∎

11institutetext: C. Evans22institutetext: European Space Agency (ESA), ESA
Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, USA
22email<EMAIL_ADDRESS>33institutetext: W. Marcolino 44institutetext:
Universidade Federal do Rio de Janeiro, Observatório do Valongo, Ladeira Pedro
Antônio, 43, CEP 20.080-090, Rio de Janeiro, Brazil 55institutetext: J.-C.
Bouret 66institutetext: Aix-Marseille Univ, CNRS, CNES, LAM, Marseille, France
77institutetext: M. Garcia 88institutetext: Centro de Astrobiología, CSIC-
INTA, Crtra. de Torrejón a Ajalvir km 4, E-28850, Torrejón de Ardoz (Madrid),
Spain

# A near-UV reconnaissance of metal-poor massive stars

Chris Evans  Wagner Marcolino  Jean-Claude Bouret  Miriam Garcia

(Accepted: 8 August 2023)

###### Abstract

We use synthetic model spectra to investigate the potential of near-
ultraviolet (3000-4050 Å) observations of massive O-type stars. We highlight
the He I $\lambda$3188 and He II $\lambda$3203 pair as a potential temperature
diagnostic in this range, supported by estimates of gravity using the high
Balmer series lines. The near-ultraviolet also contains important metallic
lines for determinations of chemical abundances (oxygen in particular) and
estimates of projected rotational velocities for O-type spectra. Using the
model spectra we present performance estimates for observations of
extragalactic massive stars with the Cassegrain U-Band Efficient Spectrograph
(CUBES) now in construction for the Very Large Telescope. The high efficiency
of CUBES will open-up exciting new possibilities in the study of massive stars
in external galaxies. For instance, CUBES will provide new insights into the
physical properties of O-type stars, including oxygen abundances, in metal-
poor irregular galaxies at $\sim$1 Mpc from integrations of just 2-3 hrs.
Moreover, CUBES will bring quantitative spectroscopy of more distant targets
within reach for the first time, such as the O-type star ($V$ $\sim$ 21.5 mag)
in Leo P (at 1.6 Mpc) in only half a night of observations.

###### Keywords:

instrumentation: spectrographs stars: early-type galaxies: Local Group
galaxies: individual: Leo P

##  1 Introduction

Ground-based spectroscopy of OB-type stars has traditionally been obtained in
the $\lambda$3950–4750Å range for spectral classification (e.g. wf00 ) and
estimates of physical parameters (temperatures, gravities), and of the
H$\alpha$ line to investigate their stellar winds (e.g. puls96 ). In contrast,
the shorter wavelengths accessible from the ground, down to the atmospheric
cut-off, have received significantly less attention, partly due to the
challenges of the reduced atmospheric transmission combined with the limited
efficiency of available instrumentation.

Development of the Cassegrain U-Band Efficient Spectrograph (CUBES) instrument
for the Very Large Telescope (VLT) provided us with the motivation to
investigate the potential of observations of massive stars in the near
ultraviolet (UV). The CUBES design offers a potential tenfold gain in end-to-
end efficiency at $\lambda$ $<$ 3400 Å (incl. the telescope and atmosphere)
compared to the existing Ultraviolet and Visible Echelle Spectrograph (UVES).
In brief, the CUBES design provides a spectral resolving power of $R$ $\geq$
20,000 over the 3000-4050 Å range, with provision of a second, lower-
resolution option with $R$ $\sim$ 7,000 zanutta22 .

Here we investigate the potential performance of CUBES for studies of massive
stars. In Sect. 2 we briefly review past studies of massive stars shortwards
of 4000 Å. Motivated by the presence of a relatively strong He II line
($\lambda$3203) and a range of He I lines, in Sect. 3 we use synthetic model
spectra to qualitatively investigate the sensitivity of near-UV lines to the
physical parameters of massive stars. In Sect. 4 we focus on the possibility
of estimating oxygen abundances in massive stars from near-UV observations. In
Sect. 5 we investigate the potential performance of CUBES to study massive
stars in Local Group galaxies and beyond, with concluding remarks given in
Sect. 6.

##  2 Near-ultraviolet spectroscopy of massive stars

An early ground-based study of massive stars at $\lambda$ $<$ 4000 Å used
photographic observations of $\epsilon$ Ori (B0 Ia), which included a detailed
linelist that extended as far bluewards as 3550 Å lamers72 . A first
quantitative investigation of the He I $\lambda$3188 and He II $\lambda$3203
lines employed photographic observations from the 2.2-m telescope on Mauna Kea
of 19 O- and B0.5-type stars m75 . This included equivalent width-measurements
of both lines in the sample of stars, and comparisons with the predictions of
non-LTE model atmospheres for He I $\lambda$3889 and He II $\lambda$3203,
finding generally good agreement for the trend of the He I lines and the
values for the He II line (except for the two hottest stars). Given the strong
response to temperature of both lines, this was a first indication of the
potential of these two lines to be used in tandem as a temperature diagnostic.
The only other example known to us of observations in this region prior to the
use of digital detectors is observations of the He I $\lambda$3188 line in
$\sim$30 early-type stars with the Copernicus satellite dufton .

