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A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| β Cephei stars in the ASAS-3 data. I. Long-term variations of periods and amplitudes
Aims.We analyse V-filter ASAS-3 photometry of 41 known βCephei-type stars. The ASAS-3 photometry is combined with the archivaldata, if available, to determine long-term stability of periods andamplitudes of excited modes. Methods: Frequencies of modes are derivedby means of Fourier periodograms with consecutive prewhitening. Theresults are examined in the context of detection threshold. Results: Wedetected amplitude changes in three β Cephei stars, BW Cru, V836Cen, and V348 Nor. Period changes were found in KK Vel and V836 Cen. Ouranalysis shows that intrinsic period changes are more common amongmultiperiodic stars, apparently because they are caused by some kind ofmode interaction. In addition, we found new modes for seven stars, andfor ten others we provide new solutions or remove ambiguities in thedetected frequencies. One candidate hybrid β Cephei/SPB star, HD133823, is discovered.Appendix A, Figs. 2-6 and Tables 2-6 are only available in electronicform at http://www.aanda.org Table V photometry for all 41 stars is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/477/907
| Observational Constraints on Interstellar Grain Alignment
We present new multicolor photopolarimetry of stars behind the SouthernCoalsack. Analyzed together with multiband polarization data from theliterature, probing the Chamaeleon I, Musca, ? Opiuchus, R CrA, andTaurus clouds, we show that the wavelength of maximum polarization(?max) is linearly correlated with the radiationenvironment of the grains. Using far-infrared emission data, we showthat the large scatter seen in previous studies of?max as a function of AV is primarily dueto line-of-sight effects causing some AV measurements to notbe a good tracer of the extinction (radiation field strength) seen bythe grains being probed. The derived slopes in ?maxversus AV, for the individual clouds, are consistent with acommon value, while the zero intercepts scale with the average values ofthe ratios of total to selective extinction (RV) for theindividual clouds. Within each cloud we do not find direct correlationsbetween ?max and RV. The positive slope isconsistent with recent developments in theory and indicating alignmentdriven by the radiation field. The present data cannot conclusivelydifferentiate between direct radiative torques and alignment driven byH2 formation. However, the small values of?max(AV=0), seen in several clouds, suggesta role for the latter, at least at the cloud surfaces. The scatter inthe ?max versus AV relation is found to beassociated with the characteristics of the embedded YSOs in the clouds.We propose that this is partially due to locally increased plasmadamping of the grain rotation caused by X-rays from the YSOs.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Photometric studies of three multiperiodic β Cephei stars: β CMa, 15 CMa and KZ Mus
We have carried out single and multisite photometry of the three βCephei stars β and 15 CMa as well as KZ Mus. For the two stars inCMa, we obtained 270h of measurement in the Strömgren uvy andJohnson V filters, while 150h of time-resolved Strömgren uvyphotometry was acquired for KZ Mus. All three stars are multiperiodicvariables, with three (β CMa) and four (15 CMa, KZ Mus) independentpulsation modes. Two of the mode frequencies of 15 CMa are newdiscoveries and one of the known modes showed amplitude variations overthe last 33yr. Taken together, this fully explains the diverse behaviourof the star reported in the literature.Mode identification by means of the amplitude ratios in the differentpassbands suggests one radial mode for each star. In addition, βCMa has a dominant l = 2 mode while its third mode is non-radial withunknown l. The non-radial modes of 15 CMa, which are l <= 3, form analmost equally split triplet that, if physical, would imply that we seethe star under an inclination angle larger than 55°. The strongestnon-radial mode of KZ Mus is l = 2, followed by the radial mode and adipole mode. Its weakest known mode is non-radial with unknown l,confirming previous mode identifications for the pulsations of the star.The phased light curve for the strongest mode of 15 CMa has a descendingbranch steeper than the rising branch. A stillstand phenomenon duringthe rise to maximum light is indicated. Given the low photometricamplitude of this non-radial mode this is at first sight surprising, butit can be explained by the aspect angle of the mode.
| Multiperiodicity in the large-amplitude rapidly-rotating β Cephei star HD 203664
Aims.We made a seismic study of the young massive β Cephei star HD203664 with the goal of constraining its interior structure.Methods.Ourstudy is based on a time series of 328 new Geneva 7-colour photometricdata of the star spread over 496.8 days.Results.The data confirm thefrequency of the dominant mode of the star, which we refined tof_1=6.02885 c d-1. The mode has a large amplitude of 37 mmagin V and is unambiguously identified as a dipole mode (&ea;=2) from itsamplitude ratios and non-adiabatic computations. Besides f_1, wediscovered two additional new frequencies in the star with amplitudesabove 4σ: f_2=6.82902 c d-1 and f_3=4.81543 cd-1, or one of their daily aliases. The amplitudes of thesetwo modes are only between 3 and 4 mmag, which explains why they werenot detected before. Their amplitude ratios are too uncertain for modeidentification. Conclusions.We show that the observed oscillationspectrum of HD 203664 is compatible with standard stellar models butthat we have insufficient information for asteroseismic inferences.Among the large-amplitude β Cephei stars, HD 203664 stands out asthe only one rotating at a significant fraction of its critical rotationvelocity (~ 40%).
