arXiv subjects

Eliza M. -R. Kempton

Publications and source records attributed to Eliza M. -R. Kempton.

At least 73 records · Page 4Linked to original sources

arXiv2022

Early Release Science of the exoplanet WASP-39b with JWST NIRCam

Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution, and high precision that, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST's Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0 - 4.0 $μ$m, exhibit minimal systematics, and reveal well-defined molecular absorption features in the planet's spectrum. Specifically, we detect gaseous H$_2$O in the atmosphere and place an upper limit on the abundance of CH$_4$. The otherwise prominent CO$_2$ feature at 2.8 $μ$m is largely masked by H$_2$O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100$\times$ solar (i.e., an enrichment of elements heavier than helium relative to the Sun) and a sub-stellar carbon-to-oxygen (C/O) ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation or disequilibrium processes in the upper atmosphere.

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arXiv2022

Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems. Access to an exoplanet's chemical inventory requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based and high-resolution ground-based facilities. Here we report the medium-resolution (R$\sim$600) transmission spectrum of an exoplanet atmosphere between 3-5 $μ$m covering multiple absorption features for the Saturn-mass exoplanet WASP-39b, obtained with JWST NIRSpec G395H. Our observations achieve 1.46x photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO$_2$ (28.5$σ$) and H$_2$O (21.5$σ$), and identify SO$_2$ as the source of absorption at 4.1 $μ$m (4.8$σ$). Best-fit atmospheric models range between 3 and 10x solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO$_2$, underscore the importance of characterising the chemistry in exoplanet atmospheres, and showcase NIRSpec G395H as an excellent mode for time series observations over this critical wavelength range.

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arXiv2022

Early Release Science of the exoplanet WASP-39b with JWST NIRISS

Transmission spectroscopy provides insight into the atmospheric properties and consequently the formation history, physics, and chemistry of transiting exoplanets. However, obtaining precise inferences of atmospheric properties from transmission spectra requires simultaneously measuring the strength and shape of multiple spectral absorption features from a wide range of chemical species. This has been challenging given the precision and wavelength coverage of previous observatories. Here, we present the transmission spectrum of the Saturn-mass exoplanet WASP-39b obtained using the SOSS mode of the NIRISS instrument on the JWST. This spectrum spans $0.6 - 2.8 μ$m in wavelength and reveals multiple water absorption bands, the potassium resonance doublet, as well as signatures of clouds. The precision and broad wavelength coverage of NIRISS-SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favoring a heavy element enhancement ("metallicity") of $\sim 10 - 30 \times$ the solar value, a sub-solar carbon-to-oxygen (C/O) ratio, and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are best explained by wavelength-dependent, non-gray clouds with inhomogeneous coverage of the planet's terminator.

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arXiv2022

The K2-3 system revisited: testing photoevaporation and core-powered mass loss with three small planets spanning the radius valley

Multi-planet systems orbiting M dwarfs provide valuable tests of theories of small planet formation and evolution. K2-3 is an early M dwarf hosting three small exoplanets (1.5-2.0 Earth radii) at distances of 0.07-0.20 AU. We measure the high-energy spectrum of K2-3 with HST/COS and XMM-Newton, and use empirically-driven estimates of Ly-alpha and extreme ultraviolet flux. We use EXOFASTv2 to jointly fit radial velocity, transit, and SED data. This constrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b and c to 13% and 30%, respectively; K2-3d is not detected in RV measurements. K2-3b and c are consistent with rocky cores surrounded by solar composition envelopes (mass fractions of 0.36% and 0.07%), H2O envelopes (55% and 16%), or a mixture of both. However, based on the high-energy output and estimated age of K2-3, it is unlikely that K2-3b and c retain solar composition atmospheres. We pass the planet parameters and high-energy stellar spectrum to atmospheric models. Dialing the high-energy spectrum up and down by a factor of 10 produces significant changes in trace molecule abundances, but not at a level detectable with transmission spectroscopy. Though the K2-3 planets span the small planet radius valley, the observed system architecture cannot be readily explained by photoevaporation or core-powered mass loss. We instead propose 1) the K2-3 planets are all volatile-rich, with K2-3d having a lower density than typical of super-Earths, and/or 2) the K2-3 planet architecture results from more stochastic processes such as planet formation, planet migration, and impact erosion.

