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Volume 457, Number 1, October I 2006
Page(s) 265 - 280
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361:20065392
The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611

A&A 457, 265-280 (2006)
DOI: 10.1051/0004-6361:20065392

The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611

P. L. Dufton1, S. J. Smartt1, J. K. Lee1, R. S. I. Ryans1, I. Hunter1, C. J. Evans2, A. Herrero3, 4, C. Trundle1, 3, D. J. Lennon5, 3, M. J. Irwin6 and A. Kaufer7

1  Department of Physics and Astronomy, Queen's University Belfast, BT7 1NN, UK
    e-mail: p.dufton@qub.ac.uk
2  UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland, UK
3  Instituto de Astrofísica de Canarias, Calle Vía Làctea, 38200 La Laguna, Tenerife, Canary Islands, Spain
4  Departamento de Astrofísica, Avda. Astrof. Francisco Sanchez s/n, Universidad de La Laguna, 38071 La Laguna, Tenerife, Canary Islands, Spain
5  The Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain
6  Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
7  European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile

(Received 7 April 2006 / Accepted 21 June 2006 )

An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The origin of the low gravity estimates for apparently near main sequence objects is discussed and is related to the stellar rotational velocity. The atmospheric parameters have been used to estimate cluster distances (which are generally in good agreement with previous determinations) and these have been used to estimate stellar luminosities and evolutionary masses. The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC 3293 and NGC 4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC 3293 and NGC 4755, the peak of the velocity distribution would be 250 km s-1 with a full-width-half-maximum of approximately 180 km s-1. For NGC 6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with $\Gamma$-values in the range of -1.5 to -1.8, which are similar to other young stellar clusters in the Milky Way.

Key words: stars: early-type -- stars: fundamental parameters -- stars: rotation -- Hertzsprung-Russell (HR) and C-M diagrams -- stars: evolution

© ESO 2006