M dwarfs in the b201 tile of the VVV survey: Colour-based selection, spectral types and light curves

Context. The intrinsically faint M dwarfs are the most numerous stars in the Galaxy, have main-sequence lifetimes longer than the Hubble time, and host some of the most interesting planetary systems known to date. Their identification and classification throughout the Galaxy is crucial to unraveling the processes involved in the formation of planets, stars, and the Milky Way. The ESO Public Survey VVV is a deep near-IR survey mapping the Galactic bulge and southern plane. The VVV b201 tile, located in the border area of the bulge, was specifically selected for the characterisation of M dwarfs. Aims. We used VISTA photometry to identify M dwarfs in the VVV b201 tile, to estimate their subtypes, and to search for transit-like light curves from the first 26 epochs of the survey. Methods. UKIDSS photometry from SDSS spectroscopically identified M dwarfs was used to calculate their expected colours in the YJHK_s VISTA system. A colour-based spectral subtype calibration was computed. Possible giants were identified by a (J-K_s,H_J) reduced proper motion diagram. The light curves of 12.8 <K_s< 15.8 colour-selected M dwarfs were inspected for signals consistent with transiting objects. Results. We identified 23 345 objects in VVV b201 with colours consistent with M dwarfs. We provided their spectral types and photometric distances, up to ~300 pc for M9s and ~1.2 kpc for M4s, from photometry. In the range 12 <K_s< 16, we identified 753 stars as possible giants out of 9232 M dwarf candidates. While only the first 26 epochs of VVV were available, and 1 epoch was excluded, we were already able to identify transit-like signals in the light curves of 95 M dwarfs and of 12 possible giants. Conclusions. Thanks to its deeper photometry (~4 mag deeper than 2MASS), the VVV survey will be a major contributor to the discovery and study of M dwarfs and possible companions towards the centre of the Milky Way.