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arXiv2022

The Roasting Marshmallows Program with IGRINS on Gemini South I: Composition and Climate of the Ultra Hot Jupiter WASP-18 b

We present high-resolution dayside thermal emission observations of the exoplanet WASP-18b using IGRINS on Gemini South. We remove stellar and telluric signatures using standard algorithms, and we extract the planet signal via cross correlation with model spectra. We detect the atmosphere of WASP-18b at a signal-to-noise ratio (SNR) of 5.9 using a full chemistry model, measure H2O (SNR=3.3), CO (SNR=4.0), and OH (SNR=4.8) individually, and confirm previous claims of a thermal inversion layer. The three species are confidently detected (>4$σ$) with a Bayesian inference framework, which we also use to retrieve abundance, temperature, and velocity information. For this ultra-hot Jupiter (UHJ), thermal dissociation processes likely play an important role. Retrieving abundances constant with altitude and allowing the temperature-pressure profile to freely adjust results in a moderately super-stellar carbon to oxygen ratio (C/O=0.75^{+0.14}_{-0.17}) and metallicity ([M/H]=1.03^{+0.65}_{-1.01}). Accounting for undetectable oxygen produced by thermal dissociation leads to C/O=0.45^{+0.08}_{-0.10} and [M/H]=1.17^{+0.66}_{-1.01}. A retrieval that assumes radiative-convective-thermochemical-equilibrium and naturally accounts for thermal dissociation constrains C/O<0.34 (2$σ$) and [M/H]=0.48^{+0.33}_{-0.29}, in line with the chemistry of the parent star. Looking at the velocity information, we see a tantalising signature of different Doppler shifts at the level of a few km/s for different molecules, which might probe dynamics as a function of altitude and location on the planet disk. Our results demonstrate that ground-based, high-resolution spectroscopy at infrared wavelengths can provide meaningful constraints on the compositions and climate of highly irradiated planets. This work also elucidates potential pitfalls with commonly employed retrieval assumptions when applied to UHJ spectra.

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arXiv2022

Identification of carbon dioxide in an exoplanet atmosphere

Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (i.e., elements heavier than helium, also called "metallicity"), and thus formation processes of the primary atmospheres of hot gas giants. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2 but have not yielded definitive detections due to the lack of unambiguous spectroscopic identification. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science Program (ERS). The data used in this study span 3.0 to 5.5 μm in wavelength and show a prominent CO2 absorption feature at 4.3 μm (26σ significance). The overall spectrum is well matched by one-dimensional, 10x solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide, and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 μm that is not reproduced by these models.

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arXiv2022

The Detectability of Rocky Planet Surface and Atmosphere Composition with JWST: The Case of LHS 3844b

The spectroscopic characterization of terrestrial exoplanets will be made possible for the first time with JWST. One challenge to characterizing such planets is that it is not known a priori whether they possess optically thick atmospheres or even any atmospheres altogether. But this challenge also presents an opportunity - the potential to detect the surface of an extrasolar world. This study explores the feasibility of characterizing the atmosphere and surface of a terrestrial exoplanet with JWST, taking LHS 3844b as a test case because it is the highest signal-to-noise rocky thermal emission target among planets that are cool enough to have non-molten surfaces. We model the planetary emission, including the spectral signal of both atmosphere and surface, and we explore all scenarios that are consistent with the existing Spitzer 4.5 $μ$m measurement of LHS 3844b from Kreidberg et al. (2019). In summary, we find a range of plausible surfaces and atmospheres that are within 3 $σ$ of the observation - less reflective metal-rich, iron oxidized and basaltic compositions are allowed, and atmospheres are restricted to a maximum thickness of 1 bar, if near-infrared absorbers at $\gtrsim$ 100 ppm are included. We further make predictions on the observability of surfaces and atmospheres, perform a Bayesian retrieval analysis on simulated JWST data and find that a small number, ~3, of eclipse observations should suffice to differentiate between surface and atmospheric features. However, the surface signal may make it harder to place precise constraints on the abundance of atmospheric species and may even falsely induce a weak H$_2$O detection.

