The MUSCLES Treasury Survey I: Motivation and Overview

FRANCE, KEVIN; PARKE LOYD, R. O.; YOUNGBLOOD, ALLISON; BROWN, ALEXANDER; SCHNEIDER, P. CHRISTIAN; HAWLEY, SUZANNE L.; FRONING, CYNTHIA S.; LINSKY, JEFFREY L.; ROBERGE, AKI; BUCCINO, A.P.; DAVENPORT, JAMES R. A.; FONTENLA, JUAN M.; KALTENEGGER, LISA; KOWALSKI, ADAM F.; MAUAS, PABLO J. D.; MIGUEL, YAMILA; REDFIELD, SETH; RUGHEIMER, SARAH; TIAN, FENG; VIEYTES, MARIELA C.; WALKOWICZ, LUCIANNE M.; WEISENBURGER, KOLBY L.

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2016 vol. 820 p. 89 - 113

0004-637X

Ground- and space-based planet searches employing radial velocitytechniques and transit photometry have detected thousands ofplanet-hosting stars in the Milky Way. With so many planets discovered,the next step toward identifying potentially habitable planets isatmospheric characterization. While the Sun?Earth system providesa good framework for understanding the atmospheric chemistry ofEarth-like planets around solar-type stars, the observational andtheoretical constraints on the atmospheres of rocky planets in thehabitable zones (HZs) around low-mass stars (K and M dwarfs) arerelatively few. The chemistry of these atmospheres is controlled by theshape and absolute flux of the stellar spectral energy distribution(SED), however, flux distributions of relatively inactive low-mass starsare poorly understood at present. To address this issue, we haveexecuted a panchromatic (X-ray to mid-IR) study of the SEDs of 11 nearbyplanet-hosting stars, the Measurements of the Ultraviolet SpectralCharacteristics of Low-mass Exoplanetary Systems (MUSCLES) TreasurySurvey. The MUSCLES program consists visible observations from Hubbleand ground-based observatories. Infrared and astrophysicallyinaccessible wavelengths (EUV and Lyα) are reconstructed usingstellar model spectra to fill in gaps in the observational data. In thisoverview and the companion papers describing the MUSCLES survey, we showthat energetic radiation (X-ray and ultraviolet) is present frommagnetically active stellar atmospheres at all times for stars as lateas M6. The emission line luminosities of C iv and Mg ii are stronglycorrelated with band-integrated luminosities and we present empiricalrelations that can be used to estimate broadband FUV and XUV(≡X-ray + EUV) fluxes from individual stellar emission linemeasurements. We find that while the slope of the SED, FUV/NUV,increases by approximately two orders of magnitude form early K to lateM dwarfs (≈0.01?1), the absolute FUV and XUV flux levels attheir corresponding HZ distances are constant to within factors of afew, spanning the range 10?70 erg cm‑2s‑1 in the HZ. Despite the lack of strong stellaractivity indicators in their optical spectra, several of the M dwarfs inour sample show spectacular UV flare emission in their light curves. Wepresent an example with flare/quiescent ultraviolet flux ratios of theorder of 100:1 where the transition region energy output during theflare is comparable to the total quiescent luminosity of the starEflare(UV) ∼ 0.3 L*Δt (Δt = 1 s).Finally, we interpret enhanced L(line)/LBol ratios for C ivand N v as tentative observational evidence for the interaction ofplanets with large planetary mass-to-orbital distance ratios(Mplan/aplan) with the transition regions of theirhost stars.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute.STScI is operated by the Association of Universities for Research inAstronomy, Inc. under NASA contract NAS 5-26555.