# UV Spectral Characterization of Low-Mass Stars With AstroSat UVIT for
Exoplanet Applications: The Case Study of HIP 23309

Sukrit Ranjan Center for Interdisciplinary Exploration and Research in
Astrophysics & Department of Physics and Astronomy, Northwestern University,
Evanston, 60201, USA  Indian Institute of Astrophysics, Bengaluru, 560034,
India  University of Arizona, Lunar and Planetary Laboratory/Department of
Planetary Sciences, Tucson, 85721, USA  Prasanta K. Nayak Institute of
Astrophysics, Pontificia Universidad Católica de Chile, Av. Vicuña MacKenna
4860, 7820436, Santiago, Chile  Department of Astronomy and Astrophysics, Tata
Institute for Fundamental Research, Mumbai, 400005, India  J. Sebastian Pineda
University of Colorado-Boulder, Boulder, 80309, USA  Mayank Narang Academia
Sinica Institute of Astronomy & Astrophysics, 11F of Astro-Math Bldg., No.1,
Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C.  Department of Astronomy
and Astrophysics, Tata Institute for Fundamental Research, Mumbai, 400005,
India

(Accepted June 23, 2023)

###### Abstract

Characterizing rocky exoplanet atmospheres is a key goal of exoplanet science,
but interpreting such observations will require understanding the stellar UV
irradiation incident on the planet from its host star. Stellar UV mediates
atmospheric escape, photochemistry, and planetary habitability, and
observations of rocky exoplanets can only be understood in the context of the
UV SED of their host stars. Particularly important are SEDs from
observationally favorable but poorly understood low-mass M-dwarf stars, which
are the only plausible targets for rocky planet atmospheric characterization
for the next 1-2 decades. In this work, we explore the utility of AstroSat
UVIT for the characterization of the UV SEDs of low-mass stars. We present
observations of the nearby M0 star HIP 23309 in the FUV and NUV gratings of
UVIT. Our FUV spectra are consistent with contemporaneous HST data and our NUV
spectra are stable between orbits, suggesting UVIT is a viable tool for the
characterization of the SEDs of low-mass stars. We apply our measured spectra
to simulations of photochemistry and habitability for a hypothetical rocky
planet orbiting HIP 23309 and elucidate the utility and limitations of UVIT in
deriving UV SEDs of M-dwarf exoplanet hosts. Our work validates UVIT as a tool
to complement HST in the characterization of exoplanet host stars and carries
implications for its successor missions like INSIST.

M dwarf stars (982), Exoplanet atmospheric composition (2021), Ultraviolet
telescopes (1743), Astrobiology (74), Habitable planets (695), Spectral energy
distribution (2129)

††journal: AJ††facilities: HST(STIS), AstroSat (UVIT)††software: MEAC (Hu et
al., 2012)

##  1 Introduction

Temperate, rocky planets orbiting other stars are abundant, and upcoming
facilities will have the ability to characterize their atmospheres (Dressing &
Charbonneau, 2015; Bryson et al., 2021; Rodler & López-Morales, 2014; Morley
et al., 2017; Lustig-Yaeger et al., 2019). Characterization of rocky exoplanet
atmospheres will probe their surface processes including potentially
constraining the presence or absence of life (Schwieterman et al., 2018), and
such constraints are high priorities for the astronomical community (National
Academies of Sciences Engineering and Medicine, 2018, 2021). However,
interpreting measurements of rocky exo-atmospheres requires understanding the
UV spectral energy distribution (SED) of their host stars. Stellar UV is a
first-order control on the surface-atmosphere system of rocky planets, and
mediates atmospheric escape, photochemistry, and habitability, meaning that
observations of rocky exo-atmospheres can only be understood in the context of
the SED of their host star (Teal et al., 2022). This observation has motivated
observational and theoretical work aimed at constraining the high-energy
emission of exoplanet host stars (e.g., France et al. 2013; Viswanath et al.
2020).

Characterization of the UV emission of low-mass ($0.08-0.6~{}M_{\sun}$)
M-dwarf stars is of particular importance to the quest to characterize rocky
exo-atmospheres. Most stars are M-dwarfs, and planets orbiting M-dwarfs are
uniquely accessible to characterization (Scalo et al., 2007; Shields et al.,
2016). Consequently, M-dwarf planets are the _only_ plausible targets for in-
depth atmospheric characterization and biosignature search with near-term
facilities such as JWST and the ELTs (Cowan et al., 2015; Morley et al., 2017;
Batalha et al., 2018; Tremblay et al., 2020). However, M-dwarf UV emission is
very different from the UV emission of higher-mass stars. These differences
are predicted to impact atmospheric escape, atmospheric photochemistry, and
habitability relative to Sun-like stars (Segura et al., 2005; Ramirez &
Kaltenegger, 2014; Luger & Barnes, 2015; Rugheimer et al., 2015; Dong et al.,
2017; Rimmer et al., 2018; Wordsworth et al., 2018; Harman et al., 2018;
Tilley et al., 2019; Schwieterman et al., 2022), motivating observational
efforts to characterize the SEDs of M-dwarf stars both directly (e.g., France
et al. 2013, 2016; Pineda et al. 2021) and by using measurements to calibrate
models (e.g., Peacock et al. 2019a; Duvvuri et al. 2021; Diamond-Lowe et al.
2021). Such observations must be done from space due to telluric absorption,
and to-date are primarily executed on the Hubble Space Telescope (HST) (e.g.,
France et al. 2016). However, the energetic emission of M-dwarfs remains
underconstrained, in part because of the need to use extremely scarce HST
time, and in part, because the considerable diversity of M-dwarfs necessitates
a large number of observations to constrain their emission as a function of
parameters such as mass and age (Loyd et al., 2021; Pineda et al., 2021).
Efforts have been made to predict M-dwarf UV SEDs from more tractably-measured
emission lines (Youngblood et al., 2017; Melbourne et al., 2020), but the use
of such approximations may lead to significant inaccuracies in models of rocky
exoplanet atmospheres. It remains preferred to characterize the UV SEDs of
planet-hosting M-dwarf stars to accurately model the atmospheres of their
orbiting planets (Teal et al., 2022).

