Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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.
| The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars
Context. High-velocity stars in the Galactic halo, e.g. the so-calledhyper-velocity stars (HVS), are important tracers of the properties ofthe dark matter halo, in particular its mass. Aims: A search forthe fastest stars among hot subdwarfs (sdB) in the halo is carried outto identify HVS, unbound to the Galaxy, and bound population II stars,in order to derive a lower limit to the halo mass. Methods: Basedon the SDSS DR6 spectral database we selected stars with high rest-framevelocities. These radial velocity (RV) measurements were verified atseveral telescopes to exclude RV variable stars. Out of 88 starsobserved in the follow-up campaign, 39 stars were found to have constantRVs. For twelve of them we measured a proper motion significantlydifferent from zero and obtained spectroscopic distances fromquantitative spectral analysis to construct the full 6D phase spaceinformation for a kinematical study. Results: All but oneprogramme sdB show halo characteristics, but can be distinguished as twokinematical groups, one (G1) with the low Galactic rotation typical ofhalo stars and a second one (G2) with rapid retrograde motion. We alsoinvestigate the possibility that the programme stars are not genuinehalo stars but ejected from the Galactic disc or bulge. The G1 objectscrossed the Galactic plane in the central bulge, whereas the G2 starsdid so in the outer Galactic disc. J1211+1437 (G2) is an HVS candidate,as it is unbound to the Galaxy if the standard Galactic potential isadopted. Conclusions: We conclude that in the ejection scenario,G1 stars might have been formed via the slingshot mechanism that invokesacceleration by tidal interaction of a binary with the centralsupermassive black hole. The G2 stars, however, would originate in theoutskirts of the Galactic disc and not in the central bulge. J1211+1437is the first unbound subdwarf B star, for which we can rule out theslingshot mechanism. Alternatively, we may assume that the stars are oldpopulation II stars and so they have to be bound. Then the kinematics ofJ1211+1437 set a lower limit of 2 × 1012M&sun; to the mass of the Galactic dark matter halo.Based on data from the Sloan Digital Sky Survey, and data collected atthe 3.5 m telescope at DSAZ observatory (Calar Alto) in Spain, the 4.2 mWilliam Herschel Telescope on La Palma, and the European SouthernObservatory in Chile according 081.D-0819(A), 082.D-0649(A) and084.D-0348(A).
| 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.
| 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.
| Catalogue of Ap, HgMn and Am stars
We present a catalogue of 8205 known or suspected Ap, HgMn and Am stars.This paper is a major update of our first edition of the catalog of Apand Am stars and includes revised identifications, additional stars andrevised information obtained from the literature.Catalogue (full Table 1) is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/498/961
| Separated before birth: pulsars B2020+28 and B2021+51 as the remnants of runaway stars
Astrometric data on the pulsars B2020+28 and B2021+51 suggest that theyoriginated within several parsecs of each other in the direction of theCyg OB2 association. It was proposed that the pulsars share their originin a common massive binary and were separated at the birth of the secondpulsar following the asymmetric supernova explosion. We consider adifferent scenario for the origin of the pulsar pair based on apossibility that the pulsars were separated before their birth and thatthey are the remnants of runaway stars ejected (with velocities similarto those of the pulsars) from the core of Cyg OB2 due to strong three-or four-body dynamical encounters. Our scenario does not require anyasymmetry in supernova explosions.
| Detection of Wolf-Rayet stars in host galaxies of gamma-ray bursts (GRBs): are GRBs produced by runaway massive stars ejected from high stellar density regions?
We have obtained deep spectroscopic observations of several nearbygamma-ray burst (GRB) host galaxies revealing for the first time thepresence of Wolf-Rayet (WR) stars and numerous O stars located in richand compact clusters or star forming regions. Surprisingly, high spatialresolution imaging shows that the GRBs and the associated supernovae didnot occur in these regions, but several hundreds of parsec away.Considering various scenarios for GRB progenitors, we do not find anysimple explanation of why they should be preferentially born in regionswith low stellar densities. All the examined GRBs and associated SNehave occurred 400 to 800 pc from very high density stellar environmentsincluding large numbers of WR stars. Such distances can be travelledthrough at velocities of 100 km s-1 or larger, assuming thetravel time to be the typical life time of WR stars. It leads us tosuggest that GRB progenitors may be runaway massive stars ejected fromcompact massive star clusters. The ejection from such super starclusters may lead to a spin-up of these stars, producing the loss of thehydrogen and/or helium envelopes leading to the origin of the type Ibcsupernovae associated with GRBs. If this scenario applies to all GRBs,it provides a natural explanation of the very small fraction of massivestars that emit a GRB at the end of their life. An alternative to thisscenario could be a binary origin for GRBs, but this still requires anexplanation of why it would preferentially occur in low stellar densityregions.
