Bibcode
Martín, E. L.; Cabrera, J.; Martioli, E.; Solano, E.; Tata, R.
Bibliographical reference
Astronomy and Astrophysics, Volume 555, id.A108, 11 pp.
Advertised on:
7
2013
Journal
Citations
14
Refereed citations
12
Description
Observations of very low-mass stars with Kepler represent an excellent
opportunity to search for planetary transits and to characterize optical
photometric variability at the cool end of the stellar mass
distribution. In this paper, we present low-resolution red optical
spectra that allow us to identify 18 very low-mass stars that have
Kepler light curves available in the public archive. Spectral types of
these targets are found to lie in the range dM4.5-dM8.5, implying
spectrophotometric distances from 17 pc to 80 pc. Limits to the presence
of transiting planets are set by modeling of the Kepler light curves. We
find that the size of the planets detectable by Kepler around these
small stars typically lies in the range 1 to 5 Earth radii within the
habitable regions (P ≤ 10 days). We identify one candidate transit
with a period of 1.26 days whose light curve resembles a planet slightly
smaller than the Moon. However, our pixel by pixel analysis of the
Kepler data shows that the signal most likely arises from a background
contaminating eclipsing binary. For 11 of these objects reliable
photometric periods shorter than 7 days are derived, and are interpreted
as rotational modulation of magnetic cool spots. For 3 objects we find
possible photometric periods longer than 50 days that require
confirmation. The Hα emission measurements and flare
rates are used as proxies for chromospheric activity, and transversal
velocities are used as an indicator of dynamical ages. These data allow
us to discuss the relationship between magnetic activity and
detectability of planetary transits around very low-mass stars. We show
that super-Earth planets with sizes around 2 Earth radii are detectable
with Kepler around about two thirds of the stars in our sample,
independently of their level of chromospheric activity.