The UltraViolet Imaging Telescope (UVIT) on the Indian Space Research
Organization (ISRO) Astronomy Satellite (AstroSat) mission is in principle
well-suited to the characterization of the UV SEDs of M-dwarf stars. UVIT is
composed of two 38-cm telescopes with a field of view of 28′ in diameter. One
telescope is dedicated to observations in the FUV region (130-180 nm), while
the other one is for NUV (200-300 nm) and visible regions (320-550 nm). The
UVIT operates in photon counting mode and observes simultaneously with both
the telescopes with a spatial resolution (FWHM) $<$1.′′5 (Tandon et al.,
2020), better than the resolution of GALEX ($\sim 5^{\prime\prime}$). Both FUV
and NUV channels incorporate multiple narrow to broad-band filters. UVIT also
has a facility for low-resolution slit-less spectroscopy in both NUV and FUV
bands: one grating is mounted in the NUV wheel (now inoperative) and two
gratings (with orthogonal dispersions axis) in the FUV wheel. The maximum
efficiency is achieved in the $-$2 order of the FUV gratings with a spectral
resolution of 15 Å and $-$1 order of the NUV grating with a spectral
resolution of 33 Å (Tandon et al., 2020; Dewangan, 2021). The NUV channel is
now inoperative and only archival studies are possible; the FUV channel
remains operative.

In this paper, we explore the utility of UVIT to constrain the SEDs of M-dwarf
stars using HIP 23309 as a case-study. HIP 23309 is a young ($\sim 24$ Myr),
rapidly rotating, early-type M-dwarf star (Pineda et al. 2021; Table 1). The
age estimate for HIP 23309 is based on its membership in the $\beta$ Pictoris
moving group (Messina et al., 2010; Pineda et al., 2021), which is $24\pm 3$
Myr old (Bell et al., 2015). We secure spectrally resolved observations of HIP
23309 in the UVIT FUV ($130-180$ nm) and NUV ($200-300$ nm) gratings. Standard
UVIT calibration pipelines are able to extract resolved spectra of HIP 23309
with no modification (Dewangan, 2021). We compare our FUV data to
contemporaneous HST measurements and find agreement at levels that we consider
satisfactory for use in exoplanet photochemistry studies. We perform
atmospheric and surficial photochemical modeling to elucidate the utilities
and limitations of such measurements for studying the atmospheres of rocky
planets and assessing their habitability. We close by commenting on the
potential of UVIT for studies of M-dwarfs and their exoplanets, and
recommendations for future facilities like INSIST (Subramaniam, 2022).

Table 1: HIP 23309 Stellar Parameters Parameter | Value | Reference
---|---|---
Spectral Type | M0 | Torres et al. (2006); Pineda et al. (2021)
Effective Temperature |  $3886^{+28}_{-27}$ K | Pineda et al. (2021)
Mass |  $0.785^{+0.009}_{-0.010}$ $M_{\sun}$ | Pineda et al. (2021)
Radius |  $0.932\pm 0.014$ $R_{\sun}$ | Pineda et al. (2021)
Luminosity |  $6.82^{+0.12}_{-1.1}\times 10^{32}$ erg s-1 | Pineda et al. (2021)
Rotation Period |  $8.6\pm 0.07$ days | Messina et al. (2010)

