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Astron. Astrophys. 322, 460-476 (1997)

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Critical tests of stellar evolution in open clusters

II. Membership, duplicity, and stellar and dynamical evolution in NGC 3680 * **

B. Nordström , J. Andersen and M.I. Andersen

Niels Bohr Institute for Astronomy, Physics, and Geophysics, Astronomical Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark

Received 2 October 1996 / Accepted 3 December 1996


Based on new, accurate photometry, radial velocities, and proper motions for the intermediate-age open cluster NGC 3680, we identify individual single and binary cluster members and field stars in the colour-magnitude diagram (CMD). This basic step turns out to be crucial for a proper understanding of the cluster CMD: [FORMULA] 60% of the stars are found to be field stars, and over 50% of the cluster stars are binaries. No [FORMULA] cluster star is found more than [FORMULA] below the turnoff, and cluster stars below 1.4 [FORMULA] are only found in binary systems. The total present mass of NGC 3680 is [FORMULA] 100 [FORMULA], excluding any as yet unseen stellar remnants, and its half-mass radius is [FORMULA] (1.2 pc). Comparison with plausible IMFs indicates that only [FORMULA] 3% of the original stars and [FORMULA] 10% of the mass now survive, [FORMULA] 30% of the initial mass being in the form of massive stars that have now completed their evolution, and [FORMULA] 60% in low-mass stars which may now be located in a distant cluster halo or perhaps have been lost entirely.

The single main-sequence cluster members form an extremely tight sequence in the CMD, with [FORMULA]  = 0.034 and [Fe/H] = +0.11. A direct fit to the Hyades main sequence yields (m-M)0  = 10.5 [FORMULA] 0.2 for NGC 3680. Isochrones from several stellar models have been fit to the cluster sequence. When based on consistent uvby  colour transformations and the above cluster parameters, these fits are very stable and show that standard models are not acceptable for stars with the turnoff mass of NGC 3680. Overshooting models perform much better, but further refinement of the overshooting formalism seems to be needed. The age derived for NGC 3680 is 1.45 [FORMULA] 0.3 Gyr. The limiting factor in a precise comparison of theory and observations is now the transformation from theoretical to observed parameters, particularly (broad-band) colours.

Key words: stars: evolution; fundamental parameters; HR diagram; kinematics; luminosity function, mass function – open clusters and associations: NGC 3680

* Based on observations obtained with the Danish 1.5-m telescope at the European Southern Observatory, La Silla, Chile.
** Table 1 is available in electronic form at the CDS via anonymous ftp to

Send offprint requests to: B. Nordström (birgitta@astro.ku.dk)

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998