arXiv subjects

Sarah Rugheimer

Publications and source records attributed to Sarah Rugheimer.

At least 19 recordsLinked to original sources

arXiv2026

Detecting habitable exoplanet atmospheres with LIFE, the Large Interferometer for Exoplanets

A key goal of astronomers with the next generation telescopes is to detect signs of life in exoplanet atmospheres. NASA's next flagship is the Habitable Worlds Observatory (HWO). In the context of ESA's Voyage 2050 program, the Senior Committee report prioritises detecting habitable exoplanet atmospheres in the mid-IR. The most suited mission for this is the Large Interferometer for Exoplanets (LIFE) which can detect an even wider range of biosignatures than HWO and at lower concentrations. LIFE is a global science collaboration based out of ETH Z\"urich. With the UK's expertise in building infrared instruments we could play a leading role in realising an ambitious European-led mission. Notably, LIFE is able to detect necessary planetary context like surface temperature and pressure, along with a key discriminator molecule for biosignature false positives, methane, which will be much harder or impossible with HWO. Also, LIFE will be able to investigate many of the nearby rocky exoplanets known from radial velocity searches that are inaccessible to HWO due to its limited spatial resolution.

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arXiv2026

The Goldilocks problem for detecting water in terrestrial planets: Constraining water abundances in the mid-IR with LIFE

We investigate how well the Large Interferometer for Exoplanets (LIFE) mission concept can detect habitable conditions on exoplanets through the presence of atmospheric water vapor as a proxy for surface oceans. We model the atmosphere of a pre-biotic Earth-like planet across a range of water concentrations, from water-poor to water-rich, with surface partial pressures from 10$^{-7}$ to 1 bar of H$_2$O. We simulate LIFE-like noise at spectral resolutions R = 50 and 100 using LIFEsim and perform Bayesian atmospheric retrievals to determine the technical requirements for LIFE to confirm habitability. We model three vertical water distributions: a vertically constant profile, a Manabe-Wetherald based Earth-like profile, and a diffusion and photochemistry profile to test how the assumed vertical structure influences the retrieved abundances. Clouds are not modeled. We find the ability for LIFE to detect water strongly depends on the vertical profile assumed. LIFE is unable to constrain the highest water cases and provides upper limits on low water planets. For the highest water abundances, absorption features saturate and reduce sensitivity to characterize precise H$_2$O levels. Water vapor is not detectable in any profile modeled for $\leq10^{-6}$ bar in surface water, comparable to Mars. For an Earth-like profile, LIFE could constrain H$_2$O concentrations from $\sim10^{-3}$ to 1 bar, spanning below and above present-day Earth concentrations of 10$^{-2}$ bar. Detectable atmospheric water may imply surface oceans, as water is highly reactive and rapidly removed by surface mineral reactions. Thus, LIFE can characterize water abundances indicative of habitable surface conditions.

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arXiv2026

The Key to Unlocking Exoplanet Biosignatures: a UK-led IR Spectrograph for the Habitable Worlds Observatory Coronagraph

The detection of life on rocky exoplanets in the habitable zones of nearby stars would be a paradigm-shifting advance, and it is one of the greatest scientific challenges of our time. There is no single spectral feature that is an unambiguous sign of life on a given exoplanet. Instead, the current state-of-the-art approach involves detecting multiple molecular atmospheric features that should not exist together in equilibrium, e.g. simultaneous detection of O$_2$ and CH$_4$. Spectra across a wide wavelength (0.3-1.7 $\mu$m) range are necessary to cover multiple spectral features per molecule of interest and to contextualise the suite of molecular features detected. While the US will lead the optical arm of the Habitable Worlds Observatory (HWO) coronagraph, a UK-led contribution of a near-infrared Integral Field Spectrograph (IFS) for the infrared arm will ensure UK leadership in the flagship scientific goal of HWO - to search for signatures of life on potentially habitable exoplanets.