##  2 Observations and Data Reduction

We observed HIP 23309 with the FUV-Grating1 (FUV-G1) and NUV-Grating of UVIT
as a part of the AO-1 proposal (PI: Lalitha Sairam) on 24-11-2017 with an
exposure time of $\sim$12 ksec in each channel. The observations were
completed in multiple orbits. We applied corrections for spacecraft drift,
flat-field, and distortion, using the software CCDLAB (Postma & Leahy, 2017)
and created images for each orbit. Then, the orbit-wise images were co-aligned
and combined to generate final science-ready images in each grating.
Astrometry on the images was also performed using CCDLAB by comparing the
Gaia-DR2 source catalogue. The zoomed-in FUV and NUV grating images are shown
in Figure 1, with boxes indicating the different spectral orders. We extracted
the source spectrum of $-$2 order from FUV-G1 image and $-1$ order from the
NUV image with a cross-dispersion width of 50 pixels (1 pixel $\sim$
$0.^{\prime\prime}41$). Background regions are selected exactly in the same
pixel range above the source spectrum along the dispersion axis for both
gratings. We used the UVITTools package written in the Julia language by
Dewangan 2021 to extract spectral files for a given grating order and cross-
dispersion width. Wavelength and flux calibration to the spectra are done
using standard UVIT source calibrators: HZ4 and NGC40. We re-ran the pipeline
on the raw HZ4 and NGC40 data and recovered results consistent with IUE
measurements and Dewangan (2021), confirming that we were not using the
pipeline erroneously. We show the FUV and NUV spectra of HIP 23309 in Figure
2.

Figure 1: Images of HIP 23309 in FUV-Grating1 (left) and NUV-Grating (right)
are shown here. Different grating orders are marked in green.

##  3 Measured Spectra of HIP 23309

###  3.1 UVIT Measurements of HIP 23309

In the NUV spectrum (right panel of Figure 2), we can clearly identify three
peaks: one at 2800 Å due to strong Mgii h and k lines, and the other two peaks
at 2400 Å and 2600 Å due to strong Feii lines. The strong Mgii h and k lines
emissions indicate that the star is chromospherically active (Blanco et al.,
1979; Doherty, 1985). In the FUV spectrum (left panel of Figure 2),
comparative poor signal-to-noise (SNR) makes it difficult to identify emission
lines. We can clearly identify the strong Civ line at 1560 Å despite low SNR,
however, the identification of Heii and Oiii are less reliable for further
analysis.

Figure 2: UVIT FUV (left) and NUV (right) spectra of HIP 23309. Multiple NUV
lines are detected; detections of FUV lines are much weaker, with only C IV
being clearly detected.

###  3.2 Robustness of UVIT M-dwarf Spectra

We next assess the accuracy of our spectra of HIP 23309 from UVIT. For the
far-UV wavelength regime, we are able to directly compare our UVIT
observations to contemporaneous FUV measurements collected by HST within 1 day
of this work. (Table 2). For the near-UV data, we evaluate the orbit-by-orbit
stability of our measured spectrum.

####  3.2.1 FUV Spectrum: Comparison to HST Data

We coordinated our UVIT observations of HIP 23309 with HST STIS observations
of HIP 23309 collected as part of the FUMES survey (PI: J. S. Pineda; Pineda
et al. 2021; Youngblood et al. 2021). The spectral coverage of the UVIT FUV
Grating1 and the HST STIS overlap from 130-170 nm, enabling us to compare
spectra derived from the two facilities.

Table 2: Summary of HIP 23309 Observations Discussed in this Work Facility | Instrument | Grism/Grating | Orbits | Observation Start Time (UT) | Total Integration Time | Reference
---|---|---|---|---|---|---
HST | STIS | G140L (115-173 nm) | 1 | 2017-11-24 01:15:22 | 2350 s | Pineda et al. (2021)
AstroSat | UVIT | FUV Grating1 (130-180 nm)aaTandon et al. (2017) | 7 | 2017-11-24 10:16:44 | 11967 s | This Work
AstroSat | UVIT | NUV (200-300 nm)aaTandon et al. (2017) | 7 | 2017-11-24 10:16:44 | 12027 s | This Work

The UVIT FUV and STIS measurements of HIP 23309 broadly agree to within a
factor of $\sim 2$, but the UVIT measurements systematically exceed the STIS
measurements (Figure 3). The 130-170 nm continuum flux measured by UVIT is
$(5.62\pm 0.34)\times 10^{-13}$ erg s-1 cm-2, compared to $(4.02\pm
0.04)\times 10^{-13}$ erg s-1 cm-2. This corresponds to a 40% overestimate by
UVIT and 4.63$\sigma$ tension relative to STIS. To compare the STIS data to
the UVIT data spectrally, we (1) convolve the STIS data against a Gaussian
with full-width at half-max of 14.63, corresponding to the resolution of the
UVIT FUV grating (Dewangan, 2021), and (2) rebin the smoothed spectrum to the
same wavelength scale as the UVIT data using the coronograph package and
conserving total SNR in each bin (Robinson et al., 2016; Lustig-Yaeger et al.,
2019). The synthetic spectrum derived from the HST data is in tension with the
UVIT spectrum by an average of 1.46$\sigma$ (Figure 3). The disagreement is
strongest in the continuum region between lines, where HIP 23309 emits very
little flux ($3.10\pm 0.09\times 10^{-16}$ erg s-1 cm-2 Å-1 in FUV; Pineda et
al. 2021). For example, integrated from 1410-1510 Å, the flux measured by UVIT
is $5.5\pm 1\times 10^{-14}$ erg cm-2 s-1 Å-1, whereas the flux measured by
HST is $3.39\pm 0.1\times 10^{-14}$ erg cm-2 s-1 Å-1, a tension of
1.9$\sigma$. The agreement between HST and UVIT is better for major lines,
i.e. Civ. The Civ line emission from UVIT is $9.8\pm 3.5\times 10^{-14}$ erg
cm-2 s-1, compared to $6.9\pm 0.2\times 10^{-14}$ erg cm-2 s-1 from HST
(Pineda et al., 2021). We conclude that the UVIT FUV data broadly reproduce
the HST data to within a factor of 2, but that there is the potential for
uncorrected systematic error in the UVIT data, including potentially an offset
of $\sim 2\times 10^{-16}$ erg s-1 cm-2 Å-1.

