Multiple Star Formation to the Bottom of the Initial Mass Function

Abstract

The frequency and properties of multiple star systems offer powerful tests of star formation models. Multiplicity surveys over the past decade have shown that binary properties vary strongly with mass, but the functional forms and the interplay between frequency and semimajor axis remain largely unconstrained. We present the results of a large-scale survey of multiplicity at the bottom of the IMF in several nearby young associations, encompassing 78 very low mass members observed with Keck laser guide star adaptive optics. Our survey confirms the overall trend observed in the field for lower-mass binary systems to be less frequent and more compact, including a null detection for any substellar binary systems with separations wider than ∼7 AU. Combined with a Bayesian re-analysis of existing surveys, our results demonstrate that the binary frequency and binary separations decline smoothly between masses of 0.5 M ⊙ and 0.02 M ⊙ , though we can not distinguish the functional form of this decline due to a degeneracy between the total binary frequency and the mean binary separation. We also show that the mass ratio distribution becomes progressively more concentrated at q ∼1 for declining masses, though a small number of systems appear to have unusually wide separations and low mass ratios for their mass. Finally, we compare our results to synthetic binary populations generated by smoothed particle hydrodynamic simulations, noting the similarities and discussing possible explanations for the differences. Subject headings: stars:binaries:visual-stars:low-mass,brown dwarfs-stars:pre-main sequence 2.2. Observations Most of the data that we summarize were obtained in 4 observing runs, totaling 10 nights, between December 2005 and March 2007. One source was observed during a time trade in December 2006. Most of our ob-

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…Todorov et al (2014) searched for companions to young brown dwarfs in Taurus and Chamaeleon I. Resolving binaries with separations larger than 10 AU, they find, combining their results with the studies of Kraus et al (2006), Konopacky et al (2007) and Kraus & Hillenbrand (2012), a binary fraction of 18 +8 −4 % for M4-M6 type binaries (0.1 − 0.3M ⊙ ) and 4 +5 −1 % for binaries in their sample with spectral type >M6 (< 0.1M ⊙ ) in Taurus. Given its youth and low-density, Todorov et al (2014) suggest that the observed reduction of binary fraction towards later spectraltypes might be primordial.…”

Discovery of wide low and very low-mass binary systems using Virtual Observatory tools

“…Burgasser et al (2007) presented an extensive review on the multiplicity of VLM objects, reporting important differences between them and the more massive stars. In particular, VLM objects show a lower multiplicity ratio, 20-25% (Kraus & Hillenbrand 2012), compared to ≈80% for B stars (Kouwenhoven et al 2005), ≈65% for G stars (Duquennoy & Mayor 1991) and ≈40% for M2-M5 dwarfs (Fischer & Marcy 1992), and the mass-ratio distribution strongly peaks at unity whereas it is more evenly distributed for more massive stars. Also, the separation between components is significantly smaller for VLM objects.…”

“…Typical separation is ∼4 AU (e.g. Close et al 2003;Burgasser et al 2007;Kraus & Hillenbrand 2012), from statistical samples with separation over 3 AU due to the resolving power of imaging programmes. If works with samples with separation under 3 AU are included the total binary fractions for VLM stars and brown dwarfs (BDs) could range between 2-50% (Bardalez Gagliuffi et al 2014).…”

Astrometric orbit of a low-mass companion to an ultracool dwarf

“…Because its last update was in July 2009, we searched the literature for new systems and revised parameters. We added 24 systems from Choi et al (2013), Burgasser et al (2011), Allers et al (2010, Gelino & Burgasser (2010), Liu et al (2010Liu et al ( , 2011, Kraus & Hillenbrand (2012), Gelino et al (2011), Chauvin et al (2012, Geyer et al (1988), Phan-Bao et al (2006), Allers et al (2009), andBurgasser et al (2009), and we updated system parameters, when applicable, using the compilations of Liu et al (2010), , and Kraus & Hillenbrand (2012).…”