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

Abstract

Quantifying the evolution of stellar extreme ultraviolet (EUV, 100–1000 Å) 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 ± 0.05 M ☉ 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.

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

“…XUV (≡ X-ray + EUV) irradiance in the HZ environments around M dwarfs is expected to be more than a factor of 10 times the average Earth-Sun value owing to the close-in habitable zone (France et al 2016), making M dwarf exoplanets particularly prone to XUV-driven atmospheric escape. Additionally, the XUV luminosity of M dwarfs is enhanced by another factor of 10-50 relative to solar-type stars during their prolonged premain-sequence evolution (Shkolnik & Barman 2014;Luger & Barnes 2015;Ribas et al 2016;Peacock et al 2020). The highenergy emission stays in the saturated regime of rotation-driven activity on M dwarf systems several gigayears longer than that of Sun-like stars (see Pineda et al 2020 and references therein), well into the era in which life emerged on Earth.…”

A Snowball in Hell: The Potential Steam Atmosphere of TOI-1266c

“…During our initial tests, we chose to empirically set the saturation timescale to when the time-dependent X-ray luminosity falls below the saturated value, resulting in τ i ∼ 0.47 Gyr, which produced a negligible change in the total mass lost. We instead chose to use the X-ray and EUV scaling relationships from Peacock et al (2020) in order to address these two discrepancies. Peacock et al (2020) note a saturation of ∼10 −2 F EUV /F bol in their simulations of ∼0.4 M e stars, slightly higher in magnitude but qualitatively consistent with work for larger stars (e.g., King & Wheatley 2020).…”

“…We instead chose to use the X-ray and EUV scaling relationships from Peacock et al (2020) in order to address these two discrepancies. Peacock et al (2020) note a saturation of ∼10 −2 F EUV /F bol in their simulations of ∼0.4 M e stars, slightly higher in magnitude but qualitatively consistent with work for larger stars (e.g., King & Wheatley 2020). Additionally, the log-linear dependence on age after the saturated period is comparable to other early M dwarf studies (e.g., Stelzer et al 2013).…”

Time-Resolved Photometry of the High-Energy Radiation of M Dwarfs with the Star-Planet Activity Research CubeSat (SPARCS)

“…The SPARCS dual-band, high-cadence, long time baseline observations (Figure 2) will enable measurements of M dwarf UV flare color, energies, occurrence rate, and duration of quiescent and flaring states for both active and inactive M dwarfs. It is also anticipated that SPARCS observations will help predict M dwarf EUV flux to better than a factor of two, and will allow for the development of improved EUV-NUV M dwarf upper-atmosphere model spectra (see Peacock et al, 2020, and references therein) that will serve as more reliable inputs to the modelling of the atmospheres of planets around M dwarfs.…”