# FUV Observations of the Inner Coma of 46P/Wirtanen

John W. Noonan Lunar and Planetary Laboratory,University of Arizona, 1629 E
University Blvd, Tucson, Arizona 85721-0092, USA  Walter M. Harris  Lunar and
Planetary Laboratory,University of Arizona, 1629 E University Blvd, Tucson,
Arizona 85721-0092, USA  Steven Bromley Physics Department, Leach Science
Center, Auburn University, Auburn, AL 36849, USA  Davide Farnocchia Jet
Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, 91109 CA, USA  Jian-Yang Li (李荐扬) Planetary Science
Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, AZ 85719, USA  Kathleen
E. Mandt  Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Rd.,
Laurel, MD, 20723, USA  Joel Wm. Parker Planetary Science Directorate,
Southwest Research Institute, Suite 300, 1050 Walnut Street, Boulder, Colorado
80302,USA  Kumar Venkataramani Physics Department, Leach Science Center,
Auburn University, Auburn, AL 36849, USA  Dennis Bodewits Physics Department,
Leach Science Center, Auburn University, Auburn, AL 36849, USA

(Received September 29, 2020; Revised November 23, 2020; Accepted December 1,
2020)

###### Abstract

Far ultraviolet observations of comets yield information about the energetic
processes that dissociate the sublimated gases from their primitive surfaces.
Understanding which emission processes are dominant, their effects on the
observed cometary spectrum, and how to properly invert the spectrum back to
composition of the presumably pristine surface ices of a comet nuclei are all
critical components for proper interpretation and analysis of comets. The
close approach of comet 46P/Wirtanen in 2018-2019 provided a unique
opportunity to study the inner most parts of a cometary coma with the Hubble
Space Telescope Cosmic Origins Spectrograph, rarely accessible with remote
observations, at length scales (100’s of km) and wavelengths (900-1430 Å)̃
previously probed only by the European Space Agency’s _Rosetta_ spacecraft.
Our observations show a complex picture for the inner coma; atomic production
rates for H and O that show water is the dominant source of both, an abundance
of atomic sulfur that is difficult to explain with the lifetimes of common
sulfur parent molecules, and a density distribution that is poorly fit with
both Haser and vectorial models.

††journal: Planetary Science Journal

##  1 Introduction

Comets are frequently described as the well-preserved primitive remnants of
our solar system’s formation, offering insight into the chemical abundances of
their formation location in the protoplanetary disk. This assumption is based
on the source regions for comets: the Kuiper-Edgeworth Belt for Jupiter-family
comets (JFCs) and the Oort Cloud for long-period comets (LPCs), both of which
have temperatures well below the sublimation point of common volatiles and
should preserve the primordial ice abundances over billion year timescales
(A’Hearn, 2011). Directly measuring the ice compositions of comet nuclei is
nearly impossible, unless icy grains are lifted from the surface and can be
studied (e.g. Protopapa et al. (2018)), and we are limited to investigating
the emissions from gas in the coma produced by sublimating cometary ices.
These emissions require modeling of the distribution of molecules and emission
mechanisms in the coma to properly derive abundance of the parent molecules
and tie them to nucleus properties (Feldman et al., 2004). Far Ultraviolet
(FUV) observations of cometary comae have been instrumental in characterizing
the atomic budget of comets (Feldman et al., 2017; Bodewits et al., 2020), and
provide a directly comparable metric to protoplanetary disks. However, our
understanding of the physical processes that drive cometary emissions, and
that are subsequently used to derive these primordial properties, is not yet
complete. The European Space Agency’s Rosetta mission revealed the prevalence
of dissociative electron impact in the coma of 67P/Churyumov-Gerasimenko at
heliocentric distances beyond 2.8 au in the near nucleus coma ($\leq$ 100 km),
which can produce different spectral signatures than the more commonly
detected resonance fluorescence and photodissociation mechanisms, and thus
affects interpretation of those observations (Feldman et al., 2015; Bodewits
et al., 2016). The presence of dissociative electron impact could allow
detection of molecules which do not have emission features, like O2, in the
extreme inner coma, as it produces direct emission from dissociation of parent
molecules with unique emission line ratios. However, the process was most
consistent at large heliocentric distances, and was more related to transient
events nearer perihelion. Such a process helped reveal the presence of faint
water emissions from Europa attributed to a plume (Roth et al., 2017) and the
presence of species that are difficult to detect via traditional
photodissociation and fluorescence, as with O2 on both Ganymede and Callisto
(Hall et al., 1998; Cunningham et al., 2015). This discovery brought forth two
particular questions: Is dissociative electron impact present in other comet
comae? What local conditions need to be met to produce significant
dissociative electron impact emissions?

The unique combination of extensive prior observation, substantial
contemporaneous observation during the 2018-2019 close approach, and the
ability to plan observations implementing the lessons learned from the Rosetta
mission provided an unprecedented opportunity to characterize the atomic and
molecular emissions of 46P in the FUV.

Figure 1: Diagram showing approximate points of the COS aperture for each of
the settings described in Table 1 relative to the coma of 46P. Each white
circle represents the field-view-of the COS aperture subtended at
$\Delta$=0.22 au and the radius of the full image is 12”, approximately 1910
km. The background image is a simple Haser model for H2O at rh = 1.18 au,
which was not observed in the COS bandpass, to provide context for the type of
flux decrease that was expected if dissociative electron impact emission were
present. The image is independent of production rate and thus has units of s
cm-2 . The offset direction is not necessarily representative of the as-
executed offset angle.

In this paper we discuss observations of 46P/Wirtanen obtained January 9-20,
2019 with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope
(HST) between wavelengths of 900 and 1430 Å. In Section 2 we review the
observing strategy, as well as the COS instrument and COS data reduction. The
reduced spectra, derived column densities, and calculated production rates for
assumed parent species are discussed in Section 3. Section 4 compares these
results to previous apparitions of 46P as well as studies from the 2018-2019
apparition. Our conclusions are presented in Section 5.

