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Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectra of Ross 825 (K3; left), Ross 1044 (M0; middle), and Kapteyn's Star (M1; right) plotted as thick black lines reproduced from Schneider et al. (2019) and Youngblood et al. (2022). An altered version of the HST Cosmic Origins Spectrograph (COS) spectrum of Kapteyn's Star from Guinan et al. (2016) is plotted as the gray curve in the third panel. This line profile was produced by mirroring the right-hand side of the stellar component of the Lyα observation. The large RVs of Ross 1044 (−169.6 km s −1 ), Ross 825 (−340.2 km s −1 ), and Kapteyn's Star (+245.2 km s −1 ) allow their stellar Lyα emission lines to be well separated from the contaminating geocoronal airglow emission, with little Lyα flux scattered by the ISM. Model ISM transmittance curves are plotted as black dotted lines (Schneider et al. 2019; Youngblood et al. 2022). The blue profiles are the intrinsic PHOENIX model profiles, which when multiplied by the ISM transmittance curves yield the red profiles. These red profiles should reproduce the solid black curves, but severely underestimate the core flux.
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Accurately measuring and modeling the Ly α (Ly α ; λ 1215.67 Å) emission line from low-mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons. Ly α also controls the photodissociation of important molecules, like wate...
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... are two additional stars, Ross 1044 (M0) and Ross 825 (K3) that have the majority of their Lyα flux shifted out of the geocoronal line core and contamination from the ISM because of their exceptionally large RVs (>150 km s −1 ; Schneider et al. 2019). These observations reveal either no self-reversals or very slight ones in the line cores, indicating that previous PHOENIX models underpredict the Lyα line core in these stars by a factor of ∼2 (Figure 1). ...
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... this initial model setup, we were able to reproduce the observed Lyα line widths but not the line core ( Figure 1). In Section 3 we detail various analyses performed to quantify their effects on the intensity of the Lyα core flux and ultimately match the complete observed line profile. ...
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... wavelength shifts expose between 63% and 95% of the intrinsic profiles. To compare to these measurements, we convolved our models to the resolution of the observations, accounted for the radial velocity shifts, and multiplied by the ISM transmittance curves shown in Figure 1 (dotted lines). ...
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... final model spectra have good agreement with the measured Lyα profiles, including core flux, and the NUV photometry. When multiplying the final model spectra by the ISM transmittance curves from Figure 1, the Lyα profiles overlay the HST observations. The comparison with initial PHOENIX models show that the adjustments made to the H I(n = 2) departure coefficients result in negligible changes to EUV or NUV wavelengths, only increasing flux in the cores of Lyα and Hα. ...
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... is because the locations in the atmosphere with the greatest divergence are of the lowest densities and would contribute minimal opacity in transit transmission observations. The anoxic case, shown in Figure 10, displays similar trends when comparing the deep self-reversal and full emission Lyα . Anoxic atmosphere comparison of full emission and deep selfreversal Lyα fill cases. ...
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... a better understanding of what the intrinsic Lyα fluxes are for main-sequence stars, we can assess the accuracy of these correlations and subsequent analyses conducted with these inputs. Figure 11. Comparison of photolysis rates in an oxic atmosphere for the Lyα fill cases considered in this work. ...
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... linear x-axis is chosen to best illustrate the differences between the various Lyα fill cases at the top of the atmosphere. These results correspond to the atmospheric mixing ratios shown in Figure 9. Figure 12. Same as Figure 11 but for our anoxic atmosphere scenario. ...
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... results correspond to the atmospheric mixing ratios shown in Figure 9. Figure 12. Same as Figure 11 but for our anoxic atmosphere scenario. These results correspond to the atmospheric mixing ratios shown in Figure 10. ...
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... as Figure 11 but for our anoxic atmosphere scenario. These results correspond to the atmospheric mixing ratios shown in Figure 10. essentially identical in the stratosphere and troposphere for all species in both atmospheric scenarios. The only exception is the O 3 photolysis rate for the deep self-reversal case in the oxic atmosphere, though this has a minimal effect on the O 3 profile as shown in Figure 9. ...
Citations
... Usually the difference in velocities is less than about 20 km/s, but in a few cases a star's very high radial velocity completely shifts the Lyman-α emission line away from the interstellar absorption. An example is the Lyman-α line of Kapteyn's star Guinan et al. 2016) which has a radial velocity of 245 km/s as shown in Fig. 6. Peacock et al. (2022) reconstructed the Lyman-α line profiles of three other stars with very high radial velocities. J.L. Linsky, S. Redfield ...
