Observations of young stars to understand how planets form

The discovery of the first Hot Jupiter (HJ, giant extrasolar planets with orbital periods shorter than about 10 days), 51 Peg b (Mayor & Queloz, 1995), challenged our assumptions about planet formation, putting in doubt also the ideas about the formation of our Solar System. Moreover, the following discoveries, that revealed the diversity of the exoplanets characteristics, clearly demonstrated that most of the gross features of the Solar System are only one result within a wide range of possibilities. This consideration raised the interest in the field of exoplanet discovery and characterization. Different theories were developed in order to explain the origins and properties of HJs. Sec. 1.2 provides a more detailed description of these formation theories. Observing young stars (pre-main sequence stars, PMS) is becoming very important in this context, because they give the opportunity to study the planetary systems in their earlier formation phases and calibrate and test the evolutionary models for planets (Sec. 1.3). While the direct imaging method can only detect massive planets at wide separation from their host stars, the radial velocity (RV) method is more suited to identify planets on close orbits, like HJs. Details on methods to search for exoplanets and their corresponding sensitivities are discussed in Sec. 1.1. Unluckily, young stars are very active and the observations are made complicated by star spots, jets, accretion, and circumstellar disks. The strong magnetic field of these stars induces intrinsic RV variations with amplitudes similar or even larger than the Keplerian signal, so that the stellar activity can mimic or cover the presence of a companion. For this reason a successful RV survey of young stars should be characterized by the capability of distinguishing between the activity-induced and planet-induced RV modulation. In last years a lot of techniques were developed in order to deal with the RV stellar noise (Dumusque et al., 2017). One of these is the multi-wavelength observations, that are a powerful tool to overcome the limits imposed by the stellar activity. In particular coupling near-infrared (NIR) and visible (VIS) observations allows to immediately disentangle the activity contribution, because the RV jitter due to the activity is reduced in the NIR respect to the VIS range. On the other hand, if the RV variation is due to a Keplerian motion it is wavelength-independent. A more detailed description of the methods used for modelling the stellar activity is reported in Sec. 1.3.1. Simultaneous multi-band spectroscopy has been evaluated from GAPS2.0, a collaboration among most of Italian exoplanetary community, leading to the realization of GIARPS, between 2014 and 2017, that allows to simultaneously observe with HARPS – N in the VIS range and GIANO – B in NIR range. In Chapter 2 the GIARPS Project is described in details. The scientific aim of GAPS2.0 is to explore the realm of planet formation and evolution with emphasis on the inner regions of planetary systems, observing young stars at different ages and identifying and characterizing their planets. From the observable exoplanetary properties (orbital parameters, age-dependent frequency, atmospheric composition) it is possible to understand the physical processes responsible for generating HJs. A pilot program, performed in Autumn-Winter 2017, allowed us to test the multi-wavelength methodology and to optimize the strategy of removal of activity signal at very young ages. This PhD work concerned the analysis of some case of claimed HJs. The observations of these targets in GIARPS mode were performed during the pilot program and also during the following Large Program started in April 2018. The first results are discussed in Chapter 3 and show two cases of disclaimed planets (Sec. 3.2 - 3.5) and one confirmed planet (Sec. 3.4), demonstrating the feasibility of our program and the potentiality of the multi-band spectroscopy technique.