Several dozen hypervelocity star (HVS) candidates have been reported based on the second data release of Gaia (Gaia DR2). However, it has been proven that the radial velocities of some Gaia HVS candidates are not reliable. In this paper, we employ refined astrometric criteria to re-examine Gaia DR2, arriving at a more reliable sample of HVS and high velocity star candidates than those found by previous authors.We develop a method called Binary Escape Probability Analysis to identify some HVS candidates. This method allows us to work with stars having only two epochs of measured radial velocity. These stars were usually discarded in previous similar studies. A scrutiny of our final results sheds light on selection effects present in our studies, which we propose to be the focus of future studies. In total, we find three late-type (2 G-type and 1 K-type) HVS and 21 high velocity star candidates, 3 and 11 of which are new, respectively. Judging by their historical trajectories, which we calculate, all three HVS candidates could not have had Galactic center origins. Further monitoring is required to confirm their status.
We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelocity Star Survey. The measurements reveal a clear pattern in the B-type stars. Halo stars dominate the sample at speeds about 100 km/s below Galactic escape velocity. Disk runaway stars have speeds up to 100 km/s above Galactic escape velocity, but most disk runaways are bound. Stars with speeds about 100 km/s above Galactic escape velocity originate from the Galactic center. Two bound stars may also originate from the Galactic center. Future Gaia measurements will enable a large, clean sample of G…
We predict the distinctive three dimensional space motions of hypervelocity stars (HVSs) and runaway stars moving in a realistic Galactic potential. For nearby stars with distances less than 10~kpc, unbound stars are rare; proper motions alone rarely isolate bound HVSs and runaways from indigenous halo stars. At large distances of 20-100 kpc, unbound HVSs are much more common than runaways; radial velocities easily distinguish both from indigenous halo stars. Comparisons of the predictions with existing observations are encouraging. Although the models fail to match observations of solar-type …
Hypervelocity stars (HVSs) are amongst the fastest objects in our Milky Way. These stars are predicted to come from the Galactic center (GC) and travel along unbound orbits across the Galaxy. In the coming years, the ESA satellite Gaia will provide the most complete and accurate catalogue of the Milky Way, with full astrometric parameters for more than $1$ billion stars. In this paper, we present the expected sample size and properties (mass, magnitude, spatial, velocity distributions) of HVSs in the Gaia stellar catalogue. We build three Gaia mock catalogues of HVSs anchored to current observ…
The third data release (DR3) of the European Space Agency satellite Gaia provides coordinates, parallaxes, proper motions, and radial velocities for a sample of $\sim 34$ million stars. We use the combined 6-dimensional phase space information to search for hypervelocity stars (HVSs), unbound stars accelerated by dynamical processes happening in the Galactic Centre. By looking at the kinematics of Gaia DR3 stars in Galactocentric coordinates and by integrating their orbits in the Galactic potential, we do not identify any HVS candidates with a velocity higher than $700$ km s$^{-1}$ and robustl…
The paucity of hypervelocity stars (HVSs) known to date has severely hampered their potential to investigate the stellar population of the Galactic Centre and the Galactic Potential. The first Gaia data release gives an opportunity to increase the current sample. The challenge is the disparity between the expected number of hypervelocity stars and that of bound background stars. We have applied a novel data mining algorithm based on machine learning techniques, an artificial neural network, to the Tycho-Gaia astrometric solution (TGAS) catalogue. With no pre-selection of data, we could exclude…
Hypervelocity stars are intriguing rare objects traveling at speeds large enough to be unbound from the Milky Way. Several mechanisms have been proposed for producing them, including the interaction of the Galaxy's super-massive black hole (SMBH) with a binary; rapid mass-loss from a companion to a star in a short-period binary; the tidal disruption of an infalling galaxy and finally ejection from the Large Magellanic Cloud. While previously discovered high-velocity early-type stars are thought to be the result of an interaction with the SMBH, the origin of high-velocity late type stars is amb…
