SCYON Abstract

Received on September 29 2009

Open clusters in the Third Galactic Quadrant. III: Alleged binary clusters

AuthorsRubén A. Vázquez (1), André Moitinho (2), Giovanni Carraro (3), and Wilton S. Dias (4)
Affiliation(1) Facultad de Ciencias Astronómicas y Geofísicas de la UNLP, IALP-CONICET, Paseo del Bosque s/n 1900, La Plata, Argentina
(2) SIM/IDL, Faculdade de Ciâncias da Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
(3) ESO, Alonso de Cordova 3107, Vitacura, Santiago, Chile
(4) UNIFEI, Instituto de Ciâncias Exatas, Universidade Federal de Itajubá, Itajubá MG, Brazil
Accepted byAstronomy & Astrophysics
Contactrvazquez@fcaglp.fcaglp.unlp.edu.ar
URL
Links Czernik 29 / Haffner 10 / Haffner 18 / Haffner 19 / NGC 2383 / NGC 2384 / Ruprecht 72 / Ruprecht 158

Abstract

Aims. We aim to determine accurate distances and ages of eight open clusters in order to: (1) assess their possible binarity (2) provide probes to trace the structure of the Third Galactic Quadrant.
Methods. Cluster reddenings, distances, ages and metallicities are derived from ZAMS and isochrone fits in UBVRI photometric diagrams. Field contamination is reduced by restricting analysis to stars within the cluster limits derived from star counts. Further membership control is done by requiring that stars have consistent positions in several diagrams and by using published spectral types.
Results. The derived distances, ages and metallicities have shown that none of the analysed clusters compose binary/double systems. Of the four candidate pairs, only NGC 2383/NGC 2384 are close to each other, but have different metallicities and ages. Ruprecht 72 and Ruprecht 158 are not clusters but fluctuations of the field stellar density. Haffner 18 is found to be the superposition of two stellar groups at different distances: Haffner 18(1) at 4.5 kpc and Haffner 18(2) between 9.5 and 11.4 kpc from the Sun. The derived distances and ages have been used to situate the clusters in the Galactic context. In particular, young stellar groups trace the spiral structure at large Galactocentric radii. At least two clusters formed during the last few 10^8 yr in an interstellar medium with less than solar abundances.
Conclusions. In contrast with the LMC, double clusters are apparently rare, or even non existent, in the undisturbed environment of the Third Galactic Quadrant. This leaves open the question of whether binary clusters form more easily toward denser and more violent regions of the Milky Way such as the inner Galaxy.