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arXiv2026

The power of polarimetry for characterising exoplanet atmospheres, clouds, and surfaces with NASA's Habitable Worlds Observatory

The Habitable Worlds Observatory (HWO), planned for launch in the 2040s, represents the next major step in exoplanet characterisation. HWO will, for the first time, enable detailed studies of the atmospheres and surfaces of Earth-like exoplanets through high-contrast reflection spectroscopy across the UV, optical, and near-infrared. These wavelength ranges provide access to key molecular absorption features, including O2, O3, H2O, CO2, and CH4, as well as potential surface biosignatures such as the vegetation red edge or ocean glint, making HWO a cornerstone mission for assessing planetary habitability. Clouds are a dominant factor in determining planetary climate and observability, yet their properties remain highly degenerate when constrained using reflected flux alone. Spectropolarimetry, a measure of the polarisation state of reflected light as a function of wavelength and orbital phase, provides a powerful complementary diagnostic. Polarisation is highly sensitive to cloud particle size, composition, shape, vertical distribution, and surface type, enabling degeneracies between atmospheric and surface models to be broken. Numerous studies have demonstrated the value of polarimetry for characterising a wide range of exoplanets, from hot Jupiters to cooler potentially habitable worlds. HWO's proposed instrument suite includes a coronagraph, a high-resolution imager, and a candidate high-resolution spectropolarimeter, offering multiple pathways to exploit polarimetry across diverse planetary regimes. This white paper argues that incorporating polarimetric capability into HWO instruments would significantly enhance the mission's scientific return. We highlight the unique opportunity for UK leadership in both instrument development and theoretical modelling, and advocate for a strong UK role in shaping HWO's polarimetric capabilities to maximise its impact on exoplanet science.

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arXiv2026

Exoplanet characterization with NASA's Habitable Worlds Observatory

Exoplanet atmosphere characterization has seen revolutionary advances over the last few years, providing us with unique insights into atmospheric chemistry, dynamics and planet formation mechanisms. However, true solar system analog planets remain inaccessible. A major goal for exoplanet science over the coming decades is to observe, and characterize, temperate rocky planets and cool gas giants in orbit around solar-type stars, with the prospect of detecting signs of habitability or even life. Characterization and categorization of these planets relies on direct spectroscopic observations capable of identifying molecular species in their atmospheres; however, these observations represent a substantial engineering challenge due to the extreme contrast between a temperate, Earth-sized exoplanet and its parent star. NASA's next flagship mission, the Habitable Worlds Observatory (HWO) - planned for launch in the mid-2040s - will boast a coronagraphic instrument capable of reaching the needed 10$^{-10}$ contrast, on an ultrastable platform enabling long integration times to achieve the required signal to noise. HWO will cover near-ultraviolet to the near-infrared wavelengths, enabling detections of key biosignature molecules and habitability indicators such as ocean glint and a vegetation `red edge'. Via early involvement in this groundbreaking observatory, including a potential UK instrument contribution, the UK exoplanet community now has an important opportunity to influence the telescope's design. To maintain our international competitiveness, we must be at the forefront of observational campaigns with HWO when it eventually launches, and this comes with the need for parallel development in laboratory astrophysics and computational modelling. Maximising our exploitation of this transformative NASA mission requires consistent financial support in these areas across the next two decades.

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arXiv2024

The Mega-MUSCLES Treasury Survey: X-ray to infrared Spectral Energy Distributions of a representative sample of M dwarfs