EUV radiation is primarily responsible for driving hydrodynamic mass loss and thus determining whether an exoplanet can retain its atmosphere and water. The Lyman-α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha $\end{document} flux is primarily responsible for photo-dissociating water and methane, and therefore plays a major role in determining the chemistry in the upper atmospheres of exoplanets. Since interstellar hydrogen absorbs much of the EUV and Lyman-α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha $\end{document} radiation, reconstruction and theoretical techniques are needed to determine the intrinsic flux levels received by exoplanets.
We describe the techniques and their limitations for estimating the extreme ultraviolet (EUV) spectral energy distribution (10–91.2 nm) and hydrogen Lyman-α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha $\end{document} flux (121.6 nm) emitted by host stars and incident on exoplanet atmospheres.
We evaluate how each reconstruction technique can match the observed solar EUV spectral energy distribution and Lyman-α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha $\end{document} flux.
Each technique has its limitations, but the techniques that can reconstruct the observed solar emission and are based on stellar activity observables that are not affected by interstellar absorption should best explain the intrinsic EUV and Lyman-α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha $\end{document} stellar emission.
... We conclude that our updated flux values might not significantly alter the Melbourne et al. (2020) Lyα scaling relations, but a recalibration of those relations is deferred to future work. Miguel et al. (2015) and Peacock et al. (2022) explored the photochemical effects of Lyα on mini-Neptune and terrestrial atmospheres, respectively. Specifically, Miguel et al. (2015) analyzed the impact of varied Lyα flux intensities on the photochemistry of a mini-Neptune's atmosphere with different metallicity compositions. ...
... The smallest flux change considered in their work was 10× the Lyα flux while the largest flux difference described in this work is a 5× flux decrease. Peacock et al. (2022) examined the impact of Lyα flux and line profile shape at different degrees of self-reversal on the chemistry of oxic and anoxic terrestrial atmospheres. For the oxic atmosphere, they found lower concentrations of atmospheric gases with Lyα displaying no self-reversal compared to deep self-reversal. ...
The Ly α emission line is the brightest UV emission line in M and K dwarf spectra and serves as an important tool for studies of stellar chromospheres, the interstellar medium, and exoplanet atmospheres. However, Ly α observations have proven difficult due to the strong absorption by the interstellar medium, necessitating a reconstruction of the intrinsic stellar line from the observed spectrum. We have performed new Ly α reconstructions on the MUSCLES Treasury Survey stars, incorporating improved parameterizations for the intrinsic line wings and line core. We present an analysis of how the updated Ly α fluxes could impact photochemical and atmospheric escape studies and flux–flux scaling relations with other chromospheric emission lines such as Ca ii H and K. We find the overall intrinsic Ly α flux of our star sample decreases by as little as 10% to as much as ∼5× fainter compared to previous findings. The exception to this flux decrease is the M dwarf GJ 581, whose Ly α flux increased by 4%. These results will likely have a limited impact on the aforementioned studies that rely on Ly α fluxes.
... The model includes NO x production by lightning (Harman et al. 2018). The Atmos code is widely used to predict the atmospheric composition of terrestrial exoplanets with secondary atmospheres (Lustig-Yaeger et al. 2019;Peacock et al. 2022;Teal et al. 2022). ...
Many past studies have predicted the steady-state production and maintenance of abiotic O$_2$ and O$_3$ in the atmospheres of CO$_2$-rich terrestrial planets orbiting M dwarf stars. However, the time-dependent responses of these planetary atmospheres to flare events - and the possible temporary production or enhancement of false positive biosignatures therein - has been comparatively less well studied. Most past works that have modeled the photochemical response to flares have assumed abundant free oxygen like that of the modern or Proterozoic Earth. Here we examine in detail the photochemical impact of the UV emitted by a single flare on abiotic O$_2$/O$_3$ production in prebiotic, CO$_2$-dominated atmospheres of M dwarf planets with CO$_2$ levels ranging from 10% to 90% of 1 bar. We find that a single flare generally destroys O$_2$ while modestly enhancing O$_3$ column densities. We simulate the spectral observables of both the steady-state atmosphere and time-dependent spectral response over the flare window for both emitted and transmitted light spectra. Over the course of the flare, the O$_3$ UV Hartley band is modestly enhanced by a maximum of 6 ppm while the CO$_2$ molecular transit depths modestly decline by 7 ppm. In both emitted and transmitted light spectra, the 9.65 $\mu$m O$_3$ band is hidden by the overlapping 9.4 $\mu$m CO$_2$ band for all scenarios considered. Overall, we find that the possible enhancements of abiotic O$_3$ due to a single flare are small compared to O$_3$'s sensitivity to other parameters such as CO$_2$ and H$_2$O abundances or the availability of reducing gases such as H$_2$.
