Accretion and dynamical interactions in small-Nstar-forming clusters:N= 5

Abstract

We present results from high‐resolution hydrodynamical simulations that explore the effects of small‐scale clustering in star‐forming regions. A large ensemble of small‐N clusters with five stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data. Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s−1, which might deprive low‐mass star‐forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and multiples is found to follow a distribution similar to that of observed long‐period (uncircularized) binaries. The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low‐mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low‐mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores.

Multiple protostellar systems

“…Global simulations of cloud collapse support this scenario and predict the formation of even higher density subclusters consisting of 10-50 protostellar seeds (Klessen et al 1998;Bate et al 2003;Bonnell et al 2003). Small-N protostellar aggregates as well as high density subclusters undergo rapid dynamical evolution on a timescale of 10 4−5 yr (Sterzik & Durisen 1998;Delgado-Donate et al 2003). The dynamical decay of initially unstable groups is thus expected to eject most of the lowest mass protostellar seeds, with a typical ejection velocity of < ∼ 1 km s −1 , while those remaining in the cloud core form stable binary and higher order multiple systems.…”

“…The frequency and orbital properties of protostellar systems are expected to evolve on various timescales due to a number of processes, such as dynamical decay of initially unstable configurations (∼10 4−5 yr, e.g., Delgado-Donate et al 2003;Goodwin et al 2004a) or disruptive encounters with other systems if located in dense environments (∼10 5−6 yr). In this context, a somewhat surprising result is the similar frequency and properties we find for protostellar systems in both Taurus and Ophiuchus.…”

“…It is often assumed that prestellar cores actually experience multiple fragmentation as they collapse thus leading to the formation of several protostellar seeds, i.e., small-N protostellar aggregates with N = 3-10, typically, on a scale of a few 100 AU (e.g. Burkert et al 1997;Sterzik & Durisen 1998;Delgado-Donate et al 2003;Goodwin et al 2004a). Global simulations of cloud collapse support this scenario and predict the formation of even higher density subclusters consisting of 10-50 protostellar seeds (Klessen et al 1998;Bate et al 2003;Bonnell et al 2003).…”

“…Numerical simulations of the dynamical decay of small-N clusters including accretion onto the protostellar seeds and circumstellar disks (e.g. Delgado-Donate et al 2003;Goodwin et al 2004a) predict the formation of an equal number of tight binaries and higher order (triple and quadruple) systems. This prediction also seems to contrast with our results, although it may simply indicate that a fraction of our sample primaries actually are tight binaries with a separation less than 100 AU and have therefore gone unresolved in this study.…”

Discovery of a Very Low Mass Triple With Late-M and T Dwarf Components: Lp 704-48/SDSS J0006–0852ab

“…However, estimates for the frequency of tightly bound VLM spectroscopic binaries (SBs; roughly 10% of currently known systems) span a broad range of 1%-25% (Maxted & Jeffries 2005;Basri & Reiners 2006;Kurosawa et al 2006;Joergens 2008;Blake et al 2010;Clark et al 2012). As such, there remains significant uncertainty in the overall VLM binary frequency, which has a direct impact on our understanding of how brown dwarfs form in the first place (e.g., Delgado-Donate et al 2003). There is also evidence that the orbital characteristics of multiples are mass dependent, with VLM binaries being on average tighter ( a ≈ 7 AU versus ≈ 30 AU) and more frequently composed of equal-mass components ( q ≡ M 2 /M 1 ≈ 1 versus ≈ 0.3) than their solar-mass counterparts (Allen 2007).…”