Gaia and the Galactic Center Origin of Hypervelocity Stars (original) (raw)
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Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The Astrophysical Journal, 2006
The disruption of binary stars by the tidal field of the black hole in the Galactic center can produce the hypervelocity stars observed in the halo. We use numerical models to simulate the full spectrum of observable velocities of stars ejected into the halo by this binary disruption process. Our models include a range of parameters for binaries with 3-4 M primaries, consideration of radial orbits of the ejected stars through an approximate mass distribution for the Galaxy, and the impact of stellar lifetimes. We calculate the spectrum of ejection velocities and reproduce previous results for the mean ejection velocity at the Galactic center. The model predicts that the full population of ejected stars includes both the hypervelocity stars with velocities large enough to escape from the Galaxy and a comparable number of ejected, but bound, stars of the same stellar type. The predicted median speeds of the population of ejected stars as a function of distance in the halo are consistent with current observations. Combining the model with the data also shows that interesting constraints on the properties of binaries in the Galactic center and on the mass distribution in the Galaxy can be obtained even with modest samples of ejected stars.
591 High-velocity Stars in the Galactic Halo Selected from LAMOST DR7 and Gaia DR2
The Astrophysical Journal Supplement Series, 2020
In this paper, we report 591 high-velocity star candidates (HiVelSCs) selected from over 10 million spectra of Data Release 7 (DR7) of the Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia data release, with three-dimensional velocities in the Galactic rest frame larger than 445 km s−1. We show that at least 43 HiVelSCs are unbound to the Galaxy with escape probabilities larger than 50%, and this number decreases to eight if the possible parallax zero-point error is corrected. Most of these HiVelSCs are metal-poor and slightly α-enhanced inner halo stars. Only 14% of them have [Fe/H] > −1, which may be the metal-rich “in situ” stars in the halo formed in the initial collapse of the Milky Way or metal-rich stars formed in the disk or bulge but kinematically heated. The low ratio of 14% implies that the bulk of the stellar halo was formed from the accretion and tidal disruption of satellite galaxies. In addition, HiVelSCs on retrograde orbits have slight...
An Alternative Origin for Hypervelocity Stars
The Astrophysical Journal, 2009
Halo stars with unusually high radial velocity ("hypervelocity" stars, or HVS) are thought to be stars unbound to the Milky Way that originate from the gravitational interaction of stellar systems with the supermassive black hole at the Galactic center. We examine the latest HVS compilation and find peculiarities that are unexpected in this black hole-ejection scenario. For example, a large fraction of HVS cluster around the constellation of Leo and share a common travel time of ∼ 100-200 Myr. Furthermore, their velocities are not really extreme if, as suggested by recent galaxy formation models, the Milky Way is embedded within a 2.5 × 10 12 h −1 M ⊙ dark halo with virial velocity of ∼ 220 km/s. In this case, the escape velocity at ∼ 50 kpc would be ∼ 600 km/s and very few HVS would be truly unbound. We use numerical simulations to show that disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system. These stars are arranged in a thinly-collimated outgoing "tidal tail" stripped from the dwarf during its latest pericentric passage. We speculate that some HVS may therefore be tidal debris from a dwarf recently disrupted near the center of the Galaxy. In this interpretation, the angular clustering of HVS results because from our perspective the tail is seen nearly "end on", whereas the common travel time simply reflects the fact that these stars were stripped simultaneously from the dwarf during a single pericentric passage. This proposal is eminently falsifiable, since it makes a number of predictions that are distinct from the black-hole ejection mechanism and that should be testable with improved HVS datasets.
Hypervelocity stars from young stellar clusters in the Galactic Centre
Monthly Notices of the Royal Astronomical Society, 2017
The enormous velocities of the so called hypervelocity stars (HVSs) derive, likely, from close interactions with massive black holes, binary stars encounters or supernova explosions. In this paper, we investigate the origin of hypervelocity stars as consequence of the close interaction between the Milky Way central massive black hole and a passing-by young stellar cluster. We found that both single and binary HVSs may be generated in a burst-like event, as the cluster passes near the orbital pericentre. High velocity stars will move close to the initial cluster orbital plane and in the direction of the cluster orbital motion at the pericentre. The binary fraction of these HVS jets depends on the primordial binary fraction in the young cluster. The level of initial mass segregation determines the value of the average mass of the ejected stars. Some binary stars will merge, continuing their travel across and out of the Galaxy as blue stragglers.
The Astronomical Journal
In this paper, we report the discovery of a new late-B type unbound hyper-runaway star (LAMOST-HVS4) from the LAMOST spectroscopic surveys. According to its atmospheric parameters, it is either a B-type mainsequence (MS) star or a blue horizontal branch (BHB) star. Its Galactocentric distance and velocity are 30.3 ± 1.6 kpc and 586±7 km s −1 if it is an MS star, and they are 13.2 ± 3.7 kpc and 590±7 km s −1 if a BHB star. We track its trajectories back, and find that the trajectories intersect with the Galactic disk and the Galactic center lies outside of the intersection region at the 3σ confidence level. We investigate a number of mechanisms that could be responsible for the ejection of the star, and find that it is probably ejected from the Galactic disk by supernova explosion or multiple-body interactions in dense young stellar clusters.
