Spectropolarimetric techniques and applications to stellar magnetism

This dissertation is aimed at the measurement and the characterization of stellar magnetic fields, which are one of the most challenging topics in the modern astrophysics. They are been detected in almost all the stellar evolutionary stages, from pre-main sequence to degenerate stars. They are the keys from the understanding of several phenomena, such as accretion on pre-main sequence stars, stellar activity and spots and they are also important in order to investigate possible false detections of exoplanets and to characterize star-planet interactions. In particular, this thesis focus on the study of upper main sequence, active solar analogs and stars hosting planet, with particular attention on evolved stars. Stellar magnetic fields can be measured through the polarization and the splitting due to Zeeman effects, from spectropolarimetric observations. In particular, this dissertation employs archive data of Narval and HARPSpol and it is based on observations obtained with the instruments CAOS (Catania Astrophysical Observatory Spectropolarimeter) in more than hundred nights, during the period of the thesis. Full Stokes observations of the magnetic Ap star BetaCrB are used in order to determine the transversal component and the angle of the magnetic fields. The observations of a sample of 22 stars with detected planets are used in order to study the impact of the presence of the field; results show that the 47% of the giants and the 40% of the main sequence stars in the sample host magnetic fields. In particular, the field strengths of the giant stars show a correlation with the rotational period, which can be connected to the presence of a dynamo process driven by rotation. Measurements are performed through the Least Square Deconvolution technique, using a code that was implemented and tested during the period of thesis. Furthermore, a new technique for the measurement of the effective magnetic field from high-resolution observations is introduced. This technique, called multi-line slope method, is tested with synthetic spectra and it is applied to a dataset of spectropolarimetric observations of the active star EpsEri, which spans 9 years from 2007 to 2016. The temporal analysis allows to determine a period of variation (P1=1099±71d) consistent with the variation of the activity index. This measurement represents the first estimation of the period of the cycle of a star obtained from direct measurements of the magnetic field.