##  2 Observations

To access the same region of the FUV probed by the Rosetta Alice ultraviolet
spectrograph (Stern et al., 2007) that escorted comet 67P/Churyumov-
Gerasimenko for over two years we executed observations with the Cosmic
Origins Spectrograph on the Hubble Space Telescope with the G130M grating
(Green et al., 2012). For these FUV observations two separate cross delay line
detector arrays are used, each with 16384 spectral and 1024 spatial pixels.
Not all pixels are exposed to incoming light. The G130M grating provides a
resolving power between 1.75-2.25$\times$104 over the bandpass between 900 to
1430 Å. The narrow aperture of the COS instrument, 2.5” in diameter, allowed a
discrete probing of the inner 1350 kilometers of 46P’s coma, as shown in
Figure 1. Two instrument settings were used, one centered at 1096 Å and the
other at 1291 Å for three pointings (centered, 2.5”, and 8” offsets) chosen to
search for near-nucleus coma emissions and dissociative electron impact like
those observed at 67P (Feldman et al., 2015, 2016; Bodewits et al., 2016;
Chaufray et al., 2017; Feldman et al., 2017), a transition zone near the
collisionopause (Mandt et al., 2016), and a radius beyond the expected
collisional zone where only photodissociation and prompt emission were
expected. Spectra taken with a central wavelength of 1096 Å were able to
capture wavelength ranges between 900 and 1230 Å in hopes to detect the
strongest transitions of the Lyman series of hydrogen atoms in the coma:
Lyman-$\alpha$ and -$\beta$, at 1215.72 and 1025.7 Å, respectively. Properties
of each COS exposure from Guest Observer (GO) program 15625 (PI: D. Bodewits)
are presented in Table 1.

For this set of observations it is important to note the low heliocentric
velocity of 46P, varying between 7.70 and 9.9 km s-1 (Table 1). This low
velocity decreases the strength of the Swings effect (Swings, 1941), by which
the redshift or blueshift of the solar spectrum relative to the comet can
enhance or decrease atomic and molecular fluorescence efficiencies, and
therefore limits variation of our calculated efficiencies from typical values
at 1 au. For the wavelength range covered by the G130M spectral element (900 -
1430 Å), the feature that benefits the most from this effect is cometary
sulfur emission at 1425 Å, which is produced by resonance scattering of the
solar S I feature by S atoms (Roettger et al., 1989; Feldman et al., 2017),
and thus increases the fluorescence efficiency, making it easier to detect.
However, the low geocentric velocity makes it difficult to resolve any
extended geocoronal emission contribution to the Lyman-$\alpha$ emission at
1215.6 Å (McCoy et al., 1992). We discuss our method of geocoronal emission
subtraction in Section 2.1 and analysis of the line shape of Lyman-$\alpha$ in
Section 3.

Observations were scheduled after the comet’s close approach between January 9
and 20 of 2019 to mitigate two factors. The first, and most pressing, was that
HST would have difficulty tracking 46P with the accuracy required to keep the
inner coma within the narrow aperture at close approach due to the high
relative motion across the sky. By scheduling observations for a period when
46P was at a larger geocentric distance the on sky motion of the comet was
within the HST maximum slew rate capability. The second issue lies with the
characteristics of observing cometary comae; comets appear brightest when
their coma are distant and therefore more concentrated on the sky. By
observing the inner coma when the comet was just $\sim$0.20 au away, much of
the light from the extended coma is excluded, rendering the object relatively
faint by typical spectroscopic standards, but significantly brighter than if
observations had been conducted at 0.07 au at closest approach. This improved
the acquisition likelihood as well as spectroscopic data.

We note that there are contemporaneous observations taken with the Space
Telescope Imaging Spectrograph (STIS) in the same observing campaign (GO-15625
PI: D. Bodewits). These will be discussed in a separate publication
(Venkataramani et al, in prep). All observations used in this publication are
available in the Mikulksi Archive for Space
Telescopes111https://archive.stsci.edu/.

Table 1: 46P/Wirtanen HST COS Observation Log

Observation ID | Date | Center | Exposure | Offset |  $\Delta$a,ba,bfootnotemark:  |  $\dot{\Delta}$ccGeocentric Range-rate | Heliocentric | Heliocentric | Offset DistanceddObservations with both identical instrument settings and offsets are co-added to create spectra presented in Figures 2 and 3
---|---|---|---|---|---|---|---|---|---
| (UTC) | Wavelength (Å) | Time (s) | Angle (”) | (AU) | (km/s) | DistanceaaQueried from JPL Horizons (AU) | Range-rateaaQueried from JPL Horizons (km/s) | at 46P (km)
ldx60901 | 2019-01-15 17:19:49 | 1096.0 | 1630.18 | 0 | 0.2 | 13.15 | 1.15 | 8.99 | 0.0
ldx61001 | 2019-01-16 13:57:17 | 1096.0 | 1630.21 | 0 | 0.21 | 13.07 | 1.15 | 9.17 | 0.0
ldx61101 | 2019-01-16 17:07:58 | 1096.0 | 1630.18 | 0 | 0.21 | 13.13 | 1.15 | 9.19 | 0.0
ldx61201 | 2019-01-17 13:48:50 | 1096.0 | 1630.18 | 0 | 0.22 | 13.9 | 1.16 | 9.36 | 0.0
ldx60101 | 2019-01-09 18:12:10 | 1291.0 | 1700.16 | 0 | 0.17 | 9.91 | 1.12 | 7.7 | 0.0
ldx60201 | 2019-01-10 21:24:45 | 1291.0 | 1660.19 | 0 | 0.18 | 11.64 | 1.13 | 7.96 | 0.0
ldx61301 | 2019-01-18 15:13:47 | 1096.0 | 1630.18 | 2.5 | 0.22 | 14.27 | 1.16 | 9.57 | 402.0
ldx61401 | 2019-01-13 11:14:44 | 1096.0 | 1630.21 | 2.5 | 0.19 | 11.37 | 1.14 | 8.52 | 345.0
ldx61501 | 2019-01-14 11:05:31 | 1096.0 | 1630.14 | 2.5 | 0.2 | 11.92 | 1.14 | 8.73 | 355.0
ldx61601 | 2019-01-14 07:54:47 | 1096.0 | 1630.18 | 2.5 | 0.2 | 11.85 | 1.14 | 8.71 | 354.0
ldx60301 | 2019-01-12 17:46:49 | 1291.0 | 1700.13 | 2.5 | 0.19 | 11.39 | 1.13 | 8.37 | 337.0
ldx60401 | 2019-01-16 10:45:23 | 1291.0 | 1700.16 | 2.5 | 0.21 | 12.84 | 1.15 | 9.14 | 377.0
ldx62901 | 2019-01-18 07:15:50 | 1096.0 | 1630.14 | 8 | 0.22 | 13.77 | 1.16 | 9.5 | 1274.0
ldx63001 | 2019-01-18 16:49:10 | 1096.0 | 1630.18 | 8 | 0.22 | 14.31 | 1.16 | 9.58 | 1289.0
ldx63101 | 2019-01-19 18:13:50 | 1096.0 | 1630.11 | 8 | 0.23 | 14.58 | 1.17 | 9.78 | 1328.0
ldx63201 | 2019-01-20 08:30:41 | 1096.0 | 1630.11 | 8 | 0.23 | 14.49 | 1.17 | 9.89 | 1350.0
ldx62501 | 2019-01-13 16:00:16 | 1291.0 | 1750.21 | 8 | 0.19 | 11.56 | 1.14 | 8.57 | 1110.0
ldx62601 | 2019-01-15 07:44:32 | 1291.0 | 1750.14 | 8 | 0.2 | 12.43 | 1.15 | 8.91 | 1167.0