Type Ia and other peculiar supernovae (SNe) are thought to originate from the thermonuclear explosions of white dwarfs (WDs). Some of the proposed channels involve the ejection of a partly exploded WD (e.g. Iax SN remnant) or the companion of an exploding WD at extremely high velocities ($>400$ km\,s$^{-1}$). Characterisation of such hyper-runaway/hypervelocity (HVS) WDs might therefore shed light on the physics and origins of SNe. Here we analyse the \textit{Gaia} DR3 data to search for HVS WDs candidates, and peculiar sub-main-sequence (sub-MS) objects. We retrieve the previously identified …
Photometric Distances for Metal-poor Giants and a Search for Hypervelocity Stars with LAMOST DR13 and Gaia DR3
Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University（北京师范大学天文与前沿科学研究院）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
Young massive stars in the halo are assumed to be runaway stars from the Galactic disk. Possible ejection scenarios are binary supernova ejections (BSE) or dynamical ejections from star clusters (DE). Hypervelocity stars (HVSs) are extreme runaway stars that are potentially unbound from the Galaxy. Powerful acceleration mechanisms such as the tidal disruption of a binary system by a supermassive black hole (SMBH) are required to produce them. Therefore, HVSs are believed to originate in the Galactic center (GC), the only place known to host an SMBH. The second Gaia data release (DR2) offers th…
We search for runaway stars in the Large Magellanic Cloud (LMC) by computing the space velocities of the visually brightest stars in the LMC that are included in the Gaia TGAS proper motion catalog. We compare with predictions from stellar dynamical models to obtain (peculiar) velocities relative to their local stellar environment. Two of the 31 stars have unusually high proper motions. Of the remaining 29 stars we find that most objects in this sample have velocities in very good agreement with model predictions of a circularly rotating disk model. Indeed the excellent fit to the model implie…
Stars slingshotted by the supermassive black hole at the Galactic centre will escape the Milky Way so quickly that their trajectories will be almost straight lines. Previous works have shown how these `hypervelocity stars' are subsequently deflected by the gravitational field of the Milky Way and the Large Magellanic Cloud (LMC), but have neglected to account for the reflex motion of the Milky Way in response to the fly by of the LMC. A consequence of this motion is that the hypervelocity stars we see on the outskirts of the Milky Way today were ejected from where the Milky Way centre was hund…
Context. Hypervelocity stars move fast enough to leave the gravitational field of their home galaxies and venture into intergalactic space. The most extreme examples known have estimated speeds in excess of 1000 km/s. These can be easily induced at the centres of galaxies via close encounters between binary stars and supermassive black holes; however, a number of other mechanisms operating elsewhere can produce them as well. Aims. Recent studies suggest that hypervelocity stars are ubiquitous in the local Universe. In the Milky Way, the known hypervelocity stars are anisotropically distributed…
The hypervelocity star (HVS) survey conducted at the Multiple Mirror Telescope (MMT) identified 42 B-type stars in the Galactic halo whose radial velocity in the Galactic rest-frame exceeds $+275\,$km$\,$s${}^{-1}$. In order to unravel the nature and origin of those high-velocity outliers, their complete six-dimensional phase space information is needed. To this end, we complemented positions and proper motions from the second data release of {\it Gaia} with revised radial velocities and spectrophotometric distances that are based on a reanalysis of the available MMT spectra of 40 objects usin…
We predict the distribution of hypervelocity stars (HVSs) ejected from the Large Magellanic Cloud (LMC), under the assumption that the dwarf galaxy hosts a central massive black hole (MBH). For the majority of stars ejected from the LMC the orbital velocity of the LMC has contributed a significant fraction of their galactic rest frame velocity, leading to a dipole density distribution on the sky. We quantify the dipole using spherical harmonic analysis and contrast with the monopole expected for HVSs ejected from the Galactic Center. There is a tendril in the density distribution that leads th…
Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One potential outcome of this "dynamically driven double-degenerate double-detonation" (D^6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its > 1000 km/s pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instrume…
Gravitational 3-body interaction among binary stars and the supermassive black hole (SMBH) at the center of the Milky Way occasionally ejects a hypervelocity star (HVS) with a velocity of ~1000 km/s. Due to the ejection location, such a HVS initially has negligible azimuthal angular momentum Lz ~ 0 kpc km/s. Even if the halo is mildly triaxial, Lz of a recently ejected nearby HVS remains negligible, since its flight time from the Galactic Center is too short to accumulate noticeable torque. However, if we make a wrong assumption about the Solar position and velocity, such a HVS would apparentl…
