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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).
| Multi-wavelength observations of the star forming region in L1616
We present the results of a multi-wavelength study of the star formingregion in L1616. Our observations include ROSAT All-Sky Survey (RASS)and High Resolution Imager (HRI) X-ray observations, optical wide-fieldimaging and near-IR imaging data and optical long-slit and multi-objectspectroscopic follow-up. 22 new low-mass pre-main sequence (PMS) starsare found to be distributed mainly to the East of the L1616 cometarycloud, in about a one-square-degree field. We find that the class-IIIinfrared sources outnumber the class-II infrared sources by a factor ofabout three. The X-ray properties of the PMS stars in L1616 are quitesimilar to those of PMS stars detected in the Orion Nebula Cluster. Thecomparison of the position of the L1616 PMS stars in the HR diagram withtheoretical PMS evolutionary tracks yields an average age of 1-2 Myr,with a very small age spread of about 1 Myr. Unlike the fossil starforming regions in Orion, L1616 appears to be a region of on-going starformation relatively far from the Orion A and B clouds. Given the smallage spread, the spatial distribution of the PMS stars relative to thehead of the cloud, as well as its cometary shape and high star formationefficiency, we conclude that the star formation in L1616 was most likelyinduced by a single event, the impact of the winds of the massive starsof the Orion OB association or a supernova explosion being the possibletriggers. The Initial Mass Function (IMF) in L1616 is roughly consistentwith that of the field in the mass range 0.3< M/Mȯ< 2.5. Several faint objects, detected in our optical images, aregood candidates for young Brown Dwarfs (BDs). We might expect the numberof BDs in L1616 to be intermediate between Taurus and the Trapezium.Based on observations carried out at the European Southern Observatory,La Silla, Chile under proposals numbers 56.E-0566 and 64.I-0355, and atthe Calar Alto observatory.
| 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.
| 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.
| Interaction between the Barnard Loop Nebula and the interstellar magnetic field
New observations of the linear interstellar polarization of starlightare used to study the geometric structure of the interstellar magneticfield in the vicinity of the Barnard Loop Nebula in Orion. The observedcomplex field structure cannot be explained by a simple radial expansionof a conducting gaseous shell into an initially parallel field. Asatisfactory fit of the observations can be obtained by assuming thatthe interstellar gas in Orion is suspended in an interstellar 'magneticpocket'. It is suggested that the formation of the observed denseinterstellar clouds and very young stars in the Orion aggregate is aresult of the local structure of the interstellar magnetic field.
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