The Observed Multiplicity of Low-mass Stars: From Embedded Protostars to Open Clusters

Summary

The multiplicity of stars is a direct tracer of the conditions under which they form and, in particular, it provides constraints on the modes of fragmentation for pre-stellar cores. For several years, we and other groups have conducted systematic surveys for the multiplicity of solar-like and lower-mass stars in nearby young open clusters (a few to a few hundred Myr) to investigate the impact of environmental conditions and/or dynamical processes. These surveys have led to the conclusion that the multiplicity of low mass stars is established by an age of a few million years at most. They have left open, however, the possibility that significant changes occur in the earlier, more embedded phases of protostellar evolution. We have recently started a statistical survey for multiplicity among embedded protostars in order to probe such a possibility. In this contribution, we summarize both types of surveys, i.e., the multiplicity of low-mass stars in young open clusters and that of embedded protostars. Using direct and high-angular resolution imaging, tens of companions have been discovered, leading to statistically significant multiplicity rates that can be compared with the predictions of (multiple) star formation models.