Figure 3: Comparison between HST and AstroSat-measured SEDs of HIP 23309 in
the FUV (1300-1700 A). Top plot shows data in linear y-scale with error bars;
bottom plot is identical to top plot except with log y-scale and omission of
error bars for easier viewing. The cyan and black data correspond to the
unbinned HST and UVIT data, respectively. The blue points correspond to HST
data degraded to the approximate quality of the AstroSat data, to enable a
closer comparison. The green points correspond to the AstroSat data binned
down by a factor of 5, to improve SNR and enable better visual comparison. We
assumed Gaussian uncorrelated noise when rebinning the AstroSat data.

####  3.2.2 NUV Spectrum: Stability of Measurement

The SNR of the NUV data is high enough to support orbit-by-orbit decomposition
(Figure 4). The band-integrated (220-290 nm) orbit-by-orbit NUV data are
consistent with the mean to within 10% but vary from it by as much as
$8.4\sigma$. We first consider potential systematic effects within our data
before discussing physical interpretations.

It is possible that the measurement errors are underestimated, e.g. due to an
uncharacterized source of red noise. Indeed, the orbit-by-orbit band-
integrated fluxes display a weak increase with orbit number, a potential
systematic effect with the observatory. Comparing the orbit-by-orbit data to
the mean on a spectral basis, we find that the average deviation from the mean
$A$:

$A=\frac{1}{N}\sum_{i=0}^{N}\frac{F_{i}-\bar{F_{i}}}{\sqrt{\sigma_{i}^{2}+\bar{\sigma_{i}}^{2}}},$
(1)

has values ranging from $1.83$ to $3.74$. This indicates that the errors are
could be underestimated by a factor of a few.

Alternately, our measurements may reflect variations in stellar flux due to
activity (Loyd & France, 2014; Loyd et al., 2018; Duvvuri et al., 2023). We do
not detect evidence of large flares (variation in total flux of $\geq
2\times$) on minute timescales. Stochastic variability on short timescales in
the Ca ii K line for HIP 23309 is on the order of 5% (Duvvuri et al., 2023),
simultaneous with our FUV data set, several hours prior to the start of our
NUV data. Loyd et al. (2018) also demonstrate $\sim$10% broadband quiescent
FUV variability in their young M-dwarf sample, with one object displaying up
to 50% variations relative to the median, which can translate to factors of a
few from minimum to maximum. Our observed NUV variability is consistent with
these data as we expect the NUV variability to fall between the optical and
FUV measurements. We also note that the fractional stochastic variability may
not be correlated with absolute activity (Duvvuri et al., 2023).

However, our NUV data show a clear trend over time (Figure 4. This might be
explained through rotational variation as an active region comes into view,
enhancing the total NUV flux. To test this idea, we fit a simple sinusoidal
model ($f(t)=f_{0}+A\cos(\phi+\omega t)$) to the 7 NUV data points, with
$\omega=2\pi/P$ for $P=8.6\pm 0.07$ d. We find that the best solution places
our data points at the rising part of the sinusoidal trough, with 50-70%
fractional variation about the model median total NUV flux. This result
appears to be similar to that of the most quiescent variable target in Loyd et
al. (2018). Although, we cannot confirm an astrophysical origin for the
observed NUV variability, it is plausible.

Figure 4: Orbit-by-orbit NUV spectral (left) and band-integrated (right) flux
of HIP 23309 as measured by UVIT. The band-integrated orbit-by-orbit fluxes
agree with the mean flux to within 10% but are in statistically significant
tension with the mean. This indicates that either our errors are
underestimated, or that there is stellar variability at the $\sim 10\%$ level.