Digital detectors transformed observational astronomy, but their performance
in the near UV has still been a limiting factor compared to longer
wavelengths, and so there remain relatively few studies of massive stars in
this region. Observations covering 3250-4750 Å of massive O-type and
Wolf–Rayet stars in NGC 3603 with the Faint Object Spectrograph on the Hubble
Space Telescope (HST) revealed some of the key features in the near UV
drissen95 . Shortwards of 4000 Å, the O-type spectra are dominated by the high
Balmer series until the Balmer limit, with a He I line at $\lambda$3820\.
Going to even shorter wavelengths, in the hottest (O3-type) stars in the HST
observations, absorption from O IV $\lambda$$\lambda$3381-85, 3412 and N IV
$\lambda$$\lambda$3479-83-85 are also seen, with the N IV blend displaying a
strong P Cygni profile in the hydrogen-rich WN-type spectra. Examples of some
of the weak He I lines present in the 3800-4000 Å range can also be seen in
slightly higher-resolution ($\sim$2 Å) spectroscopy of four late O-type
supergiants (from wh00 ).

To illustrate some of the spectral lines present in this range, in Fig. 1 we
show the far-blue UVES spectrum of HDE 269896 (taken from ecfh04 ), smoothed
and rebinned to the high-resolution mode of CUBES ($\Delta$$\lambda$ $=$ 0.14
Å, sampled by 2.3 pixels, so 0.06 Å/pixel).

The region below $\sim$3400 Å in the UVES data was of limited use because of
low signal-to-noise (S/N) in the shortest-wavelength echelle orders, but at
longer wavelengths the data give a good example of some of the features
present in massive stars in the CUBES range. In practice, degrading the
spectrum to the $R$ $\sim$ 7,000 of the low-resolution mode has little
qualitative impact on the final spectrum shown in Fig. 1 given the
(astronomical) broadening of the lines.

Classified as ON9.7 Ia$+$ (and with log($L/L_{\odot}$) $\approx$ 6) the UVES
data of HDE 269896 reveal the high Balmer series and a plethora of He I lines,
together with weak emission lines from Si III ($\lambda$3487, 3590, 3807), Si
IV ($\lambda$3762 and 3773) and Al III ($\lambda$3602) produced in its strong
stellar wind (see ecfh04 ). The presence of lines from two ionisation stages
of silicon offers a potential temperature diagnostic in the CUBES region,
albeit very dependent on the adopted stellar parameters (including the wind)
when the lines are in emission.

To illustrate the ability of sophisticated model atmospheres and spectral
synthesis to reproduce this region in O-type spectra (including the higher-
order members of the Balmer series), in red in Fig. 1 we overplot the adopted
cmfgen model (see cmfgen ) for this star from ecfh04 . The adopted parameters
were an effective temperature, $T_{\rm eff}$ $=$ 27,500 K, log$g$ $=$ 2.7, and
a rotational velocity of $v$sin$i$ $=$ 70 km s-1 (see ecfh04  for further
details of the model parameters, which necessitated a detailed treatment of
the wind in this luminous supergiant star).

Figure 1: Black spectrum: UVES spectrum of HDE 269896, an ON9.7 Ia+ star in
the Large Magellanic Cloud. The data have been smoothed and rebinned to match
the CUBES high-resolution mode (with $R$ $\sim$22,000). Identified lines in
the upper panel are: Al III $\lambda$3602; He I $\lambda$$\lambda$3479, 3488,
3499, 3513, 3531, 3554, 3614, 3634; Si III $\lambda$$\lambda$3487, 3590. In
addition to the high Balmer lines (H9-20), identified lines in the lower panel
are: He I $\lambda$$\lambda$3652, 3820; Si III $\lambda$3807; Si IV
$\lambda$$\lambda$3762, 3773. Red spectrum: Adopted cmfgen model from a
combined UV and optical analysis of the UVES data (taken fron ecfh04 , see
text for details).

##  3 Diagnostic lines in the near UV

To investigate the sensitivity of different spectral lines in the near UV to
physical parameters such as temperature and gravity, we used synthetic spectra
from the extensive OSTAR2002 grid111http://tlusty.oca.eu/Tlusty2002/tlusty-
frames-OS02.html in which abundances in the OSTAR2002 grid are scaled relative
to solar values from gs98 . of line-blanketed, non-LTE, plane-parallel,
hydrostatic model atmospheres calculated with the tlusty code tlusty .

###  3.1 LMC metallicity: Temperature trends

The metallicity of the Large Magellanic Cloud (LMC) is approximately half
solar and we have a good understanding of stellar properties from analysis of
data at other wavelengths (e.g. vfts ). We therefore first considered the 0.5
$Z_{\odot}$ models from the OSTAR2002 grid as example sub-solar models with
which to investigate the CUBES domain.

We initially considered models with log$g$ $=$ 4.0, that are typical of the
gravities estimated for dwarfs cssj17 . The tlusty grid has models from
$T_{\rm eff}$ $=$ 27,500 to 55,000 K (in steps of 2,500 K). Using the
effective temperature – spectral type relations derived from analysis of O-
and B-type dwarfs in the LMC cssj17 ; trundle07 , we can map the tlusty models
to approximate spectral types.

The near-UV and visible ranges are shown for eight model spectra in Figs. 3
and 3, respectively. Each model spectrum in the figures has been convolved
with a rotational-broadening function of $v$sin$i$ $=$ 100 km s-1 to mimic
real targets compared to the unbroadened models. Note that $v$sin$i$ of order
100 km s-1 then dominates the broadening of the line profiles, such that
convolving the models to either the high- or low-resolution modes of CUBES has
a limited impact compared to the spectra shown in the figure.

