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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.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| 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.
| 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.
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| IUE Absorption-Line Observations of the Moderately and Highly Ionized Interstellar Medium toward 164 Early-Type Stars
We present measurements of Galactic interstellar Al III, Si IV, and C IVabsorption recorded in high-resolution archival ultraviolet spectra of164 hot early-type stars observed by the International UltravioletExplorer (IUE) satellite. The objects studied were drawn from the listof hot stars scheduled to be observed with the Far UltravioletSpectroscopic Explorer (FUSE) satellite as part of observing programsdesigned to investigate absorption by O VI in the Galactic disk andhalo. Multiple IUE echelle-mode integrations have been combined toproduce a single ultraviolet (1150-1900 Å) spectrum of each starwith a spectral resolution of ~25 km s-1 (FWHM). Selectedabsorption-line profiles are presented for each star along with plots ofthe apparent column density per unit velocity for each line of the AlIII, Si IV, and C IV doublets. We report absorption-line equivalentwidths, absorption velocities, and integrated column densities based onthe apparent optical depth method of examining interstellar absorptionlines. We also determine column densities and Doppler parameters fromsingle-component curve-of-growth analyses. The scientific analysis ofthese observations will be undertaken after the FUSE satellite producessimilar measurements for absorption by interstellar O IV, Fe III, S III,and other ions. Based on archival data from observations obtained withthe International Ultraviolet Explorer (IUE) satellite sponsored byNASA, SERC, and ESA.
| The 73rd Name-List of Variable Stars
Not Available
| Evidence for an extended neutral Galactic halo
Singly ionized titanium is the dominant ionization stage in H I regionsand provides an excellent tracer of the neutral interstellar medium.Here we present new interstellar Ti II absorption line observationstoward six halo stars located at absolute value of z greater than 1 kpc.Although the gas distribution is probably patchy, significant neutralgas does exist beyond 1 kpc from the Galactic plane and the projectedcolumn density, N(Ti II) sin (b), continues to increase out to the mostdistant star observed, at z = 8.7: kpc. Along this line of sight, the TiII absorption is the strongest yet detected and also has the highestgas-phase abundance of titanium seen in interstellar gas. Ti IIabsorption observed toward halo stars may arise from a new component ofthe halo interstellar medium which contains low column density neutralgas over a wide velocity range and which may extend beyond several H Iscale heights from the plane. The scale height of this component has yetto be determined, but could be well beyond several kpc, coincident withor even farther than the highly ionized gas observed in C IV and Si IV.Observations toward a large sample of extragalactic sources are neededto determine the actual scale height of neutral halo gas.
| The distribution of neutral hydrogen in the interstellar medium. 1: The data
We compile, from the existing literature, the largest sample to date(842 data points) of hydrogen column density measurements, N(H I), ofthe gas in the interstellar medium. We include only results obtainedfrom absorption measurements toward individual stars (594 in our sample)in an effort to construct a three-dimensional picture of theinterstellar gas. We derive hydrogen column densities toward a fractionof the stars in the sample from published column density measurements ofmetal ions. A three-dimensional physical model derived from this dataset will be presented in a companion paper. The observed stars spandistances from a few parsecs to a few thousand parsecs, and more thanhalf of the sample serves to describe the local interstellar mediumwithin a few hundred parsecs of the Sun. Hydrogen column densities rangefrom 1017 to 1022/sq cm. We describe here thevarious observational methods used to estimate the hydrogen columndensities and present the table with the stellar and hydrogen columndensity data. The provided table is intended as a global reference work,not to introduce new results.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| A Search for Beta-Cephei Type Variability in a Sample of Intermediate Galactic Latitude to High Galactic Latitude B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.267.1103H&db_key=AST
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
| 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 distribution of interstellar AL III away from the Galactic plane
IUE spectra are analyzed to study the density distribution ofinterstellar Al III away from the Galactic plane. In most cases, themeasured values of the relative line strengths are consistent with onlymodest levels of line saturation. Al III is found to have an exponentialscale height and 1 sigma errors of 1.02(+0.36, -0.24) kpc. For the sameset of 70 stars, the scale height and 1 sigma errors for H I are0.67(+0.21, -0.16) kpc. The Al III scale height is similar to the valueobtained for free electrons from pulsar dispersion measures. The ionizedgas traced by Al III is somewhat more extended than the neutral gastraced by H I but less extended than the very highly ionized gas tracedby Si IV, C IV, and N V.
