Photometric transit search for planets around cool stars from the Western Italian Alps: the APACHE survey

Small-size ground-based telescopes can effectively be used to look for transiting rocky planets around nearby low-mass M stars using the photometric transit method. Since 2008, a consortium of the Astrophysical Observatory of Torino (OATo-INAF) and the Astronomical Observatory of the Autonomous Region of Aosta Valley (OAVdA) have been preparing for the long-term photometric survey APACHE (A PAthway toward the Characterization of Habitable Earths), aimed at finding transiting small-size planets around thousands of nearby early and mid-M dwarfs. APACHE uses an array of five dedicated and identical 40-cm Ritchey-Chretien telescopes and its routine science operations started at the beginning of summer 2012. Here I present the results of the `pilot study', a year-long photometric monitoring campaign of a sample of 23 nearby dM stars, and of the APACHE survey first year data. In these studies, I set out to (i) demonstrate the sensitivity to > 2 Rearth transiting planets with periods of up to a few days around our programme stars, through a two-fold approach that combines a characterization of the statistical noise properties of our photometry with the determination of transit detection probabilities via simulations; and (ii), where possible, improves our knowledge of some astrophysical properties (e.g. activity, rotation) of our targets by combining our differential photometric measurements with spectroscopic information from the long-term programme GAPS with the HARPS-N spectrograph on the Telescopio Nazionale Galileo. Furthermore, cool M dwarfs within a few tens of parsecs from the Sun are becoming the focus of dedicated observational programs in the realm of exoplanet astrophysics that will make use of astrometric measurements. I present numerical simulations to gauge the Gaia potential for precision astrometry of exoplanets orbiting a sample of known dM stars within ~ 30 pc from the Sun. I then investigate some aspects of the synergy between the astrometric data expected from the Gaia mission on nearby M dwarfs and the APACHE program.