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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.
| 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.
| 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.
| Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities(vsini) of a sample of 45 candidate field horizontal-branch (HB) starsspanning a wide range of effective temperatures, from red HB stars withTeff~=5000K to blue HB stars with Teff of 17,000K.Among the cooler blue HB stars (Teff=7500-11500 K), weconfirm prior studies showing that, although a majority of stars rotateat vsini<15kms-1, there exists a subset of ``fastrotators'' with vsini as high as 30-35 km s-1. All but one ofthe red HB stars in our sample have vsini<10kms-1, and noanalogous rotation bimodality is evident. We also identify anarrow-lined hot star (Teff~=16,000K) with enhancedphotospheric metal abundances and helium depletion, similar to theabundance patterns found among hot BHB stars in globular clusters, andfour other stars that may also belong in this category. We discussdetails of the spectral line fitting procedure that we use to deducevsini and explore how measurements of field HB star rotation may shedlight on the issue of HB star rotation in globular clusters.
| 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.
| A Large-Scale Spectroscopic Survey of Early-Type Stars at High Galactic Latitudes
We present new model atmosphere analyses of optical spectroscopy of alarge sample of B-type stellar candidates. Of a total of 298 objects,the largest sample of its kind to date, 205 were drawn from the PalomarGreen Survey of high Galactic latitude ultraviolet-excess stellarobjects and comprise a complete magnitude-limited sample. Effectivetemperatures, surface gravities, and helium abundances for the hotsubdwarf (high-gravity) component of the sample are derived from adetailed line profile analysis of the hydrogen and helium absorptionlines in intermediate-resolution (3-5 A FWHM) optical spectra. Aseparate analysis of the lower gravity component is made using a newlycalculated grid of synthetic spectra. Additional estimates of theeffective temperatures are made from wide- and intermediate-bandphotometry taken from the literature. We are currently undertaking twofollow-up programs. (1) Detailed abundance analyses of high-resolutionechelle spectra of the lower gravity component of the survey usingmodern model atmosphere and synthetic spectrum techniques willdifferentiate between massive Population I main-sequence B stars andlow-mass, lower luminosity Population II blue horizontal branch starsand post-asymptotic giant branch stars. (2) The derived atmosphericparameters for the higher gravity component, the field extendedhorizontal branch stars, will be combined with radial velocitymeasurements to determine their spatial and kinematic distributions,which will distinguish between competing evolutionary scenarios for thishot, evolved stellar population.
| Intermediate-Velocity Gas in the North Galactic Hemisphere: H i Studies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..703K&db_key=AST
| Evidence for Mass Outflow from the Nucleus of M101: Knots, Rings, and a Geyser
Hα on-line and off-line CCD images of M101 obtained with theCanada-France-Hawaii 3.6 m telescope show the presence of two Hαbright, filled knots paired linearly across the nucleus in a north-southorientation. The knots are centered 2.4" (85 pc, assuming a distance toM101 of 7.4 Mpc) from the nucleus and lie roughly perpendicular to aneast-west molecular bar. Each knot in turn is connected to an elongated,photoionized ring lying parallel along the bar. The eastern ring,connected to the southern knot, reaches outward in a well-defined 500 x200 pc oval. The 700 x 300 pc western ring, connected to the northernknot, is more broken and dissipated in the middle. An arc containingblue stars and/or significant Hα absorption lies along thesouthern side of a dust lane extending from the nucleus westward alongthe bar. Hubble Space Telescope Planetary Camera image data show pocketsof star-forming regions to the east and to the south of the nucleuswhich are associated with the knot and ring in that half. The imagingdata, together with velocity data obtained with the Coude' feedspectrometer at Kitt Peak National Observatory show that the knots andrings are likely a bipolar outflow originating from a velocity <100km s^-1^ "geyser" which has a period of approximately 22 million yearsand is located in the nucleus. The geyser may be caused by a mass <10^6^ M_sun_ black hole orbiting within the nucleus, sweeping materialfrom the molecular bar.
| IUE observations of early-type stars at high Galactic latitudes
High- and low-resolution IUE spectra of 14 apparently normalhigh-Galactic latitude B-type stars, together with visual fluxesdetermined from Stroemgren four-color photometry, are compared withtheoretical spectra deduced from LTE model atmosphere calculations. Tenof the program stars have normal flux distributions, with only four ofthe stars exhibiting UV flux deficiency. For the latter, PHL 346 hasbeen identified as a beta Cepheid variable, SB 357 is a Be-type star,and the ultraviolet flux deficiency for HD 214080 can be removed byincreasing the E(B-V) from 0.09 to 0.12. In the case of HD 100340, thefour-color photometry is inconsistent with the ultraviolet fluxdistribution, but this inconsistency vanishes when UBV photometry isemployed in the analysis, implying that the uvby measurements may be inerror. The normal flux distributions found for the program star providesupport for their being Population I objects in the Galactic halo.
| 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.
| 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.
| Interstellar absorption lines in the spectra of the high-latitude blue stars PB 166, H.O. +41°B, H.O. +23°B, and Feige 40.
Not Available
| 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.
| Low-resolution IUE observations of four unreddened, 11th magnitude Bstars at high galactic latitudes.
Not Available
| A library of stellar spectra
Spectra for 161 stars having spectral classes O-M and luminosity classesV, III, and I have been incorporated into a library available onmagnetic tape. The spectra extend from 3510 to 7427 A at a resolution ofabout 4.5 A. The typical photometric uncertainty of each resolutionelement in the spectra is on the order of 1 percent while broad-bandvariations are smaller than 3 percent. Potential uses for the libraryinclude population synthesis of galaxies and clusters, tests of stellaratmosphere models, spectral classification, and the generation of colorindices having arbitrary wavelength and bandpass.
| On the nature of early-type stars in the galactic halo
Effective temperatures and surface gravities of 46 intermediate and highgalactic latitude OB stars have been estimated from Stromgren /C1/ andH-beta photometry. In addition, helium, calcium and magnesium abundancesrelative to hydrogen have been derived for eleven stars. The typicalOB-type stellar parameters indicate that they are normal Population Istars, several of which require ejection velocities of more than 100km/s from the plane in order to reach their present z-distances duringtheir lifetimes. The observed correlation between peculiar velocity andmass for stars with /z/ not less than 400 pc is compatible with theBlaauw runaway hypothesis as the acceleration mechanism. A plot of logN(Na I) versus E(B-V) implies that the gas-to-dust ratio in the halo isprobably similar to that found for material in the galactic disc. Thisimplies that the sputtering of calcium from grain surfaces is probablyresponsible for the observed velocity dependence of the N(Na I)/N(Ca II)ratio.
| 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
| On the relationship between the apparent magnitudes given in several catalogues and the UBV system.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974A&AS...15..215O&db_key=AST
| The Evolutionary Status of the Blue Halo Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973ApJS...26...37N&db_key=AST
| A Photometric Classification of the Feige Blue Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1970PASP...82.1305G
| The Nature of the Faint Blue Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1966ApJ...144..496G
| The Colors of Some High-Latitude Blue Stars.
Not Available
| An Atlas of Indentification Charts for 113 Blue Stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1959ApJ...129..600F
| A Search for Underluminous Hot Stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1958ApJ...128..267F
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