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An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| 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.
| CaII K interstellar observations towards early-type disc and halo stars - distances to intermediate- and high-velocity clouds
We compare existing high spectral resolution(R=λ/Δλ~ 40000) CaII K observations(λair= 3933.66 Å) towards 88 mainly B-typestars, and new observations taken using the Intermediate dispersionSpectrograph and Imaging System (ISIS) on the William Herschel Telescopeat R~ 10000 towards three stars taken from the Palomar-Green Survey,with 21-cm HI emission-line profiles, in order to search for opticalabsorption towards known intermediate- and high-velocity cloudcomplexes. Given certain assumptions, limits to the gas phase abundanceof CaII are estimated for the cloud components. We use the data toderive the following distances from the Galactic plane (z). (i)Tentative lower z-height limits of 2800 and 4100 pc towards complex Cusing lack of absorption in the spectra of HD341617 and PG0855+294,respectively. (ii) A weak lower z-height of 1400 pc towards complexWA-WB using lack of absorption in EC09470-1433 and a weak lower limit of2470 pc using lack of absorption in EC09452-1403. (iii) An upperz-height of 2470 pc towards a southern intermediate-velocity cloud (IVC)with vLSR=-55 km s-1 using PG2351+198. (iv)Detection of a possible IVC in CaII absorption at vLSR=+52 kms-1 using EC20104-2944. No associated HI in emission isdetected. At this position, normal Galactic rotation predicts velocitiesof up to ~+25 km s-1. The detection puts an upper z-height of1860 pc to the cloud. (v) Tentative HI and CaII K detections towards anIVC at ~+70 km s-1 in the direction of high-velocity cloud(HVC) complex WE, sightline EC06387-8045, indicating that the IVC may beat a z-height lower than 1770 pc. (vi) Detection of CaII K absorption inthe spectrum of PG0855+294 in the direction of IV20, indicating thatthis IVC has a z-height smaller than 4100 pc. (vii) A weak lowerz-height of 4300 pc towards a small HVC with vLSR=+115 kms-1 at l, b= 200°, + 52°, using lack of absorption inthe CaII K spectrum of PG0955+291.
| Early-type stars in the Galactic halo from the Palomar-Green survey-III. Completion of a magnitude range limited sample
High-resolution (R~ 40000) echelle spectroscopic observations of 13high-latitude early-type stars are presented. These stars comprise thefinal part of a complete magnitude range limited sample based onlow-resolution spectroscopy of targets drawn from the Palomar-Greensurvey. The magnitude range under consideration is 13<=BPG<= 14.6, corresponding to an approximate distancelimit for main-sequence B-type objects of 5 <=d<= 40 kpc. Threestars are found to be apparently normal, young stars, based on theirpositions on the (Teff, log g) diagram, normal abundancepatterns and relatively large projected rotational velocities. A furtherstar, PG 1209+263, was found to belong to the chemically peculiar (CP)silicon star class of objects. The remainder are evolved subluminousstars lying on post-horizontal branch (post-HB) tracks, with theexception of PG 2120+062, which appears to be in a post-asymptotic giantbranch evolutionary stage. For the young stars in the sample, we havederived distance and age estimates through comparison of the atmosphericparameters with recent theoretical evolutionary models. We discussformation scenarios by comparing times-of-flight and evolutionarytime-scales. It is found that all stars could have formed in theGalactic disc and been ejected from there soon after their birth, withthe exception of PG 1209+263. The adopted proper motion is found to be acrucial factor in the kinematical analysis. We also present some numberdensities for young B-type halo stars, which indicate that they areextremely scarce objects.
