  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.
