Constraints on the Galactic Centre environment from Gaia hypervelocity stars II: The evolved population

Abstract

A dynamical encounter between a stellar binary and Sgr A* in the Galactic Centre (GC) can tidally separate the binary and eject one member with a velocity beyond the escape speed of the Milky Way. These hypervelocity stars (HVSs) can offer insight into the stellar populations in the GC environment. In a previous work, our simulations showed that the lack of main sequence HVS candidates with precise astrometric uncertainties and radial velocities in current data releases from the \textit{Gaia} space mission places a robust upper limit on the ejection rate of HVSs from the GC of $3\times 10^{-2}\mathrm{y}{\mathrm{r}}^{-1}$. We improve this constraint in this work by additionally considering the absence of post main sequence HVSs in \textit{Gaia} Early Data Release 3 as well the existence of the HVS candidate S5-HVS1. This evidence offers degenerate joint constraints on the HVS ejection rate and the stellar initial mass function (IMF) in the GC. For a top-heavy GC IMF as suggested by recent works, our modelling motivates an HVS ejection rate of $\eta =0.7_{-0.5}^{+1.5}\times 10^{-4}\mathrm{y}{\mathrm{r}}^{-1}$. This preferred ejection rate can be as large as $10^{-2}\mathrm{y}{\mathrm{r}}^{-1}$ for a very top-light IMF and as low as 10${}^{-4.5}\mathrm{y}{\mathrm{r}}^{-1}$ if the IMF is extremely top-heavy. Constraints will improve further with future \textit{Gaia} data releases, regardless of how many HVS candidates are found therewithin.

16 pages, 8 figures. Accepted to MNRAS following minor revisions

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• Hyper-runaway and hypervelocity white dwarf candidates in \textit{Gaia} Data Release 3: possible remnants from Ia/Iax supernova explosions or dynamical encounters
• Constraints on the Galactic Centre environment from \textit{Gaia} hypervelocity stars III: Insights on a possible companion to Sgr A*
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• On Stellar Migration from Andromeda to the Milky Way
• Gaia DR3 high radial velocity stars: Genuine fast-moving objects or outliers?
• Searching for stars ejected from the Galactic Centre in DESI