each element. This provides a strong indication that the
fitting process effectively identifies physical characteristics of the stellar
winds of these stars.

The following sections discuss features of interest discernible for individual
stars as well as highlighting some parts of individual stellar spectra which
may not be usable for the techniques presented here due to the presence of
other features. The SEI-derived model fits, observed spectra and plots of
radial optical depths of the model fits are set out in the Appendices, as
referenced in the individual descriptions. The plots of radial optical depths
for stars requiring a single SEI model fit across both doublet elements are
plots for the blue element of the relevant doublet in each case. For stars
where separate fitting of each doublet element is possible, separate spectra
and plots for each doublet element are shown.

References to the Si iv doublet refer to the 1393.76, 1402.77Å feature,
references to the Al iii doublet refer to the 1854.72, 1862.79Å feature and
references to the C iv doublet refer to the 1548.20, 1550.77Å feature.

###  4.3 Narrow absorption components

Narrow absorption components (NACs) at or near the blue edge of wind-formed
absorption profiles in Galactic OB stellar spectra have been shown by Prinja &
Howarth (1986) and Howarth & Prinja (1989) to be ubiquitous features of those
spectra. To date, no comprehensive investigation of the presence or absence of
these features in low-metallicity OB stellar spectra has been undertaken. The
spectra presented in the Appendices to this work do however provide evidence
(as has been noted by Hawcroft et al., 2023) that NACs are indeed present, and
able to be detected with current instruments and techniques, in the spectra of
many early B supergiant stars at LMC, SMC and lower metallicities.

The spectra presented here indicate that a wider-ranging and systematic
investigation of the prevalence of NACs at lower metallicities is, therefore,
a key next step in the investigation of the formation mechanisms of NACs and
of stellar wind characteristics close to the terminal velocity of the wind. As
has already been illustrated by Prinja & Massa (2013), the stellar wind within
NACs becomes “smooth” such that porosity effects are minimised. As a
consequence, measurements of mass-loss rates by reference to optical depths
and optical depth ratios in NACs are likely to be key methods to constrain to
accurate quantification of mass-loss from hot, massive stars. The persistence
of these features at lower metallicity points the way to these methods also
being applicable to stars in such environments. The discussions of individual
stellar spectra in Subsections 4.5, 4.6 and 4.7 highlight those spectra which
demonstrate the presence of NACs. Likely and possible NAC features are
identified in each relevant plot. In each case, the NAC location marked in
each portion of a doublet feature in a particular spectrum is at the same
distance (velocity) from the rest wavelength of the relevant doublet element
in that spectrum.

###  4.4 Temporal variation in a low-metallicity B supergiant

We highlight in the following one star in our sample (SMC star AzV 96,
spectral type B1 I/B1.5 Ia) which demonstrates stellar wind variability
observable within individual spectra which make up the ULLYSES high level
science product spectrum. SEI model fits to the ULLYSES spectrum for AzV 96
produced a considerable lack of consistency between the optical depth profiles
for each of the elements of the Si iv doublet. The ULLYSES spectrum is
composed of an average of individual observations, one of which was obtained
several days following the two other, contiguous, observations. Inspection of
the individual spectra discloses variation in the shape of the observed P
Cygni profile of both elements of the Si iv resonance line doublet over that
time scale. More particularly, a broad discrete absorption component (DAC)
feature can be seen to migrate bluewards in the absorption trough of both
elements of the doublet. This is consistent with other observations of
Galactic stars (Kaper et al., 1996; Massa & Prinja, 2015) and of SMC O type
stars (Rickard et al., 2022) and represents the first observed evidence for
outward migration of a DAC in the atmosphere of a low-metallicity B
supergiant. The spectra may be compared by considering Figs. 30 and 31 in
Appendix B.

###  4.5 Notes on individual stellar spectra: LMC

####  4.5.1 Sk -69 43

(Fig. 10). The Al iii profile is used due to saturation of the Si iv profile.
As expected for a B0.5 star, the Al iii profile is relatively weak. There is
some evidence for wind structure from the derived optical depth ratio, however
the uncertainties on this result are large, as a consequence of the very low
optical depth values, particularly for the red element of the doublet.

####  4.5.2 Sk -68 140

(Fig. 11). The Si iv doublet profile displays very prominent deep absorption
features close to the terminal wind velocity in each element, as well as
strong emission peaks. The absorption features are wider than a NAC, but may
show a broader DAC which has migrated close to the terminal wind velocity. A
similar morphology is seen in the Si iv profile of AzV 215 and was noted by
Evans et al. (2004a) in relation to that star, as discussed in sub-section
4.6.1 below. Time-series analysis would be helpful in determining whether this
is the case. The single profile fitting technique used due to the high
terminal wind velocity does suggest the presence of significant structure in
the stellar wind. The SEI fitting method does however have difficulty
reproducing the very strong emission profile observed in the spectrum.