###  3.3 HIP 23309 SED in Context

We compare our FUV and NUV spectra of HIP 23309 to spectra of other low-mass
stars to assess whether our measurements are sensible in context (Figure 5).
We specifically compare the spectral flux at the stellar surface of HIP 23309
to MUSCLES M-dwarf and K-dwarf spectra (v2.2, France et al. 2016; Youngblood
et al. 2016; Loyd et al. 2016), to the composite VPL spectrum of AD Leo
(Segura et al., 2005) and to StarCAT measurements of AU Mic in the FUV (Ayres,
2010). The MUSCLES targets chosen are mature stars which bracket HIP 23309 in
spectral type, whereas AD Leo is a highly active M-dwarf and AU Mic is a young
pre-main sequence (22 Myr, Mamajek & Bell 2014; Plavchan et al. 2020) early
M-dwarf star similar to HIP 23309. HIP 23309’s FUV emission is similar to AU
Mic’s, which is sensible in that AU Mic is of similar spectral type and age to
HIP 23309. HIP 23309 is brighter in the FUV and NUV compared to main-sequence
M-dwarfs of similar spectral type, consistent with the finding that pre-main
sequence M-dwarfs display elevated UV emission compared to main-sequence
M-dwarfs due to enhanced stellar activity and larger radii (Shkolnik & Barman,
2014). This additional activity flattens the slope of the spectrum across the
entire NUV band, as seen in the most active M-dwarfs (Segura et al., 2005;
Walkowicz et al., 2008), with much stronger emissions at short wavelengths
relative to the main-sequence M-dwarfs. At longer wavelengths in the NUV, the
HIP 23309 stellar flux begins to converge with main-sequence stars of similar
spectral type, which is consistent with the increasing importance of
photospheric emission at longer wavelengths. We conclude that to zeroth order,
our measured NUV and FUV spectra of HIP 23309 are sensible in the context of
other measured spectra of low-mass stars.

Figure 5: Stellar flux of HIP 23309 at the stellar surface
($F_{\star}\times\frac{d_{\star}^{2}}{r_{\star}^{2}}$) in the FUV (left) and
NUV (right), compared to other low-mass stars. Main-sequence comparison stars
are ordered by $T_{eff}$ (Loyd et al., 2016). HIP 23309’s stellar flux is much
higher than mature stars of similar spectral type at short wavelengths due to
enhanced activity because of its young age but converges with them at longer
wavelengths where photospheric emission dominates. HIP 23309’s flux in the FUV
is similar to AU Mic’s, another extremely young M-dwarf of a similar spectral
class. Overall, our spectra of HIP 23309 appear sensible in the context of
other measured spectra of low-mass stars.

##  4 Applications of UV SEDs to Rocky Exoplanet Modeling

In this section, we explore the implications of our measured SED of HIP 23309
for a hypothetical temperate terrestrial planet orbiting HIP 23309. Our intent
is to illustrate the utility of M-dwarf SEDs for studying rocky exoplanets,
and particularly for questions of astrobiological interest.

###  4.1 Implications of HIP 23309 SED for UV-Dependent Prebiotic Chemistry

UV light has been proposed as necessary to the origin of life, but the
surfaces of habitable M-dwarf planets are low-UV environments (Ranjan et al.,
2017). Rimmer et al. (2018) showed that HCN photohomologation chemistry used
in proposed prebiotic pathways for ribonucleotide synthesis (Xu et al., 2018,
2020; Rimmer et al., 2021b) require $F_{200-280nm}\geq(6.8\pm 3.6)\times
10^{10}$ cm-2 s-1 nm-1 to function, where $F_{200-280nm}$ is the surface flux
averaged from 200-280 nm. This means that if HCN photohomologation is required
for the emergence of life, then life cannot emerge on planets that have access
to less than this critical flux of UV. Rimmer et al. (2018) find that the
quiescent emission from known M-dwarfs is insufficient to meet this criterion
and that HCN photohomologation chemistry can only proceed on planets orbiting
M-dwarfs if enabled by transiently enhanced UV from flares. Laboratory efforts
to experimentally determine if flare UV can substitute for quiescent UV in HCN
photohomologation are now under
way111https://www.mrao.cam.ac.uk/~pbr27/resources.html, accessed 12/7/2022. If
flares cannot compensate for low-steady state UV, then life search on M-dwarf
exoplanets may enable tests of theories the origin of life (Rimmer et al.,
2021a; Rimmer, 2023).

We consider whether the UV emission from HIP 23309 meets the Rimmer et al.
(2018) criterion. We assembled an FUV-NIR SED of HIP 23309 by concatenating
our UVIT-measured FUV and NUV fluxes to the low-resolution GAIA spectrum of
HIP 23309 (Gaia Collaboration et al., 2022). We set the stellar flux at
wavelengths shorter than the limits of the measured SED to 0. We scale these
observed fluxes to the flux that would be experienced by a hypothetical Earth-
like planet orbiting HIP 23309 by multiplying them by $(\frac{d_{*}}{a})^{2}$,
where $d_{*}=26.8$ pc is the distance to HIP 23309 (Malo et al., 2014; Gaia
Collaboration et al., 2022),
$a=1\texttt{AU}\times(\frac{L_{*}}{L_{\sun}*0.9})^{0.5}$ is the distance such
that the bolometric flux received at distance $a$ is equivalent to the
bolometric flux received by Earth, corrected by $0.9\times$ to account for the
redder emission from M-dwarf stars (Segura et al., 2005), and
$L_{*}=6.82\times 10^{32}$ erg s-1 (Pineda et al., 2021). We then calculated
the hemispherically-integrated surface actinic flux (“surface radiance”) for
an early Earth-like planet with a clear-sky 0.9 bar N2, 0.1 bar CO2 atmosphere
as in Ranjan et al. (2017). We find that HIP 23309 comfortably meets the
Rimmer et al. (2018) criterion, with $F_{200-280nm}=3\times 10^{11}$ cm-2 s-1
nm-1. Interestingly, we also find that the quiescent flux from the extremely
active M-dwarf AD Leo also meets the Rimmer et al. (2018) criterion
($F_{200-280nm}=2\times 10^{11}$ cm-2 s-1 nm-1), and that quiescent flux from
early (high-mass) M-dwarfs like GJ 832 also marginally meet this criterion.
This contradiction with Rimmer et al. (2018) is due to their assignment of a
ceiling on single-scattering albedo $\omega_{0}$ of 0.9222P. Rimmer, personal
communication, 12/7/2020 when calculating two-stream radiative transfer, which
underestimates surface UV in the unique regime unveiled on anoxic early Earth-
like planets (Ranjan & Sasselov, 2017; Rimmer et al., 2021b). We instead find
that planets orbiting young or active M-dwarf stars, as well as some quiescent
early M-dwarf stars, can meet the Rimmer et al. (2018) criterion, illustrating
the necessity of measuring and constraining the evolution of NUV SEDs of
individual planet-hosting M-dwarfs to understand the habitability of their
worlds (Rimmer et al., 2021a).

