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Spitzer Observations of the Orion OB1 Association: Second-Generation Dust Disks at 5-10 Myr
We report new Spitzer observations of intermediate-mass stars in tworegions of the Orion OB1 association located in the subassociations OB1a(~10 Myr) and OB1b (~5 Myr). In a representative sample of stars earlierthan F5 of both stellar groups, we find a population of stars surroundedof debris disks, without excess in the IRAC bands and without emissionlines in their optical spectra, but with a varying degree of 24 μmexcess. Comparing our samples with 24 μm observations ofintermediate-mass stars in other stellar groups, spanning a range ofages from 2.5 to 150 Myr, we find that debris disks are more frequentand have larger 24 μm excess at 10 Myr (OB1a). This trend agrees withpredictions of models of evolution of solids in the outer regions ofdisks (>30 AU), where large icy objects (~1000 km) begin to form at~10 Myr; the presence of these objects in the disk initiates acollisional cascade, producing enough dust particles to explain therelatively large 24 μm excess observed in OB1a. The dust luminosityobserved in the stellar groups older than 10 Myr declines roughly aspredicted by collisional cascade models. Combining Spitzer observations,optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star (HD 36444) with a large 24 μm excess, both inOB1b. This last object could be explained as a intermediate stagebetween HAeBe and true debris systems or as a massive debris diskproduced by a collision between two large objects (>1000 km).
| 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).
| Photoelectric Photometry of Stars in the Orion Standard Region
Magnitudes and color indices in the Vilnius seven-color photometricsystem are given for 108 stars in the Orion standard region around theOrion Belt and the star lambda Orionis. New spectral and luminosityclasses, estimated from the photometric data, are given for some of thestars.
| 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.
| The Orion complex according to observations of the space telescope Glazar.
Not Available
| 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.
| Stars of Early Types in the Region of Stellar Association OB1 Observed with the Glazar Telescope
Not Available
| Kinematical studies of open clusters and OB-associations from relative radial velocity observations. II - The Orion Belt region
Relative radial velocities have been obtained for 66 stars of theclustering Cr 70 in the Orion Belt region. The peculiar frequencydistribution of the radial velocities is discussed. Their conspicuoussystematical spatial distribution corresponds perfectly to the gradientof distance moduli found by Hardie et al. (1964). Therefore the presenceof two additional subgroupings in the Ori OB 1 association is proposed.Comparison with the radial velocities in the various subgroups of theOri OB 1 complex as defined by Blaauw (1964) does not yet reveal aconsistent kinematical picture.
| 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.
| Spectral classification in the belt of ORI OB1.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..537G&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.
Not Available
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