####  4.5.3 Sk -66 35

(Fig. 12). The relatively low terminal wind velocity permits separate fitting
of each element of the Si iv doublet. There is clear evidence for extensive
structure throughout almost the entire velocity range of the stellar wind and
the mean optical depth ratio producing the best fit model profiles approaches
unity (1.10). The absorption profiles are very deep at mid-range normalised
wind velocities but rapidly become optically thin at higher velocities. The
fitting technique applied in this paper had difficulty in reproducing this
very deep central absorption feature, even when utilising unusually high
values for the acceleration parameter ($\beta$ = 2.56). Again, time-series
analysis of this star would be helpful in determining whether the deep
absorption feature is seen to migrate blueward in a similar fashion to DACs in
the winds of other stars.

####  4.5.4 Sk -68 129

(Fig. 13). A prominent NAC feature is visible in each element of the Si iv
profile, giving rise to a characteristic increased optical depth around a wind
velocity of $w$ = 0.9, with significant reduction in optical depth at slightly
lower velocities.

####  4.5.5 Sk -67 14

(Fig. 14). The Al iii profile shows deep absorption in the higher velocity
portion of the stellar wind, although this is partly obscured in the red
element of the doublet by overlap with the blue element emission peak.

####  4.5.6 Sk -68 26

(Fig. 15). The results for wind velocities greater than $w$ = 0.6 are affected
by the blue element of the Si iv profile approaching saturation. The depth of
the profile suggests a particularly optically-thick wind for a star of this
spectral type (BC2 Ia) as the terminal velocity is approached. It is to be
noted that the acceleration parameter providing the best fit is relatively
high for a supergiant ($\beta$ = 1.81), suggesting a low rate of acceleration
which potentially contributes to a concentration of material toward the
terminal velocity. The mean optical depth ratio of 1.01 suggests a high degree
of structure present in the wind or extensive coverage of the stellar disk by
optically-thick material, or both in combination. This result may be
exaggerated by the near-saturation of the blue element of the profile, however
there remains clear evidence for the presence of structure at lower velocities
where the profile is not approaching saturation. Although not presented here,
the Al iii profile is unhelpful for this purpose as it is highly saturated, as
may be expected for a star of spectral type BC2.

####  4.5.7 Sk -67 78

(Fig. 16). The profile indicates significant absorption at higher radial wind
velocities, more evidently in the blue Si iv doublet element. The region
around $w$ = 0.7 is affected by the blue-shifted Milky Way ISM absorption
feature, however optical depth ratios mostly do not show a significant
departure from a smooth wind.

####  4.5.8 Sk -70 50

(Fig. 17). Optical depth ratios indicate the presence of significant structure
in the wind of this star. The optical depth of the wind increases
substantially at velocities greater than $w$ = 0.4 (this deep absorption
trough is a feature of the wind itself, within which the Milky Way ISM
absorption line can just be discerned.

####  4.5.9 Sk -69 140

(Fig. 18). This star exhibits an unusually high terminal wind velocity for its
spectral type (B4). A shallow, but clear, NAC feature is visible in both
elements of the Si iv profile. There is evidence from the inferred oscillator
strength ratios that there is structure within the wind.

####  4.5.10 Sk -70 16

(Fig. 19). An unusual wind profile with deep absorption at lower wind
velocities and the wind becoming very optically thin at velocities above
approximately $w$ = 0.6. There appears however to be a highly prominent
absorption feature approaching the terminal wind velocity which is wider than
a typical NAC. This may be evidence of a DAC feature which has migrated close
to the terminal wind velocity. The plot of individual optical depth ratios
shows considerable scatter.

####  4.5.11 Sk -68 8

(Fig. 20). Optical depth ratios suggest that this B5 hypergiant exhibits a
generally smooth wind with a low terminal wind velocity. The wind rapidly
becomes optically thin however, meaning that meaningful data are not available
for higher normalised velocities.

####  4.5.12 NGC 2004 ELS 3

(Fig. 21). The mean optical depth ratio suggests a smooth wind overall. The
plot of individual optical depth ratios indicate however that there may be
variability in the presence of structure at different velocities. There is
some evidence of a shallow NAC feature.

###  4.6 Notes on individual stellar spectra: SMC

####  4.6.1 AzV 215

(Fig. 22). This star’s spectrum exhibits the highest terminal wind velocity in
the present sample. Very deep absorption at normalised velocities above $w$ =
0.6 and a prominent absorption feature, perhaps constituting a DAC approaching
the terminal wind velocity and in the process of becoming a NAC, is observed
in each element of the Si iv feature. This appears however to be a persistent,
or recurring, feature since it is observed also in the STIS spectrum
illustrated in Evans et al. (2004a) and Evans et al. (2004c), who also note
the unusual “shelf-like” appearance that this gives to the absorption profile.
The wind appears to be relatively smooth, with a mean inferred optical depth
ratio of 1.75 indicated by the model fitting process. A very prominent ISM
feature is visible at the lower velocity end of the Si iv absorption profile.