Figure 6: Surface radiances on early Earth-like planets orbiting different
M-dwarfs including HIP 23309, computed as in Ranjan et al. (2017). The Rimmer
et al. (2018) minimum flux criterion for HCN homologation towards
ribonucleotide synthesis is shown in pink. Extremely active stars like HIP
23309 meet the Rimmer et al. (2018) criterion, illustrating the need to
constrain UV emission of M-dwarfs over the course of their lifetimes to
understand the potential of life search on M-dwarf planets to test theories of
abiogenesis (Rimmer et al., 2021a).

###  4.2 Implications of HIP 23309 SED for Atmospheric Photochemistry

In this section, we explore the utility and limitations of AstroSat UVIT for
constraining the UV SEDs of exoplanet host stars for atmospheric
photochemistry applications. UV photochemistry controls the trace gas
composition of temperate terrestrial planet atmospheres, and accurate UV SEDs
are necessary for accurate predictions of atmospheric composition and
inferences derived thereof (Rugheimer et al., 2015; Teal et al., 2022). We
assess the impact of UV SEDs on predicted atmospheric trace gas concentrations
using a procedure established first by Kasting et al. (1997) and subsequently
extensively deployed (e.g., Segura et al. 2005; Rauer et al. 2011; Teal et al.
2022): We run a photochemical model with a fixed planetary scenario, varying
only the UV irradiation between model runs, and examine the implications for
predicted atmospheric composition. Our calculations are solely sensitivity
tests aimed at constraining the utility and limitations of using AstroSat UVIT
to constrain M-dwarf SEDs for photochemical modeling applications, as opposed
to self-consistent predictions of the atmospheric composition of temperate
terrestrial planets orbiting young M-dwarf stars.

To conduct this sensitivity test, we use the MIT Exoplanet Atmospheric
Chemistry model (MEAC, Hu et al. 2012), as updated by Ranjan et al. (2022). We
apply MEAC to the N2-dominated abiotic temperate terrestrial planet benchmark
scenario from Hu et al. (2012), which simulates a planet geologically similar
to Earth in the absence of life. We explore the effects of different estimates
of the HIP 23309 SED on the abundances of key HCO species which dominate the
atmosphere and whose remote detection could constrain surfaces processes like
the carbon cycle and the presence or absence of life (Bean et al., 2017;
Schwieterman et al., 2019; Wogan & Catling, 2020; Lehmer et al., 2020;
Thompson et al., 2022).

We begin by showing that the UVIT-measured SED alone can achieve order-of-
magnitude accuracy at best in the photochemical modeling of rocky exoplanet
atmospheres. We formulate a UVIT-only input SED for HIP 23309 as in Section
4.1, except that we do not include the luminosity correction factor of 0.9 to
maintain consistency with similar past sensitivity tests conducted with MEAC
(e.g., Seager et al. 2013). We formulate a second SED as the UVIT SED, except
that we replace the UVIT FUV measurements with the FUMES HST measurements
(Figure 7). Calculating the trace gas composition under irradiation by each of
these SEDs (Figure 8), we find that the use of the UVIT-only SED leads to
overpredictions of CH4 by $7\times$ and underprediction of CO by $6\times$
relative to the UVIT+HST SED (Table 3). Two factors drive model inaccuracy
when using the UVIT-only SED. First, the UVIT SED systematically overestimates
the continuum flux due to its lower sensitivity. This leads to overestimates
of H2O photolysis and accounts for the variation in pCO and some of the
variation in pCH4. The effects of this overestimate are amplified by the large
wavelength gap between reliable measurements in the FUV and NUV gratings,
which is interpolated in the model. The importance of the UV continuum has
been affirmed in other studies (Teal et al., 2022; Peacock et al., 2022).
Second, the UVIT SED does not reliably cover the Lyman-alpha line, where
M-dwarfs emit a substantial fraction of their flux and which is particularly
relevant to CH4 photolysis in anoxic atmospheres (France et al., 2013; Ranjan
et al., 2022). Peacock et al. (2022) reported the predicted atmospheric
composition of M-dwarf planets to be insensitive to the degree of self-
reversal in the stellar Ly$\alpha$ line, and we reproduced this finding with
our model. However, omitting Ly$\alpha$ entirely results in a much bigger
change in irradiating flux than variations in the degree of self-reversal in
the Ly$\alpha$ line, and we find this omission to result in significant
variations in predicted pCH4.

