The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. II. Kinematics and Full Sample Analysis
This paper continues the analysis of faint high-latitude B stars fromMartin. Here we analyze the kinematics of the stars and combine themwith the abundance information from the first paper to classify eachone. The sample contains 31 Population I runaways, 15 old evolved stars(including 5 blue horizontal-branch [BHB] stars, 3 post-HB stars, 1pulsating helium dwarf, and 6 stars of ambiguous classification), 1 Fdwarf, and 2 stars that do not easily fit in one of the othercategories. No star in the sample unambiguously shows thecharacteristics of a young massive star formed in situ in the halo. Thetwo unclassified stars are probably extreme Population I runaways. Thelow binary frequency and rotational velocity distribution of thePopulation I runaways imply that most were ejected from dense starclusters by the dynamic ejection scenario. However, we remain puzzled bythe lack of runaway Be stars. We also confirm that PB 166 and HIP 41979are both nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory, operated by the University of Texas at Austin.
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The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. I. Composition and Spectral Features
The existence of faint blue stars far above the Galactic plane that havespectra that are similar to nearby Population I B stars presents severalinteresting questions. Among them are the following: Can a Population IB star travel from the disk to a position many kiloparsecs above theplane in a relatively short main-sequence lifetime? Is it possible thatsingle massive star formation is occurring far from the Galactic plane?Are these objects something else masquerading as main-sequence B stars?This paper (the first of two) analyzes the abundances of a sample ofthese stars and reveals several that are chemically similar to nearbyPopulation I B stars, whereas others clearly have abundance patternsmore like those expected in blue horizontal-branch (BHB) orpost-asymptotic giant branch stars. Several of those with old evolvedstar abundances also have interesting features of note in their spectra.We also consider why this sample does not have any classical Be starsand identify at least two nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory operated by the University of Texas at Austin.
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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.
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Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
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Incidence of X-ray sources among magnetic chemically peculiar stars
Cash & Snow (1982) and Golub et al. (1983) have detected X-rayemission from 3 out of 7 observed magnetic chemically peculiar (CP)stars. Although the incidence of X-ray sources apparently is very high,these authors concluded that such emission is not unquestionable becauseof the presence of a companion. To determine the incidence of X-raysources among hot CP stars, I have checked the list of B-type starsmeasured by Grillo et al. (1992) selecting 90 stars. Of the 4 magneticCP stars showing X-ray emission 3 are members of a binary system. Thusit appears that detectable X-ray emission from magnetic CP stars is notvery common and still questionable. To find out whether HD 37017presents X-ray emission (Drake et al. 1987) or not (Grillo et al. 1992),I have analyzed an HRI frame from the ROSAT satellite finding noevidence for X-ray emission at this star's position.
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An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
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The runaway nature of distant early-type stars in the galactic halo
The kinematics of a sample of 32 distant halo B-stars with masses in therange 3-21 solar masses are investigated using the current 'runawaystar' hypotheses to establish if they belong to this group of stars.Whether postmass transfer secondaries in evolved massive close binariescan be normal low mass B-stars is discussed, but support for this ideais not found. Thus it appears that the lower mass stars could not havebeen ejected out of the disk as a result of supernova explosions inmassive close binaries, while the higher mass objects do not in generalreveal a binary nature as expected from this theory. The results ofrecent N-body simulations of the purely dynamical ejection of runawaystars from young galactic star clusters are in excellent agreement withthe observations. Thus it appears that these halo stars are the mostdistant subgroup of OB runaways produced by cluster ejection yetobserved.
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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
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Investigations of young stars in the galactic halo
High resolution optical spectra of fifteen intermediate and highlatitude blue stars have been obtained using the 2.5 m Isaac Newton andthe 3.9 m Anglo-Australian Telescopes. Three of the sample areidentified as subdwarfs. The remaining twelve are analyzed foratmospheric parameters and metal and helium abundances using modelatmosphere techniques, and appear to be normal B-type stars 1 kpc ormore from the plane. Their origin is discussed in terms of theiravailable kinematics. Eight of the stars could have been formed in thegalactic plane and reached their present z-positions in their lifetimes.From the stellar kinematics, cluster ejection seems the most likelymechanism for their presence in the halo, although close binary ejectionmay have been responsible for a small number of these stars. Four starscould not have traveled from the disk to their current halo z-distancesin their lifetimes, and the idea of star formation in the halo isbriefly discussed.
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An abundance and kinematical analysis of four early-type high latitude stars
Stellar absorption line profiles and equivalent widths are measured fromhigh resolution INT and AAT optical spectra of four high galacticlatitude targets and used in conjunction with model atmosphere codes todetermine their atmospheric parameters and chemical compositions. NormalPopulation I abundances are derived, with typical errors of + or - 0.2dex, implying z-distances in the range 1-2 kpc. Various explanations forthe existence of such stars so far from the Galactic plane arediscussed, the most likely mechanism being cluster ejection.
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The galactic reddening law - The evidence from uvby-beta photometry of B stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&A...142..189T&db_key=AST
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Photometry of faint blue stars. IV - Some 'very definitely blue' PHL stars
Photometry on the uvby system is presented for 56 faint blue stars,mostly from the Palomar-Haro-Luyten 'very definitely blue' list. Themajority of these stars are hot subdwarfs.
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Four-colour and H beta photometry of southern B stars at high galactic latitudes
Four-color and H beta photometry has been obtained for 105 early-type HDstars with galactic latitudes less than -45 deg. They are mostly late Bstars of luminosity class V to III. Two new Am stars and several Ap orBp stars are detected photometrically. Absolute magnitudes aredetermined from the photometry and from MK types where available. Theinterstellar reddening of many of the more distant stars is very small,suggesting either the existence of undetected peculiar stars in thesample or that there are areas of effectively zero reddening at highsouthern galactic latitudes.
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Lowell Proper Motion Survey - Summary Catalogue of GD and GR Stars
Not Available
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Lowell proper motions XVIII. Proper Motion Survey in the southern hemisphere with the 13-inch photographic telescope of the Lowell Observatory.
Not Available
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Photometry of faint blue stars - III. Hbeta photometry of some southern stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1977MNRAS.178..369K
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Photometry of faint blue stars. II - A second list of southern stars
Photometry in the uvby system is given for 61 faint blue stars. Thestars are classified by means of the Stroemgren indices, usingpreviously described criteria.
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Gaps in the blue horizontal branch.
Abstract image available at:http://adsabs.harvard.edu/abs/1976ApJ...204..804N
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The Nature of Faint Blue Stars in the Halo. II
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974ApJS...28..157G&db_key=AST
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On the detectability of Bp-type abundance anomalies in early main-sequence stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&A....34....1B
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Mean absolute magnitude of the blue stars at high galactic latitude.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1962AJ.....67..740K&db_key=AST
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A search for blue stars in high galactic latitudes.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958AJ.....63..484C&db_key=AST
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INHABITED SKY
