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The timescale of cluster formation is an essential parameter in order tounderstand the formation process of star clusters. Pre-main sequence (PMS)stars in nearby young open clusters reveal a large spread in brightness. If thespread were considered as a result of a real spread in age, the correspondingcluster formation timescale would be about 5 -- 20 Myr. Hence it could beinterpreted that star formation in an open cluster is prolonged for up to a fewtens of Myr. However, difficulties in reddening correction, observationalerrors, and systematic uncertainties introduced by imperfect evolutionarymodels for PMS stars, can result in an artificial age spread. Alternatively, wecan utilize Li abundance as a relative age indicator of PMS star to determinethe cluster formation timescale. The optical spectra of 134 PMS stars in NGC2264 have been obtained with MMT/Hectochelle. The equivalent widths have beenmeasured for 86 PMS stars with a detectable Li line (3500 < T_eff [K] <= 6500).Li abundance under the condition of local thermodynamic equilibrium (LTE) wasderived using the conventional curve of growth method. After correction fornon-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 agespread 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 thatthe cluster formed on a timescale shorter than 5 Myr.

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