Accretion Disc Structure of Supermassive Stars Formed by Collisions

Abstract

Globular clusters (GCs) display large variations in light elements; the main one being O-Na, C-N and Mg-Al anticorrelations. Additionally most GCs demonstrate multiple sequences in the colour-magnitude diagram, proving that globular clusters host multiple stellar populations. It has been suggested that the second population forms from the hot-hydrogen burning yields of the first population, which then also explains the chemical peculiarities via self-enrichment. One of the leading proposals for the polluter is a supermassive star (SMS) (≥ 103 Mꙩ), which forms via runaway collisions, simultaneously with the globular cluster [Gieles et al., 2018].

At the present time it is very hard to observe a SMS due to its location. The candidate forming massive clusters are located outside the Milky Way with very dense centers, where the SMS would be obscured by gas and dust. One way to detect it could be the use of MASER, where 22.2 GHz H2O, water masers, are in general associated with massive star formation. Gorski et al. [2018] found a very strong MASER, a kilomaser in the nearby galaxy NGC 253 associated with a young massive cluster.

A SMS disc is perturbed by stellar flybys, inspiralling and colliding stars. I investigate what the predicted MASER spectrum of that disc looks like using 2D hydrodynamic simulations and compare this to the W1 kilomaser in NGC 253. The simulations are modelled using the finite volume fluid dynamics code PLUTO starting from the standard Disk-Planet problem. I derive model maser spectra from the final simulations by using the general maser model from Kartje et al. [1999] for appropriate disc temperatures against velocity along the line of sight.

The model maser spectra for the most destructive case for the simulations of MSMS = 1000 Mꙩ are a good match with W1 kilomaser spectrum obtained from Gorski et al. [2018], in terms of scaling, flux values and some of the signal trends. For the more massive star of 10,000 Mʘ the spectra start to resemble megamasers from AGNs rather than stellar masers. I have also observed changes in flux values for "high-velocity" features and their outwards and inward movement due to the presence of a dense spiral arm, rotating around the central star.