Could kilomasers pinpoint supermassive stars?

TL;DR

This study explores whether kilomasers can be used to identify supermassive stars by modeling their accretion discs and comparing simulated maser spectra with observed data, supporting the hypothesis that kilomasers can pinpoint such stars.

Contribution

The paper presents hydrodynamic simulations of accretion discs around supermassive stars and compares the resulting maser spectra with observations, suggesting kilomasers as indicators of supermassive stars.

Findings

Model discs survive in dynamic environments.

Simulated spectra match observed kilomaser W1 reasonably well.

Stars of a few thousand solar masses fit the data better.

Abstract

A strong nuclear kilomaser, W1, has been found in the nearby galaxy NGC 253, associated with a forming super star cluster. Kilomasers could arise from the accretion disc around supermassive stars (>10^3 Msun), hypothetical objects that have been proposed as polluters responsible for the chemical peculiarities in globular clusters. The supermassive stars would form via runaway collisions, simultaneously with the cluster. Their discs are perturbed by stellar flybys, inspiralling and colliding stars. This raises the question if an accretion disc would at all be able to survive in such a dynamic environment and mase water lines. We investigated what the predicted maser spectrum of such a disc would look like using 2D hydrodynamic simulations and compared this to the W1 kilomaser. We derived model maser spectra from the simulations by using a general maser model for appropriate disc temperatures. All our model discs survived. The model maser spectra for the most destructive case for the simulations of M = 1000 Msun are a reasonable match with the W1 kilomaser spectrum in terms of scaling, flux values and some of the signal trends. Details in the spectrum suggest that a star of a few 1000 Msun might fit even better, with 10,000 Msun clearly giving too large velocities. Our investigations thus support the hypothesis that kilomasers could pinpoint supermassive stars.