bbfootnotetext: Geocentric Distance

###  2.1 Data Reduction

All COS data were processed with the CALCOS pipeline before any further
reduction (Rafelski, 2018). The CALCOS pipeline accounts for flatfield
correction, pulse height filtering, geometric correction, thermal corrections,
and flux calibrations of raw COS spectra. Following the CALCOS reduction each
individual spectrum had a scaled airglow spectrum subtracted. Attempts were
made to scale this spectrum by average angle to Earth-horizon, but ultimately
given the non-detection of N I 1200 Å, which was the benchmark for the
importance of airglow and geocorona features, the integrated flux between 1198
and 1202 Å was used to scale an averaged airglow spectrum which was then
subtracted from each x1dsum spectrum before any coaddition. Airglow spectra
were obtained from the HST COS deliberate airglow observation
page222https://www.stsci.edu/hst/instrumentation/cos/calibration/airglow.

For emission features with such large signal-to-noise ratios in individual
spectra we can extract integrated line brightnesses from those calibrated
spectra. To search for emission features that may have emissions too faint to
observe in the original COS data we can both bin the spectrum and co-add
spectra with the same location relative to the nucleus. Binned and co-added
spectra are shown in Figures 2 and 3, while the high signal-to-noise co-added
but unbinned Lyman-$\alpha$ and O I 1302 Å triplet emissions are shown in
Figures 4, 5, and 6.

All spectral fluxes recorded by the CALCOS pipeline are in units of ergs cm-2
Å-1 s-1 and are converted to units of Rayleighs, a surface brightness unit
useful for extended objects. One Rayleigh corresponds to 106 photons cm-2 s-1
sr-1, so a conversion function is used to convert the COS spectral data first
from an energy flux in ergs to a photon flux based on photon energy, then to a
surface brightness. This is done via the equation

$B(R)=(\frac{4\pi}{10^{6}})\Omega F$ (1)

where $\Omega$ is the solid angle of the COS aperture in steradians,
1.84$\times$10-9 sr, and F is the flux from the object in photons cm-2 s-1
Å-1.

##  3 Results

###  3.1 Spectra

Co-added spectra for each pointing offset and wavelength setting are presented
in Figures 2 and 3. We strongly detect the emission features of three atomic
species in these data: H (1215.7 and 1025.7 Å ), O ( 1302 Å triplet ) , and S
(1425 Å ).

Lyman-$\alpha$ has been used frequently to measure the hydrogen coma from
observations with much larger fields of view than HST COS (degrees as opposed
to arcseconds) and is a powerful diagnostic for determining the water
production rates of comets (McCoy et al., 1992; Combi & Feldman, 1992; Combi
et al., 1998, 2019; Mayyasi et al., 2020; Combi et al., 2020). While the
Lyman-$\beta$ transition was detected in our coadded spectra, it was at a much
lower significance due to increased detector noise blueward of 1100 Å.
Lyman-$\alpha$ is very strong but proper interpretation of the brightness
requires modeling the distribution and radiative transfer of H atoms in the
inner coma. The minimal geocoronal contribution is due to the fortunate
observing geometry for 46P’s apparition, which required nightside observations
for HST, and the narrow Field of View (FOV) of the COS instrument. Similarly,
the ecliptic longitude and latitude of 46P placed it in the ”downwind”
interstellar direction, which has a low IPM contribution to the Lyman-$\alpha$
emission of between 300 and 400 Rayleighs (Pryor et al., 2013). A high
resolution profile of the Lyman-$\alpha$ emission is shown in Figure 4 for
each co-added pointing offset, and shows the dominance of cometary hydrogen
emissions for the COS observations, with only minor contributions from both
the Earth’s geocorona and the interplanetary medium in this particular dataset
relative to the brightness of the coma. Emission from the 2-1 transition of
deuterium at 1215.33 Å is not clearly present when subtracting off a best fit
gaussian (Fig. 5). We note that the Ly-$\alpha$ profile of both the airglow
and cometary emissions is poorly fit by both Cauchy-Lorentzian and Voigt
profiles, and is instead best characterized with a Gaussian profile with a
$\sigma$ between 0.36-0.38 Å, corresponding to a velocity of 83-87.5 km/s.
This is much larger than the expected velocities of H atoms in the inner coma
(8-26 km/s) (Combi et al., 2004), and is therefore the limiting resolution of
our observations and not indicative of the actual emission profile.The
Gaussian fit overpredicts flux at the peak of the emission feature by a small
amount and underpredicts flux on the wings but the effect is nearly symmetric,
making it difficult to detect deuterium emissions or the IPM contribution
directly. We note that this is larger than the expected line spread function
of the COS instrument, due to the aperture filling observations of the comet.
The 3$\sigma$ resolution of the filled COS aperture is measured to be $\sim$1
Å in our spectra, similar to previous cometary COS observations and airglow
measurements.

The atomic oxygen triplet emission at 1302, 1304, and 1306 Å has an excellent
signal-to-noise ratio for all pointings. The spectral resolution provided by
the G130M element from COS allows this triplet to be completely resolved (Fig.
6). With no clear evidence of self absorption in the O I triplet via inversion
at the emission peaks nor redistribution of flux from the stronger O I 1302
Åf̃eature to other features that would indicate significant optical thickness
in the atomic oxygen coma, it is reasonable to allow discussion of the
diagnostic potential of the O I triplet in Section 4.

The last strongly detected atomic emission feature is the triplet of atomic
sulfur around 1425 Å in our data, shown in Figure 7. As described in Section 2
the strength of this feature is enhanced by the low heliocentric velocity of
46P, increasing the fluorescence efficiency of the cometary sulfur from the
solar sulfur emission feature.

We note that several atomic and molecular features often observed in cometary
FUV spectra (Weaver et al., 1981; Feldman et al., 2018, 2017) were not
detected; we do not see any evidence of the C I 1277 or C II 1335 Å emission
that could indicate either dissociative electron impact emission of CO2 or CO,
or from reflected sunlight from the near-nucleus dust or the nucleus itself.
This non-detection is supported by the lack of CO Fourth Positive Group
emissions between 1350 - 1430 Å which produces a number of features that would
be identifiable in our wavelength range (Fig. 8) (Lupu et al., 2007; Bodewits
et al., 2020). Another notable absentee in particular is N I 1200 Å emission,
which would have implied significant contribution of airglow emissions to the
spectrum.

