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The Nature of Hypervelocity Stars and the Time between Their Formation and Ejection
We obtain Keck HIRES spectroscopy of HVS5, one of the fastest unboundstars in the Milky Way halo. We show that HVS5 is a 3.62 ± 0.11 M&sun; main-sequence B star at a distance of 50 ± 5kpc. The difference between its age and its flight time from theGalactic center is 105 ± 18 (stat) ±30 (sys) Myr; flighttimes from locations elsewhere in the Galactic disk are similar. This108 yr "arrival time" between formation and ejection isdifficult to reconcile with any ejection scenario involving massivestars that live for only 107 yr. For comparison, we derivearrival times of 107 yr for two unbound runaway B stars,consistent with their disk origin where ejection results from asupernova in a binary system or dynamical interactions between massivestars in a dense star cluster. For HVS5, ejection during the first107 yr of its lifetime is ruled out at the 3? level.Together with the 108 yr arrival times inferred for threeother well-studied hypervelocity stars (HVSs), these results areconsistent with a Galactic center origin for the HVSs. If the HVSs wereindeed ejected by the central black hole, then the Galactic center wasforming stars sime200 Myr ago, and the progenitors of the HVSs tooksime100 Myr to enter the black hole's loss cone.
| Supernovae without host galaxies?. Hypervelocity stars in foreign galaxies
Context. Harvesting the SAI supernova catalog, the most complete list ofsupernovae (SNe) currently available, we search for SNe that apparentlydo not occur within a distinct host galaxy but lie a great distance(several arcmin) apart from the host galaxy given in the catalog or evenshow no sign of an identifiable galaxy in their direct vicinity. Aims: We attempt to distinguish between two possible explanations ofthis host-lessness of a fraction of reported SNe, namely (i) that a hostgalaxy is too faint (of too low surface brightness) to be detectedwithin the limits of currently available surveys (presumably a lowsurface brightness galaxy) or (ii) the progenitor of the SN is ahypervelocity star (HVS) that exploded kiloparsecs away from its hostgalaxy. Methods: We use deep imaging to test the firstexplanation. If no galaxy is identified within our detection limit of~27 mag arcsec-2, which is the central surface brightness ofthe faintest known LSB galaxy so far, we discard this explanation andpropose that the SN, after several other checks, had a hypervelocitystar progenitor. We focus on observations for which this is the case andgive lower limits to the actual space velocities of the progenitors,making them the first hypervelocity stars known in galaxies other thanour own Milky Way. Results: Analyzing a selected subsample offive host-less SNe, we find one, SN 2006bx in UGC 5434, is a possiblehypervelocity progenitor category with a high probability, exhibiting aprojected velocity of ~800 km s-1. SN 1969L in NGC 1058 ismost likely an example of a very extended star-forming disk visible onlyin the far-UV, but not in the optical wavebands. Therefore, this SN isclearly due to in situ star formation. This mechanism may also apply totwo other SNe that we investigated (SN 1970L and SN 1997C), but thiscannot be determined with certainty. Another SN, SN 2005 nc which isassociated with a gamma-ray burst (GRB 050525), is a special case thatis not covered by our initial assumptions. Even with deep Hubble SpaceTelescope data, a host galaxy cannot be unambiguously identified.
| Overview and current relevance of "Run-away stars as the result of the gravitational collapse of proto-stellar clusters" by Poveda, Ruiz, & Allen (1967)
The mechanism for producing massive runaway stars proposed by A. Poveda,J. Ruiz and C. Allen, 1967, BOTT, 4, 28, 86, is briefly described. Itsimpact over the years and its gradual acceptance as a viable way toaccount for such stars are traced. Its current relevance for theunderstanding of runaway stars is assessed.