####  4.6.2 AzV 235

(Fig. 23). A relatively optically thin wind with only a short section of the
Si iv absorption profile dropping below 0.5 of the normalised continuum flux.
There is however prominent emission in that profile. A NAC feature is also
apparent, the red element of which appears as an absorption feature within the
slope of the blue element emission feature. There is some evidence of wind
structure from the inferred mean optical depth ratio of 1.65.

####  4.6.3 AzV 317

(Fig. 24). The shape of the absorption trough in the Si iv profile for this
star is unusual. It shows a continuously increasing depth at higher radial
wind velocities, but commences from a very shallow absorption at low
velocities. The increase in absorption is in an almost linear relation with
radial wind velocity. The red element of this doublet shows what appears to be
a prominent NAC feature, which however cannot be confirmed due to saturation
of the blue element profile very close to the terminal wind velocity. The
inferred mean optical depth ratio from a combined SEI fit to the whole Si iv
feature suggests a relatively smooth, and moderately optically-thick, wind.

####  4.6.4 AzV 104

(Fig. 25). The wind profile has an unusual appearance and is not very well-
developed. Using published results for the terminal wind velocity suggests
that the wind becomes extremely optically thin at higher velocities. the
unusual nature of the spectrum of this star was first discussed by Evans et
al. (2004a) who noted that in some respects, the star appears more akin to
spectral type B0.5 Ib. It is noted that recent literature suggests widely-
varying measures for terminal velocity depending upon how it is measured
(Hawcroft et al., 2023). The rapid onset of a very optically thin regime
greatly limits the data that may be obtained using the techniques applied
here. This star is therefore excluded from the plots discussed in Section 5.
The spectrum is however presented here as an unusual example of an early B
supergiant with an uncharacteristically optically-thin wind. It is possible
that the characteristics observed in this spectrum arise from the line of
sight for this observation only containing an unstructured part of the stellar
wind (see the discussion in Rickard et al., 2022).

####  4.6.5 AzV 488

(Fig. 26). The Si iv profile for this star shows deep, although not saturated,
absorption as well as very strong emission peaks. A shallow NAC feature is
apparent. The fitting process produces a large acceleration parameter ($\beta$
= 1.99) in order to fit the emission peaks, likely indicating a slowly
accelerating, optically-thick wind. A considerable degree of structure appears
to be present.

####  4.6.6 AzV 242

(Fig. 27). There appears to be evidence for structure in a wide range of wind
velocities, although there is considerable scatter in the plot of individual
optical depth ratios. A significant NAC feature is present and the Milky Way
and SMC ISM absorption lines are very prominent.

####  4.6.7 AzV 264

(Fig. 28). Optical depth ratios suggest the presence of wind structure
throughout the velocity range. Only one ISM absorption feature (for the Milky
Way) is prominent.

####  4.6.8 AzV 266

(Fig. 29). Optical depth ratios suggest a mostly uniform or smooth wind, with
some evidence for structure present in the higher velocity region. There is a
very deep and significant NAC feature visible in both Si iv doublet elements.

####  4.6.9 AzV 96

(Figs. 30 and 31). Initial attempts to produce model fits for the Si iv
profiles for this star using the ULLYSES DR5 data gave very different optical
depth profiles for each of the blue and red elements of the doublet. It is to
be noted that the ULLYSES spectrum comprises an average of 3 separate
observations, two of which are contiguous and the third of which was obtained
at a remove of several days. Examination of the individual 1D spectra
available in the MAST archive shows that there are significant variations in
the shape of the absorption trough of the Si iv doublet elements between the
first two spectra on the one hand and the third spectrum on the other,
indicating blueward migration of a DAC feature. This in itself provides strong
evidence of temporal variation in the conditions within the stellar wind of
this star, consistent with the earlier results demonstrated by Prinja et al.
(2002). This variation in the central part of the absorption profile appears
to produce contradictory results in the fitting process when the different
spectra are averaged.

By model fitting against an average of only the two contiguous spectra, and
separate fitting for the third spectrum, the results illustrated here were
obtained. In addition to the foregoing issue, the effective temperature of
this star contained in earlier literature appears to be too high. The lower
temperature used here, which accords with results obtained (Bestenlehner,
priv. comm.) using the procedure introduced by Bestenlehner et al. (2023),
appears to be necessary to produce a reasonable fit with a feasible $\beta$
value. There is a prominent NAC feature visible in both elements the Si iv
profile, particularly in the later spectrum, the location of which also
suggests a slightly lower terminal wind velocity than previously published
results.

####  4.6.10 AzV 78

(Fig. 32). The Al iii profile suggests a mostly uniform wind, although this
feature becomes optically thin at higher velocities. The Si iv absorption for
this hypergiant star is saturated.

####  4.6.11 Sk 191

(Fig. 33). The Al iii profile suggests a mostly uniform or smooth wind, except
possibly in the lower velocity range. The Si iv absorption feature for this
star is saturated so cannot be used for measuring fitted optical depth ratios.
However, both the Si iv and C iv (also saturated) profiles indicate
$\varv_{\infty}=$ 460 km s-1, using the $\varv_{\rm{black}}$ measurement
technique described in Prinja et al. (1990) and Hawcroft et al. (2023). There
is a possible NAC feature present close to the terminal wind velocity in both
elements of the Al iii profile.

