On the difference between nuclear and contraction ages

Free access article

Issue		A&A Volume 453, Number 1, July I 2006


Page(s)		101 - 119
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20053894

A&A 453, 101-119 (2006)
DOI: 10.1051/0004-6361:20053894

On the difference between nuclear and contraction ages

W. Lyra^{1, 2, 3}, A. Moitinho⁴, N. S. van der Bliek¹ and J. Alves⁵

¹ Cerro Tololo Interamerican Observatory, Casilla 603 La Serena, Chile
² Observatório do Valongo/UFRJ, Ladeira do Pedro Antônio 43, 20080-090 Rio de Janeiro, Brazil
³ Department of Astronomy and Space Physics, Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden
e-mail: wlyra@astro.uu.se
⁴ Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisbon, Portugal
⁵ European Southern Observatory, Karl-Schwarzschild 2, 85748 Garching, Germany

(Received 23 July 2005 / Accepted 20 February 2006 )

Abstract
Context.Ages derived from low mass stars still contracting onto the main sequence often differ from ages derived from the high mass ones that have already evolved away from it.
Aims.We investigate the general claim of disagreement between these two independent age determinations by presenting $\it UBVRI$ photometry for the young galactic open clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning the age range ~10-150 Myr
Methods.We derived reddenings, distances, and nuclear ages by fitting ZAMS and isochrones to color-magnitudes and color-color diagrams. To derive contraction ages, we used four different pre-main sequence models, with an empirically calibrated color-temperature relation to match the Pleiades cluster sequence.
Results.When exclusively using the V vs. V-I color-magnitude diagram and empirically calibrated isochrones, there is consistency between nuclear and contraction ages for the studied clusters. Although the contraction ages seem systematically underestimated, in none of the cases do they deviate by more than one standard deviation from the nuclear ages.

Key words: Galaxy: open clusters and associations: general -- galaxies: star clusters -- stars: formation -- techniques: photometric -- Hertzsprung-Russell (HR) and C-M diagrams

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