| Ejection velocities of high Galactic latitude runaway stars
We estimate the distribution of ejection velocities for the knownpopulation of high Galactic latitude runaway stars. The initial sampleis a collection of 174 early-type stars selected from the literature.The stars are first classified according to their evolutionary status inorder to obtain a homogeneous sample of 96 genuine main-sequence stars.Their present velocities and flight times are then estimated usingproper motion data from various astrometric catalogues (includingTycho-2, UCAC2 and USNO-B) and the ejection velocities are computed bytracing their orbits back in time, based on a Galactic potential. Thepotential used is constructed from a mass density model chosen to fitthe most recent observational constraints.We find evidence for two different populations of runaway stars: a'high' velocity population, with a maximum ejection velocity of about400-500 km s-1, and a 'low' velocity population, with amaximum ejection velocity of about 300 km s-1. We argue thatthe observed limit of 500 km s-1 and the bimodality of theobserved ejection velocity distribution are natural consequences of theso-called Binary Ejection Mechanism. We discuss the connection betweenthe 'high' velocity population and the so-called hypervelocity stars,showing how previously studied hypervelocity stars are consistent withthe results obtained.We also find that some stars that were once thought to be best explainedas being formed in the halo are compatible with a runaway hypothesisonce proper motions are included in the analysis. However, three starsin the selected sample appear to be inconsistent with ejection from theGalactic disc. Possible scenarios are discussed, including a possibleformation in the Galactic halo.
| Very massive runaway stars from three-body encounters
Very massive stars preferentially reside in the cores of their parentclusters and form binary or multiple systems. We study the role of tightvery massive binaries in the origin of the field population of verymassive stars. We performed numerical simulations of dynamicalencounters between single (massive) stars and a very massive binary withparameters similar to those of the most massive known Galactic binaries,WR 20a and NGC 3603-A1. We found that these three-body encounters couldbe responsible for the origin of high peculiar velocities (?70 kms-1) observed for some very massive (?60-70M&sun;) runaway stars in the Milky Way and the LargeMagellanic Cloud (e.g. ? Cep, BD+43°3654, Sk -67°22, BI237, 30 Dor 016), which can hardly be explained within the framework ofthe binary-supernova scenario. The production of high-velocity massivestars via three-body encounters is accompanied by the recoil of thebinary in the opposite direction to the ejected star. We show that therelative position of the very massive binary R145 and the runaway earlyB-type star Sk-69°206 on the sky is consistent with the possibilitythat both objects were ejected from the central cluster, R136, of thestar-forming region 30 Doradus via the same dynamical event - athree-body encounter.
| The Nature of the Hyper-Runaway Candidate Hip 60350
Young, massive stars in the Galactic halo are widely supposed to be theresult of an ejection event from the Galactic disk forcing some stars toleave their place of birth as so-called runaway stars. Here, we presenta detailed spectroscopic and kinematic analysis of the runaway B starHIP 60350 to determine which runaway scenario—a supernovaexplosion disrupting a binary system or dynamical interaction in starclusters—may be responsible for HIP 60350's peculiar orbit. Basedon a non-local thermodynamic equilibrium approach, a high-resolutionoptical echelle spectrum was examined to revise spectroscopic quantitiesand for the first time to perform a differential chemical abundanceanalysis with respect to the B-type star 18 Peg. The results togetherwith proper motions from the Hipparcos Catalog further allowed thethree-dimensional kinematics of the star to be studied numerically. Theabundances derived for HIP 60350 are consistent with a slightlysupersolar metallicity agreeing with the kinematically predicted placeof birth ~6 kpc away from the Galactic center. However, they do notexclude the possibility of an ?-enhanced abundance patternexpected in the case of the supernova scenario. Its outstanding highGalactic rest-frame velocity of 530 ± 35 km s-1 is aconsequence of ejection in the direction of Galactic rotation andslightly exceeds the local Galactic escape velocity in a standardGalactic potential. Hence, HIP 60350 may be unbound to the Galaxy.
| Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys
Runaway stars ejected from the Galactic disk populate the halo of theMilky Way. To predict the spatial and kinematic properties of runaways,we inject stars into a Galactic potential, compute their trajectoriesthrough the Galaxy, and derive simulated catalogs for comparison withobservations. Runaways have a flattened spatial distribution, withhigher velocity stars at Galactic latitudes less than 30°. Due totheir shorter stellar lifetimes, massive runaway stars are moreconcentrated toward the disk than low mass runaways. Bound (unbound)runaways that reach the halo probably originate from distances of 6-12kpc (10-15 kpc) from the Galactic center, close to the estimated originof the unbound runaway star HD 271791. Because runaways are brighter andhave smaller velocities than hypervelocity stars (HVSs), radial velocitysurveys are unlikely to confuse runaway stars with HVSs. We estimatethat at most one runaway star contaminates the current sample. We placean upper limit of 2% on the fraction of A-type main-sequence starsejected as runaways.
