A constraint on the formation timescale of the young open cluster NGC 2264: Lithium abundance of pre-main sequence stars

Abstract

The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered as a result of a real spread in age, the corresponding cluster formation timescale would be about 5 -- 20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars, can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line (3500 < T_eff [K] <= 6500). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is A(Li) = 3.2 +/- 0.2. From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3 -- 4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr.

53 pages, 12 figures, 4 tables, Accepted for publication in the Astrophysical Journal

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• Double-peaked OI profile: a likely signature of the gaseous ring around KH 15D
• Sustaining Star Formation in the Galactic Star Cluster M 36?
• UBVI CCD Photometry of Berkeley 55 Open Cluster
• The Sejong Open cluster Survey (SOS) VI. A small star-forming region in the high Galactic latitude molecular cloud MBM 110
• A Gaia view on the star formation in the Monoceros OB1 and R1 associations
• Rotational Velocities and Radii Estimates of Low-Mass Pre-Main Sequence Stars in NGC 2264