Figure 2: Co-added spectra with 39 pixel binning to improve signal in emission
features with low flux, specifically Lyman-$\beta$ near 1026 Å. Note that only
Lyman-$\alpha$ at 1215.7 Å, is easily identifiable. An enlarged high
resolution profile of Lyman-$\alpha$ is presented in Figure 4.  Figure 3: Co-
added spectra with 13 pixel binning to improve signal in emission features
with low flux. In addition to Lyman-$\alpha$ emissions at 1215.7 Å the O I
1302 Å multiplet is easily identified and presented in Figure 6. With the
exception of S I 1425 Å, shown enlarged in Figure 7, no other significant
atomic or molecular emissions were identified.  Figure 4: Profile of the
Lyman-$\alpha$ emission for each co-added pointing and central wavelength
setting with no binning applied. The contribution of geocoronal and
interplanetary hydrogen emission to the cometary spectrum is relatively
constant for each offset and is minimal, if not negligible. Figure 5: Profile
of the Lyman-$\alpha$ emission for the two comet-centered co-added spectra and
central wavelength setting with no binning applied. A gaussian fit to the data
is plotted and the residuals resulting from subtracting the model from the
data are plotted below. Figure 6: Enlarged section of Figure 3 to show O I
triplet emission between 1302 and 1306 Å. This spectrum has only been binned
by a factor of 3 to show line profiles and relative shapes.  Figure 7:
Enlarged section of Figure 3 to show S I 1425 Å emission.  Figure 8: Enlarged
section of Figure 3 with 13-pixel binning to show lack of CO Fourth Positive
Group emissions between 1350 and 1420 Å . CO emission features at 1368, 1384,
1392, and 1420 Å are absent.

###  3.2 Coma Emission Modeling

For each observed atomic emission feature we calculate the heliocentric
velocity dependent fluorescence efficiency, or g-factor, from the Einstein
coefficients, oscillator strengths, and energy levels available through the
National Institutes of Standards and Technology Atomic Spectra
Database333https://www.nist.gov/pml/atomic-spectra-database (Kramida et al.,
2019). To properly account for the solar emission feature shapes we implement
the high-resolution _SUMER_ spectrum from Curdt et al. (2001), available
through the BASS200 archive444http://www.bass2000.obspm.fr/solar_spect.php.
The high resolution spectrum is in relative flux units, normalized to solar
emission at 680 Å . The solar spectrum from each day is then determined by
multiplying the daily averaged flux from the TIMED-SEE instrument (Woods et
al., 1998, 2000)555Available at
https://www.lasp.colorado.edu/lisird/data/timed_see_ssi_l3/ for a given day of
observations at 680 Å by the _SUMER_ -averaged relative spectrum to produce a
high resolution ($\sim$0.01 Å ) solar spectrum for calculating g-factors.
Solar flux is integrated over a bin $\pm$0.7 Å of the red/blueshifted
transition wavelength and the g-factor calculated in accordance with Equation
3 in Feldman et al. (2004).

Once g-factors have been calculated for the detected atomic emission features,
average column densities can be retrieved from the measured brightnesses. This
is done via the equation:

$\bar{N_{a}}=\frac{(10^{6}B_{i})}{g_{i}}$ (2)

where $\bar{N_{a}}$ is the atomic column density in molecules cm-2, $B_{i}$ is
the integrated brightness of feature in Rayleighs, and $g_{i}$ is the g-factor
of the atomic transition in phts s-1 molecule-1. Once a column density has
been found for the COS aperture, we can attempt to find the production rates
of the likely parent molecules for each atomic species by running the two- or
three component Haser model for each observation’s timestamp heliocentric and
geocentric distance.

$Q_{n}=\frac{\bar{N_{a}}(\pi\rho^{2})}{H(r_{h},\Delta)}$ (3)

where $Q_{n}$ is the neutral production rate, $\rho$ is the radius that the
COS aperture subtends at the comet in cm, and $H(r_{h},r_{\delta})$ is the
two- or three-component Haser model result integrated as a function of
heliocentric and geocentric distance of 46P for the subtended size of the COS
aperture at 46P. By multiplying the average column density by the aperture
area the total number of molecules in the field can be found for the
numerator. The Haser model is numerically integrated along the line of sight
for a range of aperture radii, which are then summed to produce a result with
units of s, as it has not been multiplied by a production rate. Production
rates for likely parent molecules were calculated using two- and three-
component Haser models (Haser, 1957; Festou, 1981). Given the small area
subtended by the COS aperture at the distance of 46P/Wirtanen, which is only
slightly larger than the collisional radius for the outflowing neutrals
($\sim$300 km, Eq. 5 of Festou (1981)), we only used these models for the 0”
pointing observations. Lifetimes for H2O, H2S, CS2, and S2 were taken from
Huebner & Mukherjee (2015), Meier & A’Hearn (1997), and A’Hearn et al. (1983)
for solar minimum. The velocity for water was assumed to follow the
0.85$r_{h}^{-2}$ km s-1 relation from Combi et al. (2004), while the sulfur
bearing molecules and atoms were given initial velocities of 0.59$r_{h}^{-2}$
km s-1 from Jackson et al. (1986). We note that for the velocity of OH
resulting from dissociation of H2O we use the velocity of 1.33 km s-1 found by
Fink & Combi (2004) specifically for 46P/Wirtanen. Velocities of atomic H were
set to 18 km s-1, atomic O 1.33 km s-1, and atomic S to 1 km s-1.

None of our model runs were able to produce reasonable production rates for
parent molecules, often reaching values 10-20$\times$ that of Combi et al.
(2020). A well known issue with the Haser model is its difficulty in reaching
high enough column densities near the nucleus (Festou, 1981), so this is
rather unsurprising. Similar attempts were made with a vectorial model for H
using the publicly available Web Vectorial
Model666https://www.boulder.swri.edu/wvm/, and a similar over estimation of
the water production rates found. However, the web vectorial model accurately
describes the OH column densities observed in the STIS data, so this issue is
likely limited to the narrow COS aperture. For this reason we are unable to
accurately produce parent molecule abundances from this dataset at this time,
and further work is necessary. Therefore, we will limit our analysis to the
atomic column densities and production rates.

