Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b

Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b absorbs over 50% of the stellar Ly$\alpha$ emission during transit. Given the planet's atmospheric composition and energy-limited escape rate, this hydrogen outflow is expected to entrain heavier atoms such as C and O. We searched for C and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M observations made during the planet's extended H I transit. These observations show no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\ lines integrated over [-100, 100] km s$^{-1}$, imposing 95% (2$\sigma$) upper limits of 14% (C II) and 60% (Si III) depth on the transit of an opaque disk and 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transit similar to that of H I Ly$\alpha$. C$^+$ is likely present in the outflow according to a simulation we carried out using a spherically-symmetric, photochemical-hydrodynamical model. This simulation predicts a $\sim$2% transit over the integrated bandpass, consistent with the data. At line center, we predict the C II transit depth to be as high as 19%. Our model predicts a neutral hydrogen escape rate of $1.6\times10^{9}$ g s$^{-1}$ ($3.1\times10^{9}$ g s$^{-1}$ for all species) for an upper atmosphere composed of hydrogen and helium.Comment: 7 pages, 4 figures, 1 table; accepted to ApJ Letter

The Volatility of Far-Ultraviolet Radiation from Low-Mass Stars and Planetary Implications

A low-mass star emits only 0.001-0.01% of its electromagnetic energy in the ultraviolet (100 &ndash; 1700 &Aring;), yet the implications for planets are profound. Radiation at the short-wavelength end of this range, the extreme ultraviolet (100 &ndash; 912 &Aring;), powers planetary atmospheric escape that can be observed through transit spectroscopy in the far ultraviolet (FUV; 912 &ndash; 1700 &Aring;). In addition, FUV light that penetrates further into a planet&rsquo;s atmosphere photolyzes molecules, driving nonthermal chemistry capable of producing O2 and O3 abiotically. I present results from extensive Hubble Space Telescope (HST) observations of the FUV emission from low-mass stars, focusing on its variability in time. Using all available archival data, I determined an astrophysical noise floor on detectable absorption in the C II, Si III, and Si IV FUV lines due to a transiting evaporating planet. I analyzed such a transit of the evaporating hot-Neptune GJ 436b, finding no detectable absorption in C II or Si III despite a large (&gt;50%) transit in Ly&alpha;, but placing upper limits that agree with a photochemical-hydrodynamical model of the planetary outflow. These observations of GJ 436 were part of the MUSCLES Treasury Survey, a much larger dataset covering 7 M and 4 K dwarf stars. A time series analysis of all of these data constrained FUV flares on the surveyed M dwarfs, which I augmented with archival data on well-studied flare stars. The analysis confirmed that M dwarf flares are ubiquitous. In relative units, the FUV flares of the "inactive&rsquo;&rsquo; MUSCLES M dwarfs are equally as energetic and frequent as those of the M dwarf flare stars. Both flare ~3 orders of magnitude more frequently than the sun. Indeed, flares possibly (if not probably) dominate the energy budget of FUV emission from M dwarf stars. Highly energetic flares occurring roughly yearly could annihilate most of the ozone from an Earth-like atmosphere. However, the effect is short-lived, unless additional reactions not accounted for or particle events play a dominant role.</p

Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars

We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star-planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope-COS and -STIS instruments, targeting the chromospheric and transition region emission lines of Si III, N V, C II, and Si IV. We find that the exoplanet host stars, on average, display factors of 5 - 10 lower UV activity levels compared with the non-planet hosting sample; this is explained by a combination of observational and astrophysical biases in the selection of stars for radial-velocity planet searches. We demonstrate that UV activity-rotation relation in the full F - M star sample is characterized by a power-law decline (with index $\alpha$ ~ -1.1), starting at rotation periods >~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric flux, we present a new analytic relationship to estimate the intrinsic stellar EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of ~2. Finally, we study the correlation between SPI strength and UV activity in the context of a principal component analysis that controls for the sample biases. We find that SPIs are not a statistically significant contributor to the observed UV activity levels.Comment: ApJS, accepted. 33 pages in emulateapj, 13 figures, 10 table

HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Star

Quantifying the evolution of stellar extreme ultraviolet (EUV, 100 -- 1000 $\overset{\circ}{A}$) emission is critical for assessing the evolution of planetary atmospheres and the habitability of M dwarf systems. Previous studies from the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program showed the far- and near-UV (FUV, NUV) emission from M stars at various stages of a stellar lifetime through photometric measurements from the Galaxy Evolution Explorer (GALEX). The results revealed increased levels of short-wavelength emission that remain elevated for hundreds of millions of years. The trend for EUV flux as a function of age could not be determined empirically because absorption by the interstellar medium prevents access to the EUV wavelengths for the vast majority of stars. In this paper, we model the evolution of EUV flux from early M stars to address this observational gap. We present synthetic spectra spanning EUV to infrared wavelengths of 0.4 $\pm$ 0.05 M$_{\odot}$ stars at five distinct ages between 10 and 5000 Myr, computed with the PHOENIX atmosphere code and guided by the GALEX photometry. We model a range of EUV fluxes spanning two orders of magnitude, consistent with the observed spread in X-ray, FUV, and NUV flux at each epoch. Our results show that the stellar EUV emission from young M stars is 100 times stronger than field age M stars, and decreases as t$^{-1}$ after remaining constant for a few hundred million years. This decline stems from changes in the chromospheric temperature structure, which steadily shifts outward with time. Our models reconstruct the full spectrally and temporally resolved history of an M star's UV radiation, including the unobservable EUV radiation, which drives planetary atmospheric escape, directly impacting a planet's potential for habitability.Comment: 23 pages, 15 figures, accepted to Ap

Using Lyman-α\alpha transits to constrain models of atmospheric escape

Lyman-$\alpha$ transits provide an opportunity to test models of atmospheric escape directly. However, translating observations into constraints on the properties of the escaping atmosphere is challenging. The major reason for this is that the observable parts of the outflow often comes from material outside the planet's Hill sphere, where the interaction between the planetary outflow and circumstellar environment is important. As a result, 3D models are required to match observations. Whilst 3D hydrodynamic simulations are able to match observational features qualitatively, they are too computationally expensive to perform a statistical retrieval of properties of the outflow. Here, we develop a model that determines the trajectory, ionization state and 3D geometry of the outflow as a function of its properties and system parameters. We then couple this model to a ray tracing routine in order to produce synthetic transits. We demonstrate the validity of this approach, reproducing the trajectory of the outflows seen in 3D simulations. We illustrate the use of this model by performing a retrieval on the transit spectrum of GJ 436 b. Our model constrains the sound speed of the outflow $\gtrsim 10 \text{ km s}^{-1}$, indicating that we can rule out core-powered mass loss as the mechanism driving the outflow for this planet. The bound on planetary outflow velocity and mass loss rates are consistent with a photoevaporative wind