| 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.
| Estimation of the equivalent width of the interstellar CA II K absorption line
Literature values for the equivalent width of the interstellarabsorption line of Ca II K (3933 A) are used to constrain a simpleempirical model for the strength of this line as a function of distancefrom the galactic plane. The best-fit model has an integrated Ca II Kequivalent width in the direction perpendicular to the galactic plane ofapproximately 200 mA and a 'scale height' of approximately 1100 pc. Sucha model is useful for estimating (and correcting for) the contributionof the interstellar Ca II K feature to the total observed Ca II K linestrength in the spectra of metal-deficient stars in the galactic halo.
| Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
| The density distribution of refractory elements away from the Galactic plane
The density distributions of the three refractory elements Ti II, Ca II,and Fe II away from the Galactic plane are compared with thedistribution of hydrogen and dust by examining plots of N s in b versusz. It is found that Ti II and Ca II are considerably more extended in zthan the H I and dust and that Fe II has an intermediate extension.Although the results are strongly influenced by sample bias, theindicated exponential scale heights for the data sample are h(Ti II) notless than 2 kpc, h(Ca II) = 1 kpc, h(Fe II) = 0.5 kpc, H(H I) = 0.3 kpc,and h(E/B-V) = 0.1 kpc. Furthermore, it is demonstrated that Ti II andCa II are much more smoothly distributed in space than the hydrogen ordust. The large scale heights for Ti II and Ca II and their smoothdistributions are most easily understood as the effect of a mixturealong the line of sight of two H I phases namely, a diffuse cloud phase,in which nearly all of the Ti and Ca are tied up in dust, and anintercloud medium, where refractory elements are less depleted. It isfound that Ti II and Ca II mostly trace the smoothly distributedintercloud medium. The smoothness of the distributions of Ti II and CaII makes them candidates for use as distance indicators.
| PHL 346, a Beta Cephei star situated at more than 5 KPC from the Galactic plane?
The high-latitude early-B star PHL 346 is a pulsating variable with aperiod of 0.152 days. This period, the fact that color and brightnessvary in phase, and the atmospheric parameters of PHL 346 suggest that itis a ? Cephei variable. The star would then have a mass of about 10solar masses and an age of about 107years. PHL 346 is faint(mv = 11.44) and is located at a high galactic latitude of-58°, so that it appears excluded that it originated in the galacticplane only 107yr ago. The pulsation of PHL 346 thus lendssupport to the idea that star formation far from the plane of the galaxycan occur. It is argued, more generally, that the study of thevariability of high-latitude B stars provides a strong test for thenature of these stars.
| Interstellar CA II absorption toward early-type stars at high Galactic latitudes
New high resolution AAT and INT observations of the Ca II 3933 Åinterstellar line towards nine early-type stars at high galacticlatitudes are presented. These are combined with the authors' earlierAAT data to investigate the distribution of Ca II in the galactic halo.A comparison of these results with Ca II equivalent widths forextragalactic sightlines implies that there is a significant amount ofthis ion out to z ≅ 1 kpc, but not much beyond z ≅ 2 kpc. Nocoincident high velocity Ca II absorption is detected in the spectra ofstars near known high velocity clouds (HVC). Although this may indicatethat the HVCs are either at greater distances than the stars or haveangular extents smaller than the separations of the stellar and HVCsightlines, it is also possible that they are nearby but contain toolittle Ca II to produce an observable absorption line.
| A catalog of ultraviolet interstellar extinction excesses for 1415 stars
Ultraviolet interstellar extinction excesses are presented for 1415stars with spectral types B7 and earlier. The excesses with respect to Vare derived from Astronomical Netherlands Satellite (ANS) 5-channel UVphotometry at central wavelengths of approximately 1550, 1800, 2500, and3300 A. A measure of the excess extinction in the 2200-A extinction bumpis also given. The data are valuable for investigating the systematicsof peculiar interstellar extinction and for studying the character of UVinterstellar extinction in the general direction of stars for which theextinction-curve shape is unknown.