We present 5-1x10^7 Angstrom spectral energy distributions (SEDs) for twelve M dwarf stars covering spectral types M0-M8. Our SEDs are provided for community use as a sequel to the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) survey. The twelve stars include eight known exoplanet hosts and four stars chosen to fill out key parameter space in spectral type and rotation period. The SEDs are constructed from Hubble Space Telescope ultraviolet spectroscopy and XMM Newton, Chandra and/or Swift X-ray observations and completed with various model data, including Lyman alpha reconstructions, PHOENIX optical models, APEC coronal models and Differential Emission Measure models in the currently-unobservable Extreme Ultraviolet. We provide a complete overview of the Mega-MUSCLES program, including a description of the observations, models, and SED construction. The SEDs are available as MAST High-Level Science Products and we describe the various data products here. We also present ensemble measurements from our sample that are of particular relevance to exoplanet science, including the high-energy fluxes in the habitable zone and the FUV/NUV ratio. Combined with MUSCLES, Mega-MUSCLES provides SEDs covering a wide range of M\,dwarf spectral types and ages such that suitable proxies for any M dwarf planet host of interest may be found in our sample. However, we find that ultraviolet and X-ray fluxes can vary even between stars with similar parameters, such that observations of each exoplanet host star will remain the gold standard for interpreting exoplanet atmosphere observations.

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arXiv2024

Earth-like exoplanets in spin-orbit resonances: climate dynamics, 3D atmospheric chemistry, and observational signatures

Terrestrial exoplanets around M- and K-type stars are important targets for atmospheric characterisation. Such planets are likely tidally locked with the order of spin-orbit resonances (SORs) depending on eccentricity. We explore the impact of SORs on 3D atmospheric dynamics and chemistry, employing a 3D coupled Climate-Chemistry Model to simulate Proxima Centauri b in 1:1 and 3:2 SOR. For a 1:1 SOR, Proxima Centauri b is in the Rhines rotator circulation regime with dominant zonal gradients (global mean surface temperature 229 K). An eccentric 3:2 SOR warms Proxima Centauri b to 262 K with gradients in the meridional direction. We show how a complex interplay between stellar radiation, orbit, atmospheric circulation, and (photo)chemistry determines the 3D ozone distribution. Spatial variations in ozone column densities align with the temperature distribution and are driven by stratospheric circulation mechanisms. Proxima Centauri b in a 3:2 SOR demonstrates additional atmospheric variability, including daytime-nighttime cycles in water vapour of ${+}$55% to ${-}$34% and ozone ($\pm5.2$%) column densities and periastron-apoastron water vapour cycles of ${+}$17% to ${-}$10%. Synthetic emission spectra for the spectral range of the Large Interferometer For Exoplanets fluctuate by up to 36 ppm with orbital phase angle for a 1:1 SOR due to 3D spatial and temporal asymmetries. The homogeneous atmosphere for the 3:2 SOR results in relatively constant emission spectra and provides an observational discriminant from the 1:1 SOR. Our work emphasizes the importance of understanding the 3D nature of exoplanet atmospheres and associated spectral variations to determine habitability and interpret atmospheric spectra.

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arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $\mu$m is important for decreasing the detection threshold and discriminating emission temperatures.

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arXiv2023

Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST

The search for biosignatures on exoplanets connects the fields of biology and biochemistry to astronomical observation, with the hope that we might detect evidence of active biological processes on worlds outside the solar system. Here we focus on a complementary aspect of exoplanet characterisation connecting astronomy to prebiotic chemistry: the search for molecules associated with the origin of life, prebiosignatures. Prebiosignature surveys in planetary atmospheres offer the potential to both constrain the ubiquity of life in the galaxy and provide important tests of current prebiotic syntheses outside of the laboratory setting. Here, we quantify the minimum abundance of identified prebiosignature molecules that would be required for detection by transmission spectroscopy using JWST. We consider prebiosignatures on five classes of terrestrial planet: an ocean planet, a volcanic planet, a post-impact planet, a super-Earth, and an early Earth analogue. Using a novel modelling and detection test pipeline, with simulated JWST noise, we find the detection thresholds of hydrogen cyanide (HCN), hydrogen sulfide (H2S), cyanoacetylene (HC3N), ammonia (NH3), methane (CH4), acetylene (C2H2), sulfur dioxide (SO2), nitric oxide (NO), formaldehyde (CH2O), and carbon monoxide (CO) in a variety of low mean molecular weight (<5) atmospheres. We test the dependence of these detection thresholds on M dwarf target star and the number of observed transits, finding that a modest number of transits (1-10) are required to detect prebiosignatures in numerous candidate planets, including TRAPPIST-1e with a high mean molecular weight atmosphere. We find that the NIRSpec G395M/H instrument is best suited for detecting most prebiosignatures.

