Luminosity and mass function of galactic open clusters I. NGC 4815 ^*

¹ Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
² Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
³ Astronomical Observatory, Ural State University, pr. Lenina 51, Ekaterinburg, 620083 Russia
⁴ Dominion Astrophisical Observatory, Herzberg Institute of Astrophysics, National Res earch Council of Canada, 5071 West Saanich Road, Victoria, British Columbia V8X 4M6, Canada
⁵ UCLA/ Department of Physics and Astronomy, Los Angeles, CA 90095-1562, USA

Corresponding author: G. Carraro, carraro@pd.astro.it

Received: 30 June 2000
Accepted: 23 January 2001

Abstract

We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic fields down to a limiting magnitude . These data are used to study cluster spatial extension by means of star counts, and to derive the luminosity (LF) and mass function (MF). The radius turns out to be arcmin at , whereas the mass is down to . From the color-magnitude diagram, we obtain the LFs in the V and I bands, using both the standard histogram and an adaptive kernel. After correction for incompleteness and field star contamination, the LFs were transformed into the present day mass functions (PDMF). The PDMFs from the V and I photometry can be represented as a power-law with a slope and (the (Salpeter [CITE]) MF in this notation has a slope ) respectively, in the mass range . Below this mass, the MF cannot be considered as representative of the cluster IMF, as it is the result of the combined effect of strong irregularities in the stellar background, probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic field. Unresolved binaries and mass segregation can only flatten the apparent derived IMF, so we expect that the real IMF must be steeper than the quoted slope by an unknown amount.