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Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| ROSAT PSPC/HRI observations of the open cluster NGC 2422
We present the results of a ROSAT study of NGC 2422, a southern opencluster at a distance of about 470 pc, with an age close to thePleiades. Source detection was performed on two observations, a 10-ksPSPC and a 40-ks HRI pointing, with a detection algorithm based onwavelet transforms, particularly suited to detecting faint sources incrowded fields. We have detected 78 sources, 13 of which were detectedonly with the HRI, and 37 detected only with the PSPC. For each source,we have computed the 0.2-2.0 keV X-ray flux. Using optical data from theliterature and our own low-dispersion spectroscopic observations, wefind candidate optical counterparts for 62 X-ray sources, with more than80% of these counterparts being late type stars. We have assigned to theoptical sources an astrometric and/or a photometric membership flag,depending on which data are available to us. The number of sources (38of 62) with high membership probability counterparts is consistent withthat expected for Galactic plane observations at our sensitivity. Wehave computed maximum likelihood X-ray luminosity functions (XLF) for Fand early-G type stars with high membership probability. Heavy datacensoring due to our limited sensitivity permits determination of onlythe high-luminosity tails of the XLFs; the distributions areindistinguishable from those of the nearly coeval Pleiades cluster.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Photoelectric uvby-beta photometry of the open cluster NGC 2422
New and previously observed uvby-beta photometry has been utilized todetermine membership in the cluster. Reddening and age of the clusterhas been determined.
| Equatorial coordinates of double and multiple star components measured on GPO astrographic plates
Equatorial coordinates are given for some of the multiple starcomponents having incomplete or inconsistent information in the IndexCatalog of Visual Double Stars. Two photographic plates were taken foreach system with incomplete or inconsistent information using the GPOastrograph. The plate measurement and reduction techniques used aredescribed.
| UVBY beta photometry of southern clusters. V - NGC 2422
Stromgren four-color and H-beta photometry has been obtained for 28 Band A0 members of the young cluster NGC 2422. A comparison of V0/c0diagrams clearly shows that the cluster is very similar in age to thePleiades. Distance estimates for the cluster from three luminositycalibrations of the beta index and from a direct comparison of the V0/c0diagram with that of the Pleiades are all in good agreement. If thedistance modulus of the Pleiades is taken to be 5.50, the adoptedmodulus for NGC 2422 is 8.0, corresponding to a distance of 400 pc withan estimated uncertainty of 10 pc.
| Evolved stars in open clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJS...30..451H&db_key=AST
| The frequency of peculiar A and metallic-line stars in open clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJ...205..807H&db_key=AST
| Spectroscopic study of the open cluster NGC 2422
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975AJ.....80..131D&db_key=AST
| Distance Moduli of Open Clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1965ApJS...12..215H&db_key=AST
| Polarization of Galactic Clusters M25, NGC 869, 884, 1893, 2422, 6823, 6871, and Association VI Cygni.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1965ApJ...141.1340S&db_key=AST
| Three-colour photometry of southern galactic clusters I,
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