| Young Stars far from the Galactic Plane: Runaways from Clusters
Quite recently, a significant number of OB stars far from the galacticplane have been found, situated at z-distances ranging from severalhundreds of pc to several kpc. The short lifetimes of these stars poseproblems for their interpretation in terms of the standard picture ofstar formation. Different mechanisms have been put forward to explainthe existence of these stars, either within the conventional view, orpostulating star formation in the galactic halo itself. These mechanismsrange from arguing that they are misidentified evolved or abnormalstars, to postulating powerful ejection mechanisms for young disk stars;in situ formation also admits several variants. We have collected fromthe literature a list of young stars far from the plane, for which theevidence of youth seems convincing. We discuss two possible formationmechanisms for these stars: ejection from the plane as the result ofdynamical evolution of small clusters (Poveda et al. 1967) and in situformation, via induced shocks created by spiral density waves (Martos etal. 1999). We compute galactic orbits for these stars, and identify thestars that could be explained by one or the other mechanism. We findthat about 90 percent of the stars can be accounted for by the clusterejection mechanism, that is, they can be regarded as runaway stars inthe galactic halo.
| A bright early-type star in the halo of NGC 253: Runaway or in situ formation
We present observations of J004804.8-251749, a blue object in thedirection of the halo of NGC 253 with (B-V) = -0.16 +/- 0.08, V = 22.09+/- 0.06 and a stellar spectrum showing the Balmer lines redshifted to avelocity of 279 +/- 15 km s-1. These data are consistent witha B5/B8 supergiant star with a mass M = 12 M_sun and a maximum age tau=~ 20 Myr in the halo of NGC 253. Based on its position, radialvelocity, and inferred maximum age, we consider the possibility thatJ004804.8-251749 may be a runaway star ejected from the disk of NGC 253.We derive a lower limit of 172 km s-1 for the ejectionvelocity, or 148 km s-1 if the lower limit of the distancefork to NGC 253 is adopted. While being within the range of velocitiesaccessible to runaway stars through the dynamical ejection mechanism,such high velocities are rather unlikely, especially since the inferredvalue is only a lower limit. This leads us to consider in situ formationas a possible alternative scenario, in which the formation ofJ004804.8-251749 would have been triggered by the interaction betweenthe starburst-driven superwind stemming from the center of NGC 253 and acloud of cold hydrogen in the halo. Such a formation mechanism would beconsistent with our previous finding of an excess of blue point sourcesin the direction of the halo of NGC 253, which we interpreted asindicating the existence of superwind-triggered star formation.Based on observations obtained at the European Southern Observatoryusing the Very Large Telescope in Cerro Paranal (programme69.D-0109(A)).
| Galactic starburst NGC 3603 from X-rays to radio
While NGC 3603 is often quoted as the most massive visible Giant HIIRegion in the Galaxy, there are other similar and even more massiveregions now being found towards the inner Galaxy in the near-IR.Nevertheless, NGC 3603 still retains the status of clone to the densecore-object in 30 Dor, R 136 --- but 7× closer and 49× lesscrowded! This paper summarizes the most recent findings concerning NGC3603's color-magnitude diagram (CMD), initial mass function (IMF), masssegregation and stellar content --- including its unusually luminousH-rich WNL members --- down to its pre-main-sequence stars near theH-burning limit. Of special relevance are new high-resolution X-ray andradio images as related to merging/colliding winds and three massiveproplyd-like objects. NGC 3603 is a somewhat younger, hotter,scaled-down version of typical starbursts found in other galaxies.
| Astrophysics in 2001
During the year, astronomers provided explanations for solar topicsranging from the multiple personality disorder of neutrinos tocannibalism of CMEs (coronal mass ejections) and extra-solar topicsincluding quivering stars, out-of-phase gaseous media, black holes ofall sizes (too large, too small, and too medium), and the existence ofthe universe. Some of these explanations are probably possibly true,though the authors are not betting large sums on any one. The data oughtto remain true forever, though this requires a careful definition of``data'' (think of the Martian canals).
