Searching for gas in debris disks
Creators: name: Iglesias, Daniela
Publisher: name: Zenodo
Year: 2020

description: A debris disk is commonly described as a second generation circumstellar disk com- posed of dust grains, planetesimals and possibly already formed giant planets. It was long thought that debris disks were systems fully depleted of gas, but in the last few years the presence of gas has been detected in a increasing number of them. The na- ture of this gas is still under debate; it may be residual gas (leftover from earlier stages of the disk) or second generation gas (generated by the sublimation of exocomets or collisions of icy bodies), however, both possibilities would have great implications in the process of planet formation and evolution. An efficient way to search for this gas in debris disks is monitoring gas tracers using multi-epoch high-resolution UV-optical spectroscopy. A detection of gas in the line of sight of the star shows up as a very narrow absorption feature superposed to the photospheric absorption of lines such as Ca ii or Na i, which are very sensitive gas tracers. This Thesis is devoted to the search for gas via high-resolution spectroscopy in a sample of 301 debris disks with the pur- pose of estimating the frequency of gas in debris disks. We collected high-resolution UV-optical spectroscopic data from our own observations and the ESO archive for 273 objects, completing 91% of our sample. We analysed the multi-epoch spectra of each object searching for variable and stable non-photospheric absorption features of circumstellar origin. Firstly, we performed a detailed analysis of a sub-sample of 27 objects having multiple stable absorption features with the aim of determining whether their features were of circumstellar or interstellar origin. In this group we found two objects; c Aql and HR 4796, with variable absorption features attributable to the presence of star-grazing exocomets, and one object; HD 110058 with a stable circumstellar gas absorption. We concluded from these preliminary results that the presence of gas in debris disks might be a common phenomenon. Secondly, we studied the presence of unusually large non-photospheric absorption features found in several ionized lines of the object HD 37306, that lasted for several days. After analysing different possibilities we concluded that the most likely explanation might be an ex- ocometary break-up releasing a stream of gas that remained in orbit for at least a week around the star. Then, we analyzed the variability found in the Ca ii K lines of 97 objects with the purpose of distinguishing variable absorption features most likely produced by exocometary activity from variability of a different nature, like stellar pulsations, spots or radial velocity shifts, for instance. We found five candidates with variability consistent with exocometary activity: c Aql, 49 Cet, gam Tri, HR 4796 and HD 37306, two of them with no previous reports of exocomets besides this work. Finally, we studied a sub-sample of 107 objects presenting stable non-photospheric features in their Ca ii K lines within our updated database of observations in addi- tion to those in our first sub-sample of analysis. Similarly, the absorption features observed in these objects were analysed in order to determine whether they were of circumstellar or interstellar origin. We confirmed an interstellar origin for the features in 104 of these objects, one of them, 49 Cet having a blended feature of combined circumstellar and interstellar origin. In addition, we found three candidates with features of yet inconclusive origin that might be circumstellar but need further study to confirm it. Putting together our results, we found six debris disks with presence of circumstellar gas, five of them presenting variable absorption features attributable to exocometary activity and one of them with a stable circumstellar gas component. Considering that our sample is unbiased with respect to inclination, that the gas is more likely to be detected when the disk is observed close to edge-on, more easily detected in the spectra of early-type stars, the many detection challenges related to the sporadical nature of exocometary transits, and the interlopers of gas detection, then the evidence of circumstellar gas is significant. Taking into account the currently increasing number of detections, this imply the possibility that most debris disks may harbor some gas, challenging the current paradigm of debris disks.