Hypervelocity stars (HVSs) travel so fast that they may leave the Galaxy. The tidal disruption of a binary system by the supermassive black hole in the Galactic center is widely assumed to be their ejection mechanism. To test the hypothesis of an origin in the Galactic center using kinematic investigations, the current space velocities of the HVSs need to be determined. With the advent of Gaia's second data release, accurate radial velocities from spectroscopy are complemented by proper motion measurements of unprecedented quality. Based on a new spectroscopic analysis method, we provide revis…
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 …
We predict and compare the distributions and properties of hyper-velocity stars (HVSs) ejected from the centres of the Milky Way (MW) and the Large Magellanic Cloud (LMC). In our model, HVSs are ejected at a constant rate -- equal in both galaxies -- via the Hills mechanism and are propagated in a combined potential, where the LMC orbits the MW on its first infall. By selecting $m>2\, \mathrm{M_\odot}$ HVSs well-separated from the Magellanic Clouds and Galactic midplane, we identify mock HVSs which would stand out from ordinary stars in the stellar halo in future data releases from the Gaia sa…
A recently discovered young, high-velocity giant star J01020100-7122208 is a good candidate of hypervelocity star ejected from the Galactic center, although it has a bound orbit. If we assume that this star was ejected from the Galactic center, it can be used to constrain the Galactic potential, because the deviation of its orbit from a purely radial orbit informs us of the torque that this star has received after its ejection. Based on this assumption, we estimate the flattening of the dark matter halo of the Milky Way by using the Gaia DR2 data and the circular velocity data from Eilers et a…
We present an analysis of the chemical abundances and kinematics of six low-mass dwarf stars, previously claimed to be candidate hypervelocity stars (HVSs). We obtained moderate-resolution ($R\sim$ 6000) spectra of these stars to estimate their abundances of several chemical elements (Mg, Si, Ca, Ti, Cr, Fe, and Ni), and derived their space velocity components and orbital parameters using proper motions from \gaia\ Data Release 2. All six stars are shown to be bound to the Milky Way, and in fact are not even considered high-velocity stars with respect to the Galactic rest frame. Nevertheless, …
We measure proper motions with the Hubble Space Telescope for 16 extreme radial velocity stars, mostly unbound B stars in the Milky Way halo. Twelve of these stars have proper motions statistically consistent with zero, and thus have radial trajectories statistically consistent with a Galactic center "hypervelocity star" origin. The trajectory of HE 0437-5439 is consistent with both Milky Way and Large Magellanic Cloud origins. A Galactic center origin is excluded at 3-sigma confidence for two of the lowest radial velocity stars in our sample, however. These two stars are probable disk runaway…
An Old, Low-mass, Metal-poor Hypervelocity Star Candidate Consistent with a Galactic Center Origin
University of Chinese Academy of Sciences（中国科学院大学）；China West Normal University（西华师范大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Peking University（北京大学）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）
The recently proposed "dynamically driven double-degenerate double-detonation" (D6) scenario posits that Type Ia supernovae (SNe) may occur during dynamically unstable mass transfer between two white dwarfs (WDs) in a binary. This scenario predicts that the donor WD may then survive the explosion and be released as a hypervelocity runaway, opening up the exciting possibility of identifying remnant stars from D6 SNe and using them to study the physics of detonations that produce Type Ia SNe. Three candidate D6 runaway objects have been identified in Gaia data. The observable runaway velocity of…
Little is known about the origin of the fastest stars in the Galaxy. Our understanding of the Milky Way and surrounding dwarf galaxies chemical evolution history allows us to use the chemical composition of a star to investigate its origin, and say whether a star was formed in-situ or was accreted. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. We have used high-resolution…
In the past, SDSS J160429.12+100002.2 was spectroscopically classified as a blue horizontal branch (BHB) star. Assuming a luminosity that is characteristic of BHB stars, the object's radial velocity and proper motions from Gaia Early Data Release 3 would imply that its Galactic rest-frame velocity exceeds its local escape velocity. Consequently, the object would be considered a hypervelocity star, which would prove particularly interesting because its Galactic trajectory points in our direction. However, based on the spectroscopic analysis of follow-up observations, we show that the object is …