... 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 (Peacock et al. 2022;Teal et al. 2022). Second, the UVIT SED does not reliably cover the Lyα line, where M dwarfs emit a substantial fraction of their flux and which is particularly relevant to CH 4 photolysis in anoxic atmospheres (France et al. 2013;Ranjan et al. 2022). ...
... Second, the UVIT SED does not reliably cover the Lyα line, where M dwarfs emit a substantial fraction of their flux and which is particularly relevant to CH 4 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 Lyα line, and we reproduced this finding with our model. However, omitting Lyα entirely results in a much bigger change in irradiating flux than variations in the degree of self-reversal in the Lyα line, and we find this omission to result in significant variations in predicted pCH 4 . ...
... P.K.N. 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. ...
Characterizing rocky exoplanet atmospheres is a key goal of exoplanet science, but interpreting such observations will require understanding the stellar ultraviolet (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 spectral energy distribution (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 UltraViolet Imaging Telescope (UVIT) for the characterization of the UV SEDs of low-mass stars. We present observations of the nearby M0 star HIP 23309 in the far-UV (FUV) and near-UV (NUV) gratings of UVIT. Our FUV spectra are consistent with contemporaneous Hubble Space Telescope (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.
... 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 CH 4 photolysis in anoxic atmospheres (France et al. 2013;Ranjan 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 CH 4 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 Lyα line, and we reproduced this finding with our model. However, omitting Lyα entirely results in a much bigger change in irradiating flux than variations in the degree of self-reversal in the Lyα line, and we find this omission to result in significant variations in predicted pCH 4 . ...
... 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. ...
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.
... With the growth of exoplanetary science and the associated awareness of the importance of the host star's UV radiation field on the evolution of exoplanetary atmospheres, the community resources devoted to UV characterization of cool stars have increased (Shkolnik et al. 2016;France et al. 2018;Duvvuri et al. 2023). This is particularly important since it has been shown that empirical scaling relations alone are insufficient to model the evolution of planetary atmospheres, and the extended UV continuum and UV emission lines are necessary to generate accurate models (Peacock et al. 2022;Teal et al. 2022). The MUSCLES Treasury Survey (HST Cycle 22; PI-France) began to address this dearth of observations by creating panchromatic 5 Å-5.5 μm spectral energy distributions (SEDs) of M and K dwarfs, which have since been used extensively to study the importance of the UV radiation environment on exoplanets (for example, Kawashima & Ikoma 2018;Lora et al. 2018;Chen et al. 2021). ...
X-ray through infrared spectral energy distributions (SEDs) are essential for understanding a star’s effect on exoplanet atmospheric composition and evolution. We present a catalog of panchromatic SEDs, hosted on the Barbara A. Mikulski Archive for Space Telescopes, for 11 exoplanet-hosting stars that have guaranteed JWST observation time as part of the ERS or GTO programs but have no previous UV characterization. The stars in this survey range from spectral type F4-M6 (0.14–1.57 M ☉ ), rotation periods of 4–132 days, and ages of approximately 0.5–11.4 Gyr. The SEDs are composite spectra using data from the Chandra X-ray Observatory and XMM-Newton, the Hubble Space Telescope, BT-Settl stellar atmosphere models, and scaled spectra of proxy stars of similar spectral type and activity. From our observations, we have measured a set of UV and X-ray fluxes as indicators of stellar activity level. We compare the chromospheric and coronal activity indicators of our exoplanet-hosting stars to the broader population of field stars and find that a majority of our targets have activity levels lower than the average population of cool stars in the solar neighborhood. This suggests that using SEDs of stars selected from exoplanet surveys to compute generic exoplanet atmosphere models may underestimate the typical host star’s UV flux by an order of magnitude or more, and consequently, that the observed population of exoplanetary atmospheres receive lower high-energy flux levels than the typical planet in the solar neighborhood.
... With the growth of exoplanetary science and the associated awareness of the importance of the host star's UV radiation field on the evolution of exoplanetary atmospheres, the community resources devoted to UV characterization of cool stars have increased (France et al. 2018;Shkolnik et al. 2016;Duvvuri et al. 2023). This is particularly important since it has been shown that empirical scaling relations alone are insufficient to model the evolution of planetary atmospheres, and the extended UV continuum and UV emission lines are necessary to generate accurate models (Teal et al. 2022;Peacock et al. 2022). The MUSCLES Treasury Survey (HST Cycle 22; PI-France) began to address this dearth of observations by creating panchromatic 5Å-5.5 µm SEDs of M and K dwarfs which have since been used extensively to study the importance of the UV radiation environment on exoplanets (for example, Kawashima & Ikoma 2018;Lora et al. 2018;Chen et al. 2021). ...
