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

## TL;DR

This study investigates how interstellar medium absorption affects stellar activity measurements, revealing that for stars beyond 100 pc, ISM absorption can significantly bias activity indicators, impacting exoplanet research.

## Contribution

It provides a detailed analysis of ISM absorption effects on activity indices and offers guidelines for correcting biases in stars beyond 100 pc.

## Key findings

- ISM absorption significantly biases activity measurements for stars >100 pc
- Bias in logR'$_{
m HK}$ can reach 0.05-0.1 dex due to ISM effects
- ISM CaII column density of logN$_{
m CaII}$=12 typically occurs within 100 pc

## Abstract

Past UV and optical observations of stars hosting hot Jupiters have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening ISM. Inspired by this result, we study the effect of ISM absorption on activity measurements (S and logR'$_{\rm HK}$ 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, stellar, and ISM parameters. We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km/s and for ISM CaII column densities logN$_{\rm CaII}$>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 CaII column density of logN$_{\rm CaII}$=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 logR'$_{\rm HK}$ 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.