####  4.6.12 AzV 210

(Fig. 34). The modelled optical depth ratios suggest a highly-structured wind
across almost all of the wind velocity range. A shallow NAC is discernible in
the Si iv profile.

####  4.6.13 AzV 18

(Fig. 35). Optical depth ratios suggest a highly-structured wind throughout
the velocity range. The mean optical depth ratio may however be affected by
the blue element of the Si iv line profile beginning to saturate at higher
wind velocities.

####  4.6.14 NGC 330 ELS 4

(Fig. 36). A generally smooth wind is indicated by the inferred optical depth
ratios across most of the wind velocity range. A prominent ISM line affects
the model fit at mid-lower velocities.

####  4.6.15 AzV 187

(Fig. 37). Evidence for significant wind structure is apparent in mid-range
velocities with limited usable data for this star. A prominent NAC feature is
visible in the Si iv profile.

####  4.6.16 AzV 234

(Fig. 38). A good fit for the observed spectrum is very difficult to obtain,
due principally to the very low terminal wind velocity meaning that a large
portion of the absorption troughs in the Si ivdoublet profile is significantly
affected by interstellar absorption lines. The limited useful data obtainable
are however consistent with the presence of optically-thick structure in the
stellar wind.

####  4.6.17 NGC 330 ELS 2

(Fig. 39). Limited data suggest a mostly smooth wind with some limited
evidence of structure in the lower velocity region. What may be a prominent
NAC feature is visible in the blue element of the Si iv profile, although the
apparent optically thin regime pertaining at higher velocities makes this
difficult to confirm in the red element.

####  4.6.18 AzV 324

(Fig. 40). The very low terminal wind velocity for this star results in
considerable intrusion upon the middle part of the Si iv absorption trough by
the blue shifted Milky Way ISM line. The limited usable data do however
indicate the presence of significant structure in the stellar wind. The
limited data mean that the calculated mean inferred optical depth ratio is
greater than the lower velocity part of the stellar wind would suggest.

####  4.6.19 AzV 22

(Fig. 41). The Si iv doublet profile suggests a wind that becomes optically
thin at velocities well below the terminal velocity. The absorption profile is
also significantly affected by the presence of deep interstellar absorption
lines at a significant fraction of the relatively low terminal wind velocity.
The limited data are however consistent with the presence of optically-thick
structure.

####  4.6.20 AzV 445

(Fig. 42). As with AzV 324, the amount of usable data is limited by the
presence of a very significant Milky Way ISM feature in the mid- to higher-
velocity range. Optical depth ratios at lower velocities suggest the presence
of optically-thick structure.

###  4.7 Notes on individual stellar spectra: low Z

####  4.7.1 NGC 3109 EBU07

(Fig. 43). SEI fitting has enabled a reliable estimate for stellar wind
velocity to be obtained. There is a significant broad (DAC) type absorption
feature visible at higher velocities in the Si iv doublet profile. The C iv
doublet profile shows an apparent NAC feature close to the measured terminal
wind velocity and this appears also to be present in the Si iv shown here. The
derived optical depth ratio is consistent with the presence of significant
degree of optically-thick structure in the stellar wind.

####  4.7.2 IC 1613 BUG2007 B3

(Fig. 44). The wind absorption profile is very shallow, as may be expected at
lower metallicity, and there is very little emission evident in the P Cygni
profile of the Si iv resonance line doublet. The inferred optical depth ratios
do however suggest the presence of a structured wind.

####  4.7.3 IC 1613 BUG2007 A10

(Fig. 45). The stellar wind appears to be relatively small based on the
inferred optical depth ratios. The Si iv wind-formed profiles are however
strong, with the wind exhibiting relatively high optical depth in the middle
of its velocity range. The wind becomes optically tin towards the terminal
velocity.

####  4.7.4 IC 1613 BUG2007 B4

(Fig. 46). As noted by Garcia et al. (2014), the wind is relatively weak, as
expected at this low metallicity. The inferred optical depth ratios suggest a
relatively smooth wind. There is what may be a relatively shallow NAC
approaching the terminal wind velocity of both components of the Si iv
doublet.

Figure 5: Plot of mean optical depth ratios against effective temperature for
all target stars. LMC stars are plotted in blue, SMC stars in orange and “low
Z” stars in brown. In each case, the filled circles show stars for which a
separate fitting of each element of the Si iv or Al iii doublet has been
carried out. The open circles show stars for which a single SEI model fitting
has been made to the entire doublet feature, with the inferred oscillator
strength ratio being allowed to float, as described in the text. Figure 6:
Plot of mean optical depth ratios against luminosity for all target stars.
Plot colours and symbols are as described for Figure 5 above. Figure 7: Plot
of mean optical depth ratios against terminal wind velocity for all target
stars. Plot colours and symbols are as described for Figure 5 above. Figure 8:
Plot of mean optical depth ratios against the mean optical depth of the blue
doublet element for all target stars. Plot colours and symbols are as
described for Figure 5 above. Figure 9: Plot of mean optical depth ratios
against the rotational velocity of selected target stars. Plot colours and
symbols are as described for Figure 5 above.