Our motivations to show the visible region of the models is twofold. Firstly,
to investigate if there are near-UV lines in the same models that can then be
used to delineate similar trends as in the temperature (spectral type)
sequence in Fig. 3. Secondly, the models shown illustrate some of the key
classification criteria in O- and early B-type stars (see wf00 ; sota11 ). For
instance, the weakening of the He I lines when moving to hotter temperatures
(earlier spectral types), such that the He I $\lambda$4471 line is very weak
by O3 ($T_{\rm eff}$ $\approx$ 45,000 K), and the weakening of the He II lines
as we move to cooler temperatures (later spectral types), such that the Si III
triplet is stronger than He II $\lambda$4542 by B0. The $T_{\rm eff}$ $=$
40,000 K model is also a useful classification anchor point in terms of the He
I/He II ratios for the $\lambda$4026/$\lambda$4200 and
$\lambda$4388/$\lambda$4542 pairs, corresponding to a classification of
$\sim$O6.

To complement the UVES data of HDE 269896, in Fig. 3 we show the near-UV
($\lambda$ $<$ 3450 Å) spectra for the same models as shown in Fig. 3. The
primary features of interest, in terms of their potential usefulness for
spectral classification and determination of physical parameters are the He I
$\lambda$3188 and He II $\lambda$3203 lines (previously studied by m75 ). The
He I line is absent for the hottest spectra and increases in strength down the
temperature sequence, while the He II changes in the opposite sense. The lines
are closest to being equivalent in the $T_{\rm eff}$ $=$ 32,500 model, which
would be classified as approximately O9.7 V from comparison of its blue-
visible spectrum (Fig. 3) to published spectral standards sota11 , although
the point at which the line intensities are equal is slightly cooler.

This region is also rich with O III absorption lines, as well as pairs of
lines from Si III and Si IV and several O IV lines in the hottest spectra. As
at the longer wavelengths shown in Fig. 1, the presence of two ionisation
stages in this region (from multiple species) offers alternative potential
diagnostics of temperature.

In this first exploration, with relatively modest rotational broadening
($v$sin$i$ $=$ 100 km s-1), we have followed the general classification
approach for digital spectral of considering the central line depths rather
than the line intensities (equivalent widths). Once empirical data is
available for a broad range of massive stars in this spectral range, a more
robust morphological treatment will be required in the future, which e.g.
takes into account effects such as rotational broadening (e.g. markova11 , see
also the discussion by arias16 ).

Figure 2: Synthetic near-UV spectra from the tlusty $Z$ $=$ 0.5$Z_{\odot}$
OSTAR2002 grid, which have been convolved with a rotational-broadening
function of $v$sin$i$ $=$ 100 km s-1. Identified lines are: He I
$\lambda$$\lambda$3188, 3448; He II $\lambda$3203; N III
$\lambda$$\lambda$3354, 3367 (where the former is blended with Ne II 3355); O
II $\lambda$$\lambda$3390; O III $\lambda$$\lambda$3024, 3035, 3047, 3059,
3122, 3133, 3261, 3267, 3299, 3312, 3341, 3351; O IV $\lambda$$\lambda$3063,
3072; 3381, 3385, 3390, 3397, 3404, 3410-12-14; Si III $\lambda$$\lambda$3086,
3093; Si IV $\lambda$$\lambda$3150, 3166; S IV $\lambda$$\lambda$3097, 3118.
Two blends are also identified at $\lambda$3355 (He I and Ne II) and
$\lambda$3377 (O II and Ne II).

Figure 3: Synthetic spectra from the tlusty $Z$ $=$ 0.5$Z_{\odot}$ OSTAR2002
grid for the traditional classification domain. The spectra have been
convolved with a rotational-broadening function of $v$sin$i$ $=$ 100 km s-1.
The identified lines are some of those used for classification of O-type
spectra, namely: He I $\lambda$$\lambda$4026 (blended with He II at the
hottest temperatures), 4388, 4471; He II $\lambda$$\lambda$4200, 4542, 4686;
Si III $\lambda$$\lambda$4552-68-75.

###  3.2 LMC metallicity: Stellar gravities

The $\lambda$3188/$\lambda$3203 line ratio looks potentially interesting in
the context of (approximate) spectral classification and estimates of $T_{\rm
eff}$, but if we were limited to CUBES observations alone of a given target we
must also consider how to constrain its gravity (as well as the possible
impact that it has on the $\lambda$3188/$\lambda$3203 ratio). As demonstrated
in Fig. 1, the CUBES range contains the high Balmer series lines (plus the H8
and H$\epsilon$ lines not shown in the figure). The profile wings of the
Balmer series are generally excellent diagnostics of stellar gravity, and the
high Balmer lines could be used to constrain the gravities of CUBES targets.
With crossed-dispersed instruments such as UVES and X-Shooter, correction of
the echelle blaze function and stitching together the different echelle orders
can be challenging in the 3650-3900 Å region, where the wings of the Balmer
lines can overlap and it is difficult to accurately define a continuum. An
advantage of the CUBES design is the continuous spectrum from each of its two
arms, provided by having only one dispersing element (in each arm) operating
in first order zanutta22 , i.e. no echelle orders to combine in the data
reduction.

To illustrate the diagnostic potential of the Balmer lines in this regard, in
Fig. 4 we show models for $T_{\rm eff}$ $=$ 35,000 K (typical of a late O-type
star) for three gravities: log$g$ $=$ 3.5 (a typical value for an O-type giant
in the LMC, e.g. ora17 ), 4.0 and 4.5, spanning from shortwards of the Balmer
limit up to the H8 line. We recognise the challenges of the blending of the
higher-series lines (particularly for noisy data), but note that the H8 line
is relatively isolated, and the CUBES range (which extends further redwards to
4050 Å) also includes the H$\epsilon$ line. While the latter is blended with
the interstellar Ca II $K$ line at $\lambda$3968, its redward wing could be
used to provide further constraints.

