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Spitzer Observations of the Hyades: Circumstellar Debris Disks at 625 Myr of Age
We use the Spitzer Space Telescope to search for infrared excess at 24,70, and 160 μm due to debris disks around a sample of 45 FGK-typemembers of the Hyades. We supplement our observations with archival 24and 70 μm Spitzer data of an additional 22 FGK-type and 11 A-typeHyades members in order to provide robust statistics on the incidence ofdebris disks at 625 Myr of age, an era corresponding to the late heavybombardment in the solar system. We find that none of the 67 FGK-typestars in our sample show evidence for a debris disk, while 2 out of the11 A-type stars do. This difference in debris disk detection rate islikely to be due to a sensitivity bias in favor of early-type stars. Thefractional disk luminosity, Ldust/L*, of the disksaround the two A-type stars is ~4×10-5, a level that isbelow the sensitivity of our observations toward the FGK-type stars.However, our sensitivity limits for FGK-type stars are able to exclude,at the 2 σ level, frequencies higher than 12% and 5% of disks withLdust/L*>1×10-4 andLdust/L*>5×10-4, respectively.We also use our sensitivity limits and debris disk models to constrainthe maximum mass of dust, as a function of distance from the stars, thatcould remain undetected around our targets.
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A list of candidates for high-velocity AP stars
'Reduced proper motion' is used to select from a catalog of over fifteenhundred Ap stars those that probably have a high velocity. A list of 164candidates is presented, 22 of them having very probably high velocity.Some of them could be the first specimens of late Ap stars with highvelocity. The possible relation to horizontal branch stars is alsodiscussed.
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