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arXiv2022

Confirmation of Water Absorption in the Thermal Emission Spectrum of the Hot Jupiter WASP-77Ab with HST/WFC3

Secondary eclipse observations of hot Jupiters can reveal both their compositions and thermal structures. Previous observations have shown a diversity of hot Jupiter eclipse spectra, including absorption features, emission features, and featureless blackbody-like spectra. We present a secondary eclipse spectrum of the hot Jupiter WASP-77Ab observed between $1-5$ $μ$m with the Hubble Space Telescope (HST) and the Spitzer Space Telescope. The HST observations show signs of water absorption indicative of a non-inverted thermal structure. We fit the data with both a one-dimensional free retrieval and a grid of one-dimensional self-consistent forward models to confirm this non-inverted structure. The free retrieval places a $3σ$ lower limit on the atmospheric water abundance of $\log(n_\mathrm{H_2O})>-4.78$ and can not constrain the CO abundance. The grid fit produces a slightly super-stellar metallicity and constrains the carbon-to-oxygen ratio to less than or equal to the solar value. We also compare our data to recent high-resolution observations of WASP-77Ab taken with the Gemini-South/IGRINS spectrograph and find that our observations are consistent with the best-fit model to the high-resolution data. However, the metallicity derived from the IGRINS data is significantly lower than that derived from our self-consistent model fit. We find that this difference may be due to disequilibrium chemistry, and the varying results between the models applied here demonstrate the difficulty of constraining disequilibrium chemistry with low-resolution, low wavelength coverage data alone. Future work to combine observations from IGRINS, HST, and JWST will improve our estimate of the atmospheric composition of WASP-77Ab.

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arXiv2022

Effects of UV stellar spectral uncertainty on the chemistry of terrestrial atmospheres

The upcoming deployment of JWST will dramatically advance our ability to characterize exoplanet atmospheres, both in terms of precision and sensitivity to smaller and cooler planets. Disequilibrium chemical processes dominate these cooler atmospheres, requiring accurate photochemical modeling of such environments. The host star's UV spectrum is a critical input to these models, but most exoplanet hosts lack UV observations. For cases in which the host UV spectrum is unavailable, a reconstructed or proxy spectrum will need to be used in its place. In this study, we use the MUSCLES catalog and UV line scaling relations to understand how well reconstructed host star spectra reproduce photochemically modeled atmospheres using real UV observations. We focus on two cases; a modern Earth-like atmosphere and an Archean Earth-like atmosphere that forms copious hydrocarbon hazes. We find that modern Earth-like environments are well-reproduced with UV reconstructions, whereas hazy (Archean Earth) atmospheres suffer from changes at the observable level. Specifically, both the stellar UV emission lines and the UV continuum significantly influence the chemical state and haze production in our modeled Archean atmospheres, resulting in observable differences in their transmission spectra. Our modeling results indicate that UV observations of individual exoplanet host stars are needed to accurately characterize and predict the transmission spectra of hazy terrestrial atmospheres. In the absence of UV data, reconstructed spectra that account for both UV emission lines and continuum are the next best option, albeit at the cost of modeling accuracy.

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arXiv2021

No umbrella needed: Confronting the hypothesis of iron rain on WASP-76b with post-processed general circulation models

