J.-P. Caillault

Ages of A-Type Vega-like Stars fromuvbyβ Photometry

We have estimated the ages of a sample of A-type Vega-like stars by using Stromgren uvbyβ photometric data and theoretical evolutionary tracks. We find that 13% of these A stars have been reported as Vega-like stars in the literature and that the ages of this subset run the gamut from very young (50 Myr) to old (1 Gyr), with no obvious age difference compared to those of field A stars. We clearly show that the fractional IR luminosity decreases with the ages of Vega-like stars.

AN X-RAY SURVEY OF THE OPEN CLUSTER NGC 6475 (M7) WITH ROSAT

A ROSAT x-ray survey, with complimentary optical photometry, of the open cluster NGC 6475 has enabled the detection of -50 late-F to K0 and -70 K/M dwarf new candidate members, providing the first reliable detection of low-mass stars in this low galactic latitude, 220 Myr old cluster. The x-ray observations reported here have a typical limiting sensitivity of Lx=1029 erg/s. The detection frequency of early type cluster members is consistent with the hypothesis that the x-ray emitting early type stars are binary systems with an unseen, low-mass secondary producing the x rays. The ratio between x-ray and bolometric luminosity among NGC 6475 members saturates at a spectral-type/color which is intermediate between that in much younger and in much older clusters, consistent with rotational spindown of solar-type stars upon their arrival on the ZAMS. The upper envelope of x-ray luminosity as a function of spectral type is comparable to that of the Pleiades, with the observed spread in x-ray luminosity among low-mass members being likely due to the presence of binaries and relatively rapid rotators. However, the list of x-ray selected candidate members is likely biased against low-mass, slowly rotating single stars. While some preliminary spectroscopic information is given in an appendix, further spectroscopic observations of the new candidate members will aid in interpreting the coronal activity among solar-type NGC 6475 members and their relation to similar stars in older and younger open clusters.