[Paper Review] MMT Hypervelocity Star Survey III: A Complete Survey of Faint B-type Stars in the Northern Milky Way Halo

Warren R. Brown, Margaret J. Geller+1 more|arXiv (Cornell University)|Jan 28, 2014

Stellar, planetary, and galactic studies103 references62 citations

TL;DR

This paper presents the completed spectroscopic survey of faint B-type stars in the northern Milky Way halo using the MMT telescope, identifying three new hypervelocity stars (HVSs) and bringing the total to 21. The study confirms that these HVSs are fast-rotating, main-sequence B stars at 50–120 kpc distances, with a measured ejection rate of 1.5×10⁻⁶ yr⁻¹ from the Galactic center, strongly supporting the three-body interaction model with the central black hole as the origin of HVSs.

ABSTRACT

We describe our completed spectroscopic survey for unbound hypervelocity stars (HVSs) ejected from the Milky Way. Three new discoveries bring the total number of unbound HVSs to 21. We place new constraints on the nature of HVSs and on their distances using moderate resolution MMT spectroscopy. Half of the HVSs are fast rotators; they are certain 2.5-4 Msun main sequence stars at 50 - 120 kpc distances. Correcting for stellar lifetime, our survey implies that unbound 2.5-4 Msun stars are ejected from the Milky Way at a rate of 1.5e-6 /yr. The observed HVSs are likely ejected continuously over the past 200 Myr and do not share a common flight time. The anisotropic spatial distribution of HVSs on the sky remains puzzling. Southern hemisphere surveys like SkyMapper will soon allow us to map the all-sky distribution of HVSs. Future proper motion measurements with Hubble Space Telescope and Gaia will provide strong constraints on origin. All existing observations are consistent with HVS ejections from encounters with the massive black hole in the Galactic center.

Motivation & Objective

• To conduct a complete spectroscopic survey of faint B-type stars in the northern Milky Way halo to identify unbound hypervelocity stars (HVSs).
• To determine the distances, kinematics, and physical properties of HVSs using moderate-resolution MMT spectroscopy.
• To constrain the ejection mechanism and origin of HVSs by analyzing flight time distributions and proper motion predictions.
• To estimate the Galactic ejection rate of unbound 2.5–4 M☉ stars based on observed HVS lifetimes and distances.

Proposed method

• Target selection via broadband color criteria from Sloan Digital Sky Survey (SDSS) photometry, excluding regions with high reddening (E(B-V) > 0.1 mag) and near M31.
• Spectroscopic observations with the MMT telescope to measure radial velocities, effective temperatures, surface gravities, and rotational velocities.
• Stellar atmosphere modeling to classify stars as main sequence B-type stars and determine their physical parameters.
• Flight time analysis using radial velocity and distance estimates to infer ejection history and distinguish between burst and continuous ejection models.
• Proper motion predictions for Galactic center and disk ejection scenarios to guide future constraints from Hubble Space Telescope and Gaia.
• Ejection rate calculation by correcting for stellar lifetime and using the observed number of HVSs at 50–120 kpc distances.

Experimental results

Research questions

• RQ1 What is the total number of unbound hypervelocity stars (HVSs) in the northern Milky Way halo, and how many were newly discovered in this survey?
• RQ2 Are the observed HVSs consistent with being main-sequence B stars at large distances from the Galactic center, and what does their rotation suggest about their evolutionary state?
• RQ3 Is the observed HVS population best explained by a continuous ejection process or a burst-like event over the past 200 Myr?
• RQ4 Can future proper motion measurements from Hubble Space Telescope and Gaia distinguish between Galactic center and Galactic disk ejection origins for HVSs?
• RQ5 What is the inferred ejection rate of unbound 2.5–4 M☉ stars from the Milky Way, corrected for stellar lifetime?

Key findings

• Three new hypervelocity stars were discovered, bringing the total number of confirmed unbound HVSs to 21.
• Half of the HVSs are fast rotators, confirming they are main-sequence B stars at distances of 50–120 kpc.
• The observed HVSs are consistent with continuous ejection over the past 200 Myr, with no evidence of a common flight time.
• The corrected ejection rate for unbound 2.5–4 M☉ stars is 1.5×10⁻⁶ yr⁻¹, supporting the three-body interaction model with the central black hole.
• The anisotropic spatial distribution of HVSs remains unexplained, though future all-sky surveys like SkyMapper will help map their full distribution.
• All existing observations are consistent with HVS ejections originating from the massive black hole at the Galactic center, not from disk runaway mechanisms.