The properties of brown dwarfs and low-mass hydrogen-burning stars formed by disc fragmentation

By D. Stamatellos, A. Whitworth. 2008

280 citations recorded by the source corpus.

We suggest that a high proportion of brown dwarf (BD) stars are formed by gravitational fragmentation of massive extended discs around Sun-like primary stars. We argue that such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs typically fragment within a few thousand years, and produce mainly BD stars, but also planetary-mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Subsequently most of the lower mass stars (i.e. the PM and BD stars) are ejected by mutual interactions. We analyse the statistical properties of these stars, and compare them with observations. After a few hundred thousand years the Sun-like primary is typically left with a close low-mass HB companion, and two much wider companions: a low-mass HB star and a BD star, or a BD–BD binary. The orbits of these companions are highly eccentric, and not necessarily coplanar, either with one another, or with the original disc. There is a BD desert extending out to at least ∼100 au; this is because BDs tend to be formed further out than low-mass HB stars, and then they tend to be scattered even further out, or even into the field. BDs form with discs of a few Jupiter masses and radii of a few tens of au, and they are more likely to retain these discs if they remain bound to the primary star. Binaries form by pairing of the newly formed stars in the disc, giving a low-mass binary fraction of ∼0.16. These binaries include close and wide BD/BD binaries and BD/PM binaries. Binaries can be ejected into the field and survive, even if they have quite wide separations. BDs that remain as companions to Sun-like stars are more likely to be in BD/BD binaries than are BDs ejected into the field. The presence of close and distant companions around Sun-like stars may inhibit planet formation by core accretion. We conclude that disc fragmentation is a robust mechanism for BD formation. Even if only a small fraction of Sun-like stars host the required massive extended discs, this mechanism can produce all the PM stars observed, most of the BD stars and a significant proportion of the very low-mass HB stars.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/14405412ce94bbe0e50c0c38a8e9f2882ec59612

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Citations in the corpus

ANALYTICAL THEORY FOR THE INITIAL MASS FUNCTION. II. PROPERTIES OF THE FLOW (2009, background): “It is not excluded, however, that the low mass end of the IMF might be affected by secondary mechanisms such as, for instance, dynamical ejections and radiative feedback (Bate 2009) or massive disk fragmentation (Stamatellos & Whitworth 2009).”
THE RUNTS OF THE LITTER: WHY PLANETS FORMED THROUGH GRAVITATIONAL INSTABILITY CAN ONLY BE FAILED BINARY STARS (2009, background): “While more numerical follow up will be necessary to pin down the true distribution of fragments born through GI, at present simulations show that this estimate may well be a lower limit, but is the correct order of magnitude (Boley 2009; Stamatellos & Whitworth 2009).”
Individual Dynamical Masses of Ultracool Dwarfs (2017, result): “…result, reinforcing the conclusion of Dupuy & Liu (2011) that very-low mass binaries are very inconsistent with the high eccentricity orbits found in simulations from Stamatellos & Whitworth (2009) but consistent with the more modest eccentricities predicted from the simulations of Bate (2009).”
The stellar and sub-stellar IMF of simple and composite populations (2011, background): “…formation is via the fragmentation of massive circum-stellar disks (FMCSDs) beyond about 100 AU and subsequent dynamical separation of the resulting weekly bound BD companion (Goodwin & Whitworth 2007; Stamatellos et al. 2007a; Stamatellos & Whitworth 2009; Thies et al. 2010; Basu & Vorobyov 2012).”
Dynamics of Protoplanetary Disks (2010, background): “…then suggests molecular cloud cores with sufficiently high specific angular momentum would be susceptible to fragmentation at their outer edge (Kratter, Murray-Clay & Youdin 2010; Matzner & Levin 2005; Rice, Mayo & Armitage 2010; Stamatellos, Hubber & Whitworth 2007; Stamatellos & Whitworth 2009).”
THE VLA NASCENT DISK AND MULTIPLICITY SURVEY OF PERSEUS PROTOSTARS (VANDAM). II. MULTIPLICITY OF PROTOSTARS IN THE PERSEUS MOLECULAR CLOUD (2016, background): “…rotating, and infalling core (e.g., Inutsuka & Miyama 1992; Burkert & Bodenheimer 1993; Boss & Keiser 2013, 2014), and/or (3) the fragmentation of a gravitationally unstable circumstellar disk (e.g., Adams et al. 1989; Bonnell & Bate 1994a, 1994b; Machida et al. 2008; Stamatellos & Whitworth 2009).”
THE EFFECTS OF RADIATIVE TRANSFER ON LOW-MASS STAR FORMATION (2009, background): “Gravitational instability is expected to occur only in the regime where the mass of the disks is comparable to the stellar mass (Cai et al. 2008; Stamatellos & Whitworth 2008, 2009).”
TRANSITIONAL DISKS AS SIGNPOSTS OF YOUNG, MULTIPLANET SYSTEMS (2011, background): “As a bonus, gravitational instability tends to produce hotter, more massive planets than core accretion (Clarke 2009, Stamatellos & Whitworth 2009), exactly the types of planets best suited to discovery by direct imaging and aperture masking.”
PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS (2014, background): “…low-mass stars have effective temperatures reaching early-L spectral types, so this frequency is slightly different from the substellar companion fraction. is the most likely place for giant planets to form from this mechanism (e.g., Stamatellos & Whitworth 2009; Vorobyov & Basu 2010; Boss 2011).”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, methodology): “…2005; Bate, Bonnell & Bromm 2002, 2003; Goodwin & Whitworth 2007; Offner, Klein & McKee 2008; Offner et al. 2009; Rice et al. 2003; Shen et al. 2010; Stamatellos, Hubber & Whitworth 2007; Stamatellos & Whitworth 2009; Stamatellos et al. 2011; Thies et al. 2010; Whitworth & Stamatellos 2006).”
Stellar, brown dwarf and multiple star properties from a radiation hydrodynamical simulation of star cluster formation (2011, background): “…molecular cloud cores (e.g. Delgado-Donate, Clarke & Bate 2004a; Delgado-Donate et al. 2004b; Goodwin, Whitworth & Ward-Thompson 2004a,b,c, C© 2011 The Author Monthly Notices of the Royal Astronomical Society C© 2011 RAS 2006; Stamatellos, Hubber & Whitworth 2007; Stamatellos & Whitworth 2009).”
Gravitational Instabilities in Circumstellar Disks (2016, background): “If one aims to capture more realistic thermal physics, radiative transfer must be properly accounted for (Boley et al. 2006; Stamatellos & Whitworth 2009b; Krumholz et al. 2009).”
THE FORMATION OF LOW-MASS BINARY STAR SYSTEMS VIA TURBULENT FRAGMENTATION (2010, background): “…studies suggested that Electronic address: soffner@cfa.harvard.edu this process was limited to large mass ratio systems, recent work such as Stamatellos & Whitworth (2009) and Kratter et al. (2010a) has shown that when disks continue to be fed at their outer edges, the companions can grow…”