The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b
arXiv:1906.08783 · doi:10.1051/0004-6361/201935636
Abstract
In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts planetary atmospheres. LHS 1140b is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs. We present the results of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV and NUV flux of a sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution. No significant variation of the NUV flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV flux <2% with respect to that of the present-day Earth, while the FUV/NUV ratio is ~100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the GALEX FUV band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.
6 pages, 4 figures, 2 tables. Accepted for pubblication in Astronomy & Astrophysics
References in corpus (13)
- The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
- Extreme Water Loss and Abiotic O Buildup On Planets Throughout the Habitable Zones of M Dwarfs
- Habitable planets around the star Gl 581?
- The Structure of the Local Interstellar Medium V: Electron Densities
- A Re-appraisal of the Habitability of Planets Around M Dwarf Stars
- The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present
- Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets
- The Solar Neighborhood XXXVII: The Mass-Luminosity Relation for Main Sequence M Dwarfs
- Tidal Locking of Habitable Exoplanets
- UV habitable zones around M stars
- A path towards understanding the rotation-activity relation of M dwarfs with K2 mission, X-ray and UV data
- A scenario of planet erosion by coronal radiation
- HAZMAT II: Ultraviolet Variability of Low-Mass Stars in the GALEX Archive