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arXiv2022

Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets

The Great Oxidation Event was a period during which Earth's atmospheric oxygen (O$_2$) concentrations increased from $\sim 10^{-5}$ times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O$_2$ mixing ratios between 0.1% and 150% PAL. Using these simulations, we calculate the reflection/emission spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet's reflected flux (including the reflected flux of key spectral features such as O$_2$ and H$_2$O) by up to factors of 5 and 20, respectively, for the same orbital phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O$_2$ concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O$_2$ concentrations of $<$1% PAL.

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arXiv2022

Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs

An important future goal in exoplanetology is to detect and characterize potentially habitable planets. Using nulling interferometry, LIFE will allow us to constrain the radius and effective temperature of (terrestrial) exoplanets, as well as provide unique information about their atmospheric structure and composition. We explore the potential of LIFE in characterizing emission spectra of Earth at various stages of its evolution. We perform Bayesian retrievals on simulated spectra of 8 different scenarios, which correspond to cloud-free and cloudy spectra of four different epochs of the evolution of the Earth. Assuming a distance of 10 pc and a Sun-like host star, we simulate observations obtained with LIFE using its simulator LIFEsim, considering all major astrophysical noise sources. With the nominal spectral resolution (R=50) and signal-to-noise ratio (assumed to be S/N=10 at 11.2 $\mu$m), we can identify the main spectral features of all the analyzed scenarios (most notably CO$_2$, H$_2$O, O$_3$, CH$_4$). This allows us to distinguish between inhabited and lifeless scenarios. Results suggest that particularly O$_3$ and CH$_4$ yield an improved abundance estimate by doubling the S/N from 10 to 20. We conclude that the baseline requirements for R and S/N are sufficient for LIFE to detect O$_3$ and CH$_4$ in the atmosphere of an Earth-like planet with an abundance of O$_2$ of around 2% in volume mixing ratio. This information is relevant in terms of the LIFE mission planning. We also conclude that cloud-free retrievals of cloudy planets can be used to characterize the atmospheric composition of terrestrial habitable planets, but not the thermal structure of the atmosphere. From the inter-model comparison performed, we deduce that differences in the opacity tables (caused by e.g. a different line wing treatment) may be an important source of systematic errors.

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arXiv2021

Starting Life and Searching for Life on Rocky Planets

The study of origins of life on Earth and the search for life on other planets are closely linked. Prebiotic chemical scenarios can help prioritize planets as targets for the search for life as we know it and can provide informative priors to help us assess the likelihood that particular spectroscopic features are evidence of life. The prerequisites for origins scenarios themselves predict spectral signatures. The interplay between origins research and the search for extraterrestrial life must start with lab work guiding exploratory ventures in the solar system, and the discoveries in the solar system informing future exoplanet observations and laboratory research. Subsequent exoplanet research will in turn provide statistical context to conclusions about the nature and origins of life.

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arXiv2021

The Mega-MUSCLES Spectral Energy Distribution Of TRAPPIST-1

We present a 5A-100um Spectral Energy Distribution (SED) of the ultracool dwarf star TRAPPIST-1, obtained as part of the Mega-MUSCLES Treasury Survey. The SED combines ultraviolet and blue-optical spectroscopy obtained with the Hubble Space Telescope, X-ray spectroscopy obtained with XMM-Newton, and models of the stellar photosphere, chromosphere, transition region and corona. A new Differential Emission Measure model of the unobserved extreme-ultraviolet spectrum is provided, improving on the Lyman alpha to EUV relations often used to estimate the 100-911A flux from low-mass stars. We describe the observations and models used, as well as the recipe for combining them into an SED. We also provide a semi-empirical, noise-free model of the stellar ultraviolet spectrum based on our observations for use in atmospheric modelling of the TRAPPIST-1 planets.