Figure 7: Input UV SEDs used for modeling the atmospheric composition (Figure 8) of a hypothetical abiotic Earth-like planet orbiting HIP 23309. Figure 8: Predicted atmospheric composition of a hypothetical abiotic Earth-like planet orbiting HIP 23309, subject to varying estimates of its UV irradiation (Figure 7).The different colors correspond to different chemical species. The left column corresponds to chemical species emitted by the surface, while the right column corresponds to photochemically generated species. In each row, the different linestyles correspond to different SEDs. The UVIT-only SED alone is insufficient to achieve highly precise photochemical modeling (top row), but can be used to calibrate other estimates of the stellar SED (middle row) to achieve higher precision, as done crudely here (bottom row) and with vastly more sophistication by, e.g., (Peacock et al., 2019a, b))) Table 3: Surface Mixing Ratios Predicted in MEAC Abiotic N2-dominated Benchmark Scenario SED | $r_{CH_{4}}(z=0)$ | $r_{C0}(z=0)$
---|---|---
HIP 23309 HST FUV | $1\times 10^{-6}$ | $6\times 10^{-8}$
HIP 23309 UVIT FUV | $7\times 10^{-6}$ | $1\times 10^{-8}$
GJ 832 (MUSCLES) | $4\times 10^{-7}$ | $3\times 10^{-7}$
HD85512 (MUSCLES) | $4\times 10^{-7}$ | $3\times 10^{-7}$
GJ 832 (Scaled) | $6\times 10^{-7}$ | $1\times 10^{-7}$
HD85512 (Scaled) | $4\times 10^{-7}$ | $1\times 10^{-7}$

We next consider how the UVIT-only SED performs in comparison to proxy SEDs
from stars of similar spectral type, which are sometimes employed when
accurate SEDs of planet-hosting stars are not available (e.g., Hu et al. 2020,
2021). We consider SEDs from GJ832 (M1.5) and HD 85512 (K6), which are well-
characterized by the MUSCLES survey (v2.2, France et al. 2016; Loyd et al.
2016; Youngblood et al. 2016), as proxies for HIP 23309 (M0). We normalize
their SEDs to the solar constant as for HIP 23309, and apply these SEDs to our
planetary scenario (Figure 7). We find that use of proxy SEDs matches the
HST+UVIT SED calculation better than the UVIT-only SED (Figure 8, Table 3).

We finally consider whether UVIT data can be used to calibrate other
techniques for estimating M-dwarf UV SEDs. For example, partial SED
measurements have been used to calibrate full-SED estimates derived from
stellar models (Peacock et al., 2019a, b, 2020). Such sophisticated estimates
are beyond the scope of this work; as proof-of-concept, we instead utilize our
UVIT measurements to crudely calibrate our use of similar stellar spectra as
proxies. Specifically, we scale our GJ832 and HD85512 SEDs by a constant
factor, such that they have the same flux integrated from 130-170 nm as our
UVIT-only HIP 23309 SED (Figure 7). We do not utilize the NUV data because the
UVIT NUV grating is no longer operative, and so it is of limited utility to
determine its utility at calibrating other estimates. For simplicity we scale
the SEDs across all wavelengths, but only the UV wavelengths have any impact
on our models since our photochemistry-only sensitivity test neglects climate
adjustments. We find that use of scaled proxy SEDs to marginally improve
agreement with the HIP 23309 UVIT+HST SED compared to unscaled proxy SEDs, in
the case of GJ832 reproducing the predicted surface concentrations of CO and
CH4 to within a factor of 2 (Figure 8, Table 3). That even an extremely crude
calibration with UVIT can marginally improve agreement suggests that it is
possible that more sophisticated calibrations with UVIT can significantly
improve agreement; we advocate such work.

##  5 Discussion & Conclusions

Constraining the UV SEDs of low-mass stars is crucial to interpreting
atmospheric measurements of the exoplanets they host, and to understanding the
habitability and urability of those planets (Deamer et al., 2022). The primary
instrument used for such studies, HST, is oversubscribed and well past its
designed lifetime. Here, we have examined the potential of AstroSat UVIT to
carry out such studies through spectrally resolved NUV and FUV grating
observations of the young M0 star HIP 23309. We robustly detect stellar lines
of Civ in the FUV and Feii and Mgii in the NUV. We find UVIT can reproduce
contemporaneous HST measurements in the FUV to within a factor of 2 and that
its orbit-by-orbit NUV measurements are stable to 10%, but that its errors may
be underestimated by a factor of a few. The UVIT-measured HIP 23309 SED
displays trends in the context of other measured M-dwarf SEDs that are
consistent with zeroth-order expectations. Taken together, the example of HIP
23309 suggests AstroSat UVIT is a viable tool for characterizing the UV SEDs
of bright, nearby low-mass stars to within a factor of a few.

