EDP Sciences
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Volume 374, Number 2, August I 2001
Page(s) 504 - 522
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20010642

A&A 374, 504-522 (2001)
DOI: 10.1051/0004-6361:20010642

NGC 7654: An interesting cluster to study star formation history

A. K. Pandey1, Nilakshi1, K. Ogura2, Ram Sagar1 and K. Tarusawa3

1  State Observatory, Manora Peak, Naini Tal, 263 129, India
2  Kokugakuin University, Higashi, Shibuya-ku, Tokyo 150-8440, Japan
3  Kiso Observatory, School of Science, University of Tokyo, Mitake-mura, Kiso-gun, Nagano 397-0101, Japan

(Received 8 February 2001 / Accepted 27 April 2001 )

CCD $UBVI_{\rm C}$ photometry in a wide field around the open cluster NGC 7654 has been carried out for ~17 860 stars, down to $V\sim20$. The reddening across the cluster region is found to be variable with $E(B-V)_{\rm min} = 0.46$ to $E(B-V)_{\rm max}=0.80$. The cluster is situated at a distance of 1380 $\pm$ 70 pc. The colour magnitude diagrams show a large age spread in the ages. Star formation was biased towards relatively higher masses during the early phase of star formation whereas most of the low mass stars of the cluster were formed during the later phase. The star formation seems to have been a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars. The present data do not support a uniform mass function (MF) for different regions in the cluster. Although for the whole cluster region, the MF in the observed mass range $0.8 \leq M_{\odot}
\leq 4.5$, can be represented by a single power law with a slope $\Gamma= -1.40 \pm 0.07$, however it indicates various features when examined carefully. In three subregions of the cluster the slope $\Gamma$, for the mass range ~ $1.5< M_{\odot}< 4.0$, comes out to be $-1.07\pm 0.08$ and $-1.28\pm 0.20$ for the inner and intermediate regions and it becomes steeper in the outermost region ( $\Gamma = -2.78 \pm 0.21$). For stars having masses $< \sim$1.5 $M_{\odot}$ the MF, in the inner and intermediate regions, can be represented by a power law having a steeper slope, whereas in the outer region a turnover can be seen in the MF at $M\sim 1.5 M_{\odot}$. The age of NGC 7654 is found to be comparable to its two-body relaxation time-scale; therefore this may also be a reason for the observed mass segregation in the cluster.

Key words: open clusters and associations: general -- ISM: dust extiction -- stars: formation

Offprint request: A. K. Pandey, pandey@upso.ernet.in

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