High-resolution spectra are unique indicators of three-dimensional processes in exoplanetary atmospheres. For instance, in 2020, Ehrenreich et al. reported transmission spectra from the ESPRESSO spectrograph yielding an anomalously large Doppler blueshift from the ultra-hot Jupiter WASP-76b. Interpretations of these observations invoke toy model depictions of gas-phase iron condensation in lower-temperature regions of the planet's atmosphere. In this work, we forward model the atmosphere of WASP-76b with double-gray general circulation models (GCMs) and ray-striking radiative transfer to diagnose the planet's high-resolution transmission spectrum. We confirm that a physical mechanism driving strong east-west asymmetries across the terminator must exist to reproduce large Doppler blueshifts in WASP-76b's transmission spectrum. We identify low atmospheric drag and a deep radiative-convective boundary as necessary components of our GCM to produce this asymmetry (the latter is consistent with existing Spitzer phase curves). However, we cannot reproduce either the magnitude or the time-dependence of the WASP-76b Doppler signature with gas-phase iron condensation alone. Instead, we find that high-altitude, optically thick clouds composed of $\rm Al_2O_3$, Fe, or $\rm Mg_2SiO_4$ provide reasonable fits to the Ehrenreich et al. observations -- with marginal contributions from condensation. This fit is further improved by allowing a small orbital eccentricity ($e \approx 0.017$), consistent with prior WASP-76b orbital constraints. We additionally validate our forward-modeled spectra by reproducing lines of nearly all species detected in WASP-76b by Tabernero et al. 2021. Our procedure's success in diagnosing phase-resolved Doppler shifts demonstrates the benefits of physical, self-consistent, three-dimensional simulations in modeling high-resolution spectra of exoplanet atmospheres.

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arXiv2021

A solar C/O and sub-solar metallicity in a hot Jupiter atmosphere

Measurements of the atmospheric carbon (C) and oxygen (O) relative to hydrogen (H) in hot Jupiters (relative to their host stars) provide insight into their formation location and subsequent orbital migration. Hot Jupiters that form beyond the major volatile (H2O/CO/CO2) ice lines and subsequently migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen ratios (C/O) near 1 and subsolar metallicities, whereas planets that migrate through the disk before dissipation are predicted to be heavily polluted by infalling O-rich icy planetesimals, resulting in C/O < 0.5 and super-solar metallicities. Previous observations of hot Jupiters have been able to provide bounded constraints on either H2O or CO, but not both for the same planet, leaving uncertain the true elemental C and O inventory and subsequent C/O and metallicity determinations. Here we report spectroscopic observations of a typical transiting hot Jupiter, WASP-77Ab. From these, we determine the atmospheric gas volume mixing ratio constraints on both H2O and CO (9.5$\times 10^{-5}$ - 1.5$\times 10^{-4}$ and 1.2$\times 10^{-4}$ - 2.6$\times 10^{-4}$, respectively). From these bounded constraints, we are able to derive the atmospheric C/H (0.35$^{+0.17}_{-0.10}$ $\times$ Solar) and O/H (0.32 $^{+0.12}_{-0.08}$ $\times$ Solar) abundances and the corresponding atmospheric carbon-to-oxygen ratio (C/O=0.59$\pm$0.08; the solar value is 0.55). The sub-solar (C+O)/H (0.33$^{+0.13}_{-0.09}$ $\times$ Solar) is suggestive of a metal-depleted atmosphere relative to what is expected for Jovian-like planets while the near solar value of C/O rules out the disk-free migration/C-rich atmosphere scenario.

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arXiv2021

A unique hot Jupiter spectral sequence with evidence for compositional diversity

The emergent spectra of close-in, giant exoplanets ("hot Jupiters") are expected to be distinct from those of self-luminous objects with similar effective temperatures because hot Jupiters are primarily heated from above by their host stars rather than internally from the release of energy from their formation. Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions, and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H- ion. Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters, and population-level trends have been observed in photometric measurements. However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters such as has been done for cooler brown dwarfs and transmission spectra of hot Jupiters. Here we show that hot Jupiter secondary eclipse spectra centered around a water absorption band at 1.4 microns follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with model predictions for how the thermal structures of solar-composition planets vary with irradiation level. Nevertheless, the ensemble of planets exhibits some degree of scatter around the mean trend for solar composition planets. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models. (abridged abstract)

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arXiv2021

Spitzer phase curve observations and circulation models of the inflated ultra-hot Jupiter WASP-76b