The adopted gravity does have an impact on the strengths of the He I
$\lambda$3188 and He II $\lambda$3203 lines (as shown in Fig. 5), with the
former being stronger and the latter being weaker at higher gravities. As
such, good S/N across the full CUBES range will be critical to ensure the
maximum information is available, but the key point is that there are
potential diagnostics available of both temperature and gravity.

A caveat of our approach in using the tlusty models is that the effects of
stellar winds are not included, which would be expected to modify the
appearance of the emergent spectra. However, we note that the winds of O-type
giants and dwarfs at sub-solar metallicity are generally weak, and the tlusty
models are sufficient for our qualitative objectives here.

Figure 4: Synthetic spectra from the tlusty $Z$ $=$ 0.5$Z_{\odot}$ OSTAR2002
grid for the high Balmer lines for $T_{\rm eff}$ $=$ 35,000 and three
gravities: log$g$ $=$ 3.5 (black spectrum), 4.0 (blue) and 4.5 (red).
Identified lines are the same as those in Fig. 1, plus the H8 line
($\lambda$3889). Figure 5: Synthetic tlusty spectra ($Z$ $=$ 0.5$Z_{\odot}$)
for the He I $\lambda$3188 and He II $\lambda$3203 lines for $T_{\rm eff}$ $=$
35,000 and three gravities: log$g$ $=$ 3.5 (black spectrum), 4.0 (blue) and
4.5 (red).

###  3.3 Low-metallicity models ($Z$ $=$ 1/30 $Z_{\odot}$)

The LMC-like models described above will be relevant for future CUBES targets
in external galaxies, but the real push is to extend studies to lower
metallicities than currently possible (see voyage2050 ). A significant effort
over the past 20 years has gone into quantifying the impact that metallicity
has on the evolution of massive stars, so that we can improve stellar-
evolution and population-synthesis models to more accurately reproduce the
massive-star populations seen in both the local Universe and high-redshift,
star-forming galaxies. Necessarily, most efforts have focused on massive stars
in the Milky Way, LMC and the Small Magellanic Cloud (SMC), spanning a range
of metallicities from that of the solar neighbourhood down to approximately
one-fifth solar in the SMC. This puts limits on our ability to test model
predictions at lower metallicities, and requires (uncertain) extrapolations.

High-efficiency, multi-object spectrographs, such as the FOcal Reducer/low
dispersion Spectrograph 2 (FORS2) on the VLT, have provided first insights (at
$R$ $\sim$ 1,000) into limited numbers of massive stars in more distant
systems, with nebular oxygen abundances of $\sim$0.15$Z_{\odot}$, namely: IC
1613 at 0.7 Mpc, WLM at 0.9 Mpc, and NGC 3109 at 1.2 Mpc. However, a key
motivation to improve our understanding of the physical properties of massive
stars, and their contribution to galaxies in the early Universe, is to
directly observe stars at even lower metallicites. Example targets in this
context include: Sextans A (0.1$Z_{\odot}$ at 1.3 Mpc), SagDIG
(0.05$Z_{\odot}$ at 1.1 Mpc), Leo P (0.03$Z_{\odot}$ at 1.6 Mpc) and,
ultimately, I Zw18 (0.02$Z_{\odot}$ at 18.9 Mpc).

To investigate the spectral lines in such extremely metal-poor stars, we have
used the 0.03$Z_{\odot}$ models from the tlusty OSTAR2002 grid. As a first
comparison with the LMC-like models, the near-UV and visible regions for the
0.03$Z_{\odot}$ models are shown in Figs. 7 and 7, respectively. Echoing the
shift to higher temperatures for a given zero-age main-sequence mass at lower
metallicity (e.g. IZw18 , and references therein), note the He I/He II line
ratios in the $T_{\rm eff}$ $=$ 40,000 K model compared to that in Fig. 3. The
$T_{\rm eff}$ $=$ 40,000 K model for 0.03$Z_{\odot}$ would be classified as a
slightly later type than that for the 0.5$Z_{\odot}$ model. Similarly, the He
I $\lambda$4471 line is stronger in the 0.03$Z_{\odot}$ model. In short, a
spectrum classified as, e.g. O3 or O6, at very low metallicity (e.g. in Leo P
or I Zw18) would have a hotter temperature than its morphological counterpart
in the LMC.

The 3000-3450 Å near-UV region for the 0.03$Z_{\odot}$ models is shown in Fig.
7. As expected, the metallic lines are now significantly weaker, but there are
still several weak O III lines present in the models corresponding to later
O-types ($T_{\rm eff}$ $=$ 32,500 & 35,000). As with the He I/He II line
ratios in the visible (Fig. 7), there is a shift in temperature of when the He
I $\lambda$3188 and He II $\lambda$3203 lines have equivalent intensities at
lower metallicity. They are roughly equivalent at $T_{\rm eff}$ $=$ 32,500 in
the 0.03$Z_{\odot}$ models (Fig. 7), compared to somewhere between $T_{\rm
eff}$ $=$ 32,500 and 30,000 in the 0.5$Z_{\odot}$ models (Fig. 3). Again this
reflects the hotter models needed to reproduce a given line ratio at lower
metallicity (i.e. the temperature for a given spectral type, on the basis of
using Galactic morphological criteria, would be higher).

