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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.
We present an in-depth analysis of stellar activity and its effects on radial velocity (RV) for the M2 dwarf GJ 176 based on spectra taken over 10 years from the High Resolution Spectrograph on the Hobby-Eberly Telescope. These data are supplemented
Stellar wind and photon radiation interactions with a planet can cause atmospheric depletion, which may have a potentially catastrophic impact on a planets habitability. While the implications of photoevaporation on atmospheric erosion have been rese
Atmospheric escape from close-in exoplanets is thought to be crucial in shaping observed planetary populations. Recently, significant progress has been made in observing this process in action through excess absorption in transit spectra and narrowba
Young Moving Groups (YMGs) are close (<100pc), coherent collections of young (<100Myr) stars that appear to have formed in the same star-forming molecular cloud. As such we would expect their individual initial mass functions (IMFs) to be similar to
Long-term stellar activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five activity indicators: log$R_mathrm{{HK}}$, the cross-correlation funct