| SDSS J013655.91+242546.0 - an A-type hyper-velocity star from the outskirts of the Galaxy
Context: Hyper-velocity stars (HVS) are moving so fast that they areunbound to the Galaxy. Dynamical ejection by a supermassive black holeis favoured to explain their origin. Aims: Locating the place ofbirth of an individual HVS is of utmost importance to understanding theejection mechanism. Methods: SDSS J013655.91+242546.0(J0136+2425 for short) was found amongst three high-velocity stars(drawn from a sample of more than 10 000 blue stars), for which propermotions were measured. A kinematical as well as a quantitative NLTEspectral analysis was performed. When combined with the radial velocity(RV) and the spectroscopic distance, the trajectory of the star in theGalactic potential was reconstructed. Results: J0136+2425 isfound to be an A-type main-sequence star travelling at ?590 {kms-1}, possibly unbound to the Galaxy and originating in theouter Galactic rim nowhere near the Galactic centre. Conclusions: J0136+2425 is the second HVS candidate with measuredproper motion, besides the massive B star HD 271791, and also the secondfor which its proper motion excludes a Galactic centre origin and,hence, the SMBH slingshot mechanism. Most known HVS are late B-typestars of about 3 M_&sun;. With a mass of 2.45 M_&sun;, J0136+2425resembles a typical HVS far more than HD 271791 does. Hence, this is thefirst time that a typical HVS is found not to originate in the Galacticcentre. Its ejection velocity from the disk is so high (550 {kms-1}) that the extreme supernova binary scenario proposed forHD 271791 is very unlikely.Based on data collected at the European Southern Observatory, Chile.Program ID: 082.D-0649.Based on observations at the 3.5 m telescope at DSAZ observatory (CalarAlto) in Spain. Program ID: H09-3.5-028.
| On the origin of high-velocity runaway stars
We explore the hypothesis that some high-velocity runaway stars attaintheir peculiar velocities in the course of exchange encounters betweenhard massive binaries and a very massive star (either an ordinary50-100Msolar star or a more massive one, formed throughrunaway mergers of ordinary stars in the core of a young massive starcluster). In this process, one of the binary components becomesgravitationally bound to the very massive star, while the second one isejected, sometimes with a high speed. We performed three-body scatteringexperiments and found that early B-type stars (the progenitors of themajority of neutron stars) can be ejected with velocities of>~200-400kms-1 (typical of pulsars), while3-4Msolar stars can attain velocities of>~300-400kms-1 (typical of the bound population of halolate B-type stars). We also found that the ejected stars canoccasionally attain velocities exceeding the Milky Ways's escapevelocity.
| HD271791: dynamical versus binary-supernova ejection scenario
The atmosphere of the extremely high-velocity (530-920kms-1)early B-type star HD271791 is enriched in ?-process elements,which suggests that this star is a former secondary component of amassive tight binary system and that its surface was polluted by thenucleosynthetic products after the primary star exploded in a supernova.It was proposed that the (asymmetric) supernova explosion unbind thesystem and that the secondary star (HD271791) was released at itsorbital velocity in the direction of Galactic rotation. In this Letter,we show that to explain the Galactic rest-frame velocity of HD271791within the framework of the binary-supernova scenario, the stellarremnant of the supernova explosion (a <~10Msolar blackhole) should receive an unrealistically large kick velocity of>=750-1200kms-1. We therefore consider thebinary-supernova scenario as highly unlikely and instead propose thatHD271791 attained its peculiar velocity in the course of a strongdynamical three- or four-body encounter in the dense core of the parentstar cluster. Our proposal implies that by the moment of encounterHD271791 was a member of a massive post-supernova binary.