| H I Spectra and Column Densities toward HVC and IVC Probes
We show 21 cm line profiles in the direction of stars and extragalacticobjects, lying projected on high- and intermediate-velocity clouds (HVCsand IVCs). About half of these are from new data obtained with theEffelsberg 100 m telescope, about a quarter are extracted from theLeiden-Dwingeloo Survey (LDS), and the remaining quarter were observedwith other single-dish telescopes. H I column densities were determinedfor each HVC/IVC. Paper I of this series uses these in combination withoptical and ultraviolet high-resolution measurements to deriveabundances. Here an analysis is given of the difference and ratio of N(HI) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratioN(H I-9')/N(H I-35') lies in the range 0.2-2.5. For low-velocity gasthis ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, butit appears that the correction for stray radiation is slightly off). Thesmaller range for the low-velocity gas may be caused by confusion in theline of sight, so that a low ratio in one component can be compensatedby a high ratio in another-for 11 low-velocity clouds fitted by onecomponent the distribution of ratios has a larger dispersion. Comparisonwith higher angular resolution data is possible for 16 sight lines.Eight sight lines with H I data at 1'-2' resolution show a range of0.75-1.25 for N(H I-2')/N(H I-9'), while in eight other sight lines N(HI-Lyα)/N(H I-9') ranges from 0.74 to 0.98.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| Origin and possible birthplace of the extreme runaway star HIP 60350
Using the recently determined spatial velocity components of the extremerunaway star HIP 60350 and a gravitation potential model of the Galaxy,we integrate the orbit of HIP 60350 back to the plane of the Galaxy. Inthis way, a possible location of the formation of the star isdetermined. We estimate the uncertainty of the result due to theuncertainties of the gravitational potential model and the errors in thespatial velocity components. The place of birth lies (within the errors)near the position of the open cluster NGC 3603. However, the ejectionevent which occurred about 20 Myr ago is in contradiction with thecluster mean age of 3-4 Myr. We suggest that it occurred at an earlierphase in sequential star formation in that region. We discuss alsoejection mechanisms. Due to the rather high mass of the star (about 5Msun), the most probable model is that of dynamical ejection.
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| HIP 60350: an extreme runaway star
Using Hipparcos proper motion as well as published and our own radialvelocity data we identify HIP 60350 as fastest young B-type runaway starejected 20 Myr ago or slightly less from the galactic plane, probably ata distance from the galactic center corresponding to spiral arm -II.Both the LSR velocity (417 kms(-1) ) and the mass of the star point todynamical cluster ejection rather than to a supernova scenario.Published and our own photometric data as well as our spectroscopicevidence reveal no noticeable peculiarity and a spectral type B4/5 V. Nosignificant radial velocity variations have been found during the lasttwo decades. Based on data obtained with the Multiple Mirror Telescope,a joint facility of the University of Arizona and the SmithsonianInstitution and from the ESA Hipparcos astrometry satellite
| The distance to Complex M and the Intermediate Velocity Arch
The Intermediate Velocity Arch is an association of Hi clouds lyingwithin the broad limits l~110 deg-210 deg,b~50 deg-80 deg. We usehigh-resolution optical, and single-dish 21-cm, observations of 10early-type halo star sightlines in this region to establish distancelimits (0.4<=z<=3.5kpc) for seven IVCs that are part of the Arch.We re-examine a previous determination of upper and lower distancelimits on the IVC Complex M using the closely aligned halo starsBD+382182 and HD93521. Our Hi spectra show no evidence of emission fromComplex M, leading us to conclude that, while an upper distance limit ofz<=3.5 kpc may be established, no useful lower distance determinationcan be made. Our new Hi observations allow new limits to be establishedon the metallicity of Complex M, and show C, O and Si variations withrespect to solar abundances in the range +/-0.3 dex.
| HIC 60350 = SS59 II 135, a runaway early type chemically peculiar star.
Not Available
| The Case Low-Dispersion Northern Sky Survey. XV. A Region in Ursa Major and Canes Venatici
Positions, estimated magnitudes, and finding charts (when necessary) areprovided for 228 blue and/or emission-line galaxies, H II regions ingalaxies, 114 unresolved blue and/or emission-line objects, includingQSO candidates, and 47 known and suspected blue stars in a ~148 deg^2^region in southeastern Ursa Major and western Canes Venatici in theregion 11^h^00^m^ < R.A. < 12^h^30^m^ and +38^deg^ < Decl. <+47^deg^ ( 1950). The objects, whose blue magnitudes are mostly withinthe range 15-18, were identified on low-dispersion objective-prismplates taken with the Burrell Schmidt telescope at Kitt Peak.
