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 population in the inner parsecs of the Milky Way. In a previous work, our simulations showed that the lack of main sequence HVS candidates in current data releases from the Gaia space mission with precise astrometry and measured radial velocities places a robust upper limit on the ejection rate of HVSs from the GC of 3 × 10 −2 yr −1 . We improve this constraint in this work by additionally considering the absence of post main sequence HVSs in 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 η = 0.7 +1.5 −0.5 × 10 −4 yr −1 . This preferred ejection rate can be as large as 10 −2 yr −1 for a very top-light IMF and as low as 10 −4.5 yr −1 if the IMF is extremely top-heavy. Constraints will improve further with future Gaia data releases, regardless of how many HVS candidates are found therewithin.

Gaia DR3 in 6D: The search for fast hypervelocity stars and constraints on the Galactic Centre environment

“…Finally, we note that Evans et al (2022a) show that including the positive detection of S5-HVS1 shifts the contours in 𝜂 and 𝜅 towards the right in Fig. 3.…”

“…Specifically, Evans et al (2022b,a), assuming the Hills mechanism, showed that the population of HVSs predicted to be included in the Gaia radial velocity catalogue(s) strongly depends on the HVS ejection rate 𝜂 and on the slope 𝜅 of the initial mass function for the primary star in GC binaries, 𝑑𝑁/𝑑𝑚 ∝ 𝑚 𝜅 . The absence of HVSs with radial velocities and precise parallaxes in Gaia DR2 and EDR3 (see Marchetti et al 2019;Marchetti 2021), together with the detection of S5-HVS1 in the S 5 survey (Li et al 2019;Koposov et al 2020), constrains 𝜂 ∼ 10 −4 yr −1 (Evans et al 2022a) for a topheavy mass function in the GC (Lu et al 2013). The two parameters are strongly degenerate, with 𝜂 ∼ 10 −3 yr −1 being preferred for a canonical Salpeter (1955) mass function.…”

“…In Evans et al (2022b) and Evans et al (2022a) we used the absence of unbound GC-ejected HVSs with precise parallaxes in the Gaia DR2 and EDR3 radial velocity catalogues (Marchetti et al 2019;Marchetti 2021) to constrain assumptions concerning the ejection of HVSs, assuming a Hills mechanism ejection scenario. We found that the predicted number of HVSs in these surveys depends principally on the HVS ejection rate 𝜂 and on the initial mass function (IMF) of the primary star among HVS progenitor binaries, described by a single power law with slope 𝜅, i.e.…”