The large radii of many hot Jupiters can only be matched by models that have hot interior adiabats, and recent theoretical work has shown that the interior evolution of hot Jupiters has a significant impact on their atmospheric structure. Due to its inflated radius, low gravity, and ultra-hot equilibrium temperature, WASP-76b is an ideal case study for the impact of internal evolution on observable properties. Hot interiors should most strongly affect the non-irradiated side of the planet, and thus full phase curve observations are critical to ascertain the effect of the interior on the atmospheres of hot Jupiters. In this work, we present the first Spitzer phase curve observations of WASP-76b. We find that WASP-76b has an ultra-hot day side and relatively cold nightside with brightness temperatures of $2471 \pm 27~\mathrm{K}$/$1518 \pm 61~\mathrm{K}$ at $3.6~\micron$ and $2699 \pm 32~\mathrm{K}$/$1259 \pm 44~\mathrm{K}$ at $4.5~\micron$, respectively. These results provide evidence for a dayside thermal inversion. Both channels exhibit small phase offsets of $0.68 \pm 0.48^{\circ}$ at $3.6~\micron$ and $0.67 \pm 0.2^{\circ}$ at $4.5~μ\mathrm{m}$. We compare our observations to a suite of general circulation models that consider two end-members of interior temperature along with a broad range of frictional drag strengths. Strong frictional drag is necessary to match the small phase offsets and cold nightside temperatures observed. From our suite of cloud-free GCMs, we find that only cases with a cold interior can reproduce the cold nightsides and large phase curve amplitude at $4.5~\micron$, hinting that the hot interior adiabat of WASP-76b does not significantly impact its atmospheric dynamics or that clouds blanket its nightside.

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arXiv2021

Haze Evolution in Temperate Exoplanet Atmospheres Through Surface Energy Measurements

Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles is currently unknown under exoplanetary conditions. Here we provide experimentally-measured surface energies for a grid of temperate exoplanet hazes to characterize haze removal in exoplanetary atmospheres. We found large variations of surface energies for hazes produced under different energy sources, atmospheric compositions, and temperatures. The surface energies of the hazes were found to be the lowest around 400 K for the cold plasma samples, leading to the lowest removal rates. We show a suggestive correlation between haze surface energy and atmospheric haziness with planetary equilibrium temperature. We hypothesize that habitable zone exoplanets could be less hazy, as they would possess high-surface-energy hazes which can be removed efficiently.

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arXiv2021

The Hubble PanCET program: Transit and Eclipse Spectroscopy of the Strongly Irradiated Giant Exoplanet WASP-76b

Ultra-hot Jupiters with equilibrium temperature greater than 2000K are uniquely interesting targets as they provide us crucial insights into how atmospheres behave under extreme conditions. This class of giant planets receives intense radiation from their host star and usually has strongly irradiated and highly inflated atmospheres. At such high temperature, cloud formation is expected to be suppressed and thermal dissociation of water vapor could occur. We observed the ultra-hot Jupiter WASP-76b with 7 transits and 5 eclipses using the Hubble Space Telescope (HST) and $Spitzer$ for a comprehensive study of its atmospheric chemical and physical processes. We detect TiO and H$_2$O absorption in the optical and near-infrared transit spectrum. Additional absorption by a number of neutral and ionized heavy metals like Fe, Ni, Ti, and SiO help explain the short wavelength transit spectrum. The secondary eclipse spectrum shows muted water feature but a strong CO emission feature in Spitzer's 4.5 $μ$m band indicating an inverted temperature pressure profile. We analyzed both the transit and emission spectrum with a combination of self-consistent PHOENIX models and retrieval models (ATMO $\&$ PLATON). Both spectra are well fitted by the self-consistent PHOENIX forward atmosphere model in chemical and radiative equilibrium at solar metallicity, adding to the growing evidence that both TiO/VO and NUV heavy metals opacity are prominent NUV-optical opacity sources in the stratospheres of ultra-hot Jupiters.