##  5 Discussion

The data reductions undertaken as part of this work, in particular, the SEI-
fitting processes carried out have produced some new results for stellar
radial velocities and the terminal wind velocities of some early B supergiants
in the LMC, SMC and in low-metallicity local group dwarf galaxies IC 1613 and
NGC 3109. These new results are highlighted in Table 1. In addition, some
amendments to existing literature values of terminal wind velocities and of
stellar radial velocities are presented.

We provide plots of the derived mean radial optical depth ratios against a
range of physical stellar parameters in order to determine whether there is
evidence of any clear correlation between those ratios and any of those
physical parameters.

Figure 5 plots the derived mean optical depth ratios for all target stars
against stellar effective temperature. Consistently with the findings for
Galactic stars discussed in Prinja & Massa (2010), we do not observe evidence
of correlation between the mean optical depth ratio and stellar effective
temperature. We do however observe an apparent increase in the scatter of
optical depth ratios at effective temperatures around 22,000 K, just above
that at which a “bi-stability jump” in the ratio of terminal wind velocity to
escape velocity is observed; from $\varv_{\infty}\simeq$ 2.6$\varv_{\rm{esc}}$
for supergiants of spectral type earlier than B1 to ${\varv_{\infty}\simeq}$
1.3${\varv_{\rm{esc}}}$ for later-type supergiants (Lamers et al., 1995;
Markova & Puls, 2008).

Whether this potential increased scatter in the prevalence of optically-thick
wind structure affects the predictions of Vink et al. (1999, 2000, 2001) that
mass loss rates increase across the bi-stability jump requires further
investigation, particularly in the light of more recent studies which have
suggested that such a jump in mass-loss rates may not be evident (see, for
example, Krtička et al., 2021; Björklund et al., 2023). A similar increase in
scatter of optical depth ratios can also be observed at effective temperatures
around 14,500 K. It may therefore be the case that both observations are
primarily the result of uneven sampling as a result of the limited data
available at present.

Figures 6, 7 and 8 respectively plot mean optical depth ratios for all target
stars against stellar luminosity, measured terminal wind velocity and the
derived mean optical depth of the blue component of the relevant doublet
feature for each star. There does not appear to be clear evidence of
correlation of the extent of the observed wind structure with any of these
other features. We do observe that, at least at higher luminosities, lower
metallicity stars generally exhibit, through the inferred mean optical depth
ratios plotted, less evidence of optically-thick structure in their winds.
This result is consistent with the theoretical predictions of Driessen et al.
(2022). There is little evidence shown in our data at the lower end of the
luminosity scale, however this may be a consequence of the limited data
available.

With limited exceptions, we do observe in Figure 8 that, for stellar winds of
a given mean optical depth, the winds of stars in lower metallicity
environments (i.e. SMC and “low Z” stars) appear to be smoother and disclose
less evidence of structure. There is some evidence for a similar metallicity-
based differential when we examine stars of a given luminosity (see Figure 6),
i.e. that, at any given luminosity, the stars in a lower metallicity
environment appear to be more likely to exhibit smoother stellar winds. If
clumping in the stellar winds is indeed driven by the presence of sub-
photospheric iron convection zones in the outer envelopes of hot massive
stars, as proposed by Cantiello et al. (2009) and Cantiello & Braithwaite
(2011), some positive correlation between metallicity and the presence of wind
structure is to be expected. The results shown here are consistent with that.

Figure 9 plots the derived mean optical depth ratios for selected target stars
against rotational velocity of those stars for which such data are available
in earlier literature. If observed large-scale structure in stellar winds
arises from the existence, and persistence, of so-called co-rotating
interaction regions (“CIRs”, see Owocki et al., 1995; Cranmer & Owocki, 1996;
Fullerton et al., 1997; de Jong et al., 2001; Prinja et al., 2002, 2007;
David-Uraz et al., 2017), we may expect to see some relationship between the
extent of observed wind structure and stellar rotational velocity. The
recurrence and migration timescales of large-scale structure, evidenced by
broad DACs observable in the troughs of UV resonance line P Cygni profiles in
stellar spectra, have been demonstrated to be related to the rotation
timescales of such stars (Prinja et al., 2002). CIRs are expected to form
where differing conditions in regions of the stellar photosphere (potentially
from features such as star spots or magnetic disturbances generally) result in
the entrainment of material into the stellar wind at differing velocities and
rates of acceleration (Cranmer & Owocki, 1996; Fullerton et al., 1997). The
higher velocity stream(s) will overtake those of lower velocity, producing an
archimedean spiral structure in the wind as the star’s rotation imparts
transverse angular momentum of the stellar wind material. The ability to
detect evidence of this will, of course, depend to a significant degree upon
the viewing angle at which the system is observed.

