[Paper Review] Hypervelocity Stars and the Galactic Center

Warren R. Brown|ArXiv.org|Nov 4, 2008

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

This paper establishes hypervelocity stars (HVSs) as probes of the Galactic Center's massive black hole (Sgr A*) by analyzing their ejection mechanism, velocity distribution, and spatial distribution. It demonstrates that HVSs, ejected via three-body interactions with Sgr A*, provide unique constraints on the black hole's mass, the stellar initial mass function, and the Milky Way's dark matter halo potential, with observed HVSs favoring a continuous ejection process over a burst from an in-spiraling intermediate-mass black hole.

ABSTRACT

A summary of the current knowledge on hypervelocity stars (HVSs). HVSs are fascinating because their properties are linked to Sgr A* and the stellar environment of the Galactic Center. Observing the distribution of HVSs can address: 1) the nature of the black hole ejection mechanism, 2) the in-fall history of stars onto Sgr A*, 3) the types of stars orbiting Sgr A*, and 4) a unique measurement the shape of the Galaxy's dark matter potential. The challenge to observers is to find new HVSs and strengthen the connection between HVSs and the Galactic Center.

Motivation & Objective

• To establish hypervelocity stars (HVSs) as probes of the Galactic Center's massive black hole (Sgr A*) and its environment.
• To distinguish HVSs from other fast-moving stars (e.g., hyper-runaways) using velocity, origin, and spatial distribution.
• To constrain the nature of the central black hole and the stellar initial mass function using observed HVS statistics and ejection models.
• To test whether HVSs originate from a single massive black hole or a binary system via velocity and spatial distribution patterns.
• To use HVSs to probe the shape of the Milky Way’s dark matter halo through their escape velocity and travel time.

Proposed method

• Define HVSs by their origin in three-body interactions with a massive black hole and their unbound velocities exceeding the local escape velocity.
• Use radial velocity and distance measurements to determine if stars are unbound, applying escape velocity models from Kenyon et al. (2008) and Xue et al. (2008).
• Model HVS ejection via the Hills mechanism, where a binary is disrupted by a massive black hole, with one star ejected at high velocity and the other captured on a highly eccentric orbit.
• Compare theoretical HVS velocity distributions and ejection rates for single versus binary massive black hole scenarios to discriminate between models.
• Use isochrones of travel time from the Galactic center to estimate ejection epochs, assuming observed radial velocities represent full space motion.
• Analyze the spatial distribution of HVSs to test for coherent ejection bursts (e.g., from in-spiraling intermediate-mass black holes) versus continuous ejection.

Experimental results

Research questions

• RQ1 What fraction of unbound stars in the Milky Way are hypervelocity stars ejected by Sgr A* rather than hyper-runaways from supernova kicks?
• RQ2 Does the observed velocity and spatial distribution of HVSs support a continuous ejection process or a burst from an in-spiraling intermediate-mass black hole?
• RQ3 How do the observed HVSs constrain the initial mass function of stars near Sgr A*?
• RQ4 Can the HVS velocity distribution distinguish between ejection by a single massive black hole versus a binary massive black hole system?
• RQ5 What can the travel time and spatial distribution of HVSs reveal about the shape of the Milky Way’s dark matter halo?

Key findings

• The observed HVSs favor a continuous ejection process from Sgr A*, with no evidence for a coherent burst of ejection from an in-spiraling intermediate-mass black hole over the past few hundred million years.
• The number of 3–4 M☉ HVSs within 100 kpc is estimated at 96 ± 20, consistent with a Salpeter initial mass function if the ejection rate is enhanced by massive perturbers such as giant molecular clouds.
• HVSs ejected from the Galactic Center are estimated to be ~100 times more common than hyper-runaways of the same mass, supporting their origin in the central black hole's gravitational interaction.
• The velocity distribution of HVSs shows weak dependence on stellar mass for a single massive black hole, but no such dependence for a binary massive black hole, providing a key discriminant between models.
• The spatial distribution of HVSs does not show a ring-like structure, ruling out significant contribution from a binary massive black hole system in the Galactic Center.
• The observed HVSs are consistent with an initial mass function that may be top-heavy near Sgr A*, as suggested by the low observed density of F-type HVSs compared to expectations from a standard Salpeter function.