Figure 6: Synthetic near-UV spectra from the tlusty $Z$ $=$ 0.03$Z_{\odot}$
OSTAR2002 grid, which have been convolved with a rotational-broadening
function of $v$sin$i$ $=$ 100 km s-1. Identified lines are: He I
$\lambda$$\lambda$3188, 3355, 3448; He II $\lambda$3203; O III
$\lambda$$\lambda$3024, 3035, 3047, 3059, 3261, 3267, 3299, 3312, 3341, 3351;
O IV $\lambda$$\lambda$3063, 3072; Si III $\lambda$$\lambda$3086, 3093; Si IV
$\lambda$$\lambda$3150, 3166; S IV $\lambda$$\lambda$3097, 3118.

Figure 7: Synthetic spectra from the tlusty $Z$ $=$ 0.03$Z_{\odot}$ OSTAR2002
grid for the traditional classification domain. The spectra have been
convolved with a rotational-broadening function of $v$sin$i$ $=$ 100 km s-1.
Identified lines are the same as in Fig. 3.

##  4 Stellar properties in Local Group galaxies

###  4.1 Oxygen abundances

The key properties of a (single) massive star are defined by its initial mass
and metallicity, but its path in the Hertzsprung–Russell diagram and ultimate
fate depend critically on both mass loss and rotation puls08 ; langer12 .
Studies have shown that the effects of rotation are enhanced at low
metallicity (e.g. mm01 ; brott11 ), and rotationally-enhanced mixing is
predicted to bring CNO-processed material to the stellar surface. To test the
various implementations and prediction of rotation and mixing in stellar
evolution models we require good empirical constraints of CNO abundances as a
function of temperature, luminosity, metallicity and rotation rates.

As illustrated in Fig. 3, the near UV spectra of O-type stars contain a
plethora of oxygen lines, with several O IV lines in the hottest spectra, a
large number of O III lines in mid-late types, and weak O II lines in the
coolest spectra (e.g. $\lambda$3390). These lines are the best available
diagnostics of O abundances in massive stars, because those at other
wavelengths are generally saturated and strongly affected by stellar winds,
making abundance determination both complicated and uncertain. Moreover, the
use of multiple lines is also critical to arrive at accurate abundances with
reliable error bars (see Fig. 1 from martins15 ). We add that accurate oxygen
abundances are required to use the O V $\lambda$1371 line from far-UV
observations to estimate stellar wind densities around the sonic point and to
explore the effect of clumping in the wind to arrive at reliable mass-loss
rates. There are also useful lines from C and N in the near-UV range (e.g. C
III $\lambda$3609 and N III $\lambda$$\lambda$3354-67-74) that appear to free
of wind effects in relevant cmfgen models (e.g. mb22 ), but these warrant
further investigation.

To illustrate the sensitivity of the O III lines to abundance changes and the
S/N of the observations, in Fig. 8 we show a cmfgen model cmfgen  with Teff
$=$ 31,000 K and log($g$) $=$ 3.1 for HD 269702 (classified as O8 I(f)p, see
walborn2010 ) in the LMC. The baseline oxygen abundance is 12 $+$ log(O/H) $=$
8.39 (shown in green), compared with models for 12 $+$ log(O/H) $=$ 8.74 (in
red) and 7.78 (comparable to results for metal-poor irregulars, in black). We
then introduced model noise to each spectrum to reproduce continuum S/N levels
of 50, 100 and 150 as shown in the figure.

As expected given past observational studies (e.g. martins15 ), the spectra in
Fig 8 suggest that S/N $\gtrsim$ 100 is required to estimate O abundances in
the LMC and more metal rich targets as the lines becomes saturated. For
instance, there is little sensitivity to changes in abundance over this
abundance range in the O III $\lambda$3312 line. However, the situation is
less challenging at lower abundances – although S/N remains critical, the
lines are more responsive to abundance changes. As noted above, the precision
on the estimated O abundance can also be improved by analysing multiple lines
together (recalling the large number of O III lines in the mid-late O-type
spectra in Fig. 3, and the O IV lines in the hottest spectra).

To date, estimates of O abundances in B-type supergiants have been possible in
metal-poor galaxies at the fringes of the Local Group, e.g. 12 $+$ log(O/H)
$=$ 7.83 $\pm$ 0.12 in WLM bresolin06  and 7.76 $\pm$ 0.07 in NGC 3109 evans07
. Similar techniques were also used to estimate light-element abundances (C,
N, O, Si, Mg) of B-type supergiants at $\sim$2 Mpc in NGC 55 castro55 .
However, while the supergiants are a useful reference point for the
evolutionary models (and for comparisons with nebular abundances), for true
insights into the physical processes on the main sequence, we need similar
studies of O-type stars. Even though we have confirmed O stars in WLM and NGC
3109 bresolin06 ; evans07 , as well as in IC 1613 garcia13  and Sextans A
Cal16 , abundance estimates are out of reach of present observations (e.g.
tramper11 ; tramper14 ).

Figure 8: The impact of noise on the O III lines in cmfgen model spectra of an
O8-type star in the LMC. Three oxygen abundances are shown, a LMC model with
12 $+$ log(O/H) $=$ 8.39 (blue), and models with 12 $+$ log(O/H) $=$ 8.74
(red) and 7.78 (black, comparable to results in WLM and NGC 3109) to
illustrate the sensitivity of the lines. Identified lines are O III
$\lambda$$\lambda$3261, 3267 (blend), 3299, 3312, 3341. Noise has been added
to the models to simulate different signal-to-noise (S/N) ratios in the
continuum as indicated.

###  4.2 Stellar rotational velocities

Another important observational property of massive stars is their projected
rotational velocity ($v$sin$i$). As noted above, stellar rotation is important
in the context of chemical enrichment (e.g. CNO abundances), and observational
estimates of $v$sin$i$ are also powerful diagnostics of rapidly-rotating stars
that have experienced chemically-homogeneous evolution or that have previously
undergone binary interaction.

