Physical parameters of pre-main sequence stars in open clusters^⋆

¹ Instituto de Astrofísica de Andalucía, CSIC, Apdo 3004, 18080 - Granada, Spain
e-mail: [email protected]; [email protected]
² Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
e-mail: [email protected]

Received: 10 January 2011
Accepted: 17 May 2011

Abstract

Aims. The aims are twofold: we search and analyse pre-main sequence (PMS) stars in Galactic young open clusters (YOCs) to determine the ages, masses, and spatial distribution of PMS members in YOCs, and to check and compare the performances of different model isochrones.

Methods. We compare UBVRI photometric observations to theoretical isochrones in the photometric diagrams. The comparison simultaneously provides membership assignments for both main sequence (MS) and PMS stars and estimates for the physical properties of the cluster candidate members: masses, ages, and spatial distribution inside the cluster.

Results. The photometric measurement of an average (U − V) excess, supposedly indicative of accretion disks, is considered prior to membership assignment. This photometric excess is correlated with cluster age, suggesting that disks able to show up in (U − V) excess vanish at ages around 5 Myr. We find that comparing photometry to models in the colour–magnitude (CM) diagram leads to better results for the masses than performing the comparison in the theoretical HR plane. The obtained cluster mass functions show a marginal steepening with cluster age. Significant variations in the mass function slopes are found with the models used in member selection. The clusters NGC 3293 and NGC 2362 are found to have mass functions flatter than the Salpeter slope for all models considered. The relation between the calculated dispersion of PMS age and the characteristic clustering scale of the cluster shows interesting agreement with previous findings in star-forming regions in a wide range of scales. Finally, the ratio of characteristic clustering scales for PMS candidate members in different mass ranges can be interpreted as suggesting mass segregation, in the sense of a relatively wider spatial distribution for the lower mass members in older clusters.

Conclusions. The relations between the different cluster parameters show that the procedure applied to assign cluster membership and to measure physical parameters for the selected members is well founded.