PLANETS AROUND LOW-MASS STARS AND STELLAR ACTIVITY EFFECTS

In the last years the field of exoplanet research has focused its interest in M dwarfs. These stars have became the favourite targets in radial velocity surveys, specially when looking for small planets in the habitable zones of their parent stars. Not only for being the M dwarfs the most common objects in our Galaxy also because the Doppler signals due to small planets orbiting around them are larger and more easily detectable than those around FGK stars. However, stellar magnetic activity and rotation affect the measured radial velocities as surface inhomogeneities rotating with the stellar surface can cause periodic changes in the spectral line centroid. Disentangle these stellar activity effects from the planetary signals is a challenge, specially for M dwarfs, which retain high magnetic activity levels for long time and, on average are more active than solar-like stars. For these reasons understanding stellar activity, in particular for M dwarfs, is crucial for the detection of exoplanets through the radial velocity technique. Therefore, in the first part of this thesis we use a sample of 78 late-K/early-M dwarfs with relatively low activity levels observed with the HARPS-N high-resolution échelle spec- trograph at the Telescopio Nazionale Galileo (Spain) in the framework of the HArps-N red Dwarf Exoplanet Survey (HADES), in order to test whether the relations between activity, rotation and stellar parameters and flux-flux relationships investigated in the literature for main-sequence FGK stars and for pre-main-sequence M stars also hold for early-M dwarfs. Our results show that early-M dwrafs follow the same relations of more massive stars and, more important, our data allow us to explore the slow rotation regime, poorly covered by previous studies. This regime for low mass stars is particularly interesting since the involved relatively long periods overlap with the periods corresponding to the habitable zone around these stars. The results of this study are part of several published papers within the HADES collaboration: Scandariato et al. (2017), Maldonado et al. (2017) and Suárez Mascareño et al. (A&A, in press). In the second part of this thesis, we discuss in detail the analysis developed to search for planets in the low-mass star domain. In particular we focus on the analysis of GJ 720 A, an M0.5 M dwarf, that presents an interesting high significant periodic signal suggesting the presence of a companion. Our candidate planet would have a minimum mass around 13 M and an orbital period of 19.487 ± 0.008 days. With this characteristics our planet would occupy an interesting region of super-Earths poorly populated. To definitively verify our hypothesis further studies such as the photometric light curve or Gaussian processes analysis will be performed. Also in this part we will discuss the analysis of GJ 521 A, an M1.5 dwarf, that show some of the problems that arise when the radial velocity variations are due to stellar activity, stressing the crucial interest on understanding the stellar activity in M dwarfs that can be easily frustrate the identification of planetary signals. Finally, in the last part of this thesis we present a different approach applied to a red giant star, TYC-4282-605-1. We discovered an orbiting low-mass companion with a period of 101 days using a multi-wavelength approach. This work was a pilot programme for the GIARPS project, the new observing mode at the TNG using at the same time HARPS-N (visible) and GIANO (NIR) spectrographs. Our study let us to confirm for the first time a planet around a giant star using quasi-simultaneous RVs in VIS and NIR. All these results have been published by González-Álvarez et al. (2017).