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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).
| CCD photometry and astrometry of visual double and multiple stars of the HIPPARCOS Catalogue. II. CCD photometry and differential astrometry of 288 southern ``Intermediate'' systems
We present photometric and astrometric data of about 280 visual doublestars of the ``intermediate'' class, i.e. with angular separationsmainly in the range 2arcsec < rho < 12arcsec . The observationshave been obtained in 1991-92 with a CCD camera attached to the 91 cmDutch telescope at ESO La Silla, Chile. Differential magnitudes of thedouble star components as well as magnitudes and colour indices of theindividual components have been determined in the Cousins V and Ipassbands with an internal error of about 0.005 mag and an externalaccuracy of less than 0.03 mag. In addition, angular separations havebeen secured to an internal accuracy of 0.004arcsec and position anglesto about 0.05degr. Tables 1 to 4 are only available in electronic format the CDS via ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html. Based on observationscollected at the European Southern Observatory, La Silla, Chile.
| UBV absolute CCD photometry and differential astrometry of a sample of visual double stars with A-type primaries
We present V magnitudes, (B-V) and (U-B) colours of the components of111 visual double stars and their relative position measurements rho andtheta . The primaries are of spectral class A; the angular separation isin the 2farcs5 -8arcsec range. The observations were made at the 61-cmreflector of University of Toronto Southern Observatory, Chile. Theobtained photometric results are of a moderate quality for absolutevalues (0.04() m (mag) in V and about 0.02() m in the (B-V) and (U-B)colour indices) and more precise in Delta V and Delta (U-B), Delta (B-V)(0.023() m and 0.015() m, respectively). We find that about 60% ofsecondaries are also of spectral type A; 56% of pairs have Delta (B-V)less than 0.1() m. The two-colour diagram reveals significant offset ofobjects from the curve of unreddened Main Sequence stars. Acomprehensive survey of stars in the close vicinity to the objects isalso presented. The final astrometric accuracy of ~ 0farcs01 in rho and0fdg1 in theta provides additional measurements for study of relativemotion of the components. Based on observations made at University ofToronto Southern Observatory, Las Campanas, Chile. Tables\arabic{tabptm}--\arabic{tabast} are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Speckle Interferometry at the US Naval Observatory. IV.
The results of 1314 speckle interferometric observations of 625 binarystars, ranging in separation from 0.2" to 5.2" with a limiting secondarymagnitude of V=11, are tabulated. These observations were obtained usingthe 66 cm refractor at the US Naval Observatory in Washington, DC, withan intensified CCD detector. This is the fourth in a series of paperspresenting measures obtained with this equipment and covers the period1997 January 1 through December 31. Random errors for all measures areestimated to be 18 mas in separation and 0.57d/rho in position angle,where rho is the separation in arcseconds.
| Speckle Interferometry at the US Naval Observatory. III.
Position angles and separations resulting from 2578 speckleinterferometric observations of 590 binary stars are tabulated. This isthe third in a series of papers presenting measures obtained using the66 cm refractor at the US Naval Observatory in Washington, DC, andcovers the period from 1995 June through 1996 December. Program starsrange in separation from 0.2" to 4.3", with a limiting magnitude ofV=11. Random errors are estimated to be 17.0 mas in separation and0.56d/rho in position angle, where rho is the separation in arcseconds.These are the first results acquired using an improved intensified CCDdetector. The new detector, in concert with an intensity-filteringtechnique applied in software, has permitted a 1 mag increase in dynamicrange, to 3.5 mag, for pairs separated by about 2". The instrumentationand calibration are briefly described, with an emphasis on thecharacteristics of the new detector. The software filter used toincrease the dynamic range is also described.
| 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.
| Spectroscopic studies of stars in ORI OB1 /Belt/
MK spectral classifications are presented for 120 B- and A-type stars inthe Belt subgroup of the Ori OB1 association. Combined with otherpublished spectroscopic data, and the assumed absolute magnitudes givenby Schmidt-Kaler (1965) and Lesh (1979), a mean corrected distancemodulus of 8.56 + or - 0.11 is computed for the eastern region, whilefor the remainder of the Belt a modulus of 8.20 + or - 0.05 is derived.Several B9-A2 subgiants and A7-F0 main-sequence stars were observed inthe western and central regions of the Belt. The ages of these starsimply a spread in formation ages for these subgroups from about 1million yr to at least 10 million yr.
| 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
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Observation and Astrometry data
| Constellation: | Ωρίων |
| Right ascension: | 05h31m21.21s |
| Declination: | -02°05'57.1" |
| Apparent magnitude: | 8.603 |
| Distance: | 684.932 parsecs |
| Proper motion RA: | 0.6 |
| Proper motion Dec: | 0.2 |
| B-T magnitude: | 8.665 |
| V-T magnitude: | 8.609 |
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