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arXiv2021

Reconstructing the Extreme Ultraviolet Emission of Cool Dwarfs Using Differential Emission Measure Polynomials

Characterizing the atmospheres of planets orbiting M dwarfs requires understanding the spectral energy distributions of M dwarfs over planetary lifetimes. Surveys like MUSCLES, HAZMAT, and FUMES have collected multiwavelength spectra across the spectral type's range of Teff and activity, but the extreme ultraviolet flux (EUV, 100 to 912 Angstroms) of most of these stars remains unobserved because of obscuration by the interstellar medium compounded with limited detector sensitivity. While targets with observable EUV flux exist, there is no currently operational facility observing between 150 and 912 Angstroms. Inferring the spectra of exoplanet hosts in this regime is critical to studying the evolution of planetary atmospheres because the EUV heats the top of the thermosphere and drives atmospheric escape. This paper presents our implementation of the differential emission measure technique to reconstruct the EUV spectra of cool dwarfs. We characterize our method's accuracy and precision by applying it to the Sun and AU Mic. We then apply it to three fainter M dwarfs: GJ 832, Barnard's Star, and TRAPPIST-1. We demonstrate that with the strongest far ultraviolet (FUV, 912 to 1700 Angstroms) emission lines, observed with Hubble Space Telescope and/or Far Ultraviolet Spectroscopic Explorer, and a coarse X-ray spectrum from either Chandra X-ray Observatory or XMM-Newton, we can reconstruct the Sun's EUV spectrum to within a factor of 1.8, with our model's formal uncertainties encompassing the data. We report the integrated EUV flux of our M dwarf sample with uncertainties between a factor of 2 to 7 depending on available data quality.

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arXiv2021

Characterizing Atmospheres of Transiting Earth-like Exoplanets Orbiting M Dwarfs with James Webb Space Telescope

A number of transiting, potentially habitable Earth-sized exoplanets have recently been detected around several nearby M dwarf stars. These worlds represent important targets for atmospheric characterization for the upcoming NASA James Webb Space Telescope. Given that available time for exoplanet characterization will be limited, it is critically important to first understand the capabilities and limitations of JWST when attempting to detect atmospheric constituents for potentially Earth-like worlds orbiting cool stars. Here, we explore coupled climate-chemistry atmospheric models for Earth-like planets orbiting a grid of M dwarf hosts. Using a newly-developed and validated JWST instrument model - the JWST Exoplanet Transit Simulator (JETS) - we investigate the detectability of key biosignature and habitability indicator gaseous species for a variety of relevant instruments and observing modes. Spectrally-resolved detection scenarios as well as cases where the spectral impact of a given species is integrated across the entire range of an instrument/mode are considered and serve to highlight the importance of considering information gained over an entire observable spectral range. When considering the entire spectral coverage of an instrument/mode, detections of methane, carbon dioxide, oxygen and water at signal-to-noise ratio 5 could be achieved with observations of several tens of transits (or less) for cloud-free Earth-like worlds orbiting mid- to late-type M dwarfs at system distances of up to 10-15 pc. When compared to previous results, requisite exposure times for gas species detection depend on approaches to quantifying the spectral impact of the species as well as underlying photochemical model assumptions. Thus, constraints on atmospheric abundances, even if just upper limits, by JWST have the potential to further our understanding of terrestrial atmospheric chemistry.