We have demonstrated the utility of such measurements for planetary
applications using HIP 23309 as a case study. Archival measurements of stellar
UV from the UVIT NUV grating can inform studies of planetary habitability
because they constrain emission at the $\geq 190$ nm wavelength range relevant
to habitable planet surfaces (Ranjan & Sasselov, 2017). For example, HIP 23309
meets the Rimmer et al. (2018) criterion for sufficient UV output to power HCN
photohomologation in its habitable zone, demonstrating the necessity of
characterizing the detailed evolution of M-dwarf SEDs to understand the
potential of biosignature search on M-dwarf planets to test theories of the
origin of life (Rimmer et al., 2021a; Rimmer, 2023). Our FUV and NUV spectra
of HIP 23309 on their own permit limited accuracy in photochemical modeling of
planetary atmospheres, because the UVIT FUV grating does not reliably cover
the crucial Lyman alpha line and because the limited sensitivity and
wavelength coverage of the FUV grating leads to overestimates of the continuum
flux. However, our sensitivity tests suggest that UVIT FUV observations can be
used to calibrate estimates of M-dwarf SEDs, thereby supporting photochemical
modeling. Our work suggests a role for archival NUV observations and archival
and future FUV observations in supporting studies of exoplanet habitability
and photochemistry.

Our study bears implications for the proposed Indian Spectroscopic and Imaging
Space Telescope (INSIST; Subramaniam, 2022). The INSIST mission design
features a 1m mirror ($\sim 7\times$ the size of UVIT), suggesting a factor of
$\sim$7 improvements in signal-to-noise relative to UVIT, and continuous
wavelength coverage from the FUV to the NUV (150-300 nm). This means that
INSIST would likely outperform UVIT at characterizing the important UV
continuum. However, the termination of its wavelength coverage at 150 nm means
that INSIST would not be able to directly constrain the 121.6 nm Lyman alpha
line which can be important to temperate exoplanet photochemistry. We advocate
for the extension of INSIST’s spectral coverage to 120 nm or shorter, to
enable direct characterization of the full wavelength range relevant to
exoplanet photochemistry. However, higher resolution modes ($R\sim 10,000$)
may be necessary to accurately measure Lyman alpha fluxes accounting for the
impact of the intervening ISM (Youngblood et al., 2021). Precise measurements
of chromospheric and transition region lines like Mgii, Civ, Siiv, and/or Nv
can provide useful proxies within potential INSIST data to estimate the total
FUV flux of interesting stellar targets (Pineda et al., 2021). Nevertheless,
even if INSIST is unable to directly measure full exoplanet host star UV SEDs,
measurements from INSIST will still be able to calibrate other estimates of
M-dwarf SEDs like stellar models (Peacock et al., 2019a, b), and its coverage
of NUV wavelengths will enable planetary habitability studies (e.g., Rugheimer
et al. 2015; Ranjan et al. 2017; Rimmer et al. 2018; O’Malley-James &
Kaltenegger 2019).

JSP acknowledges support from NASA via grant HST-GO-14640. SR thanks Sujan
Sengupta for his mentorship in developing the project idea, and the Indian
Institute of Astrophysics, Northwestern University, and the University of
Arizona for their support while carrying out this project. PKN acknowledges
TIFR’s postdoctoral fellowship and support from the Centro de Astrofisica y
Tecnologias Afines (CATA) fellowship via grant Agencia Nacional de
Investigacion y Desarrollo (ANID), BASAL FB210003. We thank Sarah Peacock and
Edward Schwieterman for sharing TRAPPIST-1 spectra from Peacock et al. (2022)
as an additional validation test. We thank Gulab Dewangan for sharing the raw
data for HZ4 to enable us to verify correct functioning of our pipeline. We
thank the anonymous referee for their constructive critical feedback, which
has materially improved the quality of this manuscript. Author contribution
statement: SR conceived the project, served as co-I on the UVIT proposal,
assessed the reduced data, performed the modeling, and wrote the paper. PN led
to the reduction of the UVIT data. JSP served as co-I on the UVIT proposal,
advised on data reduction, supplied pointers to other M-dwarf spectra utilized
in this paper, and commented on fundamental stellar physics. MN was involved
with the data validation by developing an alternate reduction pipeline (Narang
2023) with PN to reduce the UVIT data. All authors reviewed and edited the
paper. Code availability: The code underlying Sections 3.2.1-3.3 and Figures
3, 4, 5, and 7 is available at https://github.com/sukritranjan/uvit_mdwarf.
The code underlying Section 4.1 and Figure 6 is modified from Ranjan et al.
(2017) and is available at
https://github.com/sukritranjan/ranjanwordsworthsasselov2017b/tree/for-uvit-
paper. The input files and simulation outputs from the MEAC code underlying
Section 4.2 and Figure 8 are available by request.

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