| A high galactic latitude survey of far-ultraviolet excess objects
This study presents optical spectra obtained for a selection of objectsincluded in a catalog of far UV-bright, high-galactic-latitude objectsdetected with a balloon-borne survey telescope. The observed objectsprovide a sample of subdwarf O and B stars, white dwarfs, and binarysystems including a hot subluminous member. Model-atmospheres analysisof the subdwarf sample is used to determine the temperature, gravity,and helium-to-hydrogen ratio of the individual objects. A smoothdistribution of objects is found on the gravity-versus-temperaturediagram near the theoretical location of the extended horizontal branch.A break between the helium-rich and helium-poor objects is found tooccur at 40,000 K. Suspected binary objects were identified and analyzedto determine the temperature and gravity of the hot subluminous memberin each system. The number of subdwarf stars contained in the binariesis determined at 65 to 100 percent. The proportion of white dwarfs thatexperience the subdwarf phase of evolution is found to be 0.94 percent.
| General catalogue of AP and AM stars
A catalog of all the known Ap and Am stars, containing a total of 6684objects, was prepared. The few stars that were wrongly considered as Apor Am star are noted as well as some others for which their identity asAp or Am stars is not yet well established. The catalog gives theidentificators and the main observational data for each star, includingthe magnitude, color, spectral type, duplicity, periodicity, magneticfield, and projected rotational velocity.
| Runaway stars from young star clusters containing initial binaries. II - A mass spectrum and a binary energy spectrum
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99..608L&db_key=AST
| The nature of the four blue halo stars of Tobin
Model atmosphere techniques presently used to analyze visual spectra ofTobin's (1986) four blue halo star IUE spectra have yielded resultsconfirming the population I character of Feige 40. H.O.+23 deg B, andH.O.+41 deg B. A 2-3 kpc distance from the galactic plane corresponds tothese results. The fourth star, PB 166, is likely to be a little-evolvedhorizontal-branch star with a distance of only 1 kpc. Radial velocitiesdetermined from coude spectra are consistent with the evolutionary statederived.
| Low-resolution IUE observations of four unreddened, 11th magnitude B stars at high galactic latitudes
Low-resolution IUE spectra are presented for the four faint, unreddened,high-latitude, mid-B stars Feige 40 (≡FB 72), H.O. +23°B, H.O.+41°B (≡SS II 135) and PB 166. By comparison with stars in theHeck et al. atlas, the spectra are like those of normal disc stars ofPopulation I metallicity. This suggests that the programme stars aremassive, luminous objects, which, if true, means that they are situatedseveral kiloparsecs outside the galactic plane. Effective temperaturesand upper limits on the Population I ages are derived. Radial velocitiesare known for two of the stars: they are consistent with stellar birthin the galactic plane followed by ejection, at high speed, to thepresent locations within the evolutionary lifetimes. Predictions aremade for the radial velocities of the other two stars.
| Four-colour and H-beta photometry of blue stars selected from a balloon-ultraviolet survey and other sources
New uvby and/or H-beta photometry is obtained at the Chiranhigh-altitude outstation for 105 stars. Certain of the program stars areselected from a comparison of the SCAP 2000 balloon-ultraviolet skysurvey of the Laboratoire d'Astronomie Spatiale with the correspondingblue and red prints of the Palomar Observatory sky survey. Only a smallportion of these stars turn out to be B stars, whereas, the rest of thestars, which are selected from a variety of other sources, are mostly Bstars and if of normal luminosity are in many cases many Population Iscale heights from the galactic plane. It is shown that theidentification of B stars is practicable only at a good observing siteand that if H-beta photometry is also obtained, sdOB, sdB, and hothorizontal branch B stars can be distinguished from normal stars bytheir position in the beta/c(0) diagram.
| Further Studies of A-Stars and F-Stars in the Region of the North Galactic Pole - Part Four - a Catalogue of Uvbyr Photometry and Derived Quantities
Not Available
| Further Studies of A-Stars and F-Stars in the Region of the North Galactic Pole - Part Three - a Catalogue of Star Names and Positions
Not Available
| Lowell Proper Motion Survey - Summary Catalogue of GD and GR Stars
Not Available
| Photoelectric photometry of stars near the north galactic pole
Abstract image available at:http://adsabs.harvard.edu/abs/1973A&AS...12..381H
| A finding-list of stars of spectral type F2 and earlier in a north galactic pole region.
Not Available
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Ловачки пси |
| Ректацензија: | 12h22m29.60s |
| Deклинација: | +40°49'35.6" |
| Apparent магнитуда: | 11.831 |
| Proper motion RA: | -12 |
| Proper motion Dec: | 17.1 |
| B-T magnitude: | 11.431 |
| V-T magnitude: | 11.798 |
Каталог и designations:
|
