The radial distribution of the two stellar populations in NGC 1851^*

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, Padova 35122, Italy e-mail: antonino.milone@unipd.it
² Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ INAF – Osservatorio Astronomico di Collurania, via M. Maggini, Teramo 64100, Italy
⁵ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK

Received: 2 April 2009
Accepted: 26 May 2009

Abstract

We have analyzed ACS/WFC and WFPC2 images from HST, as well as ground-based data to study the radial distribution of the double sub-giant branch (SGB) recently discovered in the Galactic globular cluster NGC 1851. We found that the SGB split can be followed all the way from the cluster center out to at least 8´ from the center. Beyond this distance out to the tidal radius at ~11.7 arcmin, there are simply too few SGB stars to identify the sequences. The number ratio of the bright SGB to the faint SGB stars shows no significant radial trend. Furthermore, we have found that the ratio of blue horizontal-branch (HB) stars plus RR Lyrae to the red HB stars also remains constant from the cluster center to the outer envelope.