The Young Cluster IC 348

Abstract

CCD photometry in BVRI was obtained for about 260 stars in and around IC 348, and multiobject spectroscopy for 80 of these. A somewhat larger region was surveyed for stars having Hα in emission; over 110 emission-line stars brighter than about R = 19 were discovered. Because Hα emission could be detected to a limit near W = 3 A, division into weak-line (WTTSs) and classical T Tauri stars (CTTSs) was possible on purely spectroscopic grounds. There is a steep rise in the number of emission-line stars below W(Hα) = 10 A; the proportion of WTTSs to CTTSs in the area surveyed is 58:51. ROSAT detected only about 58% of the spectroscopic WTTSs and about 65% of the CTTSs, although these numbers are sensitive to the survey thresholds. The bulk of the ages of about 100 stars, read off the theoretical tracks of D'Antona & Mazzitelli, range between about 0.7 and 12 Myr, but the emission-line stars, which are most likely to be members of IC 348, have a mean age of 1.3 Myr. Allowance for unresolved binaries would increase this somewhat, but there is a firm upper limit at 2.95 Myr. There is no indication that the ages of the emission-line stars depend upon W(Hα): the IC 348 WTTSs as a population are not systematically older than the CTTSs, but there is a tendency for the WTTSs to be concentrated toward the center of IC 348, while the CTTSs are more widely distributed. There is a scattering of emission-Hα stars over the entire area surveyed. There are too many to be explained as low-mass members of an earlier generation of star formation in Per OB2 or as foreground dMe stars. The mass frequency function, based on some 125 stars fitted to theoretical tracks, rises from 1.5 M☉ to about 0.2 M☉, with a slope very much like that of the Scalo initial mass function. The optical cluster IC 348 radius is about 40, or 0.37 pc. The total mass of optically detectable stars in this volume is 57 M☉, while the mean space density is about 520 stars pc-3. The amount of interstellar material remaining within the cluster is small in comparison. Star formation in the Per OB2/IC 348 region cannot be characterized by one unique age; it appears that stars have been forming in the region now occupied by the association for 10-20 Myr.