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arXiv2020

Estimating the Ultraviolet Emission of M dwarfs with Exoplanets from Ca II and H$\alpha$

M dwarf stars are excellent candidates around which to search for exoplanets, including temperate, Earth-sized planets. To evaluate the photochemistry of the planetary atmosphere, it is essential to characterize the UV spectral energy distribution of the planet's host star. This wavelength regime is important because molecules in the planetary atmosphere such as oxygen and ozone have highly wavelength dependent absorption cross sections that peak in the UV (900-3200 $\r{A}$). We seek to provide a broadly applicable method of estimating the UV emission of an M dwarf, without direct UV data, by identifying a relationship between non-contemporaneous optical and UV observations. Our work uses the largest sample of M dwarf star far- and near-UV observations yet assembled. We evaluate three commonly-observed optical chromospheric activity indices -- H$\alpha$ equivalent widths and log$_{10}$ L$_{H\alpha}$/L$_{bol}$, and the Mount Wilson Ca II H&K S and R$'_{HK}$ indices -- using optical spectra from the HARPS, UVES, and HIRES archives and new HIRES spectra. Archival and new Hubble Space Telescope COS and STIS spectra are used to measure line fluxes for the brightest chromospheric and transition region emission lines between 1200-2800 $\r{A}$. Our results show a correlation between UV emission line luminosity normalized to the stellar bolometric luminosity and Ca II R$'_{HK}$ with standard deviations of 0.31-0.61 dex (factors of $\sim$2-4) about the best-fit lines. We also find correlations between normalized UV line luminosity and H$\alpha$ log$_{10}$ L$_{H\alpha}$/L$_{bol}$ and the S index. These relationships allow one to estimate the average UV emission from M0 to M9 dwarfs when UV data are not available.

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arXiv2020

The relative emission from chromospheres and coronae: dependence on spectral type and age

Extreme-ultraviolet and X-ray emissions from stellar coronae drive mass loss from exoplanet atmospheres, and ultraviolet emission from stellar chromospheres drives photo-chemistry in exoplanet atmospheres. Comparisons of the spectral energy distributions of host stars are, therefore, essential for understanding the evolution and habitability of exoplanets. The large number of stars observed with the MUSCLES, Mega-MUSCLES, and other recent HST observing programs has provided for the first time a large sample (79 stars) of reconstructed Lyman-alpha fluxes that we compare with X-ray fluxes to identify significant patterns in the relative emission from these two atmospheric regions as a function of stellar age and effective temperature. We find that as stars age on the main sequence, the emissions from their chromospheres and coronae follow a pattern in response to the amount of magnetic heating in these atmospheric layers. A single trendline slope describes the pattern of X-ray vs. Lyman-alpha emission for G and K dwarfs, but the different trendlines for M dwarf stars show that the Lyman-alpha fluxes of M stars are significantly smaller than warmer stars with the same X-ray flux. The X-ray and Lyman-alpha luminosities divided by the stellar bolometric luminosities show different patterns depending on stellar age. The L(Lyman-alpha)/L(bol) ratios increase smoothly to cooler stars of all ages, but the L(X)/L(bol) ratios show different trends. For older stars, the increase in coronal emission with decreasing T(eff) is much steeper than chromospheric emission. We suggest a fundamental link between atmospheric properties and trendlines relating coronal and chromospheric heating,

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arXiv2020

The High-Energy Radiation Environment Around a 10 Gyr M Dwarf: Habitable at Last?

High levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the liquid water habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. We present new HST and Chandra observations of Barnard's Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard's star, we observe two FUV ($\delta_{130}$ $\approx$ 5000s; $E_{130}$ $\approx$ 10$^{29.5}$ erg each) and one X-ray ($E_{X}$ $\approx$ 10$^{29.2}$ erg) flares, and estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of $\sim$ 25\%. A 5 A - 10 $\mu$m SED of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone ($r_{HZ}$ $\sim$ 0.1 AU). Both thermal and non-thermal escape modeling indicate (1) the $quiescent$ stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the $flare$ environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of $\approx$ 87 Earth atmospheres Gyr$^{-1}$ through thermal processes and $\approx$ 3 Earth atmospheres Gyr$^{-1}$ through ion loss processes, respectively. These results suggest that if rocky planet atmospheres can survive the initial $\sim$ 5 Gyr of high stellar activity, or if a second generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars.

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