The helium lines in visible spectra are often used to estimate rotational
velocities of O-type stars (e.g. rss13 ). Although they are affected by Stark
broadening (typically with an equivalent FWHM of 50-100 km s-1), the helium
lines can be used to study the overall distribution of projected rotational
velocities for samples of stars, while also allowing identification of rapid
rotators. However, working with low-resolution ($R$ $\sim$ 1000) spectroscopy
from, e.g. FORS2 or OSIRIS (on the Gran Telescopio Canarias), at S/N $\sim$ 50
we are unable to put even modest constraints on $v$sin$i$; the minimum
combination required is $R$ $>$ 2000 and S/N $>$ 100.

A more robust probe of rotational velocities is provided by metallic lines in
the spectra of massive stars. The metallic lines do not suffer from Stark
broadening, nor do they suffer nebular contamination which can also hamper the
use of helium lines in extragalactic targets. The visible spectra of B-type
stars are replete with isolated metal lines that can be used to investigate
$v$sin$i$ (e.g. Si III, Si IV, Mg II etc) but the only comparably useful probe
for O-type stars is the (less commonly observed) O III $\lambda$5591 line.

The capabilities of CUBES are particularly compelling in this context. The
combination of its sensitivity, improved spectral resolution (compared to e.g.
FORS2) and access to the broad range of metallic lines in the near UV (see
Fig. 3) will enable robust estimates of $v$sin$i$ for metal-poor stars at the
edge of the Local Group. High S/N observations would also enable investigation
of the contribution of macroturbulent broadening in (sub-SMC) metal-poor
O-type stars for the first time (e.g. a09 ; ssd10 ). For instance, for stars
with relative narrow lines, S/N $\gtrsim$ 100 is sufficient to use Fourier
transform analysis to estimate the contribution of macroturbulence ssd14 .

###  4.3 Alternative temperature diagnostics

Beyond the O-type stars discussed here, the coverage of the Balmer jump
provided by CUBES could also provide valuable constraints on effective
temperature for other extragalactic targets, as used for e.g. A-type
supergiants kud08 , B-type supergiants z09 , and Be-type stars s18 . This
technique typically requires accurate determination of the flux levels at
either side of the Balmer limit, rather than precise absolute flux
calibration. We do not explore this application further here, but note it for
completeness and as an example of where flux calibration of the spectra will
be important to have a good understanding of the wavelength-dependent
properties of the spectra (response function, slit-losses etc).

##  5 CUBES Performances

In contrast to lower-mass stars, the spectral energy distributions of massive
stars peak in the far-UV. This means that observations with CUBES will probe
the rising part of the flux distribution, potentially opening-up observations
of targets that are otherwise too faint to observe with other facilities. For
instance, the intrinsic $U-V$ colour for a mid O-type star is $(U-V)_{0}$
$\sim$ $-$1.5 mag (e.g. johnson ), representing a potentially significant gain
compared to observations at visible wavelengths.

One caveat to this potential gain is the challenge of line-of-sight extinction
towards potential targets, as its effects become more significant at shorter
wavelengths (with $A(U)$ $\approx$ 1.5 $A(V)$ cardelli ). Nonetheless, most
potential extragalactic targets are sufficiently far from the Galactic plane,
such that foreground extinction will not be too much of a limiting factor,
although targets in external galaxies might have to be carefully selected to
avoid those with a significant local contribution to the line-of-sight
extinction.

To estimate the potential performance of CUBES in the studies of O-type stars
in galaxies such as IC 1613, WLM and NGC 3109 we used some of the model
spectra discussed above as inputs to the Exposure Time Calculator (ETC)
developed during the conceptual design phase genoni22 . The confirmed O-type
stars in these irregular galaxies have $V$ $=$ 19 to 20.5 mag. The S/N
predictions from the ETC for 2 hr exposures of three of the tlusty
0.5$Z_{\odot}$ dwarf models are summarised in Table 1; adopted parameters for
each calculation were: airmass $=$ 1.2 (a reasonable assumption for the
example galaxies), seeing $=$ 0.8′′, spatial binning $\times$2, spectral
binning $\times$4.

Given the similarity in the spectral energy distributions, the same ETC
calculations using the 0.03$Z_{\odot}$ are nearly identical in the predicted
S/N, so the use of the LMC-like models here does not unduly influence the
results. The S/N values are quoted for two wavelengths: 3195 Å as
representative of the continuum S/N near the He I $\lambda$3188 and He II
$\lambda$3203 lines, and at 3640 Å to indicate the S/N shortwards of the
Balmer limit.

The image slicer for the low-resolution mode generates a wider effective slit
such that the resulting spectra are oversampled (with $>$9 pixels per
resolution element), hence the adopted $\times$4 spectral binning in the
calculations to bolster the resulting S/N without loss of resolution. Equally,
for the faintest massive stars, we could bin by a further factor of two to
improve the S/N (i.e. $\sqrt{2}$ $\times$ S/NETC), degrading the spectrum to
an effective resolving power of $R$ $\sim$ 3,500 (still more than three times
that obtained for 3 hr integrations of massive stars beyond 1 Mpc with FORS2,
e.g. evans07 ). In the low-resolution mode the slicer has six 1′′ slices, so
in the ETC calculations we only extracted the central two slices (to optimise
the S/N).

