Astron. Astrophys. 324, 549-555 (1997)

1. Introduction

The young open cluster IC 348 (C0341+321) is located in the eastern part of the Perseus dark cloud complex, south of the bright star o  Persei. Most of the cluster is embedded in warm dust and gas and it is only viewed through a partially transparent region between two dark clouds near the centre of Perseus OB2 association. Using Vilnius photometry Cernis (1993) studied the relationship between interstellar extinction and distance, postulating two absorbing dust layers in the direction of IC 348. The first layer, at pc, covers the whole area of the cluster and is probably related with the Taurus dark cloud complex. The second layer, at pc has the form of a chain of condensations named L 1468, L 1470 and L 1471; IC 348 is physically associated with L 1470. The bright stars illuminate surrounding dust and gas forming a group of reflection nebulae, the best known associated with the brightest star BD of B5 V spectral type. Dark clouds in the region were analysed in radio wavelengths by Sancisi et al. (1974), Bachiller & Cernicharo (1986) and Bachiller et al. (1987). IRAS sources in the area were studied by Ladd et al. (1993).

The first studies in the region were carried out by Gingrich (1922), Hubble (1922) and Greenstein (1948). In 1952, Blaauw pointed out that the cluster could be a part of the Perseus OB2 association. Harris et al. (1954) observed the central part of IC 348 and obtained UBV photometry of 12 stars and spectra of 8 of them, thus allowing the MK classification. Strom et al. (1974) performed UBVHKL measurements in a larger area which included some embedded stars, suggesting that the visible cluster may represent only a small part of the group. Herbig (1954) detected 16 emission stars in the IC 348 region that could be classified as PMS stars.

Recently, Lada & Lada (1995) observed the region in the near infrared JHK, covering an area of 385 square arc minutes, and estimated a total of 380 star members, 20% of them with infrared excess. They also identified 8 small sub-clusters (each one contained 10-20 stars in a radius of 0.1-0.2 pc) outside the half-mass radius of the cluster. From comparison with the stellar evolutionary models (containing PMS stars) and their observations, they conclude that star formation in IC 348 has been a continuous process over the last 5-7   yr. Former evaluations of the age, made by Strom et al. (1974), range from 5 to  yr.

The paper by Preibisch et al. (1996) reports the first X-ray analysis of the cluster. They detected 116 sources in very deep pointings with ROSAT PSPC and HRI. Apart from previously known members, they considered as probable cluster members all the emission line stars and 56 additional X-ray sources. The observations by Lada & Lada (1995) were used to estimate individual extinctions of the sources and hence X-ray luminosities. The derived properties of the sources detected were rather similar to those of other young clusters. The authors also conclude that the ionization fraction of the molecular cloud can be considerably enhanced by the X-ray sources, and so they influence the evolution of the cluster.

Historically, clusters have provided important information about the process of star formation and the initial mass function (IMF) because they are statistically significant groups of stars assumed to have a common origin. However, mass segregation and evaporation of low mass members make the determination of the IMF uncertain since a significant fraction of the initial mass can be lost in the process of emerging from molecular clouds in which the stars are formed. So the observations of very young embedded clusters are crucial in IMF determinations.

In this paper we report UBVRI -CCD observations of the central region of IC 348 down to visual magnitude 21. These observations are analysed in order to determine individual stellar relationship to the cluster, and faint members are identified. The structure and sub-clustering in the densest region of IC 348 are analysed. A redetermination of the age of the cluster is performed using recent PMS evolutionary models on the assumption that all the stars were born at the same time.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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