It is expected that rapidly rotating stars exhibit a greater degree of
detectable coherent macro-scale wind structure compared with slower rotators
(Cranmer & Owocki, 1996). We have limited the data set used in Figure 9 to
those stars for which there are reasonably precise $\varv$ sin $i$
determinations in existing literature. We do not observe clear evidence of the
postulated correlation from these limited data. It should be noted that the
available sample size used here may be too small to permit $\varv$ sin $i$ to
be used as an effective proxy for true rotational velocity (c.f. the
discussion of this issue in Penny & Gies, 2009), rather placing only a lower
bound on that measure. The lack of correlation shown here in a relatively
small available sample may be dominated by differing viewing angles to
rotational axes of the stars observed (Rickard et al., 2022).

##  6 Conclusions

###  6.1 Temporal variability

The distinct variations observed in individual spectral observations of SMC
star AzV 96 shows that, in this example at least, there is clear evidence of
temporal variation in the structure and optical depth of the stellar wind,
observable over a period of several days. Further analysis of individual 1D UV
spectra underlying the ULLYSES data set is likely, therefore, to be of value
in indicating suitable target stars for follow-up time series observations at
low metallicity. AzV 96 is now established as a suitable candidate star for
this purpose.

###  6.2 Wind structure

Using the SEI method applied to unsaturated P Cygni resonance line profiles in
early B supergiant stellar spectra at sub-solar metallicity, we have
demonstrated that single-epoch UV spectra may be used to find the presence of
structure within the stellar winds of those stars. Most stars in our target
sample show clear evidence of a significant degree of wind structure,
consistent with earlier studies at solar metallicity. Our findings demonstrate
that even at low metallicities, wind variability needs to be taken in to
account when determining mass-loss rates of massive stars.

The results shown here confirm the conclusion reached by Prinja & Massa (2010)
for Galactic early B type supergiants that there is widespread evidence for
macro-scale, optically-thick, clumping in the winds of B supergiants. We have
extended this result to the lower metallicity environments of the Large and
Small Magellanic Clouds and, to a limited degree, to very low metallicity
environments elsewhere in the Local Group. We confirm, in the context of these
other environments, the earlier conclusion of Prinja & Massa (2010) that there
does not appear to be evidence for a direct correlation between stellar
effective temperature and the degree of such structure evident in those
stellar winds. This conclusion also therefore suggests that, in these
environments, there are likely to be time-variable factors at work, with
transient events contributing to the scatter in the results. This conclusion
further emphasises the desirability of obtaining time series UV observations
particularly of selected SMC and lower metallicity stars from the ULLYSES
sample.

###  6.3 NACs at low metallicity

The observable presence of NACs in wind profiles within the UV spectra of many
early B supergiant stars at low metallicity, as illustrated in a significant
proportion of the appended spectra, shows that a comprehensive investigation
of this phenomenon at low metallicity is warranted. As shown by Prinja & Massa
(2013), this is likely to play an important role in better constraining
quantification of mass-loss rates from hot, massive stars at lower
metallicity, leading to more accurate models of stellar and galactic evolution
in the early Universe.

###  6.4 Relationship with other stellar parameters

We similarly find no evidence for correlation between the mean optical depth
ratios inferred by the fitting process adopted here and each of stellar
luminosity, terminal wind velocity and the mean optical depth of the wind
itself. We do however find limited evidence suggesting a stratification of the
prevalence of wind structure by reference to metallicity for any given stellar
luminosity and for any given mean optical depth of the stellar wind, with some
indication that a smoother wind is more likely to be found in lower
metallicity environments based on these measures.

Finally, we have sought to test the hypothesis that faster rotating stars are
likely to exhibit more coherent structures in their stellar winds when
compared with slower rotators.

These results also suggest target stars, such as AzV 96, that are likely to
reveal the most useful results from further detailed investigation by way of
obtaining high-cadence time series UV spectra. This information may be
contained within the underlying spectra forming the ULLYSES high level science
products, from which further evidence of suitable candidates for time-series
observation may be obtained. Such observations will be of key importance to
establishing tight constraints upon variations in rates of mass-loss from
stars in low-metallicity environments exhibiting significant structure in
their stellar winds, thereby enabling more meaningful determination of the
effects of metallicity upon variation in such mass-loss rates.

In conclusion, there is some evidence from the data presented here of the
following relationships:

  1. 1.

Evidence of wind structure is generally clearer in cases where the mean
optical depth of the blue doublet element is greater - this is perhaps to be
expected as variations in optical depths are more clearly identifiable at
greater average optical depths provided the medium is not reaching an optical
depth that results in saturation;

  2. 2.

With some exceptions, evidence of the existence of macro-scale structure
appears to be clearer, and to suggest a greater degree of clumping, at higher
metallicities for a given effective temperature. At lower effective
temperatures however the scatter in the LMC data is larger than at higher
effective temperatures, so this conclusion is made only tentatively.

