NGC\,2587: a sparse open cluster projected onto a populous star field

PIATTI, ANDRÉS E.; CLARIA OLMEDO, JUAN JOSÉ; AHUMADA, A.V.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2009 vol. 397 p. 1073 - 1083

0035-8711

We present CCD photometry in the Johnson $U$, $B$ and $V$ and Kron-Cousins $I$ passbands for the open cluster NGC\,2587. The sample consists of 4406 stars reaching down to $V$ $\sim$ 21.0. We developed a new method to clean statistically the colour-magnitude diagrams. NGC\,2587 appears to be a sparse, relatively bright open cluster, with a few tens of members projected onto a populous star field. The comparatively bright F7/8 II type star HD\,70927, located close to the cluster centre, seems not to be a member. Our analysis suggests that NGC\,2587 is slightly younger than the Hyades and probably of solar metallicity. A cluster radius of roughly 8$\arcmin$ was estimated from the radial stellar density profile. From 18 probable cluster members with measured proper motions, we derive the following mean values for NGC\,2587: $\mu_{\alpha}$ = -4.3 $\pm$ 3.6 mas/year and $\mu_{\delta}$ = -2.5 $\pm$ 3.4 mas/year. Adopting the theoretical metal content $Z$ = 0.02, which provides the best global fit, we derive a cluster age of 500$^{\rm +60}_{\rm -50}$. Simultaneously, colour excesses $E(B-V)$ = 0.10 and $E(V-I)$ = 0.15 and an apparent distance modulus of $V$-$M_{V}$ = 12.50 are obtained. The interstellar extinction in the cluster direction is found to follow the normal law. NGC\,2587 is located at a distance of (2.70 $\pm$ 0.70) kpc from the Sun and $\sim$ 9.8 kpc from the Galactic centre.