With additional binning spectrally, the results in Table 1 demonstrate that it
should be possible to obtain sufficient S/N ($\gtrsim$ 100) for studies of the
physical parameters and oxygen abundances of O-type stars in galaxies at the
edges of the Local Group (i.e. 1.2-1.3 Mpc), with slightly longer ($\sim$2.5-3
hr) exposures of the fainter known O stars in these systems.

Table 1: Signal-to-noise (S/N) predictions for CUBES observations of massive stars with the low-resolution mode for illustrative LMC-like ($Z$ $=$ 0.5$Z_{\odot}$) tlusty models. Adopted parameters for the Exposure Time Calculator (ETC) are: exposure time $=$ 2 hr, airmass $=$ 1.2, seeing $=$ 0.8′′, spatial binning $\times$2, spectral binning $\times$4 (given that the low-resolution mode is sampled with $>$9 pixels per resolution element). $V$ |  $T_{\rm eff}$ $=$ 45.0 kK |  $T_{\rm eff}$ $=$ 37.5 kK |  $T_{\rm eff}$ $=$ 27.5 kK
---|---|---|---
(mag.) | S/Nλ3195 | S/Nλ3640 | S/Nλ3195 | S/Nλ3640 | S/Nλ3195 | S/Nλ3640
19.0 |  95 | 113 |  93 | 111 |  86 | 104
19.5 |  74 |  88 |  72 |  86 |  66 |  81
20.0 |  57 |  67 |  55 |  66 |  50 |  62
20.5 |  43 |  51 |  42 |  50 |  38 |  47

###  5.1 ‘ELT science’ with the VLT

As an exciting example of where CUBES can bring observations into reach that
are beyond our current capabilities, we highlight the case of star LP 26 in
the dwarf Leo P galaxy leoP . Deep (8 hr) observations with the Multi-Unit
Spectroscopic Explorer (MUSE) instrument revealed weak He II
$\lambda$$\lambda$4686, 5411 absorption lines, providing the first direct
evidence of an O-type star in Leo P. This is particularly interesting as the
significantly low oxygen abundance (3% solar) of the H II region in Leo P
suggests it has a near-primordial composition skillman13 .

Although relatively nearby (1.6 Mpc mcquinn ) for such a metal-poor system,
Leo P is sufficiently far away that further visible spectroscopy of its hot
stars is beyond our current capability. For instance, the HST magnitudes for
LP 26 are $F475W$ $=$ 21.5 and $F814W$ $=$ 21.8 (which, taking these as
proxies for $V$ and $I$ filters compared to the anticipated intrinsic colours
for O-type stars, suggest a low line-of-sight extinction). This puts it beyond
the reach of (feasible) observing proposals with existing visible
spectrographs, and further visible spectroscopy of the stellar population of
Leo P was thought to have to wait until the Extremely Large Telescope (ELT) is
operational.

First constraints on the properties of LP 26 have recently been provided by
analysis of far-UV HST spectroscopy, reporting a fast rotational velocity
($v$sin$i$ $=$ 370 $\pm$ 90 km s-1) and, as expected given the lower
metallicity, weaker wind lines than in comparison templates of SMC stars
telford21 . Using the same OSTAR2002 tlusty grid as in our calculations, the
far-UV spectroscopy was combined with multiband photometry to estimate its
temperature as $T_{\rm eff}$ $=$ 37,500 K, with uncertainties of order $\pm$6
kK.

The near-UV lines identified in the CUBES range offer the prospect of
characterising the physical properties of LP 26. We used the 3% solar $T_{\rm
eff}$ $=$ 37,500 K tlusty model to estimate the performance for $V$ $=$ 21.5
mag. with the ETC. The same parameters were used as in Table 1, including
airmass $=$ 1.4, as this corresponds to the maximum altitude that Leo P (with
a declination of $+$18∘) reaches from Paranal. Extracting only the central two
slices again, the ETC predicts S/N $=$ 34 at 3640 Å in a total integration of
4 hrs; binning this (spectrally) by a further factor of two would provide S/N
$\sim$ 50 in a relatively modest half night of observations; the same
calculation for 3195 Å (central slice only, binning the ETC results by a
further factor of two yields S/N $\sim$ 40.

Such an observation would enable estimates of the physical parameters
(temperature, gravity) of LP 26 in a relatively modest amount of observing
time (half a night). This nicely illustrates the potential of CUBES in this
scientific area – we expect more candidate O-type stars to be discovered in
the coming years at 1 Mpc and beyond, and CUBES could provide a powerful
capability to constrain their physical parameters.

Direct determination of the oxygen abundance at the low metallicity of Leo P
will, however, remain challenging. The metal lines are sufficiently weak that
greater S/N ($>$ 100) is required for secure detections. To explore this
further, in Fig. 9 we show the same tlusty model spectrum for the O III lines
in the 3250-3350 Å region. A continuum S/N in excess of 100 is required to
tentatively detect the O III $\lambda$3341 line, with S/N $=$ 150 being a more
realistic goal if equivalent widths (or even firm upper limits) were to be
measured. From the ETC, recovering a S/N of order 150 at 3350 Å (assuming the
same binning as above in the ETC, and a further factor of $\sqrt{2}$) would
require a total integration of $\sim$45 hr. Such an ambitious observation is
unlikely to be feasible, particularly given e.g. systematics that might limit
performance from combining a large number of exposures together, although we
will re-assess this case later in the construction phase of CUBES.

Figure 9: Synthetic tlusty spectra ($Z$ $=$ 0.03$Z_{\odot}$, $T_{\rm eff}$ $=$
37,500, log$g$ $=$ 4.0) to investigate the potential of recovering oxygen
abundances for different levels of simulated signal-to-noise (S/N) in the test
case for Leo P (see Section 5). The spectra with added noise are shown in
black, with the original model spectrum overplotted on each in red. The
identified O III lines are the same as those in Fig. 8.

