Martinavarro Armengol, S; (2015) A Herschel study of AGB stars in the Milky Way. Doctoral thesis , UCL (University College London).

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Abstract

The Asymptotic Giant Branch (AGB) stars are low to intermediate-mass post-MS stars that undergo thermal pulses characterized by a strong wind-driven mass loss from its surface into the interstellar medium. They are responsible for the majority of the dust contained within the Galactic plane and thus, their study is crucial to understand the life cycle of dust in our Galaxy. The Hi-GAL Survey constitutes the perfect survey to detect and study AGB stars since it uses the PACS and SPIRE Herschel instruments in parallel mode to map the |l|< 60°; |b| <1° region of the Galactic plane in the 70 µm, 160 µm, 250 µm, 350 µm and 500 µm bands, which trace the emission of the dust in which AGB stars are embedded. In this thesis I use the Hi-GAL 1 maps processed with the ROMAGAL Pipeline to create a total of four catalogues: The SMA Hi-GAL 1 Point Source Catalogue - this is, to date, the most reliable Hi-GAL 1 PSC published. It contains the position and fluxes of all the Hi-GAL 1 compact sources detected by CUTEX in the 70 µm Herschel band and the corresponding counterparts found in the other Herschel bands. The SMA Hi-GAL 1 PSC AGB star candidate catalogue - this is the first catalogue including a list of possible Hi-GAL 1 AGB star candidates. The source classification was done by means of colour cuts that efficiently separate 95 reference Hi-GAL 1 AGB stars from 205 reference YSOs, 178 HII regions and 59 PNe. This is, to date, the largest sample of reference Hi-GAL 1 sources used on colour-colour separation methods. These colour cuts involve the Herschel 70 µm band and prove that this band plays a decisive role in identifying AGB stars and that it is highly effective in distinguishing AGB stars from other stellar types. The SMA Hi-GAL 1 PSC C-rich and O-rich AGB star candidate catalogues - these are the first two catalogues including a list of possible Hi-GAL 1 PSC C-rich and O-rich AGB star candidates and constitute the largest compilation of AGB stars detected within the Hi-GAL 1 Galactic region. The source classification was done by means of colour cuts that successfully separate 101 reference OH/IR-stars from 56 reference C-stars. These colour cuts involve the MSX 8.28 µm band, the AKARI 9 µm and 18 µm bands and the WISE 12 µm and 22 µm bands, which cover the silicate and carbonaceous features of O-rich and C-rich AGB stars, and prove that these bands are highly efficient in distinguishing C-rich from O-rich AGB stars. The analysis of the stellar properties derived for the Hi-GAL 1 PSC C-rich and O-rich AGB star candidates allowed us to define a series of equations that establish the relation between the stars' mass-loss rates and their colour temperatures, NIR [J]-[K] colour, luminosities and outflow velocities. These equations are of great importance since they enable us to derive the mass-loss rates of unknown AGB stars using only their colour temperatures.

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