| 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.
| Asteroseismology of the β Cep star HD 129929. II. Seismic constraints on core overshooting, internal rotation and stellar parameters
We perform a detailed seismic study of the β Cep star HD 129929.Our analysis is based on the recent derivation of six pulsationfrequencies. These frequencies are unambiguously identified from theseismic modelling and the photometric amplitudes to be the radialfundamental, the ℓ=1, p1 triplet, two consecutivecomponents of the ℓ=2, g1 quintuplet. A non-adiabaticanalysis allows us to constrain the metallicity of the star to Z \in[0.016, 0.022]. In addition, the fitting of three independentfrequencies, two of which correspond to axisymmetric (m=0) modes, allowsus to constrain the core overshooting parameter toαov=0.10 ± 0.05, as well as the other globalparameters of the star. Finally, from the observation of the ℓ=1triplet and part of the ℓ=2 quintuplet, we derive constraints on theinternal rotation of this star.
| Asteroseismology of the β Cephei star ν Eridani - II. Spectroscopic observations and pulsational frequency analysis
We undertook a multisite spectroscopic campaign for the β Cepheistar ν Eridani. A total of 2294 high-resolution spectra were obtainedfrom telescopes at 11 different observatories around the world. The timebase of dedicated multisite observations is 88 d. To this data set wehave added 148 older, previously unpublished spectra, such that theoverall time-span of the 2442 spectra is 430 d. The analysis of theradial velocity variations derived from the SiIII triplet centred on4560Å leads to 19 significant frequencies, of which sevencorrespond to independent pulsation frequencies. Five of these aremembers of multiplets with an average spacing of 0.018 +/- 0.002cd-1. Our spectroscopic results agree well with those derivedfrom a simultaneous multisite photometric campaign of the star, albeitthat we do not recover their low frequency at 0.43218 cd-1.We find three different candidate frequencies below 1 cd-1instead. We also find that the radial velocity amplitude of the mainmode has increased by some 30 per cent over the last 15 years, which isconsistent with the photometry data. We derive a relative equivalentwidth variation of 6.5 per cent, which is completely dominated by themain radial mode. The phase difference between the radial velocity andlight variations for the main frequency is , which is clearly deviantfrom the adiabatic value and confirms the radial nature of the dominantmode. The spectral line broadening leads to an upper limit of 20 kms-1 for vsini, which is consistent with the long rotationperiod derived from the frequency splittings.
| Asteroseismological studies of three β Cephei stars: IL Vel, V433 Car and KZ Mus
We have acquired between 127 and 150 h of time-resolved multicolourphotometry for each of the three β Cephei stars IL Vel, V433 Carand KZ Mus over a time-span of 4 months from two observatories. Allthree objects are multiperiodic with at least three modes of pulsation.Mode identification from the relative colour amplitudes is performed. Weobtain unambiguous results for the two highest-amplitude modes of IL Vel(both are l= 1) and the three strongest modes of KZ Mus (l= 2, 0 and 1),but none for V433 Car. Spectroscopy shows the latter star to be a fastrotator (v sin i= 240 km s-1), whereas the other two havemoderate v sin i (65 and 47 km s-1, respectively).We performed model calculations using the Warsaw-New Jersey stellarevolution and pulsation code. We find that IL Vel is an object of about12 Msolar in the second half of its main-sequenceevolutionary track. Its two dipole modes are most probably rotationallysplit components of the mode originating as p1 on thezero-age main sequence; one of these modes is m= 0. V433 Car issuggested to be an unevolved 13-Msolar star just entering theβ Cephei instability strip. KZ Mus seems less massive (~12.7Msolar) and somewhat more evolved, and its radial mode isprobably the fundamental one. In this case its quadrupole mode would bethe one originating as g1, and its dipole mode would bep1.Two of our photometric comparison stars also turned out to be variable.HD 90434 is probably a new slowly pulsating B-type star, the dominantmode of which is a dipole, whereas the variability of HD 89768 seems tobe a result of binarity.It is suggested that mode identification of slowly rotating βCephei stars based on photometric colour amplitudes is reliable; weestimate that a relative accuracy of 3 per cent in the amplitudes issufficient for unambiguous identifications. Owing to the good agreementof our theoretical and observational results we conclude that theprospects for asteroseismology of multiperiodic slowly rotating βCephei stars are good.
| 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.
| Statistics of the Instability Strip of β Cephei Stars
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
| 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.
