The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851

Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by ~1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.

[1]  On the origin of the high helium sequence in ω Centauri , 2006, astro-ph/0601425.

[2]  P. Ventura,et al.  Yields from low metallicity, intermediate mass AGB stars:. Their role for the CNO and lithium abundances in Globular Cluster stars , 2002 .

[3]  Jay Anderson,et al.  Toward High‐Precision Astrometry with WFPC2. I. Deriving an Accurate Point‐Spread Function , 2000, astro-ph/0006325.

[4]  The Origin of the Double Main Sequence in ω Centauri: Helium Enrichment due to Gas Fueling from Its Ancient Host Galaxy? , 2005, astro-ph/0512385.

[5]  Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system , 2004, astro-ph/0412328.

[6]  S. Djorgovski,et al.  The peculiar horizontal branch morphology of the Galactic globular clusters ngc 6388 and NGC 6441 : new insights from UV observations , 2007, 0708.1736.

[7]  S. Majewski,et al.  The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy , 2007, 0708.0027.

[8]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[9]  F. D’Antona,et al.  NGC 6441: another indication of very high helium content in globular cluster stars , 2006, astro-ph/0610406.

[10]  R. Cannon,et al.  Strong CN stars in the globular cluster NGC 1851 , 1982 .

[11]  J. Norris.  The Helium Abundances of ω Centauri , 2004 .

[12]  The Early Evolution of Globular Clusters: The Case of NGC 2808 , 2004, astro-ph/0405016.

[13]  Giampaolo Piotto,et al.  Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement , 2004, astro-ph/0412016.

[14]  P. Ventura,et al.  Toward a Working Model for the Abundance Variations in Stars within Globular Clusters , 2005, astro-ph/0511603.

[15]  Paolo Ventura,et al.  Predictions for Self-Pollution in Globular Cluster Stars , 2001 .

[16]  P. Harding,et al.  Newly Identified Star Clusters in M33. I. Integrated Photometry and Color-Magnitude Diagrams , 2006, astro-ph/0610015.

[17]  A. Walker.  A BV COLOR-MAGNITUDE DIAGRAM FOR THE GALACTIC GLOBULAR CLUSTER NGC 1851 , 1992 .

[18]  Stefano Casertano,et al.  THE PHOTOMETRIC PERFORMANCE AND CALIBRATION OF WFPC2 , 1995 .

[19]  E. Baron,et al.  The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models , 2007, 0706.0847.

[20]  J. Anderson,et al.  The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation , 2007, astro-ph/0703208.

[21]  R. Gratton,et al.  Abundance Variations within Globular Clusters , 2004 .

[22]  J. Meléndez,et al.  Abundances in a Large Sample of Stars in M3 and M13 , 2004, astro-ph/0409725.

[23]  J. Lattanzio,et al.  The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario , 2006, astro-ph/0608366.

[24]  G. Carraro,et al.  The absolute motion of the peculiar cluster NGC 6791 , 2006 .

[25]  G. Costa,et al.  Correlated cyanogen and sodium anomalies in the globular clusters 47 Tuc and NGC 6752 , 1981 .

[26]  Ata Sarajedini,et al.  Photometry of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: The Age-Metallicity Relation , 2000 .

[27]  N. Benı́tez,et al.  The Photometric Performance and Calibration of the Hubble Space Telescope Advanced Camera for Surveys , 2000, astro-ph/0507614.

[28]  S. Cassisi,et al.  A triple main sequence in the globular cluster NGC 2808 , 2007 .

[29]  M. Catelán,et al.  Hot Horizontal-Branch Stars: The Ubiquitous Nature of the “Jump” in Strömgren u, Low Gravities, and the Role of Radiative Levitation of Metals , 1999, astro-ph/9903120.