Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4305d&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #43.05; Bulletin of the American Astronomical Society, Vol. 24, p.1187
Other
Scientific paper
HST Faint Object Camera images of the core of the galactic globular cluster 47 Tucanae (NGC 104) in the F342W filter have been used to count stars in the most crowded region of the cluster down to the main sequence turn-off and to determine the position of the gravity center along with the number density profile (Calzetti et al. 1992, ApJ Lett., in press). Applying the mirror autocorrelation technique to star counts, rather than to luminous fluxes, we have found that the center of gravity of 47 Tuc does not coincide with the center defined by the light profile: in fact, the two are separated by 5('') . Furthermore, the density profile obtained from star counts with respect to the gravity center clearly shows the presence of a central cusp. The observed density profile can be satisfactorily fitted either by combining two King models of core radii r_c=25('') and rcc=8('') , respectively, or by the expected density profile of a globular cluster with a central compact, massive object of about 3000 Msun. These results, in addition to other evidence such as the presence of high velocity red stars in the core of the cluster (Meylan et al., 1991, ApJ, 383, 587), of blue stragglers (Paresce et al., 1991, Nature, 352, 297), of the brightest known low-luminosity X-ray source (Hertz & Grindlay, 1983, ApJ, 267, L83; Paresce et al. 1992, Nature, in press) and of millisecond pulsars (Manchester et al., 1991, Nature, 352, 219), support the idea that the core of 47 Tuc is on the verge of, or undergoing collapse. Additional studies are required to investigate whether this result pertains only to 47 Tuc or, instead, it applies to many other globular clusters characterized by a King surface brightness profile. In this latter case, core collapse could be a more common dynamical state than previously thought.
Calzetti Daniela
de Marchi Guido
Paresce Francesco
Shara Michael M.
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