Hypervelocity Stars. II. The Bound Population
The Astrophysical Journal, 2007
Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body interactions with the massive black hole in the Galactic center. We describe 643 new spectroscopic observations from our targeted survey for HVSs. We find a significant (3.5σ) excess of B-type stars with large velocities +275 < v rf < 450 km s −1 and distances d > 10 kpc that are most plausibly explained as a new class of HVSs: stars ejected from the Galactic center on bound orbits. If a Galactic center ejection origin is correct, the distribution of HVSs on the sky should be anisotropic for a survey complete to a fixed limiting apparent magnitude. The unbound HVSs in our survey have a marginally anisotropic distribution on the sky, consistent with the Galactic center ejection picture.
The Astrophysical Journal, 2018
We investigate the nature of the double color-magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically-hot tail of the thick disk population. Information from Nissen & Schuster (2010) and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low starformation efficiency during its formation. In dynamical and orbital spaces, such as the 'Toomre diagram', the two sequences show a significant overlap, but with a tendency for stars on the blue-color sequence to dominate regions with no or retrograde rotation and high total orbital energy. In the plane defined by the maximal vertical excursion of the orbits versus their apocenters, stars of both sequences redistribute into discrete wedges. We conclude that stars which are typically assigned to the halo in the solar vicinity are actually both accreted stars lying along the blue sequence in the HR diagram, and the low rotational velocity tail of the old Galactic disk, possibly dynamically heated by past accretion events. Our results imply that a halo population formed in situ and responsible for the early chemical enrichment prior to the formation of the thick disk is yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may be in fact fossil records of its last significant merger.
The Astrophysical Journal, 2019
We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with mass of 8.3 M ⊙ and super-Solar metallicity, ejected from the inner stellar disk of the Milky Way ∼ 33 Myr ago with the intrinsic ejection velocity of 568 +19 −17 km s −1 (corrected for the streaming motion of the disk), based on the proper motion data from Gaia Data Release 2 (DR2) and high-resolution spectroscopy. The extremely large ejection velocity indicates that this star was not ejected by the supernova explosion of the binary companion. Rather, it was probably ejected by a 3-or 4-body dynamical interaction with more massive objects in a high-density environment. Such a high-density environment may be attained at the core region of a young massive cluster with mass of 10 4 M ⊙. The ejection agent that took part in the ejection of LAMOST-HVS1 may well be an intermediate mass black hole (100 M ⊙), a very massive star (100 M ⊙), or multiple ordinary massive stars (30 M ⊙). Based on the flight time and the ejection location of LAMOST-HVS1, we argue that its ejection agent or its natal star cluster is currently located near the Norma spiral arm. The natal star cluster of LAMOST-HVS1 may be an undiscovered young massive cluster near the Norma spiral arm.
TheGaia-ESO Survey: A globular cluster escapee in the Galactic halo
Astronomy & Astrophysics, 2015
A small fraction of the halo field is made up of stars that share the light element (Z ≤ 13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey. One halo star at the base of the red giant branch, here referred to as 22593757-4648029 is found to have [Mg/Fe] = −0.36 ± 0.04 and [Al/Fe] = 0.99 ± 0.08, which is compatible with the most extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029 to GCs of similar metallicity and find it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters. However, both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC ω Centauri within the last few billion years. We cannot rule out other progenitor GCs, because some may have disrupted fully, and the abundance and orbital data are inadequate for many of those that are still intact.
Runaway and Hypervelocity Stars in the Galactic Halo: Binary Rejuvenation and Triple Disruption
The Astrophysical Journal, 2009
Young stars observed in the distant Galactic halo are usually thought to have formed elsewhere, either in the Galactic disk or perhaps the Galactic center, and subsequently ejected at high velocities to their current position. However, some of these stars have apparent lifetimes shorter the required flight time from the Galactic disk/center. We suggest that such stars have evolved in close runaway or hypervelocity binaries. Stellar evolution of such binaries can drive them into mass transfer configurations and even mergers. Such evolution could then rejuvenate them (e.g. blue stragglers) and extend their lifetime after their ejection. The extended lifetimes of such stars could then be reconciled with their flight times to the Galactic halo. We study the possibilities of binary runaway and hypervelocity stars and show that such binaries could have been ejected in triple disruptions and other dynamical interactions with stars or with massive black holes. We show that currently observed "too young" star in the halo could have been ejected from the Galactic disk or the Galactic center and be observable in their current position if they were ejected as binaries. Specifically it is shown that the hypervelocity star HE 0437-5439 could be such a rejuvenated star. Other suggestions for its ejection from the LMC are found to be highly unlikely. Moreover, it is shown that its observed metallicity is most consistent with a Galactic origin and a Galactic center origin can not currently rule out. In addition, we suggest that triple disruptions by the massive black hole in the Galactic center could also capture binaries in close orbits near the MBH, some of which may later evolve to become more massive rejuvenated stars.