Observation ID | Geocentric Distance (au) | Offset Angle (”) | NH (mol cm-2) | NO (mol cm-2) | NS (mol cm-2)
---|---|---|---|---|---
ldx60101 | 0.1691 | 0.0 | 1.62e+13 | 4.62e+13 | 1.27e+14
ldx60201 | 0.1756 | 0.0 | 3.74e+13 | 1.47e+14 | 2.23e+14
ldx60301 | 0.1865 | 2.5 | 1.94e+13 | 5.73e+13 | 1.12e+14
ldx60401 | 0.2089 | 2.5 | 2.04e+13 | 3.79e+13 | 2.16e+14
ldx60901 | 0.2159 | 0.0 | 3.61e+13 | N/A | N/A
ldx61001 | 0.2106 | 0.0 | 3.66e+13 | N/A | N/A
ldx61101 | 0.2097 | 0.0 | 3.56e+13 | N/A | N/A
ldx61201 | 0.2205 | 8.0 | 2.94e+13 | N/A | N/A
ldx61301 | 0.192 | 8.0 | 1.38e+13 | 2.63e+13 | 1.39e+13
ldx61401 | 0.1908 | 2.5 | 3.11e+13 | N/A | N/A
ldx61501 | 0.196 | 2.5 | 3.31e+13 | N/A | N/A
ldx61601 | 0.1968 | 2.5 | 3.20e+13 | N/A | N/A
ldx62501 | 0.202 | 8.0 | 1.95e+13 | 3.84e+13 | 8.18e+13
ldx62601 | 0.2226 | 2.5 | 3.53e+13 | N/A | N/A
ldx62901 | 0.2045 | 0.0 | 4.10e+13 | N/A | N/A
ldx63001 | 0.223 | 8.0 | 3.16e+13 | N/A | N/A
ldx63101 | 0.2297 | 8.0 | 2.72e+13 | N/A | N/A
ldx63201 | 0.2336 | 8.0 | 2.86e+13 | N/A | N/A
Table 2: Derived column densities for H, O, and S in the near-nucleus coma of
46P/Wirtanen from COS data taken between January 8 and 20, 2019. 1-$\sigma$
error bars on the column densities are dominated by variability in the solar
flux and are placed at 30% of each value. Values for NO and NS are listed as
N/A when the O I and S I emissions were not captured in the CENWAVE 1096
spectra.

##  4 Discussion

Due to the higher than expected activity of 46P in past apparitions, it may be
suggested that 46P could be analogous to 103P/Hartley 2, where CO2 gas drove
out large amounts of water ice grains which resulted in water production rates
that exceeded what was expected if water was released by the surface of the
comet alone (Bertaux et al., 1999; Fink & Combi, 2004; A’Hearn et al., 2011;
Kelley et al., 2013; Protopapa et al., 2014; Combi et al., 2019; Lis et al.,
2019). These icy grains are short lived ($\sim$104 s at 1 au) and therefore
likely to sublimate throughout the COS aperture for each of the pointings if
present in the coma of 46P (Yang et al., 2009, 2014; Protopapa et al., 2018).
The close approach to Earth of 46P/Wirtanen in December 2018 just over two
years after the end of the Rosetta mission offered a timely opportunity to
investigate this question, as well as probe the electron impact plasma
environment. Comet 46P/Wirtanen was the primary target of the Rosetta mission
before a launch delay necessitated changing the target to comet 67P/Churyumov-
Gerasimenko. As such, there is a substantial amount of literature dedicated to
46P published between 1996 and 2004 regarding the activity and volatile
abundances in previous apparitions. In contrast to 67P, which had a relatively
low water production rate for its size (3$\times$4$\times$5 km (Sierks et al.,
2015)) of 5-7$\times$1027 s-1 (Hansen et al., 2016), 46P has been more active
than expected for its relatively small size (r=0.63
km777https://news.arizona.edu/story/ua-researcher-captures-rare-radar-images-
comet-46pwirtanen ), (Lamy et al., 1998; Lis et al., 2019) , with peak water
production rates between 0.7-5$\times$1028 s-1 (Groussin & Lamy, 2003;
Groussin et al., 2007; Kobayashi & Kawakita, 2010; Combi et al., 2020). The
chemical compositions of the two comae are exceedingly different as well;
while the coma of 67P was shown to contain significant levels of CO2 and CO
(Bockelée-Morvan et al., 2016; Feldman et al., 2017), neither molecule has
been directly detected in 46P’s coma despite detections of CO2+ in UV spectra
from 1998 (Stern et al., 1998; Altwegg et al., 1999) and observations of the
same mid-UV bandpass in Venkataramani et al. (in prep). A summary of
production rates for common cometary species previously measured in the coma
of 46P/Wirtanen is given in Table 3. Given the previously known differences
between the two comets it’s intriguing to find that close examination of our
FUV spectra yield even more discrepancies between them.

Species | Production Rate(s) (1027 mol s-1) | Dates | Refs.
---|---|---|---
H2O | 3-50 | 1998,2008 | 1,2,3,4,5
CN | 0.00069 - 0.039 | 1998 | 1,2,3,4
C2 | 0.0005 - 0.065 | 1998 | 1,2,3,4
CS2 | 0.02 | 1998 | 5
Table 3: Measured production rates of common volatiles for 46P/Wirtanen during
previous apparitions. References are 1) Fink et al. (1998), 2) Schulz et al.
(1998), 3) Farnham & Schleicher (1998), 4) Altwegg et al. (1999), and 5) Stern
et al. (1998).

The measured spectra of 46P are noticeably devoid of many of the emission
features observed by the Alice ultraviolet spectrograph in the inner coma of
67P/Churyumov-Gerasimenko (Feldman et al., 2016, 2017) or those reported in
the review of COS comet observations by Feldman et al. (2018). We focus the
discussion here on the Lyman $\alpha$, O I triplet, and S I 1425 Å emissions
and what they indicate about our models of the near-nucleus emissions as well
as the implications for future observations. We note that our observations
only overlap with the four comets reported in Feldman et al. (2018) between
1400 and 1430 Å in this particular paper, limiting comparison to atomic sulfur
and CO abundances.

###  4.1 Upper limits on CO Abundance

The non-detection of the CO Fourth Positive group emissions between 1350 and
1420 Å allows us to place an upper limit on both the column density and
production rate of carbon monoxide (Fig. 8). Using wavelengths, Einstein
coefficients, and oscillator strengths for the first seven vibrational
transitions from Morton & Noreau (1994) and Beegle et al. (1999) we calculate
a total fluorescence efficiency for the (4-0) band of 1.44$\times$10-7 phts
s-1 for the average heliocentric distances and velocities for the comet-
centered pointings (Table 1). Given observational sensitivities down to 1
Rayleigh for the integrated band wavelength range between 1419 and 1421 Å, we
can then calculate an upper limit for the average column density of CO within
the COS aperture using Equation 2. We find that our observations were
sensitive to $\bar{N}_{\ce{CO}}$ of 6.9$\times$1012 cm2. Due to the high
spatial resolution of the observations we follow the same model for production
of CO as Weaver et al. (2011). This model argues that the long lifetime of CO
at 1 AU of 1.5$\times$106 s can be ignored and the production rate derived
from $Q=N_{CO}vd$, where $v$ is the velocity of CO molecules and $d$ is the
diameter of the COS aperture in cm at 46P. Using the same $v$ of
7.8$\times$104 cm s-1 as Weaver et al. (2011) and a $d$ of 3.37$\times$107 cm
we place an upper limit on the total production rate of CO at 1.8$\times$1025
mol s-1. This upper limit is near the production rate measured for
103P/Hartley 2 by Weaver et al. (2011). Taking into account the 30% error on
these g-factors, this would put the 3-$\sigma$ upper limit for the CO/H2O
ratio for the period of our observations at $<$8$\times$10-2. This upper limit
would place 46P/Wirtanen in the middle of the range of CO/H2O values in Biver
& Bockelée-Morvan (2019).

