Exocomets: A study of the gaseous environment of A-type main sequence stars

In main sequence stars, infrared excesses attributed to the presence of circumstellar dust in debris discs has been  the empirical indirect probe of the existence of minor bodies for more than 40 years, as they were needed to explain the dust emission. But it was not until 1987 that the first evidence of the presence of exocomets was found. It was around the Beta Pictoris star and it came with the detection of non-photospheric variable absorptions superimposed to the photospheric Ca II lines. Since then, the number of known stars with exocomet-like absorptions has grown slowly, through sporadic works, up to ~20 stars, all of them A-type. The growth of the exoplanetary field, and the possible relevance of minor bodies in the configuration, building-up and architecture of planetary systems, have turned the attention of the scientific community again to the study of small bodies around stars other than the Sun. Encouraging are the first detections of exocometary bodies in photometric light curves from data taken by exoplanet search missions such as Kepler and TESS. Photometric detection using the exquisite light curves from these space observatories, in addition of providing a new methodology, has demonstrated that exocomets, as was expected from the early detection of infrared excesses, are present around stars of different spectral types in the main sequence. 
This thesis carries out a systematic spectroscopic survey in the time domain with the aim of detecting exocometary activity around main-sequence stars. The main objectives are, first, to enlarge the sample of known exocomet-host stars, and second, determine the possible environmental characteristics of the exocomet-host systems that could help to understand and detect them. 
A large survey to search for exocometary signals was conducted, where around 1500 high-resolution optical spectra of 117 stars with spectral types G to B were inspected. Although it was performed on a biased sample, this is, to our knowledge, the largest exocometary survey that allows for the first time to evaluate the possible dependence of the presence of exocomets with the characteristics of the circumstellar environment. In this thesis, exocomet-like variability has been found in six new stars, increasing from 20 to 26 -a 30%- the number of exocomet host stars known prior to this work. We have also detected non-photospheric variations in the Ca II, and in some cases in the Na I lines, in another 12 stars that were already known in the literature to present spectroscopic variability. Moreover, we do not find any strong evidence that relates the presence of a debris disc with exocometary activity.
The presence of narrow non-photospheric stable absorptions in those systems where cold gas had been previously detected in far-infrared or (sub-)mm wavelengths was analysed. A dependency with the inclination angle of the system is found, proving that hot and cold gas are most likely co-existing, both having a common origin in sublimating or colliding small bodies. 
Finally, we have performed a deeper analysis of three objects, namely Phi Leo, HR 10 and HD 37306, that we single out from the original sample because they showed non-photospherical variable absorptions that could originate from different mechanisms.
For Phi Leo we find that is the most variable star in our sample, with changes in the line profiles occurring in time scales of hours, a frequency comparable to that of the well-studied star $\beta$ Pic. We have found that the origin of its variability is more likely the presence of exocometary bodies. In the case of HR 10, that was known as a classical exocomet-host star, a strict periodicity in its variations was observed, consistent with the star being actually a binary and not an exocomet active star. Finally, we find that HD 37306 shows variability of longer duration and larger equivalent widths and line depths than the typical exocometary signatures, and with a shape similar to those observed in shell stars. The origin of this variation remains unclear, although it seems to originate in a close-in circumstellar disc, as is the case for most shell stars.