The Inner Velocity Dispersion Profile of M15

Details The Inner Velocity Dispersion Profile of M15 The Inner Velocity Dispersion Profile of M15

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...184.5814d&link_type=abstract

American Astronomical Society, 184th AAS Meeting, #58.14; Bulletin of the American Astronomical Society, Vol. 26, p.956

Astronomy and Astrophysics

Astronomy

Scientific paper

We report a new measurement of the velocity dispersion profile within 1' (3 pc) of the center of the globular cluster M15 (NGC 7078), using long-slit spectra from the 4.2-m William Herschel Telescope at La Palma Observatory. Spectra for a total of 23 slit positions were obtained during two observing runs. Each run used a set of parallel positions to map out the central region of the cluster; the position angle used during the second run was orthogonal to that used for the first. The spectra are centered near the Ca II infrared triplet at 8650 Angstroms, with a spectral range of 400 Angstroms. We determined radial velocities by cross-correlation techniques for 130 cluster members. A total of 32 stars were observed more than once. Internal and external comparisons indicate a velocity accuracy of about 4 km s(-1) . The velocity dispersion profile rises from about sigma =8 {km s^{-1}} near 1' from the center of the cluster to about sigma =12 {km s^{-1}} at 20''. Inside of 20'' the dispersion remains approximately constant with no evidence for a sharp rise near the center. This last result stands in contrast with that of Peterson et al. \ (1989, ApJ, 347, 251) who found a central velocity dispersion of 25+/-7 {km s^{-1}}, based on a line broadening measurement. Our velocity dispersion profile is in good agreement with those determined in the recent studies of Gebhardt et al. \ 1994 (ApJ, in press) and Dubath & Meylan (1994, A&A, in press). The behavior of the central velocity dispersion profile of M15 is consistent with the predictions of Fokker-Planck models developed by Grabhorn et al. \ (1992, ApJ, 392, 86) and Phinney (1993, ASP Conf. Ser., 50, 141) for globular clusters undergoing core collapse. These models predict the presence of central populations of a few times 10(4) degenerate remnants with masses exceeding about 1 M_sun, of which a few times 10(3) are 1.4 M_sun neutron stars. There appears to be no need to invoke the presence of a massive central black hole in M15.

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