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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).
| Oscillations in the PMS delta Scuti star V346 Ori
We report the discovery of periodicities in the light curve of thePre-Main Sequence (PMS) Herbig Ae star V346 Ori. We interpret variationsin the light curve of the star as the superposition of at least twoperiodic signals that result from delta Scuti-type stellaroscillations. The computation of linear non-adiabatic radial pulsationmodels for PMS stars reproduces these periods for a 1.55 Msunstar with Teff = 7410 K and log L/Lsun = 0.74pulsating in the fundamental and in the second overtone. If ourassumption of radial oscillations is indeed correct, the star's locationon the HR diagram places it at a distance significantly smaller thanthat of the Orion star forming region, hence raising questions regardingthe evolutionary status of V346 Ori. Observations with better timesampling and spanning a longer period of time are needed if the latterquestion is to be addressed fully.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/271
| 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.
| The Meridian Proper Motions of 161 Stars in the Region of the Belt of Orion.
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| Сазвежђа: | Орион |
| Ректацензија: | 05h24m13.01s |
| Deклинација: | +01°46'42.0" |
| Apparent магнитуда: | 9.017 |
| Proper motion RA: | 3.6 |
| Proper motion Dec: | -0.1 |
| B-T magnitude: | 9.036 |
| V-T magnitude: | 9.019 |
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