We search for the fastest stars in the subset of stars with radial velocity measurements of the second data release (DR2) of the European Space Agency mission Gaia. Starting from the observed positions, parallaxes, proper motions, and radial velocities, we construct the distance and total velocity distribution of more than $7$ million stars in our Milky Way, deriving the full 6D phase space information in Galactocentric coordinates. These information are shared in a catalogue, publicly available at http://home.strw.leidenuniv.nl/~marchetti/research.html. To search for unbound stars, we then fo…
The fastest moving stars provide insight into several fundamental properties of the Galaxy, including the escape velocity as a function of Galactocentric radius, the total mass, and the nature and frequency of stellar encounters with the central supermassive black hole. The recent second data release of Gaia has allowed the identification of new samples of stars with extreme velocities. Discrimination among the possible origins of these stars is facilitated by chemical abundance information. We here report the results from our high-resolution spectroscopic followup, using the Apache Point Obse…
Since the discovery of hypervelocity stars in 2005, it has been widely believed that only the disruption of a binary system by a supermassive black hole at the Galactic center (GC), that is, the so-called Hills mechanism, is capable of accelerating stars to beyond the Galactic escape velocity. In the meantime, however, driven by the Gaia space mission, there is mounting evidence that many of the most extreme high-velocity early-type stars at high Galactic latitudes do originate in the Galactic disk and not in the GC. Moreover, the ejection velocities of these extreme disk-runaway stars exceed …
In this paper, we report 591 high velocity star candidates (HiVelSCs) selected from over 10 million spectra of the data release seven (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. 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 alpha-enhanced inner halo stars. …
High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of unbound and bound Population II stars permits us to better estimate the mass of the halo.} {We carried out a detailed spectroscopic and kinematic study and have significantly refined the distance and the evolutionary state of the star. Its atmospheric parameters, chemical abundances and kinematical properties were determined using high-resolution optical spectroscopy and employing the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code {\…
Evolution of binaries containing a hot subdwarf and a white dwarf to double white dwarfs, and double detonation supernovae with hypervelocity runaway stars
Max Planck Institut für Astrophysik（马克斯·普朗克天体物理学研究所）；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森尼天体物理中心）
Among about 1.8 million Gaia DR2 high proper motion stars with $μ>60$ mas/yr and moderately significant parallaxes ($\varpi>3σ_{\varpi}$), we have selected 109 high-speed star candidates with Galactic rest frame tangential velocities (corrected for solar motion) $v_{t,g}$$>$700 km/s, well above the local Galactic escape velocity. After applying various Gaia DR2 astrometric quality criteria, only 39 candidates remained, including all three hypervelocity runaway white dwarfs (HVR WDs) found by Shen et al. (2018) but no additional such candidates (falling in the same region of the colour-magnitud…
The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey will provide positions and line-of-sight velocities of ~20,000 evolved, maser bearing stars in the Galactic plane. Although this Galactic region is affected by optical extinction, BAaDE targets may have Gaia cross-matches, eventually providing additional stellar information. In an initial attempt to cross-match BAaDE targets with Gaia, we have found more than 5,000 candidates. Of these, we may expect half to show SiO emission, which will allow us to obtain velocity information. The cross-match is being refined to avoid false positive…
In this work, starting from the well-accepted relations in literature, we introduce a new formalism to compute the astrometric membership probabilities for sources in star clusters, and we provide an application to the case of the open cluster M 37. The novelty of our approach is a refined -- and magnitude-dependent -- modelling of the parallax distribution of the field stars. We employ the here-derived list of members to estimate the cluster's mean systemic astrometric parameters, which are based on the most recent Gaia's catalog (EDR3).