| An Alternative Origin for Hypervelocity Stars
Halo stars with unusually high radial velocity (hypervelocity stars, orHVS) are thought to be stars unbound to the Milky Way that originatefrom the gravitational interaction of stellar systems with thesupermassive black hole at the Galactic center. We examine the latestHVS compilation and find peculiarities that are unexpected in this blackhole ejection scenario. For example, a large fraction of HVS clusteraround 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 isembedded within a 2.5 × 1012 h -1 Msun dark halo with virial velocity of ~220 km s-1.In this case, the escape velocity at ~50 kpc would be ~600 kms-1, and very few HVS would be truly unbound. We usenumerical simulations to show that disrupting dwarf galaxies maycontribute halo stars with velocities up to and sometimes exceeding thenominal escape speed of the system. These stars are arranged in a thinlycollimated outgoing "tidal tail" stripped from the dwarf during itslatest pericentric passage. We speculate that some HVS may, therefore,be tidal debris from a dwarf recently disrupted near the center of theGalaxy. In this interpretation, the angular clustering of HVS resultsbecause, from our perspective, the tail is seen nearly "end on," whereasthe common travel time simply reflects the fact that these stars werestripped simultaneously from the dwarf during a single pericentricpassage. This proposal is eminently falsifiable, since it makes a numberof predictions which are distinct from the black hole ejection mechanismand which should be testable with improved HVS datasets.
| MMT Hypervelocity Star Survey
We describe a new survey for unbound hypervelocity stars (HVSs), starstraveling with such extreme velocities that dynamical ejection from amassive black hole is their most likely origin. We investigate thepossible contribution of unbound runaway stars, and show that thephysical properties of binaries constrain low-mass runaways to boundvelocities. We measure radial velocities for HVS candidates with thecolors of early A-type and late B-type stars. We report the discovery ofsix unbound HVSs with velocities and distances exceeding theconservative escape velocity estimate of Kenyon and collaborators. Weadditionally report four possibly unbound HVSs with velocities anddistances exceeding the lower escape velocity estimate of Xue andcollaborators. These discoveries increase the number of unbound HVSs by60%-100%. Other survey objects include 19 newly identified z ~ 2.4quasars. One of the HVSs may be a horizontal branch star, consistentwith the number of evolved HVSs predicted by Galactic center ejectionmodels. Finding more evolved HVSs will one day allow a probe of thelow-mass regime of HVSs and will constrain the mass function of stars inthe Galactic center.
| HD 271791: An Extreme Supernova Runaway B Star Escaping from the Galaxy
Hypervelocity stars (HVSs) were first predicted by theory to be theresult of the tidal disruption of a binary system by a supermassiveblack hole (SMBH) that accelerates one component to beyond the Galacticescape velocity (the Hills mechanism). Because the Galactic center hostssuch a SMBH it is the suggested place of origin for HVSs. However, theSMBH paradigm has been challenged recently by the young HVS HD 271791because its kinematics point to a birthplace in the metal-poor rim ofthe Galactic disk. Here we report the atmosphere of HD 271791 to indeedshow a subsolar iron abundance along with an enhancement of the?-elements, indicating capture of nucleosynthesis products from asupernova or a more energetic hypernova. This implies that HD 271791 isthe surviving secondary of a massive binary system disrupted in asupernova explosion. No such runaway star has ever been found to exceedthe Galactic escape velocity; hence HD 271791 is the first hyperrunawaystar. Such a runaway scenario is an alternative to the Hills mechanismfor the acceleration of some HVSs with moderate velocities. The observedchemical composition of HD 271791 puts invaluable observationalconstraints on nucleosynthesis in a supernova from the core collapse ofa very massive star (MZAMS>~55 Msolar), whichmay be observed as a gamma-ray burst of the long-duration/soft-spectrumtype.Based on observations obtained at the European Southern Observatory,proposal 073.D-0495(A).