| CaII K interstellar observations towards early-type disc and halo stars
We present high-resolution (R=λ/Δλ~ 40000) CaII Kinterstellar observations (λair= 3933.66Å)towards 88 mainly B-type stars, of which 74 are taken from theEdinburgh-Cape or Palomar-Green surveys, and 81 have |b| > 25°.The majority of the data come from previously existing spectroscopy,although also included are 18 new observations of stars with echellespectra taken with UVES on the Very Large Telescope UT2 (Kueyen). Some49 of the sample stars have distance estimates above the Galactic plane(|z|) >= 1 kpc, and are thus good probes of the halo interstellarmedium. Of the 362 interstellar Ca K components that we detect, 75 (21per cent) have absolute values of their LSR velocity values exceeding 40km s-1. In terms of the deviation velocity for the sightlineswith distance estimates, 46/273 (17 per cent) of components havevelocity values exceeding those predicted by standard Galactic rotationby more than 40 km s-1. Combining this data set with previousobservations, we find that the median value of the reduced equivalentwidth (REW) of stars with |z| >= 1 kpc (EW×sin|b|) is ~115mÅ (n= 80), similar to that observed in extragalactic sightlinesby Bowen. Using data of all z distances, the REW at infinity is found tobe ~130 mÅ, with the scaleheight (l) of the CaII K column densitydistribution being ~800 pc (n= 196) and reduced column density atinfinity of log[N(CaII K) cm-2]~12.24. This implies that ~30per cent of CaII K absorption occurs at distances exceeding ~1 kpc. Fornine sightlines with distance exceeding 1 kpc and with a companionobject within 5°, we find that all but two have values of CaIIreduced equivalent width the same to within ~20 per cent, when the REWof the nearest object is extrapolated to the distance of the further ofthe pair, and assuming l= 800 pc. For 29 of our sightlines with |z|>= 1 kpc and a HI detection from the Leiden-Dwingeloo survey(beamsize of 0.5°), we find log(N(CaII K)/N(HI)) ranging from -7.4to -8.4. Values of the CaII K abundance relative to neutral hydrogen(log[N(CaIIK)cm-2]-log[N(HI)cm-2]) are found to bemore than ~0.5dex higher in stars with distances exceeding ~100 pc, whencompared with the (log[N(CaII K) cm-2]-log[N(Htot) cm-2]) values found in nearbysightlines such as those in Wakker & Mathis (2000). Finally, stellarCaII K equivalent widths of the sample are determined for 26 objects.
| 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.
| FAUST observations near the North Galactic Pole
We analyse a UV observation with FAUST in the direction of the NorthGalactic Pole. The region includes a cirrus cloud (G251.2+73.3) and adark globule, and the FAUST image contains 75 UV sources. We discuss theUV source detection and their identification with optical counterparts.We use, for the first time, low-resolution spectral information as theprimary means of identifying possible optical counterparts. This iscomplemented, and sometimes modified, by optical information availablefrom existing data bases. The results are interpreted with the help ofmaps of the distribution of far-infrared emission and of the neutralhydrogen gas. We discuss the types of objects found, the degree ofmatching with the predictions of our UV Galaxy model, and the generalbehaviour of the Galactic UV extinction in this Milky Way part. Wecompare the UV results for this region with similar observations in thesame neighbourhood, which are less affected by dust, and attempt toexplain the peculiar distribution of UV magnitudes as a result of apeculiar distribution of foreground dust, which does not follow theaccepted dust-to-gas relation.
| Strömgren uvby photometry of B-type stars from the Palomar-Green Survey
We present Strömgren uvby photometry for a sample of 31 highGalactic latitude stars selected from the Palomar-Green Survey. The datainclude photometric magnitudes accurate to <= 0.01 mag in most cases,plus colours and the reddening free [c_1] and [u-b] indices, whichpossess a precision of better than 0.02 and 0.04 mag, respectively. Thelatter should be suitable for the reliable determination of stellarphotometric temperatures.
| Early-type stars in the Galactic halo from the Palomar-Green Survey II: A sample of distant, apparently young Population I stars
We present échelle (R ~ 40 000) spectroscopic observations for asample of apparently normal, high Galactic latitude, early-type starsdrawn from the Palomar-Green Survey. The metal-line spectra showevidence for rotational velocity broadening with values of vsini<=300 km s(-1) . In conjunction with Kurucz model atmospheres, wederive stellar photospheric abundances that are consistent with aPopulation i chemical composition; differential abundances with respectto Galactic disk Population i stars indicate no abundance differencesoutside the estimated errors. From a comparison of the derivedatmospheric parameters with recent theoretical evolutionary models, wederive distance and age estimates for individual stars. Usingkinematical considerations, we conclude that all these objects are`runaway' stars, formed in the Galactic disk and subsequently ejected,possibly by supernovae explosions or dynamical interactions. Tables 4and 5 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| A catalog of far-ultraviolet point sources detected with the fast FAUST Telescope on ATLAS-1
We list the photometric measurements of point sources made by the FarUltraviolet Space Telescope (FAUST) when it flew on the ATLAS-1 spaceshuttle mission. The list contains 4698 Galactic and extragalacticobjects detected in 22 wide-field images of the sky. At the locationssurveyed, this catalog reaches a limiting magnitude approximately afactor of 10 fainter than the previous UV all-sky survey, TDl. Thecatalog limit is approximately 1 x 10-14 ergs A sq cm/s,although it is not complete to this level. We list for each object theposition, FUV flux, the error in flux, and where possible anidentification from catalogs of nearby stars and galaxies. Thesecatalogs include the Michigan HD (MHD) and HD, SAO, the HIPPARCOS InputCatalog, the Position and Proper Motion Catalog, the TD1 Catalog, theMcCook and Sion Catalog of white dwarfs, and the RC3 Catalog ofGalaxies. We identify 2239 FAUST sources with objects in the stellarcatalogs and 172 with galaxies in the RC3 catalog. We estimate thenumber of sources with incorrect identifications to be less than 2%.