We report 23 stars having Galactocentric velocities larger than $450~\mathrm{km\,s}^{-1}$ in the final data release of the APOGEE survey. This sample was generated using space velocities derived by complementing the high quality radial velocities from the APOGEE project in Sloan Digital Sky Survey's Data Release 17 (DR17) with distances and proper motions from Gaia early Data Release 3 (eDR3). We analyze the observed kinematics and derived dynamics of these stars, considering different potential models for the Galaxy. We find that three stars could be unbound depending on the adopted potential…
Fundamental parameters and physical processes leading to the formation of white dwarfs (WDs) may be constrained and refined by discovering WDs in open clusters (OCs). Cluster membership can be utilized to establish the precise distances, luminosities, ages, and progenitor masses of such WDs. We compile a list of probable WDs that are OC members in order to facilitate WD studies that are impractical or difficult to conduct for Galactic field WDs. We use recent catalogs of WDs and OCs that are based on the second data release of the \textit{Gaia} satellite mission (GDR2) to identify WDs that are…
We search for runaway former companions of the progenitors of nearby Galactic core-collapse supernova remnants (SNRs) in the Tycho-Gaia astrometric solution (TGAS). We look for candidates for a sample of ten SNRs with distances less than $2\;\mathrm{kpc}$, taking astrometry and $G$ magnitude from TGAS and $B,V$ magnitudes from the AAVSO Photometric All-Sky Survey (APASS). A simple method of tracking back stars and finding the closest point to the SNR centre is shown to have several failings when ranking candidates. In particular, it neglects our expectation that massive stars preferentially ha…
A systematic survey for hypervelocity runaways from thermonuclear supernovae
California Institute of Technology（加州理工学院）；University of Tübingen（蒂宾根大学）；University of California, Berkeley（加州大学伯克利分校）；Michigan State University（密歇根州立大学）；Stanford University（斯坦福大学）；Institute of Science and Technology Austria (ISTA)（奥地利科学技术研究所）；The Observatories of the Carnegie Institution for Science（卡内基科学机构天文台）；University of Potsdam（波茨坦大学）；University of Warwick（华威大学）
We begin the search for extremely-low mass ($M\leq0.3M_{\odot}$, ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar color-selection method as the Hypervelocity star survey. We switched to an astrometric selection once Gaia Data Release 2 became available. We use the previously known sample of ELM white dwarfs to demonstrate that these objects occupy a unique parameter space in parallax and magnitude. We use the SOAR 4.1m telescope to test the Gaia-based selection, and identify more than two dozen low-mass white dwarfs, inc…
We have refined the census of stars and brown dwarfs in the Upper Sco association (~10 Myr, ~145 pc) by 1) updating the selection of candidate members from our previous survey to include the high-precision astrometry from the second data release of Gaia, 2) obtaining spectra of a few hundred candidate members to measure their spectral types and verify their youth, and 3) assessing the membership (largely with Gaia astrometry) of 2020 stars toward Upper Sco that show evidence of youth in this work and previous studies. We arrive at a catalog of 1761 objects that are adopted as members of Upper …
Context. Most flybys in the Galactic disk are distant, beyond 10,000 AU, and have characteristic velocities of about 70 km/s. However, deep and fast encounters also take place, albeit with lower probability, particularly if one of the objects involved is a stellar remnant ejected during a supernova. WD 0810-353 might be a high velocity white dwarf, and it was recently identified as heading straight for the Solar System; however, the Gaia DR3 data that support its future deep and fast flyby are regarded as suspicious. Aims. Here, we reanalyze the Gaia DR3 data set associated with WD 0810-353 to…
Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejecti…
Atomic diffusion and mixing processes in stellar interiors influence the structure and the surface composition of stars. Some of these processes cannot yet be modelled from the first principles. This limits their applicability in stellar models used for studies of stellar populations and Galactic evolution. Our main goal is to put constrains on the stellar structure and evolution models using new refined measurements of chemical composition in stars of Galactic open cluster. We use medium-resolution, 19 200 <= R <= 21 500, optical spectra of the stars in the open cluster NGC 2420 obtained with…