| HVS 7: a chemically peculiar hyper-velocity star
Context: Hyper-velocity stars are suggested to originate from thedynamical interaction of binary stars with the supermassive black holein the Galactic centre (GC), which accelerates one component of thebinary to beyond the Galactic escape velocity. Aims: Theevolutionary status and GC origin of the hyper-velocity star SDSSJ113312.12+010824.9 (aka HVS 7) is constrained from a detailed study ofits stellar parameters and chemical composition. Methods:High-resolution spectra of HVS 7 obtained with UVES on the ESO VLT wereanalysed using state-of-the-art NLTE/LTE modelling techniques that canaccount for a chemically-peculiar composition via opacity sampling. Results: Instead of the expected slight enrichments of?-elements and near-solar iron, huge chemical peculiarities of allelements are apparent. The helium abundance is very low (<1/100solar), C, N and O are below the detection limit, i.e they areunderabundant (<1/100, ?1/3 and <1/10 solar). Heavierelements, however, are overabundant: the iron group by a factor of ~10,P, Co and Cl by factors ~40, 80 and 440 and rare-earth elements andmercury even by ~10 000. An additional finding, relevant also for otherchemically peculiar stars are the large NLTE effects on abundances of TiII and Fe II (~0.6-0.7 dex). The derived abundance pattern of HVS 7 ischaracteristic for the class of chemical peculiar magnetic B stars onthe main sequence. The chemical composition and high projected rotationvelocity v sin i = 55 ±2 km s-1 render a low massnature of HVS 7 as a blue horizontal branch star unlikely. Conclusions: Such a surface abundance pattern is caused by atomicdiffusion in a possibly magnetically stabilised, non-convectiveatmosphere. Hence all chemical information on the star's place of birthand its evolution has been washed out. High precision astrometry is theonly means to validate a GC origin for HVS 7.Based on observations collected at the European Southern Observatory,Paranal, Chile, proposal 079.D-0756(A).Appendix A is only available in electronic form at http://www.aanda.org
| The B-type giant HD 271791 in the Galactic halo. Linking run-away stars to hyper-velocity stars
Context: Young, massive stars have been found in the distant Galactichalo. Dynamical ejection from the Galactic disc has been suggested asthe origin of these “run-away stars”. The so-calledhyper-velocity stars have been found to travel so fast that they areunbound to the Galaxy. Only a supermassive black hole (SMBH) appears tobe able to accelerate the stars to such high velocities, which suggeststhat the Galactic centre is their place of origin. Aims: Werevisit the run-away B star HD 271791 to determine its nature andorigin. Methods: High-resolution optical echelle spectra areanalysed using LTE model atmospheres. Based on proper-motionmeasurements, the 3D kinematic of the star is investigated by means ofnumerical experiments. Results: HD 271791 is found to be amassive (11 M&sun;), rapidly-rotating B-type star. Itschemical composition is found to be consistent with that of the sun. ItsGalactic rest-frame velocity exceeds the Galactic escape velocity. Conclusions: According to its space velocity, HD 271791 qualifies as ahyper-velocity star. Its kinematic constrains the place of birth to theouter Galactic disc and rules out the Galactic centre. HD 271791 is alsotoo young (<30 Myr) to have originated in the Galactic centre. Thischallenges the SMBH paradigm for the origin of hyper-velocity stars.Based onobservations collected at the European Southern Observatory at La Silla,Chile,ESO proposal No. 073.D-0495(A).
| Photometry of faint blue stars - IX
Stromgren uv by photometry is presented for 129 `faint blue' stars takenfrom various catalogues. The photometry is used to estimate photometric`classifications' for the stars, which indicate a mixture of hotsubdwarfs, horizontal-branch stars, metal-weak subdwarfs and so on.Attention is drawn to stars (from this paper and previous papers in theseries) which appear to be somewhat reddened. Some are probablybinaries, and others might be objects with peculiar colours, such ascataclysmic variables. One star, LB 9963, almost certainly falls intothe latter category. Two stars which, from their colours, are PopulationII A-F stars are variable; one of these, OM 89, is the known RR Lyraestar, VW Dor.
| Radial velocities and spectral types for a sample of faint blue stars.
Not Available
| Analysis of high-latitude blue stars. II
A simple analysis of five high-latitude early-type stars is presented.Reticon spectra and Strömgren photometry are used to determineTeff and log g from "Kiel diagrams" and values of V sin i areestimated from He I lines. The results are checked by fitting Kurucz(1979) profiles to the observed H? and H? profiles. Fourstars appear spectroscopically normal although one might be subluminous;implied distances from the galactic plane are between 3 and 15 kpc. Onestar, TONS 308, could be a spectrum variable.
| Non-supergiant high-velocity stars near the Magellanic Clouds.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&AS...16..277C
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