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An X-Ray Survey of the Open Cluster NGC 6475 (M7) with ROSATA ROSAT x-ray survey, with complimentary optical photometry, of the open cluster NGC 6475 has enabled the detection of approx. 50 late-F to K0 and approx. 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 L(sub x) approx. equal to 10(exp 29) 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.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Prosser, Charles F. (Harvard-Smithsonian Center for Astrophysics Cambridge, MA United States)
Stauffer, John R. (Harvard-Smithsonian Center for Astrophysics Cambridge, MA United States)
Caillault, J.-P. (Georgia Univ. Athens, GA United States)
Balachandran, Suchitra (Ohio State Univ. Columbus, OH United States)
Stern, Robert A. (Lockheed Martin Corp. Palo Alto, CA United States)
Randich, Sofia (Max-Planck-Inst. fuer Extraterrestrische Physik Garching, Germany)
Date Acquired
September 6, 2013
Publication Date
September 1, 1995
Publication Information
Publication: The Astronomical Journal
Volume: 110
Issue: 3
ISSN: 0004-6256
Subject Category
Report/Patent Number
NAS 1.26:205301
Funding Number(s)
Distribution Limits
Public Use Permitted.

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