Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
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Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
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A study of some stars with circumstellar dust envelopes. I
We present the results of a study of circumstellar dust envelopes of 36stars of early(O-B-A) types in the directions of the associations CasOB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640Å, the linear radius of the dust envelopes, the mean value of thecoefficient k, and the masses of the envelopes. They differsignificantly from one another.
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A new list of effective temperatures of chemically peculiar stars. II.
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Far-ultraviolet stellar photometry: A field in Orion
Far-ultraviolet photometry for 625 objects in Orion is presented. Thesedata were extracted from electrographic camera images obtained duringsounding rocket flights in 1975 and 1982. The 1975 images were centeredclose to the belt of Orion while the 1982 images were centeredapproximately 9 deg further north. One hundred and fifty stars fell inthe overlapping region and were observed with both cameras. Sixty-eightpercent of the objects were tentatively identified with known starsusing the SIMBAD database while another 24% are blends of objects tooclose together to separate with our resolution. As in previous studies,the majority of the identified ultraviolet sources are early-type stars.However, there are a significant number for which no such identificationwas possible, and we suggest that these are interesting objects whichshould be further investigated. Seven stars were found which were brightin the ultraviolet but faint in the visible. We suggest that some ofthese are nearby white dwarfs.
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The Orion complex according to observations of the space telescope Glazar.
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Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
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Stars of Early Types in the Region of Stellar Association OB1 Observed with the Glazar Telescope
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IUE observations of the broad continuum feature at 1400 A in the silicon and related stars
The upper main sequence chemically peculiar stars have previously beenstudied spectrophotometrically using low-resolution TD-1 spectra. Theseshow that the broad continuum feature at 1400 A is a useful indicator ofSi anomaly. In this paper the observations are extended to IUElow-resolution spectra and it is shown that this feature is indeed wellcorrelated with Si and also with optical chemical peculiarity indices.Its utility in selecting silicon stars is further demonstrated. Thisfeature is likely due to autoionization of Si II and is the bestavailable discriminator of the overabundance of a key element in the Apstars.
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Statistical Investigation of Chemically Peculiar Stars - Part Four - Luminosity of Different Type Stars
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Statistical Investigation of Chemically Peculiar Stars - Part Three - Acceleration of Gravity and Mass of Stars from Hydrogen Line Profiles
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On the Effective Temperatures of Chemically Peculiar Stars
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Statistical Investigation of Chemically Peculiar Stars - Part Two - the Stars with the Dispersion of Continuum Spectrum at Lambda 5200A
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Observational problems of investigation of stars with anomalous helium lines.
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Absolute photometry of the southern Orion region in the vacuum ultraviolet (1300-2000 A)
Absolute photometry of the southern Orion region was carried out forfive passbands in the vacuum ultraviolet (1300-2000 A, Delta Lambda = 84A) with a rocket-borne spectrometer. More than 60 early-type stars andthe diffuse background radiation were observed within the raster-scannedfield of 8 x 8 deg centered at alpha = 5 h 30 m and delta = -4 deg.Stellar data suggest a revision of the current absolute scale by 10-30percent, which relatively suppresses the flux toward shorterwavelengths. The present data of the background radiation have thehighest spectral resolution among those published and indicate a steepsystematic increase of the flux toward shorter wavelengths. Directconsequences of the new calibration are briefly discussed.
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Photometric evidence for the appearance of the lambda 5200 flux depression in helium-weak stars
Photoelectric photometry in the system defined by Maitzen (1976) wascarried out for a sample of 20 population I He-weak and He-strong starsin order to look for the presence of the 5200 A flux depression featurewhich is typical for cooler magnetic Ap-stars. The majority of theHe-weak stars show a modest, but definite enhancement of thephotoelectric depression parameter, while the He-rich stars areobviously too hot for exhibiting this feature.
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A photometric study of the Orion OB 1 association. III - Subgroup analyses
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...36..497W&db_key=AST
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Spectral types for early-type stars observed by SKYLAB
MK spectral types are presented for 246 early-type stars observed withthe S-019 ultraviolet stellar astronomy experiment on Skylab. K-linetypes are also given where applicable, and various peculiar stars areidentified. The peculiar stars include five silicon stars, a shell star,a helium-rich star, a silicon-strontium star, a chromium-europium star,and two marginal metallic-line stars.
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A photometric study of the Orion OB 1 association. I - Observational data.
A catalog of observational data is presented for stars in the region ofthe young stellar association Orion OB 1. Photoelectric observationsobtained in the uvby-beta and UBV systems are compiled along withprevious photoelectric and spectroscopic data for all these stars aswell as for several bright members of the association with availablephotometric indices. Mean weighted values are computed for thephotometric data and summarized in tables expected to be reasonablycomplete for association members earlier than spectral type A0.Membership criteria are derived, and qualitative membershipprobabilities summarized, for the 526 stars in the final program. Theanalytical procedures are discussed for association stars of B,intermediate, and AF types. Effects of the nebular environment andvarious calibrations of Balmer-line and four-color indices areconsidered for the determination of absolute magnitudes for the B-typestars.
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Evidence of helium abundance differences between young groups of stars
Seven groups of B-type stars - Sco-Cen, NGC 6231, LacOBI,OriOBIb, h + chi Per, CepOBIII, and field stars - wereinvestigated photoelectrically to determine whether helium abundance isuniform in the solar neighborhood and in a region of the spiral arm ofthe Galaxy. Observations were performed using an echelle spectrometer inthe narrow band index of the I (4026) of the He I-4026 line. Amodel-atmosphere analysis of I (4026) and the beta index of the H betaline shows that in the spectral range B O-B 2, the relation between I(4026) and beta is insensitive to differences in the surface gravity butsensitive to differences in the helium to hydrogen ratio. It is foundthat stars in the two groups most distant from the sun - h + chi Per andCepOBIII - have a lower helium abundance than stars in the solarneighborhood.
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Helium abundance in atmospheres of stars appearing in OB associations and young clusters
An improved version of Liubimkov's (1974) method is used to determinethe relative helium abundance, N(He)/N(H), in the atmospheres of 80stars of spectral types O6.5 to B2 belonging to OB associations andyoung clusters. Specific results are presented for Mon OB1 and OB2, CepOB2 and OB3, Sco OB2, Lac OB1, Ori OB1, and Cas OB6. It is shown thatthe mean value of N(He)/N(H) increases with increasing stellar age andincreasing cluster or association size. A relative helium abundance of0.09 to 0.10 is found to be appropriate for stars that are severalmillion years old, and a relative abundance of 0.13 is obtained forstars older than 10 million years. It is noted that the latter value isalso characteristic of hot field stars.
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The Helium-Weak Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972ApJ...175..453M&db_key=AST
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On magnetic stars.
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Neutral-Helium Line Strengths. V. The Weak-Helium Stars of Population I
Abstract image available at:http://adsabs.harvard.edu/abs/1971ApJS...23..213N
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Zeeman Measures of Sharp-Lined Early B Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1970ApJ...159..723C
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Helium-Weak Stars
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Interstellar extinction in the Orion association
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1968ApJ...152..913L&db_key=AST
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On the spectrum-color discrepancy of some late B stars in Orion.
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The Existence of Magnetic Fields in Two Peculiar B Stars in Orion
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Photoelectric Hbeta photometry for B stars in the Orion belt region
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966AJ.....71..610C&db_key=AST
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