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arXiv2021

Understanding the Effects of Systematics in Exoplanetary Atmospheric Retrievals

Retrieval of exoplanetary atmospheric properties from their transmission spectra commonly assumes that the errors in the data are Gaussian and independent. However, non-Gaussian noise can occur due to instrumental or stellar systematics and merging discrete datasets. We investigate the effect of correlated noise and constrain the potential biases incurred in the retrieved posteriors. We simulate multiple noise instances of synthetic data and perform retrievals to obtain statistics of goodness-of-retrieval for varying noise models. We find that correlated noise allows for overfitting the spectrum, thereby yielding better goodness-of-fit on average but degrading the overall accuracy of retrievals. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. We also find that higher precision causes correlated results to be further off from the input values in terms of estimated errors. As such, we emphasize that caution must be taken in analyzing retrieved posteriors and that estimated parameter uncertainties are best understood as lower limits. Finally, we show that while correlated noise cannot be be reliably distinguished with HST observations, inferring its presence and strength may be possible with JWST observations.

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arXiv2021

The high-energy spectrum of the nearby planet-hosting inactive mid-M dwarf LHS 3844

To fully characterize the atmospheres, or lack thereof, of terrestrial exoplanets we must include the high-energy environments provided by their host stars. The nearby mid-M dwarf LHS 3844 hosts a terrestrial world which lacks a substantial atmosphere. We present a time series UV spectrum of LHS 3844 from 1131-3215A captured by HST/COS. We detect one flare in the FUV, which has an absolute energy of 8.96+/-0.79e28 erg and an equivalent duration of 355+/-31 s. We extract the flare and quiescent UV spectra separately. For each spectrum we estimate the Ly-alpha flux using correlations between UV line strengths. We use Swift-XRT to place an upper limit on the soft X-ray flux and construct a differential emission model (DEM) to estimate flux that is obscured by the interstellar medium. We compare the DEM flux estimates in the XUV to other methods that rely on scaling from the Ly-alpha, Si IV, and N V lines in the UV. The XUV, FUV, and NUV flux of LHS 3844 relative to its bolometric luminosity is log10(Lband/LBol) = -3.65, -4.16, and -4.56, respectively, for the quiescent state. These values agree with trends in high-energy flux as a function of stellar effective temperature found by the MUSCLES survey for a sample of early-M dwarfs. Many of the most spectroscopically accessible terrestrial exoplanets orbit inactive mid- to late-M dwarfs like LHS 3844. Measurements of M dwarf high-energy spectra are preferable for exoplanet characterization, but are not always possible. The spectrum of LHS 3844 is a useful proxy for the current radiation environment for these worlds.

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arXiv2021

The Featureless HST/WFC3 Transmission Spectrum of the Rocky Exoplanet GJ 1132b: No Evidence For A Cloud-Free Primordial Atmosphere and Constraints on Starspot Contamination

Orbiting a M dwarf 12 pc away, the transiting exoplanet GJ 1132b is a prime target for transmission spectroscopy. With a mass of 1.7 Earth masses and radius of 1.1 Earth radii, GJ 1132b's bulk density indicates that this planet is rocky. Yet with an equilibrium temperature of 580 K, GJ 1132b may still retain some semblance of an atmosphere. Understanding whether this atmosphere exists and its composition will be vital for understanding how the atmospheres of terrestrial planets orbiting M dwarfs evolve. We observe five transits of GJ 1132b with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We find a featureless transmission spectrum from 1.1--1.7 microns, ruling out cloud-free atmospheres with metallicities <300x Solar with >4.8$σ$ confidence. We combine our WFC3 results with transit depths from TESS and archival broadband and spectroscopic observations to find a featureless spectrum from 0.7--4.5 microns. GJ 1132b has a high mean molecular weight atmosphere, possesses a high-altitude aerosol layer, or has effectively no atmosphere. Higher precision observations are required to differentiate between these possibilities. We explore the impact of hot and cold starspots on the observed transmission spectrum GJ 1132b, quantifying the amplitude of spot-induced transit depth features. Using a simple Poisson model we estimate spot temperature contrasts, spot covering fractions, and spot sizes for GJ 1132. These limits, and the modeling framework, may be useful for future observations of GJ 1132b or other planets transiting similarly inactive M dwarfs.

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