On December 18th 2020, Sarah Massalkhi has defended in the Autonomous University of Madrid her PhD work entitled “An Observational Study of Molecular Dust Precursors in Circumstellar Envelopes” supervised by professors Marcelino Agúndez and José Cernicharo (IFF-CSIC). Congratulations!
Circumstellar envelopes around evolved stars of AGB type are probably the main factories of dust in the Universe. However, we still do not understand the process that operates in these environments to convert simple gas-phase molecules into large dust grains. In this thesis, we carried out an observational study to determine the abundance of molecules that could potentially act as precursors of dust in a large sample of AGB stars using the radiotelescope IRAM 30m.
Among the main results, in C-rich AGB envelopes we find a clear trend in which the fractional abundance of SiC2, SiO, y CS decreases as the envelope density increases, which we interpret as an evidence of efficient incorporation of these molecules onto dust grains, suggesting that that they are strong candidates to act as precursors of dust in C-rich envelopes. Likewise, SiO shows the same trend in O-rich envelopes, suggesting that it actively contributes to the formation of dust in these environments. We also find that the studied molecules have different behaviors with respect to the C- or O-rich character of the envelope, with molecules like CS and SiS showing a clear differentiation between these two types of envelopes while SiO does not seem to be sensitive to the C/O ratio.