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Protostellar Discs (protostellar + disc)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Protostellar discs formed from rigidly rotating cores

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009
S. Walch
ABSTRACT We use three-dimensional smoothed particle hydrodynamic simulations to investigate the collapse of low-mass pre-stellar cores and the formation and early evolution of protostellar discs. The initial conditions are slightly supercritical Bonnor,Ebert spheres in rigid rotation. The core mass and initial radius are held fixed at MO= 6.1 M, and RO= 17 000 au, and the only parameter that we vary is the initial angular speed ,O. Protostellar discs forming from cores with ,O < 1.35 × 10,13 s,1 have radii between 100 and 300 au and are quite centrally concentrated; due to heating by gas infall on to the disc and accretion on to the central object, they are also quite warm, , and therefore stable against gravitational fragmentation. In contrast, more rapidly rotating cores form discs which are less concentrated and cooler, and have radii between 400 and 1000 au; as a consequence they are prone to gravitational fragmentation and the formation of multiple systems. We derive a criterion that predicts whether a rigidly rotating core having given MO, RO and ,O will produce a protostellar disc which fragments whilst material is still infalling from the core envelope. We then apply this criterion to core samples for which MO, RO and ,O have been estimated observationally. We conclude that the observed cores are stable against fragmentation at this stage, due to their low angular speeds and the heat delivered at the accretion shock where the infalling material hits the disc. [source]

Limits on the location of planetesimal formation in self-gravitating protostellar discs

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009
C. J. Clarke
ABSTRACT In this Letter, we show that if planetesimals form in spiral features in self-gravitating discs, as previously suggested by the idealized simulations of Rice et al., then in realistic protostellar discs, this process will be restricted to the outer regions of the disc (i.e. at radii in excess of several tens of au). This restriction relates to the requirement that dust has to be concentrated in spiral features on a time-scale that is less than the (roughly dynamical) lifetime of such features, and that such rapid accumulation requires spiral features whose fractional amplitude is not much less than unity. This in turn requires that the cooling time-scale of the gas is relatively short, which restricts the process to the outer disc. We point out that the efficient conversion of a large fraction of the primordial dust in the disc into planetesimals could rescue this material from the well-known problem of rapid inward migration at an approximate metre-size scale and that in principle the collisional evolution of these objects could help to resupply small dust to the protostellar disc. We also point out the possible implications of this scenario for the location of planetesimal belts inferred in debris discs around main sequence stars, but stress that further dynamical studies are required in order to establish whether the disc retains a memory of the initial site of planetesimal creation. [source]

Protostellar discs formed from rigidly rotating cores

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009
S. Walch
ABSTRACT We use three-dimensional smoothed particle hydrodynamic simulations to investigate the collapse of low-mass pre-stellar cores and the formation and early evolution of protostellar discs. The initial conditions are slightly supercritical Bonnor,Ebert spheres in rigid rotation. The core mass and initial radius are held fixed at MO= 6.1 M, and RO= 17 000 au, and the only parameter that we vary is the initial angular speed ,O. Protostellar discs forming from cores with ,O < 1.35 × 10,13 s,1 have radii between 100 and 300 au and are quite centrally concentrated; due to heating by gas infall on to the disc and accretion on to the central object, they are also quite warm, , and therefore stable against gravitational fragmentation. In contrast, more rapidly rotating cores form discs which are less concentrated and cooler, and have radii between 400 and 1000 au; as a consequence they are prone to gravitational fragmentation and the formation of multiple systems. We derive a criterion that predicts whether a rigidly rotating core having given MO, RO and ,O will produce a protostellar disc which fragments whilst material is still infalling from the core envelope. We then apply this criterion to core samples for which MO, RO and ,O have been estimated observationally. We conclude that the observed cores are stable against fragmentation at this stage, due to their low angular speeds and the heat delivered at the accretion shock where the infalling material hits the disc. [source]

Substellar companions and isolated planetary-mass objects from protostellar disc fragmentation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2003
W. K. M. Rice
ABSTRACT Self-gravitating protostellar discs are unstable to fragmentation if the gas can cool on a time-scale that is short compared with the orbital period. We use a combination of hydrodynamic simulations and N -body orbit integrations to study the long-term evolution of a fragmenting disc with an initial mass ratio to the star of Mdisc/M*= 0.1. For a disc that is initially unstable across a range of radii, a combination of collapse and subsequent accretion yields substellar objects with a spectrum of masses extending (for a Solar-mass star) up to ,0.01 M,. Subsequent gravitational evolution ejects most of the lower mass objects within a few million years, leaving a small number of very massive planets or brown dwarfs in eccentric orbits at moderately small radii. Based on these results, systems such as HD 168443 , in which the companions are close to or beyond the deuterium burning limit , appear to be the best candidates to have formed via gravitational instability. If massive substellar companions originate from disc fragmentation, while lower-mass planetary companions originate from core accretion, the metallicity distribution of stars which host massive substellar companions at radii of ,1 au should differ from that of stars with lower mass planetary companions. [source]

Limits on the location of planetesimal formation in self-gravitating protostellar discs

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009
C. J. Clarke
ABSTRACT In this Letter, we show that if planetesimals form in spiral features in self-gravitating discs, as previously suggested by the idealized simulations of Rice et al., then in realistic protostellar discs, this process will be restricted to the outer regions of the disc (i.e. at radii in excess of several tens of au). This restriction relates to the requirement that dust has to be concentrated in spiral features on a time-scale that is less than the (roughly dynamical) lifetime of such features, and that such rapid accumulation requires spiral features whose fractional amplitude is not much less than unity. This in turn requires that the cooling time-scale of the gas is relatively short, which restricts the process to the outer disc. We point out that the efficient conversion of a large fraction of the primordial dust in the disc into planetesimals could rescue this material from the well-known problem of rapid inward migration at an approximate metre-size scale and that in principle the collisional evolution of these objects could help to resupply small dust to the protostellar disc. We also point out the possible implications of this scenario for the location of planetesimal belts inferred in debris discs around main sequence stars, but stress that further dynamical studies are required in order to establish whether the disc retains a memory of the initial site of planetesimal creation. [source]