###  5.2 Impact of O3 absorption

Shortwards of $\sim$3400 Å the extinction due to atmospheric ozone (O3)
becomes a significant factor, with a steep dependence of the extinction as a
function of airmass (see e.g. Fig. 2 from patat11 ). Within this region there
are also discrete O3 bands, which can impact on the study of potential stellar
features. In the context of the potential diagnostic lines discussed here, it
is notable that one of the bands spans the (rest-frame) He II $\lambda$3203
line (e.g. Fig. 1 from m75 ). Although weaker at the longer wavelengths,
careful correction for the O3 bands will also be required to study the O III
features discussed above,

On this topic we note the study by griffin05  who used historic stellar
spectroscopy to recover information on the intensity of past O3 features; this
highlighted that with careful modelling (helped in this particular case by
near-UV HST observations) the stellar features could be removed successfully
to reveal the O3 band absorption; the reverse would also be true in terms of
recovering the intrinsic stellar spectra.

The operational concept for CUBES is under development as the project
progresses, but careful subtraction of the O3 features will be an important
consideration. The current plans include observations of flux standards (with
high signal-to-noise) to help reconstruct the atmospheric absorption features,
combined with using theoretical tools such as molecfit molecfit .

##  6 Summary

We have used model atmospheres to investigate the spectral diagnostics
available over the 3000-4050 Å range that will be accessible with the new
CUBES instrument now in development for the VLT. There have been relatively
few studies of OB-type stars shortwards of the Balmer limit at (ground) near
UV wavelengths to date, but CUBES will provide an exciting new capability to
study massive stars in metal-poor systems at the edge of the Local Group and
beyond.

Following the pioneering work from m75 , the He I $\lambda$3188 and He II
$\lambda$3203 pair of lines appear to be a compelling diagnostic of stellar
temperature, with the high Balmer series of lines providing constraints on
gravity. The near UV is also rich with metallic lines (O III, O IV, Si III, Si
IV, N III) that can provide further temperature diagnostics (where more than
one ionisation stage is present for a given element) as well as estimates of
chemical abundances in O-type stars, particularly for oxygen where we lack
robust diagnostics at visible wavelengths.

Our results from the ETC presented here demonstrate that it should be possible
with CUBES to obtain high-quality (S/N $>$ 100, $R$ $\sim$ 7,000) near-UV
spectra of massive O-type stars in metal-poor systems at $\sim$1 Mpc in
$\sim$3 hr integrations. The CUBES analysis will probably be informed by
initial estimates of stellar parameters from observations at other
wavelengths, and will provide further constraints on stellar temperatures and
gravities, together with estimates of projected rotational velocities and the
first oxygen abundances for O-type stars in these extragalactic systems. Such
results will enable much needed comparisons with theoretical predictions from
low-metallicity evolutionary models and perhaps the first observational
evidence for chemically-homogeneous evolution. Furthermore, using very metal-
poor model spectra (with $Z$ $=$ 0.03$Z_{\odot}$) we have shown that CUBES
could obtain spectra with S/N $\sim$ 50 of the candidate O-type star in Leo P
at 1.6 Mpc in approximately half a night of observations.

Our qualitative consideration of the models here was intended as a first study
of the potential diagnostics available in the CUBES domain. Data from the
ongoing XShootU ESO Large Programme (in support of the HST ULLYSES Legacy
Survey) will soon enable quantitative investigation of O-type spectra in the
CUBES region from high S/N ($>$ 100) X-Shooter observations in the Magellanic
Clouds. Analysis of the XShootU data will provide an important test of the
temperatures estimated using the He I $\lambda$3188/He II $\lambda$3203 ratio
and log$g$ from the high Balmer lines compared to diagnostics in the visible.
Our expectation is that for the extragalactic metal-poor dwarfs envisaged as
future CUBES targets, the impact of stellar winds on the determination of
physical parameters from the near-UV region alone should be relatively minor,
and the XShootU data in the Clouds will provide an important test of this.

More generally, the greater throughput and resolving power delivered by CUBES
compared to X-Shooter will give a unique capability for studies of massive
stars in the ground-UV region. Moreover, the absence of cross-dispersion in
the CUBES design will avoid the challenges of blaze correction and order
recombination that affect the analysis of data from echelle instruments such
as UVES and X-Shooter; the CUBES design will give a smooth, continuous
spectral response for the two optical channels. This will enable more robust
definition of the stellar continuum for normalization, particularly for broad
features in the Balmer line series, as well as analysis of weaker absorption
lines if they would otherwise be in the reconnection regions. The latter will
be important for more rapidly-rotating massive stars ($v$sin$i$ $\gtrsim$ 100
km s-1), which will also benefit from the greater resolving power compared to
X-Shooter.

###### Acknowledgements.

We thank the reviewers for their suggestions on the manuscript, which helped
clarify details as well as helping place this study in the wider context of
previous work and some of the challenges of ground-based observations at these
short wavelengths. MG acknowledges financial support from grants
ESP2017-86582-C4-1-R and PID2019-105552RB-C41, and from the Unidad de
Excelencia “Marıá de Maeztu” – Centro de Astrobiologıá (CSIC-INTA) project,
MDM-2017-0737.

## Conflict of interest

The authors declare that they have no conflict of interest.

## Data availability statement

The tlusty models used in this article are freely available online. The UVES
data of HDE 269896 and the cmfgen models shown in Figs. 1 and 8 are available
on request.

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