We have, in addition, demonstrated evidence for the following:

  1. 1.

Temporal variation observable of a period of several days within the stellar
winds of stars at low- (i.e. SMC-) metallicity;

  2. 2.

That separate fitting of each element of the P Cygni profiles of resonance-
line doublet features is an appropriate and reliable tool for the discernment
of structure within stellar winds of massive stars and, in particular, that
treating these elements as radiatively decoupled in circumstances where the
terminal velocity of the stellar wind is sufficiently low is a suitable
technique for this purpose;

  3. 3.

The presence of NAC features, important as a diagnostic of mass-loss rates, in
many early B supergiant stellar winds at low metallicity.

## Acknowledgements

Based on observations obtained with the NASA/ESA Hubble Space Telescope,
retrieved from the Mikulski Archive for Space Telescopes (MAST) at the Space
Telescope Science Institute (STScI). STScI is operated by the Association of
Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

The STARLINK software (Currie et al., 2014) is currently supported by the East
Asian Observatory.

AACS and MBP acknowledge support by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) in the form of an Emmy Noether Research Group –
Project-ID 445674056 (SA4064/1-1, PI Sander). AACS further acknowledges
support from the Federal Ministry of Education and Research (BMBF) and the
Baden-Württemberg Ministry of Science as part of the Excellence Strategy of
the German Federal and State Governments.

DP acknowledges financial support by the Deutsches Zentrum für Luft und
Raumfahrt (DLR) grant FKZ 50OR2005.

The authors also wish to thank the anonymous referee for the review of, and
the very helpful comments provided on, the draft of this paper.

## Data Availability

The spectral data used in this work may be obtained from the Mikulski Archive
for Space Telescopes (MAST) at the Space Telescope Science Institute (STScI),
accessible at: https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html
in particular, the ULLYSES data contained therein.

DIPSO spectral analysis software is available as part of the STARLINK package,
accessible at: https://starlink.eao.hawaii.edu/starlink.

TLUSTY model grids and associated materials are accessible at:
http://tlusty.oca.eu/, with thanks to Ivan Hubeny and Thierry Lanz.

The SEI-fitting code and results therefrom will be made available upon
reasonable request to the corresponding author of this work.

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##  Appendix A SEI model line fits for LMC B supergiants

As described in the introduction to Section 3, the lower panels of each plot
in this and the following Appendices respectively show the radial optical
depths of the stellar wind at differing normalised velocities which produce
the best-fit model line (lower left panels) and the photospheric contribution
to the spectrum as derived from the model photospheric spectrum selected based
upon the physical parameters of the star (lower right panels). A detailed
explanation of each of the parameters derived and displayed with the plots is
set out in Subsection 2.2. Rest wavelengths of the blue and red elements of
the doublet are highlighted in those colours. Note also that for those plots
prepared using a decoupled treatment of each doublet element the model fit for
each element “ignores” the presence of the other element. This can be seen
most clearly in the blue element model fits, however it does not affect the
model fit within the relevant doublet element.

Figure 10: SEI-derived model combined fit for both components of the Al iii
doublet feature in the UV spectrum of LMC star Sk -69 43 (spectral type B0.5
Ia). The rest wavelengths for each element of the doublet are highlighted in
the relevant colours and a magenta colour is used to show a “combined” model
fit. Figure 11: SEI-derived model combined fit for both components of the Si
iv doublet feature in the UV spectrum of LMC star Sk -68 140 (spectral type
B0.7 Ib-Iab Nwk).

(a)  (b)

Figure 12: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -66 35 (spectral type
BC1 Ia). Figure 13: SEI-derived model combined fit for both components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -68 129 (spectral type
B1 I). The location of likely narrow absorption components (NAC) in each
doublet element are indicated, each at the same distance from the relevant
rest wavelength. Figure 14: SEI-derived model combined fit for both components
of the Al iii doublet feature in the UV spectrum of LMC star Sk -67 14
(spectral type B1.5 Ia).

(a)  (b)

Figure 15: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -68 26 (spectral type
BC2 Ia).

(a)  (b)

Figure 16: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -67 78 (spectral type
B3 Ia).

(a)  (b)

Figure 17: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -70 50 (spectral type
B3 Ia). Figure 18: SEI-derived model combined fit for both components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -69 140 (spectral type
B4 I). The location of likely narrow absorption components (NAC) in each
doublet element are indicated, each at the same distance from the relevant
rest wavelength.

(a)  (b)

Figure 19: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -70 16 (spectral type
B4 I).

(a)  (b)

Figure 20: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star Sk -68 8 (spectral type
B5 Ia+).

(a)  (b)

Figure 21: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of LMC star NGC 2004 ELS 3 (spectral
type B5 Ia).

