European Southern Observatory, Casilla 19001, Santiago, Chile
2 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
3 Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
4 Dipartimento di Fisica, Università di Roma "Tor Vergata" , 00173 Roma, Italy
Corresponding author: G. Marconi, email@example.com
Accepted: 2 October 2001
We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B, V, I CCD photometry for the stars located within 200″ (2 half-mass radii, ) from the center of the cluster NGC 6101. Color-Magnitude Diagrams (CMDs) extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO () have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a sizeable population of 73 "blue stragglers" (BSS), which had been already partly detected (27). They are considerably more concentrated than either the subgiant branch (SGB) or the main sequence (MS) stars, and have the same spatial distribution as the horizontal branch (HB) stars (84% probability from K-S test). An hypothesis on the possible BSS progeny is also presented. b) The HB is narrow and the bulk of stars is blue, as expected for a typical metal-poor globular cluster. c) The derived magnitudes for the HB and the MSTO, , , coupled with the values , , yield a distance modulus and an age similar to other "old" metal-poor globular clusters. In particular, from the comparison with theoretical isochrones, we derive for this cluster an age of 13 Gyrs. d) By using the large statistical sample of Red Giant Branch (RGB) stars, we detected with high accuracy the position of the bump in the RGB luminosity function. This observational feature has been compared with theoretical prescriptions, yielding a good agreement within the current theoretical and observational uncertainties.
Key words: globular clusters: individual: NGC 6101 / stars: fundamental parameters / Hertzsprung-Russell diagram / stars: abundances / stars: horizontal branch
Based on observations collected at the European Southern Observatory, La Silla Chile and on HST observations.
© ESO, 2001