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

TL;DR

This study uses lithium abundance measurements in pre-main sequence stars of NGC 2264 to constrain the cluster's formation timescale, suggesting it occurred within less than 5 million years.

Contribution

It introduces lithium abundance as a more reliable relative age indicator for PMS stars, providing a tighter constraint on cluster formation timescale compared to traditional methods.

Findings

Li abundance indicates an age spread of 3-4 Myr among PMS stars.

Cluster formation in NGC 2264 likely occurred within less than 5 Myr.

The initial Li abundance of NGC 2264 is A(Li) = 3.2 +/- 0.2.

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.