##  Appendix B SEI model line fits for SMC B supergiants

Figure 22: SEI-derived model combined fit for both components of the Si iv
doublet feature in the UV spectrum of SMC star AzV 215 (spectral type B0
I(b)/BN0 Ia). Figure 23: SEI-derived model combined fit for both components of
the Si iv doublet feature in the UV spectrum of SMC star AzV 235 (spectral
type B0 Iaw). The location of likely narrow absorption components (NAC) in
each doublet element are indicated, each at the same distance from the
relevant rest wavelength. Figure 24: SEI-derived model combined fit for both
components of the Si iv doublet feature in the UV spectrum of SMC star AzV 317
(spectral type B0 Iw). Note that the apparent narrow absorption component
(NAC) visible in the red component profile is, unfortunately, lost due to the
part saturation of the highest velocity part of the blue profile.

(a)  (b)

Figure 25: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 104 (spectral type
B0.5 Ia). The deep absorption feature is the Milky Way ISM line, blue-shifted
by an amount equal to the radial velocity of the star. Note that the wind
appears to become very optically thin at higher velocities. Figure 26: SEI-
derived model combined fit for both components of the Si iv doublet feature in
the UV spectrum of SMC star AzV 488 (spectral type B0.5 Iaw). The location of
likely narrow absorption components (NAC) in each doublet element are
indicated, each at the same distance from the relevant rest wavelength.

(a)  (b)

Figure 27: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 242 (spectral type
B0.7 Iaw). The location of likely narrow absorption components (NAC) in each
doublet element are indicated, each at the same distance from the relevant
rest wavelength.

(a)  (b)

Figure 28: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 264 (spectral type B1
Ia).

(a)  (b)

Figure 29: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 266 (spectral type B1
I). There appears to be a very prominent, and deep, narrow absorption
component (NAC) feature in both elements of this doublet.

(a)  (b)

Figure 30: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 96 (spectral type B1
I/B1.5 Ia). Note that this spectrum is taken from 2 contiguous observation of
this star and excludes a third observation which forms part of the ULLYSES
spectrum, as discussed in the main text. The third spectrum is presented in
Fig. 31 for comparison.

(a)  (b)

Figure 31: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 96 (spectral type B1
I/B1.5 Ia). Note that this spectrum is taken from a separate observation
obtained several days after those shown in Fig. 30. The blueward migration of
an apparent discrete absorption component (DAC) feature may be observed in
both doublet elements. The location of likely narrow absorption components
(NAC) in each doublet element are indicated, each at the same distance from
the relevant rest wavelength. This feature appears in the blue, but not the
red component of the preceding plots for this star.

(a)  (b)

Figure 32: SEI-derived model fits for (a) blue and (b) red components of the
Al iii doublet feature in the UV spectrum of SMC star AzV 78 (spectral type
B1.5 Ia+).

(a)  (b)

Figure 33: SEI-derived model fits for (a) blue and (b) red components of the
Al iii doublet feature in the UV spectrum of SMC star Sk 191 (spectral type
B1.5 Ia). The location of likely narrow absorption components (NAC) in each
doublet element are indicated, each at the same distance from the relevant
rest wavelength.

(a)  (b)

Figure 34: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 210 (spectral type
B1.5 Ia). The location of possible narrow absorption components (NAC) in each
doublet element are indicated, each at the same distance from the relevant
rest wavelength.

(a)  (b)

Figure 35: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 18 (spectral type B2
Ia).

(a)  (b)

Figure 36: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star NGC 330 ELS 4 (spectral
type B2.5 Ib).

(a)  (b)

Figure 37: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 187 (spectral type B3
Ia). The location of likely narrow absorption components (NAC) in each doublet
element are indicated, each at the same distance from the relevant rest
wavelength.

(a)  (b)

Figure 38: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 234 (spectral type B3
Iab).

(a)  (b)

Figure 39: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star NGC 330 ELS 2 (spectral
type B3 Ib). The location of possible narrow absorption components (NAC) in
each doublet element are indicated, each at the same distance from the
relevant rest wavelength.

(a)  (b)

Figure 40: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 324 (spectral type B4
Iab).

(a)  (b)

Figure 41: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 22 (spectral type B5
Ia).

(a)  (b)

Figure 42: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of SMC star AzV 445 (spectral type B5
(Iab)).

##  Appendix C SEI model line fits for “low Z” B supergiants

Figure 43: SEI-derived model combined fit for both components of the Si iv
doublet feature in the UV spectrum of NGC 3109 star NGC 3109 EBU 07 (spectral
type B0-1 Ia). The location of possible narrow absorption components (NAC) in
each doublet element are indicated, each at the same distance from the
relevant rest wavelength.

(a)  (b)

Figure 44: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of IC 1613 star BUG2007 B3 (spectral
type B0 Ia).

(a)  (b)

Figure 45: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of IC 1613 star BUG2007 A10 (spectral
type B1 Ia).

(a)  (b)

Figure 46: SEI-derived model fits for (a) blue and (b) red components of the
Si iv doublet feature in the UV spectrum of IC 1613 star BUG2007 B4 (spectral
type B1.5 Ia). The location of possible narrow absorption components (NAC) in
each doublet element are indicated, each at the same distance from the
relevant rest wavelength.