Candidate hypervelocity stars of spectral type G and K revisited

Abstract

Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy. When they were first discovered in 2005, dynamical ejection from the supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as their origin. The two dozen HVSs known today are young massive B stars, mostly of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in the Segue G and K dwarf sample, but none of them originates from the GC. We embarked on a kinematic analysis of the Segue HVS candidate sample using the full 6D phase space information based on new proper motion measurements. Their orbital properties can then be derived by tracing back their trajectories in different mass models of our Galaxy. We present the results for 14 candidate HVSs, for which proper motion measurements were possible. Significantly lower proper motions than found in the previous study were derived. Considering three different Galactic mass models we find that all stars are bound to the Galaxy. We confirm that the stars do not originate from the GC. The distribution of their proper motions and radial velocities is consistent with predictions for runaway stars ejected from the Galactic disk by the binary supernova mechanism. However, their kinematics are also consistent with old disk membership. Moreover, most stars have rather low metallicities and strong $\alpha$-element enrichment as typical for thick disk and halo stars, whereas the metallicity of the three most metal-rich stars could possibly indicate that they are runaway stars from the thin disk. One star shows halo kinematics.

arXiv commentA&A letter accepted

References in corpus (19)

• Discovery of an Unbound Hyper-Velocity Star in the Milky Way Halo
• An extensive library of 2500-10500 Ang synthetic spectra
• The binary progenitor of Tycho Brahe's 1572 supernova
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Milky Way Mass Models for Orbit Calculations
• US 708 - An unbound hyper-velocity subluminous O star
• HE 0437-5439 -- an unbound hyper-velocity main-sequence B-type star
• MMT Hypervelocity Star Survey III: A Complete Survey of Faint B-type Stars in the Northern Milky Way Halo
• The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
• 3D kinematics of white dwarfs from the SPY project II
• Maximum speed of hypervelocity stars ejected from binaries
• Ejection velocities of high Galactic latitude runaway stars
• Predicted Space Motions for Hypervelocity and Runaway Stars: Proper Motions and Radial Velocities for the GAIA Era
• Characterizing the High-Velocity Stars of RAVE: The Discovery of a Metal-Rich Halo Star Born in the Galactic Disk
• The Nature of the Hyper-Runaway Candidate HIP 60350
• Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys
• The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars
• Hypervelocity Star Candidates in the SEGUE G & K Dwarf Sample
• The Nearest High-Velocity Stars Revealed by LAMOST Data Release 1

Cited by in corpus (14)

• Revisiting hypervelocity stars after Gaia DR2
• Hypervelocity stars in the Gaia era: Runaway B stars beyond the velocity limit of classical ejection mechanisms
• Constraining Milky Way mass with Hypervelocity Stars
• Predicting the hypervelocity star population in Gaia
• New nearby hypervelocity stars and their spatial distribution from Gaia DR2
• The sdA problem I: Physical Properties
• An artificial neural network to discover Hypervelocity stars: Candidates in Gaia DR1/TGAS
• Impact of the Galactic Disk and Large Magellanic Cloud on the Trajectories of Hypervelocity Stars Ejected from the Galactic Center
• On the origins of extreme velocity stars as revealed by large-scale Galactic surveys
• Red Runaways: Hypervelocity Stars, Hills Ejecta and Other Outliers in the F-M Star Regime
• Hypervelocity star candidates from Gaia DR2 and DR3 proper motions and parallaxes
• A spectroscopic and kinematic survey of fast hot subdwarfs
• Red Runaways II: Low mass Hills stars in SDSS Stripe 82
• Three New Late-type Hypervelocity Star Candidates from Gaia DR2 with Refined Selection Criteria