| Interstellar CA II and NA I line profiles towards halo OB stars
High resolution Ca II and Na I interstellar line profiles towardsseveral halo OB stars are presented. The profiles have been correctedwhere necessary for the presence of stellar features, and analyzed usingmulticomponent models to derive information on the radial velocities,internal velocity dispersions, and column densities of individualinterstellar clouds. A method is described for estimating peculiarvelocities for the clouds, and a significant trend of decreasing R =N(Na I)/N(Ca II) with increasing peculiar velocity is found. The ratio,R, is also shown to be generally smaller in the halo than in the plane,implying that many of the halo clouds may have peculiar velocities. Thedata indicate that there are both few clouds and a small Ca II densitybeyond the absolute z value of 2 kpc.
| A study of interstellar absorption at high galactic latitudes. I - Highly ionized gas
IUE interstellar absorption line data for C IV, Si IV, N V and Fe II, ina sample of 24 distant, early-type stars at high galactic latitudes, areanalyzed with other published results in order to study the distributionand physical conditions of highly ionized gas in the halo of the Galaxy.The C IV and Si IV lines, which are detected in front of approximatelytwo-thirds of the stars observed, are significantly stronger that thoseobserved in the spectra of disk stars of similar spectral types, andtheir velocities show no correlation with the velocities of thebackground stars. The observed variation of the column densities of C(3+) and Si (3+) with distance z from the plane is consistent with thesuggested existence of a Galactic transition layer between cool gas inthe disk and outer halo regions at temperatures greater than thoseobservable with IUE. Attention is drawn to the fact that the observed C(3+)/Si (3+) ratio may be a characteristic signature of hot gas ingalactic halos.
| Atmospheric parameters and chemical compositions of eighteen halo OB stars
Stellar equivalent widths and He line profiles, measured from 5 and 10A/mm IPCS spectra obtained at the AAT, are presented for 18 halo OBstars. Effective temperatures and gravities have been estimated usingStromgren and H-beta photometry in conjunction with these data. Alsoderived are the abundances relative to hydrogen of helium, carbon,nitrogen, oxygen and calcium. From the normal chemical compositions andatmospheric parameters obtained, it is concluded that the stars are notsubluminous and show no evidence of any other peculiarities. Usingconservative and nonconservative evolutionary tracks, masses, ages anddistances are deduced for the stars, implying that several of them havebeen ejected from the galactic plane with velocities in excess of 100km/s.
| List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry
A list is presented of 333 stars, excluded from the GCVS and itssupplements, whose probability of variability ranges from high tocertain. The standard deviations observed in the V magnitude togetherwith the known spectral types, however, only allow speculation as to thetype of variable in question pending supplementary observations whichreveal the individual characteristics of these stars.
| On the apparent normality of O and B stars far from the galactic plane
Results of a search for subluminosity or other peculiarities in thespectra of apparently normal OB stars located at large distances fromthe galactic plane are presented. Photographic spectrograms werecompared for stars of type B5 and earlier located within 0.5 kpc of andfurther than 1.5 kpc from the galactic plane, and for three subdwarfstars. Values of the projected axial rotation, surface gravity, absolutemagnitude and the strengths of certain He I and Balmer lines determinedfor both groups of OB stars are found to be similar, and distinct fromthe spectral characteristics of the subdwarfs. The results suggest that,unless the normality of the distant OB stars is illusory, either OBstars can be formed outside the galactic plane, or a mechanism mustexist to eject them from the plane with large velocities, unless theirmain sequence lifetimes are substantially longer than currentlybelieved.
| 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.
| 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.
| 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.
| Photometry and spectral classification of early-type stars away from the galactic plane.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970MNRAS.150...23H&db_key=AST
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| Yükselim: | -31°56'20.5" |
| Görünürdeki Parlaklýk: | 8.765 |
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