Low-mass binaries in the young cluster IC 348: implications for binary formation and evolution

We report on a near-infrared adaptive optics survey of a sample of 66 low-mass members of the premain sequence stellar cluster IC 348. We find 12 binary systems in the separation range 0. 1–8, excluding 3 probable background projected companions. An estimate of the number of faint undetected companions is derived, before we evaluate the binary frequency in this cluster. In the range logP =5.0–7.9days, the binary fraction in IC 348 is 19± 5 %. This is similar to the values corresponding to Gand M-dwarfs in the solar neighbourhood population (23±3 % and ∼ 18%, respectively). Furthermore, the distribution of orbital periods of IC 348 binaries in this range is consistent with that of field binaries. We conclude that there is no binary excess in IC 348. Substellar companions are found to be rare, or even missing, as companions of low-mass stars in the separation range we surveyed. Also, the mass ratio distribution is not peaked at q ≈ 1 in IC 348, and it is unlikely that an observational bias can account for that. We do not find any evidence for an evolution of the binary frequency with age within the age spread of the cluster of about 10Myr. Comparing the binary frequency in IC 348 with that of other star forming regions (SFRs) and young open clusters, we conclude that there is no significant temporal evolution of the binary fraction between a few Myrs after the formation process and the zero-age main sequence (ZAMS) and field populations. We find instead a trend for the binary fraction to be inversely correlated with stellar density, with dense clusters having a binary fraction similar to that of field dwarfs and loose associations exhibiting an excess of binaries. Two scenarios can be suggested to explain these differences: either all SFRs, clusters and associations alike, initially host a large number of binaries, which is subsequently reduced only in dense clusters on a timescale of less than 1 Myr due to numerous gravitational encounters, or specific iniSend offprint requests to: G. Duchêne ⋆ Based on observations made with the Canada-FranceHawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii tial conditions in the parental molecular clouds impact on the fragmentation process leading to intrinsically different binary fractions from one SFR to the other.