###  4.2 Upper limits on dissociative electron impact emission

The lack of any O I 1356 Å is a robust indicator that dissociative electron
impact is not a significant source of emissions for the inner coma of 46P.
Sampling the noise in the co-added spectra near the expected O I 1356 Å
feature shows that we were sensitive to approximately 1 Rayleigh integrated
over the 1352-1358 Å region. Our calculations for the emission rate of O2 and
H2O, from Kanik et al. (2003) and Makarov et al. (2004), respectively, show
that for an expected Maxwellian electron distribution characterized by a
temperature of 25 eV and an electron density of 50 electrons cm-3 the COS co-
added spectrum brightness upper limit translates to a column density of
$\sim$4$\times$1014 cm-2 for O2 and $\sim$4$\times$1016 cm-2 for H2O. If
dissociative electron impact occurs in the near nucleus coma the total
affected column of either molecule must be less than either of these stated
values. We note that this upper limit for the O2 column density is of similar
magnitude to the column densities of O2 detected by the Alice ultraviolet
spectrograph onboard the _Rosetta_ mission, but was acquired over a much
larger FOV subtended at the comet (Noonan et al, in prep.). With observations
sensitive down to 0.1 R/Å, this means that the portion of the inner coma of
46P susceptible to large scale dissociative electron impact (dc $<$ $\sim$ 50
km) could not have had a total integrated brightness greater than 1 R. This
strongly implies that dissociative electron impact in the inner coma is
unobservable from HST even with an extremely favorable apparition. The
relevant scale is simply too small to be captured adequately, even with a 2.5”
FOV and a low geocentric distance.

###  4.3 Atomic emission

Given the robust detections of three atomic emission features we investigate
the properties available from each.

####  4.3.1 Lyman-$\alpha$ emissions

Emission from the 2-1 transition of the hydrogen atom is easy to detect but
often difficult to analyze, especially on small spatial scales. Line profiles
of the Lyman-$\alpha$ emission from each of the co-added spectra show
deviation from standard Voigt profiles, indicating the presence of effects
from optically thick column densities near the nucleus convolved with the line
spread function of the COS instrument at 1215 Å. We do not detect the
deuterium 2-1 electron transition (rest wavelength 1215.33 Å) due to the
$\sim$1 Å resolution of the COS data from which a direct D/H ratio could be
calculated. However, we can place an upper limit on the abundance of deuterium
by subtracting a best fit gaussian profile from the Lyman-$\alpha$ emission
and summing the remaining emission between 1215.0 and 1215.7 Å to find
integrated brightnesses between 68 and 330 R for the co-added spectra. Given
the similar g-factors for the D (1-0) transition and the H (1-0) transition,
this corresponds to an upper limit on deuterium column density abundance
between 0.29 and 1.43$\times$1011 cm2, for a conservative upper limit on the
D/H ratio of 46P/Wirtanen of 0.005, approximately an order of magnitude larger
than is typical for Jupiter family comets and measured for 46P/Wirtanen
(Altwegg et al., 2015; Lis et al., 2019). A more involved effort to model the
emission profile of Lyman-$\alpha$ emission will be attempted in future work.

Deriving a water production rate from the Lyman $\beta$ emission is useful to
compare with the near daily water production rates calculated by Combi et al.
(2020) for the 2018-2019 apparition of 46P/Wirtanen. Finding integrated
brightnesses of the Lyman-$\beta$ feature in coadded spectra, between 65 and
170 Rayleighs, and the associated calculated g-factor between
5.0-5.33$\times$10-6 s-1, allows us to calculate aperture averaged hydrogen
column densities between 1.2 and 3.2$\times$1013 cm2. These column densities
are in agreement with those derived from Lyman-$\alpha$, but our two- and
three-component Haser models have difficulty matching these column densities.
Combi et al. (2020) use the Solor Wind ANisotropies (SWAN) instrument and a
more involved physical model of the H atom distribution, in addition to large
coma images within 8 degrees of the nucleus; here we are specifically focusing
on the coma within 2.5” of the nucleus, a radius that is approximately 23,000
times smaller. We recognize that the two-component Haser model will not
accurately represent the environment within the $\sim$340 km diameter aperture
of COS at 46P/Wirtanen, and a simple model was unable to produce production
rates within the uncertainty range of the values reported in Combi et al.
(2020). This discrepancy discourages us from using the Haser model for the
offset pointings; a hybrid model is required to properly analyze both the
emission feature profile and spatial profiles and will be described in a
future publication.

####  4.3.2 O I 1302 Å triplet emissions

The O I resonance triplet is resolved here for the first time in a cometary
coma, offering insight into the interplay between collisional- and photo-
excitation of fragment species. As illustrated in Fig. 6 each individual
transition from ${}^{3}S^{0}$ to the ${}^{3}P_{J=2,1,0}$ states is resolved at
wavelengths of 1302.2, 1304.9, and 1306.0 Å. One expects the contribution of
each transition to the total triplet emission (e.g.
$\frac{F_{1302}}{F_{1302}+F_{1304}+F_{1306}}$) to follow the ratios between
known Einstein $A$-values of the three transitions. However, this does not
seem to be the case for the O I $(^{3}S_{1})$ triplet emission observed in the
inner coma of 46P. For co-added spectra at each of the three pointing angle
offsets the normalized ratio of each transition feature was determined and is
shown in Figure 9. Both the contribution of the ${}^{3}S_{1}$ to ${}^{3}P_{2}$
($1302$ Å) and ${}^{3}S_{1}$ to ${}^{3}P_{0}$ ($1306$ Å) transitions increase
at offset 2.5”, with the $1302$ Å line further increasing at 8” offset.
Interestingly, the largest deviation from the expected triplet contribution is
for the $1302$ Å transition to ground. Both $1304$ and $1306$ Å transitions
decay to the metastable ${}^{3}P_{1}$ and ${}^{3}P_{0}$ levels, with the
largest deviations seen between the 0” and 8” offsets. All three transitions
have $A$ values in order $10^{7}-10^{8}$ s-1 (Kramida et al., 2019), leading
to a lifetime around 1.6 ns, indicating that travel and de-excitation of O I
outside the FOV contributes negligibly to the observed ratios. Additionally,
uncertainty in the $A$ values cannot be the cause; the $A$ values for the O I
triplet lines are known to within $\sim$3% (Kramida et al., 2019). In all
cases, the uncertainty in the intensity changes is considerably less than the
observed change in contribution.