| Follow-up photometry of six new beta Cephei stars discovered from the HIPPARCOS mission
We present ground-based follow-up photometry in the Geneva seven-colourphotometric system of six new candidate beta Cep stars. The variabilityof these objects was originally found from the HIPPARCOS mission. Fromthe new data we are able to confirm their beta Cep-type nature, todetect the pulsational frequencies and to identify most of the degreesof the pulsation modes. All six stars turn out to have frequencies inthe expected range for beta Cep stars and the degrees of the main moderange from l =0 up to l=2, which is compatible with theoreticalexpectations. Based on observations collected with the Swiss PhotometricTelescope of the Geneva Observatory situated at La Silla, Chile.Tables~5--10 are only in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5)
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Study of an unbiased sample of B stars observed with Hipparcos: the discovery of a large amount of new slowly pulsating B stars
We present a classification of 267 new variable B-type stars discoveredby Hipparcos. We have used two different classification schemes and theyboth result in only a few new beta Cephei stars, a huge number of newslowly pulsating B stars, quite some supergiants with alpha Cyg-typevariations and variable CP stars, and further some new periodic Be starsand eclipsing binaries. Our results clearly point out the biased naturetowards short-period variables of earlier, ground-based surveys ofvariable stars. The position of the new beta Cephei stars and slowlypulsating B stars in the HR diagram is determined by means of Genevaphotometry and is confronted with the most recent calculations of theinstability strips for both groups of variables. We find that the newbeta Cephei stars are situated in the blue part of the instability stripand that the new slowly pulsating B stars almost fully cover thetheoretical instability domain determined for such stars. Thesupergiants with alpha Cyg-type variations are situated between theinstability strips of the beta Cephei and the slowly pulsating B starson the one hand and previously known supergiants that exhibitmicrovariations on the other hand. This suggests some connection betweenthe variability caused by the kappa mechanism acting in a zone ofpartially ionised metals and the unknown cause of the variations insupergiants.
| The Spacelab-1 Very Wide Field Survey of UV-excess objects. IV. The performance of the instrument in combination with optical photometry as a means of identifying stars with peculiar properties.
UV (195 nm) and Stroemgren uvby photometry of a 110 square degree fieldat high southern galactic latitudes are analyzed through a comparison ofi) UV magnitudes for 57 stars of various types common to the publishedTD1 catalogue and the Very Wide Field Camera (VWFC); and ii) observedand theoretical two-colour diagrams. The higher sensitivity of the VWFC(=~0.5magnitude) and its more complete survey are exemplified by thedetection and UV measurement of a series of objects with moderateUV-excess in addition to detection of some very blue objects of variousnature down to fainter than 12th magnitude in the optical domain. Adeeper survey with a VWFC-type instrument could provide a completesample for studies of the group properties of faint blue stars. Duringthe uvby reductions it was found that the usual procedure of plottingresiduals as functions of declination, hour angle and airmass can be apowerful and diagnostic test of photometer rigidity.
| Multicolour Polarization and Co/ Observations Towards a Dark Filament in Musca
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.265....1A&db_key=AST
| UBV photometry for southern OB stars
New UBV photometry of 1227 OB stars in the southern Milky Way ispresented. For 1113 of these stars, MK spectral types have been reportedpreviously in a comprehensive survey to B = 10.0 mag.
| Catalog of Luminous Stars in the Southern Coalsack Zone
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1982RMxAA...5..183M&db_key=AST
| On the 4430 A interstellar band - A visual classification
A system of visual classification of the strength of the interstellarband 4430 A has been developed, based on spectra taken at spectralclassification dispersion. The intensity of the band was divided into 8main classes, defined by a group of selected standard stars. The systemwas applied to 1,111 southern OB stars contained in the catalog ofspectral types by Garrison, Hiltner, and Schild (1977). The mean errorin the classification was estimated to be plus or minus 1 class.Equivalent widths were measured for 100 stars; an excellent correlationwith the visual estimations was obtained. A calibration between thesetwo parameters is given.
| O I lambda 7773 absorption in optical counterparts of X-ray sources
The existence of a correlation between the absorption equivalent widthof the O I absorption triplet at 7773 A and stellar luminosity isconfirmed by using a sample of 14 B1-B5 stars. This correlation isemployed to investigate the luminosities of six X-ray sourcecounterparts of known spectral type. It is shown that measurement of theequivalent width of the O I 7773-A absorption feature offers a valuabletechnique for determining a star's luminosity class, that this techniquerequires only knowledge of a star's spectral type and is applicable totypes in the range from B1 to about G7, and that X-ray luminosities andsource distances may be obtained independently of other methods. Theidentification of the star MMV (= 4U 1258-61 = 2S 1258-613) as a shellstar is confirmed.
| 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.
| MK spectral classifications for southern OB stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977ApJS...35..111G&db_key=AST
| 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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