Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets around Low-mass Stars: GJ 832, GJ 176, and GJ 436

Abstract

Correct estimates of stellar extreme ultraviolet (EUV; 100 -1170Å) flux are important for studying the photochemistry and stability of exoplanet atmospheres, as EUV radiation ionizes hydrogen and contributes to the heating, expansion, and potential escape of a planet's upper atmosphere. Contamination from interstellar hydrogen makes observing EUV emission from M stars particularly difficult, and impossible past 100 pc, and necessitates other means to predict the flux in this wavelength regime. We present EUV -infrared (100Å -5.5 µm) synthetic spectra computed with the PHOENIX atmospheric code of three early M dwarf planet hosts: GJ 832 (M1.5 V), GJ 176 (M2.5 V), and GJ 436 (M3.5 V). These one-dimensional semiempirical nonlocal thermodynamic equilibrium models include simple temperature prescriptions for the stellar chromosphere and transition region, from where ultraviolet (UV; 100 -3008Å) fluxes originate. We guide our models with Hubble Space Telescope far-and near-UV spectra and discuss the ability to constrain these models using Galaxy Evolution Explorer UV photometry. Our models closely reproduce the observations and predict the unobservable EUV spectrum at a wavelength resolution of <0.1Å. The temperature profiles that best reproduce the observations for all three stars are described by nearly the same set of parameters, suggesting that early M type stars may have similar thermal structures in their upper atmospheres. With an impending UV observation gap and the scarcity of observed EUV spectra for stars less luminous and more distant than the Sun, upper-atmosphere models such as these are important for providing realistic spectra across short wavelengths and for advancing our understanding of the effects of radiation on planets orbiting M stars.

The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?

“…observation of fluorescent H 2 originating from the lowerexcitation v″=1 state and the absence of fluorescent H 2 originating from the higher-excitation v″=2 as evidence for a low-temperature (T(H 2 )1500 K) molecular component to the atmosphere of Barnard's Star. The physical location within the stellar atmosphere of this cool component is not clear and may provide interesting constraints on future models of M dwarf atmospheres (Fontenla et al 2016;Peacock et al 2019). While 1500K is cooler than the 2600 K chromospheric temperature minimum estimated from the M1.5V model atmosphere of Fontenla et al (2016), it is consistent with the chromospheric minimum found in the M8V model atmosphere calculated by Peacock et al (2019).…”

“…The physical location within the stellar atmosphere of this cool component is not clear and may provide interesting constraints on future models of M dwarf atmospheres (Fontenla et al 2016;Peacock et al 2019). While 1500K is cooler than the 2600 K chromospheric temperature minimum estimated from the M1.5V model atmosphere of Fontenla et al (2016), it is consistent with the chromospheric minimum found in the M8V model atmosphere calculated by Peacock et al (2019).…”

TOI-1231 b: A Temperate, Neptune-sized Planet Transiting the Nearby M3 Dwarf NLTT 24399

“…TOI-1231ʼs fundamental stellar properties are very similar to GJ 436ʼs, and our PFS spectra indicate that TOI-1231ʼs log 10 ¢ = -R 5.06 HK is nearly equivalent to GJ 436ʼs (−5.09;Boro Saikia et al 2018), indicating the same level of magnetic activity. Because ¢ R HK is known to correlate well with UV emission (Youngblood et al 2017), we assume GJ 436ʼs synthetic X-ray and UV spectrum from Peacock et al (2019) as a proxy for TOI-1231ʼs. The integrated flux from 100 to 912 Å at 0.13 au is 172 erg cm −2 s −1 but could be as low as ∼53 erg cm −2 s −1 , according to the estimates from the MUSCLES Treasury Survey (France et al 2016;Loyd et al 2016;Youngblood et al 2016) based on Linsky et al (2014).…”

“…See García Muñoz et al (2020) and references within for more details about the method. The model simulations shown in Figure 8 are specific to the stellar spectrum from Peacock et al (2019) and assume supersonic conditions at r/R p ∼ 10. We take the stellar spectrum in its original format, 64 correcting only for orbital distance.…”

The CARMENES search for exoplanets around M dwarfs

“…This means that we obtain realistic F EUV /F bol within our models despite this inadequacy. For a recent detailed analysis of a synthetic EUV PHOENIX spectrum computed for the inactive M dwarf Ross 905, we refer to the study by Peacock et al (2019).…”