Integrated BVJHK parameters and luminosity functions of 650 Galactic open clusters^*

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: rdscholz@aip.de
² Main Astronomical Observatory, 27 Academica Zabolotnogo Str., 03680 Kiev, Ukraine
³ Astronomisches Rechen-Institut, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
⁴ Institute of Astronomy of the Russian Acad. Sci., 48 Pyatnitskaya Str., 109017 Moscow, Russia

Received: 3 March 2009
Accepted: 20 May 2009

Abstract

Aims. We determine the integrated magnitudes and colours of 650 clusters in optical (BV) and the near-infrared (JHK_s) passbands and construct the luminosity functions of the Galactic open clusters in these passbands.

Methods. The magnitudes are based on accurate and uniform cluster membership parameters derived from the ASCC-2.5 catalogue and were computed by adding the individual luminosities of the most secure cluster members. To put the computed magnitudes into a uniform and unbiased system, they were corrected for the effect of unseen stars in the ASCC-2.5 . Comparison of the derived parameters with published optical data shows that our integrated magnitudes and colours are accurate within 0.6 and 0.2 mag, respectively. Comparison of cluster distributions over apparent integrated magnitudes with the prediction of a model of cluster counts shows that the sample can be regarded as magnitude-limited down to 8.1, 7.7, 6.3, 5.3, and 5.3 mag in BVJHK_s.

Results. Out of 650 clusters, 422 (or about 2/3) have received optical integrated magnitudes for the first time. This increases the data bank of BV integrated magnitudes to about 780 clusters, compared to about 350 clusters with BV-magnitudes available in the literature. In the near-infrared, data on cluster-integrated parameters were not available before this study. The cluster sample is found to be magnitude-limited both in the optical and in the near-infrared. This enabled us to construct cluster luminosity functions in five (BVJHK_s) photometric passbands. We find that both in the optical and in the NIR the luminosity functions show similar behaviour: a linear increase to fainter magnitudes, which stops at about -2.5 mag in the optical and at -4.0 mag in the NIR. At the brightest magnitudes the luminosity functions exhibit a deficiency with respect to the linear relation. The youngest clusters have flatter luminosity functions in all five passbands with a slope of about , while the total cluster sample (all ages) produces luminosity functions with significantly steeper slopes of the order of .