Abstract
Abstract We present results constraining the multiplicity of the very low mass stars and substellar objects in the Orion Nebula Cluster (ONC). Our sample covers primary masses 0.012–0.1 M ⊙ using archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys using multiple filters. Studying the binary populations of clusters provides valuable constraints of how the birth environment affects binary formation and evolution. Prior surveys have shown that the binary populations of high-mass, high-density star clusters like the ONC may substantially differ from those in low-mass associations. Very low mass stellar and substellar binaries at wide separations, >20 au, are statistically rare in the Galactic field and have been identified in stellar associations like Taurus-Auriga and Ophiuchus. They also may be susceptible to dynamical interactions, and their formation may be suppressed by feedback from ongoing star formation. We implement a double point-spread function (PSF) fitting algorithm using empirical, position-dependent PSF models to search for binary companions at projected separations >10 au (0.″025). With this technique, we identify seven very low mass binaries, five of which are new detections, resulting in a binary frequency of 12 − 3.2 + 6 % over mass ratios of 0.5–1.0 and projected separations of 20–200 au. We find an excess of very low mass binaries in the ONC compared to the Galactic field, with a probability of 10−6 that the populations are statistically consistent. The substellar population of the ONC may require further dynamical processing of the lowest binding energy binaries to resemble the field population.
Highlights
Developing a predictive theory of star formation requires an explanation for the formation of low-mass stars and sub-stellar objects (< 0.1 M )
For the companion only identified in the F775W filter (J053509570519426), the primary was too faint to detect in F435W and F555W, and the companion was too close in separation to detect in the F850LP filter
We found that the lowmass stellar (0.1 M < Mprim < 0.6 M ) binary frequency was consistent with the Galactic field, leaving no need for further dynamical interactions
Summary
Developing a predictive theory of star formation requires an explanation for the formation of low-mass stars and sub-stellar objects (< 0.1 M ). They dominate in number and represent a limiting case of star formation at the low mass end of the initial mass function (IMF). The initial population of binaries within young clusters precedes the eventual makeup of the field binary population Various processes, such as dynamical interactions and stellar feedback, can influence the formation and evolution the binary populations of clusters (Kroupa et al 2001a). We can study the primordial binary populations of diverse star forming regions to search for any differences relative to the Galactic field and as a function of environmental properties such as cluster mass and density
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