The effect of ISM absorption on stellar activity measurements and its relevance for exoplanet studies

Abstract

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R ′ HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca ii lines, broadening b-parameter, and Ca ii column density). We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km s −1 and for ISM Ca ii column densities log N CaII 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca ii column density of log N CaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R ′ HK value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars.

The Kepler-19 System: A Thick-envelope Super-Earth with Two Neptune-mass Companions Characterized Using Radial Velocities and Transit Timing Variations

“…26 Note, however, that this value could be affected by interstellar medium absorption and could be higher than measured (Fossati et al 2017). 27 TRADES is available at https://github.com/lucaborsato/trades.…”

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

“…0.023±0.009 and 0.008 mag) to estimate the range of ISM column densities for H I, C I, C II, C III, C IV, Si IV, Mg II, Ca II, and Na I. Following the results and considerations of Fossati et al (2015bFossati et al ( , 2017, we derived the column densities employing the N HI -E(B − V) conversion 5 provided by Savage & Mathis (1979) and the ISM abundances and ionization mix given by Frisch & Slavin (2003, their model 2 - Table 5). The results are listed in Table 1.…”

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…The intrinsic narrowness of the M dwarf emission lines may mean that for some sightlines, the ISM absorption coincides with the stellar emission line, but for others the ISM absorption is shifted away from the emission, and the line's flux is not attenuated. Ca + from the ISM can significantly attenuate Ca IIH and K, but only for distant stars (d>100 pc; Fossati et al 2017). Because all of our targets are within 15 pc, Ca IIISM absorption is likely negligible, and we do not apply a correction.…”