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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.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| 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.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Model atmosphere and kinematical analyses of early-type stars from the Edinburgh-Cape Survey
We present high-resolution spectroscopic observations of 21 B-typestars, selected from the Edinburgh-Cape Blue Object Survey. Modelatmosphere analyses confirm that 14 of these stars are young,main-sequence B-type objects with Population I chemical compositions.The remaining seven are found to be evolved objects, includingsubdwarfs, horizontal branch and post-AGB objects. A kinematicalanalysis shows that all 14 young main-sequence stars could have formedin the disc and subsequently been ejected into the halo. These resultsare combined with the analysis of a previous subsample of stars takenfrom the Survey. Of the complete sample, 31 have been found to be young,main-sequence objects, with formation in the disc, and subsequentejection into the halo, again being found to be a plausible scenario.
| Rotational velocities of B-type stars from the Edinburgh-Cape survey
The projected rotational velocity distribution for a sample of 34 highGalactic latitude B-type stars from the Edinburgh-Cape Faint Blue ObjectSurvey is presented to investigate the evolutionary status of the groupas a whole. Statistical analyses of the distribution show it to besimilar to that expected if the sample contained mainly normalPopulation i early B-type stars, although a contamination of up to 20 %by evolved stars cannot be ruled out. This implies that a large fractionof the sample consists of normal Population i B-type stars similar tothose found in the Galactic disk. Possible mechanisms explaining thepresence of these stars in the halo are briefly discussed.
| Apparently normal, high-latitude B stars from the Edinburgh-Cape survey - II
Low-dispersion spectrograms and UBV photometry are used to carry out asimple analysis of a second sample of apparently normal, high galacticlatitude B stars detected in the Edinburgh-Cape faint blue objectsurvey. The sample is bigger than that in Paper I and contains morefaint stars. Of the 30 stars, eight are classified as `weak-line' starsand appear very similar to stars in the blue horizontal branch ofglobular clusters. A further eight stars are sdB stars (including twopossible binaries), and 11 stars appear to be normal B stars. Nine ofthese have distances from the Galactic plane in the approximate range1<=z<=4 kpc, and two are probably much moredistant. These results, and those of Paper I, indicate that B stars atlarge distances from the Galactic plane (z<~5kpc) must be very rare. Three stars do not fall into obvious categories;two of them are helium-rich and all may be hot subdwarfs.
| An Extension of the Case-Hamburg OB Star Surveys
We have extended the Case-Hamburg OB star surveys to b = +- 30 degreesfor l = +- 60 degres using the Curtis Schmidt telescope and 4 degreeobjective prism at the Cerro Tololo Inter-American Observatory. Acatalog of 234 OB stars and other objects with peculiar spectra ispresented along with finding charts for those objects too faint to beincluded on the BD or CD charts. (SECTION: Stars)
| 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.
| On the origin of intermediate-latitude OB stars
An attempt is made to trace the origin of early-type stars observed atappreciable distances from the galactic plane. Because uncertainties inthe proper motions make space motions and hence dynamical lifetimesrather inaccurate, a theory of oscillations normal to the plane has beenused to compute radial velocities for 138 intermediate-latitude OBstars. These theoretical values are then compared with the observedradial velocities, and it is found that the low-velocity stars wereprobably ejected from the plane some time after formation, while thehigh-velocity stars were ejected very soon after formation. Velocitiesof ejection perpendicular to the plane are computed and show a narrowdistribution with a mean absolute value of 7 km/s together with a spreadof velocities from about 40 to over 200 km/s. The data are in reasonableagreement with a 'sling' effect and 'runaway' origin for the stars inthe sample.
| Radial velocities of southern early-type stars at intermediate galactic latitudes.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975MNRAS.172..649K&db_key=AST
| H-beta photometry of southern early-type stars and galactic structure away from the plane
H-beta photoelectric photometry is reported for 165 early-type stars atintermediate and high galactic latitudes. The data are combined withearlier UBV and spectroscopic results to determine the stellar spacedistribution. Stars of type B2 and earlier, at distances of up to 1 kpcfrom the galactic plane, appear to follow spiral structure in the plane.The available material, particularly the derived color excesses, is usedto select a number of blue stars which may be subluminous.
| UBV photometry of southern early-type stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974MNRAS.168..451H&db_key=AST
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| קבוצת-כוכבים: | אוקטנט |
| התרוממות ימנית: | 19h14m10.72s |
| סירוב: | -76°38'14.2" |
| גודל גלוי: | 10.247 |
| תנועה נכונה: | 16.4 |
| תנועה נכונה: | -21.9 |
| B-T magnitude: | 9.991 |
| V-T magnitude: | 10.226 |
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