We investigated the possibility that the O I triplet features may contain
contributions from the increasing density of S atoms, which have electronic
transitions from the ${}^{3}P^{o}$ to the ${}^{3}P$ state and have a series of
emissions present between 1302 and 1308 Å. We find the g-factor for the
strongest S I transitions between 1302 and 1305 Å to be $\sim$1.1$\times$10-8
s-1 for the dates in question, and using the atomic sulfur column density
calculated from the S I 1425 Å emission of $\sim$1.7$\times$1013 cm-2 we find
that less than 0.1 Rayleighs can be attributed to S I emission in the
1302-1306 Å region.

We note that the line shape of the O I triplets is well fit by Gaussian rather
than Voigt profiles, with $\sigma$ = 0.33 - 0.35 Å, in the co-added spectra in
Figure 6. The goodness of fit with Gaussian profiles is not unexpected given
that the O coma is an extended source. There is no clear evidence that would
indicate that the O I transitions are optically thick.

Assuming a water production rate of $\sim 7\times 10^{27}$ mol. s-1, the
density of molecules in our FOV (100s km from the nucleus) is in order
$7\times 10^{17}$ m-3. Assuming a collisional cross section in order
$10^{-15}$ cm-2, the mean free path $\lambda_{\textrm{MFP}}=1/\sigma\rho$
between collisions is $\sim$3 mm. The collisional frequency,
$\nu=v_{\textrm{rms}}/\lambda_{\textrm{MFP}}$, is then found from the root-
mean-square velocity of the gas and the local mean free path. Assuming an O I
gas temperature of 100 K yields $v_{\textrm{rms}}\sim$ 220 m/s, from which the
collisional rate in the near coma follows as $\sim 7.6\times 10^{4}$ s-1.
Thus, following population of the ${}^{3}S_{0}$ state, the collisional
frequency is too small by 5 - 6 orders of magnitude to begin contributing via
collisional de-excitation, indicating that collisional effects are
insufficient for explaining the triplet ratios.

An alternative explanation for the triplet emission contributions may be the
incident solar radiation, with stimulated emission enhancing the observed line
intensities. The assumed incident solar flux from SUMER (see Sec. 3.2) at the
O I triplet wavelengths is approximately 1:1:1 between the 3 triplet lines,
which suggests negligible contributions of heterogenous stimulated emission
from ${}^{3}S_{1}$. However, there is extensive literature on understanding O
I triplet emission in the solar spectrum, including detailed radiative
transfer models of O I resonance line excitation (Bhatia & Kastner, 1995) and
(Carlsson & Judge, 1993) (who also found non-Voigt line profiles),
polarization (Anusha et al., 2014), and frequency cross redistribution effects
(Miller-Ricci & Uitenbroek, 2002). Our co-added spectra were observed over a
period of 9 days, during which it may be possible that changes in the solar
spectrum preferentially enhanced the $1302$ Å emission, though this
proposition conflicts with the SUMER solar spectra. Given the negligible
contribution from collisional effects, the most likely cause of the deviations
in relative triplet emission intensities (Fig. 9) is the incident solar flux.
As these lines are also sensitive diagnostics at the source of the O I solar
flux, one can expect similar diagnostic potential when observed in an O-rich
environment such as cometary comae. In particular, these lines may offer an
additional way to distinguish between photon- and electron-dominated
environments in future observations.

Understanding atomic oxygen emissions and its implications for high resolution
spectroscopy of the near-nucleus coma are a critical component for improving
the scientific return from comet spectroscopy. Given the observed differences
in O I triplet emission with offset angle, these transitions may prove to be
useful diagnostics of the conditions in the inner coma. At present, developing
a time-dependent atomic model of the incident solar radiation producing
${}^{3}S_{1}\rightarrow^{3}P_{J=0,1,2}$ emission is beyond the scope of this
work. Further studies of the O I transitions in the near- and extended-comae
of comets is required to fully understand the diagnostic potential of these
lines.

Figure 9: Normalized ratio of O I triplet emission at 1302, 1304, and 1306 Å
for co-added spectra at each pointing offset in arcseconds. In general the O
${}^{3}S^{0}$ J=2 to the O 3P J=1 transition at 1302 Å becomes more populated
as the offset increases, but the line ratios are never in good agreement with
the ratios defined by the known $A$ values (Kramida et al., 2019). The
expected ratios for $A_{I}$/$\Sigma A_{n}$ are shown with a solid black line
for O I 1302 Å, a dot-dash line for O I 1305 Å, and a dotted line for O I 1306
Å. Errorbars are smaller than the markers.

####  4.3.3 S I 1425 Å emissions

The detection of S I 1425 Å emission was unexpected given the narrow FOV
(Figure 7). Such a detection was possible because of the relatively low
heliocentric velocity of 46P/Wirtanen during the observations, enabling
efficient resonance scattering of the solar S I feature (Roettger et al.,
1989; Feldman et al., 2018). Our derived column densities for atomic sulfur in
46P in Table 2 are similar to those derived by Feldman et al. (2018) for both
C/2014 Q2 (Lovejoy) and 153P/Ikeya-Zhang, which were measured to have NS of
1.2 and 2.0$\times$1014 cm-2, respectively. Given the high abundance of S in
the inner nucleus it is necessary to identify potential parent molecules from
the nucleus.

A favorite parent molecule for cometary atomic sulfur is S2, due to both its
short lifetime and single atomic components (A’Hearn et al., 1983; Meier &
A’Hearn, 1997). For production rates on the order of 1025 mol s-1 the S2 band
emission between 2800-3100 Å should be easily detectable (A’Hearn et al.,
1983). Therefore, the lack of S2 band emission in STIS observations taken as
part of the same campaign (Venkataramani et al. 2020, in prep) suggests that
consideration of CS2 is a reasonable source of S I 1425 Å emission, via
photodissociation into CS and S, is necessary. CS2 has a relatively short
lifetime of 590 s (Jackson et al., 1986) compared to the 82000 s lifetime of
H2O (Combi et al., 2004), and is just approximately 150 s longer than that of
S2 (Meier & A’Hearn, 1997). However, the previously measured CS2 production
rates between 2-5$\times 10^{-2}$ that of H2O (Stern et al., 1998), and a
similar measurement of CS production rates 1-2$\times 10^{-2}$ that of H2O
from the concurrent STIS observations (Venkataramani et al., in prep) are
insufficient to explain the observed abundance of atomic sulfur in the
observations. Other molecules identified in Feldman et al. (2018) like H2S and
SO2 could also contribute to the atomic sulfur column density, but require two
dissociations to produce a sulfur atom. This makes them rather unfavorable as
dominant sources for the inner coma, but necessary to consider in future
modeling. As stated in other sections it is difficult to derive reasonable
production rates of the likely parent molecules from these extreme inner coma
column densities with empirical Haser and physical vectorial models, and a
more robust modeling approach is required.