X-ray through infrared spectral energy distributions (SEDs) are essential for understanding a star's effect on exoplanet atmospheric composition and evolution. We present a catalog of panchromatic SEDs, hosted on the Barbara A. Mikulski Archive for Space Telescopes (MAST), for 11 exoplanet hosting stars which have guaranteed JWST observation time as part of the ERS or GTO programs but have no previous UV characterization. The stars in this survey range from spectral type F4-M6 (0.14-1.57 M$_\odot$), rotation periods of ~4-132 days, and ages of approximately 0.5-11.4 Gyr. The SEDs are composite spectra using data from the Chandra X-ray Observatory and XMM-Newton, the Hubble Space Telescope, BT-Settl stellar atmosphere models, and scaled spectra of proxy stars of similar spectral type and activity. From our observations, we have measured a set of UV and X-ray fluxes as indicators of stellar activity level. We compare the chromospheric and coronal activity indicators of our exoplanet-hosting stars to the broader population of field stars and find that a majority of our targets have activity levels lower than the average population of cool stars in the solar neighborhood. This suggests that using SEDs of stars selected from exoplanet surveys to compute generic exoplanet atmosphere models may underestimate the typical host star's UV flux by an order of magnitude or more, and consequently, that the observed population of exoplanetary atmospheres receive lower high-energy flux levels than the typical planet in the solar neighborhood.
... We find that the FUV continuum has a crucial impact on the M-dwarf N 2 O photolysis rates; it is only very weakly a function of the Lyα flux (Grenfell et al. 2014;Peacock et al. 2022), so the scaling or estimation of stellar spectra that do not account for the FUV continuum will yield divergent results from those presented here. This finding is consistent with the recent work of Teal et al. (2022), who focused on the impact of differences in the input stellar spectra on the signatures of CH 4 and haze on Archean-like planets. ...
Nitrous oxide (N 2 O)—a product of microbial nitrogen metabolism—is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N 2 O as a biosignature have examined scenarios using Earthlike N 2 O mixing ratios or surface fluxes, or those inferred from Earth’s geologic record. However, biological fluxes of N 2 O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N 2 from N 2 O had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models to systematically quantify the limits of plausible N 2 O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. We examine N 2 O buildup over a range of oxygen conditions (1%–100% present atmospheric level) and N 2 O fluxes (0.01–100 teramole per year; Tmol = 10 ¹² mole) that are compatible with Earth’s history. We find that N 2 O fluxes of 10 [100] Tmol yr ⁻¹ would lead to maximum N 2 O abundances of ∼5 [50] ppm for Earth–Sun analogs, 90 [1600] ppm for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N 2 O concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N 2 O spectral features for high-flux scenarios for TRAPPIST-1e with JWST. We review potential false positives, including chemodenitrification and abiotic production via stellar activity, and identify key spectral and contextual discriminants to confirm or refute the biogenicity of the observed N 2 O.
... Testing the absolute accuracy of the Lyα reconstruction routines is challenging, as the ground truth of occulted Lyα profiles cannot be obtained. In rare cases, the Lyα line can be fully observed for stars with sufficiently high radial velocities to shift the line out of the airglow and the deepest ISM absorption, the best example being Kapteynʼs star at 245 km s −1 (Guinan et al. 2016;Schneider et al. 2019;Peacock et al. 2022;Youngblood et al. 2022). Unfortunately, highvelocity stars cannot also be observed with the line occulted. ...
The 1215.67 Å H i Ly α emission line dominates the ultraviolet flux of low-mass stars, including the majority of known exoplanet hosts. Unfortunately, strong attenuation by the interstellar medium (ISM) obscures the line core in most stars, requiring the intrinsic Ly α flux to be reconstructed based on fits to the line wings. We present a test of the widely used Ly α emission-line reconstruction code lyapy using phase-resolved, medium-resolution STIS G140M observations of the close white dwarf–M dwarf binary EG UMa. The Doppler shifts induced by the binary orbital motion move the Ly α emission line in and out of the region of strong ISM attenuation. Reconstructions of each spectrum should produce the same Ly α profile regardless of phase, under the well-justified assumption that there is no intrinsic line variability between observations. Instead, we find that the reconstructions underestimate the Ly α flux by almost a factor of 2 for the lowest velocity, most attenuated spectrum, due to a degeneracy between the intrinsic Ly α and ISM profiles. Our results imply that many stellar Ly α fluxes derived from G140M spectra reported in the literature may be underestimated, with potential consequences for, for example, estimates of extreme-ultraviolet stellar spectra and ultraviolet inputs into simulations of exoplanet atmospheres.