| The pulsations of yellow semiregular variables. I - Double-mode behaviour of UU Herculis
Long series of visual and photoelectric observations of UU Her since1908 has been analyzed. It is shown that UU Her is a double-modesemiregular variable, the two excited modes of pulsation are probablythe fundamental and the first overtone. A simple model is presented toexplain the amplitude variations observed in both modes. The model whichassumes pure sinusoidal amplitude variation is quite successful inexplaining the power spectrum of the variable. The question of the massof UU Her is also discussed. It is argued that no observation of thestar so far points unambiguously to its having a low mass.
| 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.
| Infrared observations of high galactic latitude early-type stars
Infrared photometry of four relatively faint (V = 8.5 - 11.0) early-typestars at high galactic latitude has been made in the J, H, K, L-primeand narrow-band M wavelengths. This has been combined with previouslypublished ultraviolet and visible observations to enable a comparisonwith LTE model atmosphere fluxes. The results imply that at least threeof the stars are normal main-sequence B-stars at large distances fromthe galactic plane.
| 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.
| 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
| Peculiar and normal early-type stars in the galactic halo
High resolution optical spectra of eight high latitude blue stars havebeen obtained. Three of the sample are identified as subdwarfs. Theremaining five are analyzed for atmospheric parameters and metal and Heabundances using model atmosphere techniques. Four of the sample appearto be normal B type stars 2 kpc or more from the plane. With alogarithmic surface gravity estimated to be 4.6 + or - 0.3 dex, PB 166could be either a main sequence, horizontal branch or cool subdwarf. Thenature of this star is discussed in view of its approximately normalPopulation I chemical composition.
| 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.
| A camera for astronomical CCD spectroscopy
A 100-mm aperture, visible-UV transmitting f/3.6 camera is described foruse with detectors requiring a substantial back-focal distance. Thiscamera has been installed on the Cassegrain spectrograph of the 1.6-mtelescope of the Mount Mégantic Observatory with atwo-dimensional CCD detector. Examples of spectra taken with this systemare presented.
| 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.
| The Palomar-Green catalog of ultraviolet-excess stellar objects
The Palomar-Green Catalog of Ultraviolet Excess Stellar Objects ispresented, with data consisting of positions on the sky accurate toabout 8 arcsec in each coordinate, photographic B-magnitudes accurate to0.29 mag, spectral types, some cross references, and photoelectricbroadband, multichannel, and Stromgren colors when available. Extensivediscussion is given on magnitude calibration using a combination ofphotoelectric measurements and star count modeling, and on thestatistical completeness of the sample based on internal and externaltests. Of the 1874 objects in the catalog, 1715 comprise a statisticallycomplete sample covering 10,714 square degrees from 266 fields taken onthe Palomar 18-inch (46-cm) Schmidt telescope. Limiting magnitudes varyfrom field to field, distributed around B = 16.1 mag, ranging from 15.49to 16.67. The dominant population is that of the hot, hydrogenatmosphere subdwarfs, the sdB stars, which comprise nearly 40 percent ofthe sample. At 16th magnitude, the hot subdwarf (sdB, sdO) number countsare increasing by about a factor of 2 per mag, the hot white dwarfs by 4per mag, and quasars by 8 per mag. The result is a very different mix ofspectral types expected at 18th mag than is found at 16th mag. Thesuggestion is made that the sdB stars are part of the old disk ratherthan a halo population.
| 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.
| Photoelectric photometry of stars near the north galactic pole
Abstract image available at:http://adsabs.harvard.edu/abs/1973A&AS...12..381H
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