####  4.3.4 Atomic Production Rates of the near-nucleus coma

With detections of H, O, and S we can place some constraints on the atomic
production rates of the inner coma of 46P from UV observations. However, we
note that these values are representative of the near-nucleus coma, not of the
overall cometary abundance, and are therefore difficult to directly compare
with other comets. For this reason, we derive the atomic production rates for
the near-nucleus coma from the comet-centered observations. From the column
densities in Table 2 we derive QH, QO, and QS with a simple Haser model for
column densities, with lifetimes of 1.5$\times$106, 1.8$\times$106, and
9.1$\times$106 s for H, O, and S (Huebner & Mukherjee, 2015; Meier & A’Hearn,
1997). We use a velocity of 18 km/s for H and for O and S a velocity more
reflective of the OH velocity for 46P/Wirtanen as found by Fink & Combi
(2004), 1.33 km/s. We can then use the equation for column density from a
simple Haser model:

$Q=\bar{N}2\pi\rho ve^{\rho\beta}$ (4)

where N is the average columnn density across the aperture, $\rho$ is the
distance from the nucleus, and $\beta=(v\tau)^{-1}$, where $v$ is the velocity
of the atoms and $\tau$ is the lifetime.

We find that the average atomic production rates for H and O are
3.4$\pm$0.3$\times$1027 and 1.3$\pm$0.7$\times$1027, respectively. The
similarity between QH and the water production rates of Combi et al. (2020)
suggest that the dominant source of H in the inner coma is indeed the first
dissociation of H2O, and that both Haser and vectorial models are unable to
accurately represent the densities observed. The production rate of O is
approximately two times lower than that of H. Given that two dissociations of
H2O are needed to produce a single O atom the discrepancy between QH and QO is
not unexpected. H2O is therefore capable of producing all observed O and no
other abundant oxygen-bearing molecules are needed to explain the observed
column densities. This is in agreement with our non-detection of CO and CO2+
in Venkataramani et al., in prep.

The production rate of sulfur is less easily explained. The inner coma of
46P/Wirtanen has QS of 2.3$\pm$0.5$\times$1027 s-1, rivaling the production of
hydrogen atoms. As described in the previous section this number is difficult
to obtain with the known production rates of sulfur-bearing CS2, and the
preferred parent S2 does not have the corroborating emission in the 2600-2900
Å range in STIS data for the needed production rates. Calmonte et al. (2016)
provide a variety of sulfur-bearing molecules detected in the near-nucleus
environment of 67P/Churyumov-Gerasimenko, but to match the COS observations
there are two clear constraints. Whatever the unknown sulfur parent, it must
have a lifetime on the order of 100’s of seconds and be produced directly
either from the nucleus itself or from sublimating grains within the first
10’s of km from the nucleus. In addition, the non-sulfur daughter products of
the dissociation cannot contribute substantially to either the H or O columns.
This is similar to one particular finding of Calmonte et al. (2016); 27% of
atomic S in the inner coma of 67P/Churymov-Gerasimenko could not be linked to
a parent molecule. The Alice UVS also reported significant amounts of atomic
sulfur in the coma, with no clearly identifiable parent molecule (Feaga et
al., 2015; Feldman et al., 2017). A similar situation may be present in the
inner coma of 46P/Wirtanen, where a large component of S atoms has no clearly
identifiable parent molecule, and may be sublimating off of the cometary
surface or from grains in the inner coma. This conclusion also warrants a
closer look at the production of sulfur in the inner coma of other comets
observed with COS, especially C/2014 Q2 (Lovejoy) and 153P/Ikeya-Zhang
(Feldman et al., 2018).

##  5 Summary

In this paper we have presented spectra of 46P/Wirtanen from 900 - 1430 Å
taken when the comet was between 0.16 and 0.23 au from Earth and 1.12 and 1.17
au from the Sun. During this period the 2.5” diameter aperture of HST COS
subtends between 337 and 402 km at the comet, allowing portions of the near-
nucleus coma to be observed. Our results can be summarized as follows:

  1. 1.

We found no evidence of CO Fourth Positive Group emission between 1350 and
1430 Å, and use the (4-0) band at 1420 Å to place a 3$\sigma$ upper limit on
the production rate of CO at 3.6$\times$1025 mol s-1, approximately 8% that
measured for H2O.

  2. 2.

No evidence of dissociative electron impact was detected via the semi-
forbidden O I 1356 Å emission feature. We place upper limits on the aperture
averaged column densities of H2O and O2 susceptible to dissociative electron
impact at $\sim$4$\times$1014 cm-2 for O2 and $\sim$4$\times$1016 cm-2 for
H2O.

  3. 3.

The O I 1302 - 1306 Å triplet was resolved for the first time in a cometary
coma, yielding relative line ratios which change with offset and are
inconsistent with known $A$ values. Future observations will be required to
uncover the diagnostic potential of these lines.

  4. 4.

Derived atomic production rates of H and O imply that the only substantial
source of H and O in the coma is H2O. This suggests that CO2 and O2 are not
abundant in the coma of 46P/Wirtanen for the 2018-2019 apparition.

  5. 5.

The derived production rates of atomic sulfur are only slightly less than that
of H. This production rate is difficult to explain with the known sulfur-
bearing molecules on 46P and suggests that atomic sulfur may be entering the
near-nucleus coma directly from the nucleus or grains very near the surface,
similar to 67P/Churyumov-Gerasimenko (Calmonte et al., 2016).

Attempts to derive parent molecule production rates using Haser and vectorial
modeling were unable to produce values within reasonable agreement of water
production rates from Combi et al. (2020). Monte Carlo modeling of the inner
coma is required to properly interpret observations taken at offsets and
perform full analysis of the information contained within the Lyman-$\alpha$
and O I 1302 Å emission profiles. Given the continued observed abundance of
sulfur in the inner nucleus of comets (Feldman et al., 2018) we recommend a
re-examination of past comets observed with COS and STIS with improved
modeling as well as further study of possible pathways for atomic sulfur to be
introduced into the inner coma.

## Acknowledgements

Based on observations with the NASA/ESA/CSA Hubble Space Telescope obtained at
the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy. All authors extend their sincere
thanks to Alison Vick, Tom Brown, Tony Sohn, and William Fischer for helping
schedule and execute these challenging observations. Incorporated, under NASA
contract NAS5-26555. All authors acknowledge support by HST program number
GO-15625 (PI D. Bodewits), which was provided through a grant from the STScI
under NASA contract NAS5-26555. Part of this research was conducted at the Jet
Propulsion Laboratory, California Institute of Technology, under a contract
with NASA.

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