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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).
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Apsidal Motion in the Eclipsing Binary GG Ori
High-precision WBVR photoelectric observations of the eclipsing binaryGG Ori (B9.5V+B9.5V), which has an eccentric orbit (e=0.22), werecarried out in 1988 2001 at the Moscow and high-altitude Tian-ShanObservatories of the Sternberg Astronomical Institute. The aim of theseobservations was investigation of the apsidal motion of the system.Analysis of the resulting 12-year series of observations enabled us forthe first time to accurately (to within 11%) measure the rate ofrotation of the orbit and to appreciably improve estimates of thephotometric and absolute parameters. The observed value of is 28% higherthan the theoretical prediction of . The relativistic part of theapsidal motion in this system is a factor of 2.5 greater than theclassical term due to the tidal and rotational deformations of thecomponents. The interstellar extinction in the direction of the star (ata distance of r=425 pc) is very large (A v =1.75 m ). A number ofrecently published results (in particular, the conclusion that thecomponents of this eclipsing binary are young) are confirmed.
| Absolute Dimensions of the Unevolved B-Type Eclipsing Binary GG Orionis
We present photometric observations in B and V, as well as spectroscopicobservations of the detached, eccentric 6.6 day double-lined eclipsingbinary GG Ori, a member of the Orion OB1 association. Absolutedimensions of the components, which are virtually identical, aredetermined to high accuracy (better than 1% in the masses and betterthan 2% in the radii) for the purpose of testing various aspects oftheoretical modeling. We obtain MA=2.342+/-0.016Msolar and RA=1.852+/-0.025 Rsolar forthe primary, and MB=2.338+/-0.017 Msolar andRB=1.830+/-0.025 Rsolar for the secondary. Theeffective temperature of both stars is 9950+/-200 K, corresponding to aspectral type of B9.5. GG Ori is very close to the zero-age mainsequence, and comparison with current stellar evolution models givesages of 65-82 Myr or 7.7 Myr, depending on whether the system isconsidered to be burning hydrogen on the main sequence or still in thefinal stages of pre-main-sequence contraction. Good agreement is foundin both scenarios for a composition close to solar. We have detectedapsidal motion in the binary at a rate of ω=0.00061d+/-0.00025dcycle-1, corresponding to an apsidal period ofU=10,700+/-4500 yr. A substantial fraction of this (~70%) is due to thecontribution from general relativity, and our measurement is entirelyconsistent with theory. The eccentric orbit of GG Ori is well explainedby tidal evolution models, but both theory and our measurements of therotational velocity of the components are as yet inconclusive as towhether the stars are synchronized with the orbital motion. Some of theobservations reported here were obtained with the Multiple MirrorTelescope, a joint facility of the Smithsonian Institution and theUniversity of Arizona.
| 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.
| 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.
| An Analysis of the Hauck / Mermilliod Catalogue of Homgeneous Four-Color Data - Part Two
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&AS...40..199P&db_key=AST
| Photometric studies of stars in ORI OB1 /belt/
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979AJ.....84.1846G&db_key=AST
| 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
| 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.
| A search for He-weak stars in very young clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....19..482B&db_key=AST
| 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
| 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
| The Meridian Proper Motions of 161 Stars in the Region of the Belt of Orion.
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