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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).
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Diffuse interstellar bands in the spectra of massive young stellar objects
We have compared the B - V color excess, E(B - V), obtained for a sampleof five optically visible massive young stellar objects (YSOs) both fromdiffuse interstellar bands (DIBs) in their spectra and from theiroptical continuum slopes. Our targets are HD 200775, BD+40.4124 deg, MWC1080, MWC 297 and MWC 349A. First, E(B - V) towards each of the targetsis derived by dereddening the observed continua to match those of B-typestandard stars. A survey of DIBs in the spectra of the massive YSOs, anda control field star, then reveals that the DIBs are significantlyweaker in the former than would be expected based on the total E(B - V)values. This result is strengthened by the finding that the DIBs in thecontrol field star, HD 154445, have on average the strength expectedfrom its continuum E(B - V). A rough estimate of the foregroundreddening of intervening diffuse interstellar medium shows it to besmaller than the DIB E(B - V), implying that at least part of the DIBcarriers are formed within the parental molecular clouds in which theYSOs are embedded. The DIB E(B - V) compares favorably with the totalE(B - V) towards BD+40.4124 deg, but is almost negligible in the line ofsight towards MWC 297. Despite this general, but not unexpected,deficit, we provide evidence that the DIB at 5849 A is a good tracer oftotal extinction in these lines of sight.
| MWC 297, B1.5Ve: a zero-age main-sequence star in the Aquila Rift
Moderate-resolution optical spectra of the significantly reddened HerbigBe star MWC 297 are presented. The data are of sufficient quality thatit has been possible to obtain a spectral type for this star bycomparing heavy element absorption features with those present in earlyB field stars of known spectral type. The best fit is to B1.5V, with anuncertainty of half a sub-type. It is estimated that vsini for MWC 297is about 350 +/- 50 km s^-1, suggesting that the star's rotation axis isalmost in the plane of the sky. The reddening towards this object isreviewed and is derived afresh from spectra at wavelengths spanning theB-R optical bands. Using A_v~=8 and the absolute magnitude correspondingto B1.5V, it is argued that the distance to MWC 297 is 250 +/- 50 pcrather than ~500 pc as has recently been quoted in the literature. Atthis distance the star can be located very plausibly in the Aquila Rift. We go on to present a MERLIN 5-GHz radio map of the source which showsit to be markedly elongated in the north-south direction (to a dimensionof 125 au at 250 pc). Existing data on the optical linear polarizationof MWC 297 suggest a complicated picture that leaves open the questionof whether the radio axis might trace an ionized electron-scatteringequatorial disc. We also present and briefly discuss high-resolutionBralpha and Hei1.083-μm line profiles obtained within a week of theoptical spectra. The Hei profile is very complex, but confirms that MWC297 is an outflow source. It is noted that the Hi emission lines areundoubtedly highly variable, although the time-scales for this are notyet clear.
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| Absolute magnitudes of B emission line stars - Correlation between the luminosity excess and the effective temperature
A new determination of the visual absolute magnitude of Be stars iscarried out. For this, a new calibration of visual absolute magnitudesof B stars of luminosity classes, V, IV, and III is first obtained froma sample of 215 stars. The absolute luminosity excess in the visual isdetermined for a sample of 49 Be stars. It is found that this excess iscorrelated with the effective temperature of the underlying stars. Awell defined correlation between this excess and the emission in thefirst two Balmer lines is established. From these results, using asimple model of circumstellar envelope, it is inferred that the zones ofthe circumstellar envelope contributing to the emission in the continuumand in the lines have to be rather small. It is also deduced that theemission measure of the envelope is correlated with the temperature ofthe central star and that the irregular photometric variations of Bestars are an envelope-opacity phenomenon.
| Selection of standard stars for photometric observations with the 91-cm reflector at Okayama
Not Available
| Radial velocity measurements. II - Ground-based observations of the program stars for the HIPPARCOS satellite
New radial velocities for 446 stars of magnitude 9.0 or brighter in 1616-sq-deg fields of the Northern Hemisphere are determined by automaticPDS measurement of 80-A/mm-dispersion spectra obtained at theObservatoire de Haute Provence using a 17-cm-diameter objective prism.The fields were selected to provide data for the input catalog of theESA Hipparcos astrometric satellite. The measurement techniques andprecision are discussed, and the results are presented in extensivetables and graphs.
| 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.
| The fundamental physical parameters of main-sequence and near main-sequence B type stars as derived from uvby,beta photometry
From spectrophotometric measurements of hydrogen line intensities, uvbyphotometry and photoelectrically determined H-beta and H-gamma indicesof 75 B2-B6 main-sequence stars, effective temperature and surfacegravity are derived by using the model atmospheres computed by Kurucz(1974). Comparisons between the g values and the MK luminosityclassification show good agreement for the MK standard stars but ratherserious disagreement for the average stars in such a way that most ofthe stars classified in luminosity class V really belong to class IV.Using the evolutionary model sequences calculated by Hejlesen et al.(1972) the stellar mass, age and luminosity are determined from the uvbyand beta data through effective temperature and surface gravity. Thetheoretically derived luminosities are in excellent agreement with theluminosities derived by calibrations.
| Observational studies of 12DD Lacertae. III - Secular variation and the stability
All the observational data of 12DD Lacertae obtained have been examinedto find a clue to the problem of secular variation or pulsationalstability. From the statistical consideration it is concluded that themean brightness is almost constant within small irregular variation of afew hundredth of a magnitude and the gamma-velocity also shows smallirregular variation of a few km/sec during more than a half century; buta result from the least-squares solution is also discussed. Noperiodicity of the variation in mean magnitude and gamma-velocity hasbeen detected, and an expectation that the small variation in thegamma-velocity might be caused by orbital motion is not confirmed.
| Four-color photometry for the association Lac OB1.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976PASP...88..930C&db_key=AST
| Spectral classification of stars in Lac OB1
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81.1120G&db_key=AST
| 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.
| Spectral types in the Lac OB1 association.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976PASP...88..141L&db_key=AST
| Some Spectroscopic Characteristics of the OB Stars: an Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971ApJS...23..257W&db_key=AST
| Photoelectric distances of 461 Northern OB-stars and galactic structure from Hγ- luminosities Author: Beer, A.
Abstract image available at:http://adsabs.harvard.edu/abs/1964MNRAS.128..261B
| Radial Velocities of B-Type Stars in the Nearest Associations.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963ApJ...137..791B&db_key=AST
| Stellar Rotation in Galactic Clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1962ApJ...136..381A&db_key=AST
| Hβ Photometry for the Association i Lacertae.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1961ApJ...133..860C&db_key=AST
| A Study of the i Lacertae Association.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1959ApJ...129..601H&db_key=AST
| Coordonnées moyennes de 86 étoiles 0 et B déterminées à l'Observatoire de Paris et réduites sans mouvements proper à l'équinoxe 1950,0
Not Available
| Photometry of the Lacerta Aggregate.
Not Available
| The Nebula Near 10 Lacertae.
Not Available
| Expanding Motions in the Lacerta Aggregate.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1953ApJ...117..256B&db_key=AST
| Intensities of the Interstellar Band at λ 4430.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1951ApJ...113..100D&db_key=AST
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