Updated properties of the old open cluster Melotte 66: Searching for multiple stellar populations^⋆,⋆⋆

¹ ESO, Alonso de Cordova 3107, 19001 Santiago de Chile Chile
e-mail: gcarraro@eso.org, lmonaco@eso.org
² Australian Astronomical Observatory, 105 Delhi Rd, NSW 2113 North Ryde, Australia
e-mail: gdesilva@aao.gov.au
³ Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Observatory, Cotter Rd, Weston, ACT 2611, Australia
e-mail: antonino.milone@anu.edu.au
⁴ Departamento de Astronomía, Universidad de Concepción, Casilla 169 Concepcion, Chile
e-mail: rmateluna@astro-udec.cl

Received: 27 February 2014
Accepted: 17 April 2014

Abstract

Context. Multiple generations of stars are routinely encountered in globular clusters but no convincing evidence has been found in Galactic open clusters to date.

Aims. In this paper, we use new photometric and spectroscopic data to search for multiple stellar population signatures in the old, massive open cluster, Melotte 66. The cluster is known to have a red giant branch wide in color, which could be an indication of metallicity spread. Also the main sequence is wider than what is expected from photometric errors only. This evidence might be associated with either differential reddening or binaries. Both hypothesis have, however, to be evaluated in detail before recurring to the presence of multiple stellar populations.

Methods. New, high-quality, CCD UBVI photometry have been acquired to this aim with high-resolution spectroscopy of seven clump stars that are complemented with literature data; this doubles the number of clump star members of the cluster for which high-resolution spectroscopy is available. All this new material is carefully analyzed in search for any spectroscopic or photometric manifestation of multiple populations among the cluster stars.

Results. Our photometric study confirms that the width of the main sequence close to the turn off point is entirely accounted for by binary stars and differential reddening with no need to advocate more sofisticated scenarios, such as metallicity spread or multiple main sequences. By constructing synthetic color−magnitude diagrams, we infer that the binary fraction has to be as large as 30% and their mass ratio in the range 0.6−1.0. As a by-product of our simulations, we provide new estimates of the cluster fundamental parameters. We measure a reddening E(B − V) = 0.15 ± 0.02, and confirm the presence of a marginal differential reddening. The distance to the cluster is 4.7_-0.1^+0.2 kpc and the age is 3.4 ± 0.3 Gyr, which is somewhat younger and better constrained than previous estimates.

Conclusions. Our detailed abundance analysis reveals that, overall, Melotte 66 looks like a typical object of the old thin disk population with no significant spread in any of the chemical species we could measure. Finally, we perform a photometric study of the blue straggler star population and argue that their number in Melotte 66 has been significantly overestimated in the past. The analysis of their spatial distribution supports the scenario that they are most probably primordial binaries.