Star formation through gravitational collapse and competitive accretion

Competitive accretion, a process to explain the origin of the initial mass function (IMF), occurs when stars in a common gravitational potential accrete from a distributed gaseous component. Stars located near the centre of the potential benefit from the gravitational attraction of the full potential and accrete at much higher rates than do isolated stars. We show that concerns recently raised on the efficiency of competitive accretion are incorrect as they use globally averaged properties which are inappropriate for the detailed physics of a forming stellar cluster. A full treatment requires a realistic treatment of the cluster potential, the distribution of turbulent velocities and gas densities. Accreting gas does not travel at the global virial velocity of the system due to the velocity-sizescale relation inherent in turbulent gas and due to the lower velocity dispersion of small-N clusters in which much of the accretion occurs. Accretion occurs due to the effect of the local potential in funnelling gas down to the centre. Stars located in the gas-rich centres of such systems initially accrete from low relative velocity gas attaining larger masses before needing to accrete the higher velocity gas. Stars not in the centres of such potentials, or that enter the cluster later when the velocity dispersion is higher, do not accrete significantly and thus retain their low masses. In competitive accretion, most stars do not continue to accrete significantly such that their masses are set from the fragmentation process. It is the few stars which continue to accrete that become higher-mass stars. Competitive accretion is therefore likely to be responsible for the formation of higher-mass stars and can explain the mass distribution, mass segregation and binary frequency of these stars. Global kinematics of competitive accretion models include large-scale mass infall, with mean inflow velocities of the order of ≈0.5 km s -1 at scales of 0.5 pc, but infalling signatures are likely to be confused by the large tangential velocities and the velocity dispersion present. Finally, we discuss potential limitations of competitive accretion and conclude that competitive accretion is currently the most likely model for the origin of the high-mass end of the IMF.

[1]  C. Clarke,et al.  The Jeans mass and the origin of the knee in the IMF , 2006, astro-ph/0603444.

[2]  N. Bastian,et al.  Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar Clusters , 2006, astro-ph/0602465.

[3]  Zhi-Yun Li,et al.  Cluster Formation in Protostellar Outflow-driven Turbulence , 2005, astro-ph/0512278.

[4]  N. Peretto,et al.  Probing the formation of intermediate- to high-mass stars in protoclusters - A detailed millimeter study of the NGC 2264 clumps , 2005, astro-ph/0508619.

[5]  R. Klein,et al.  The formation of stars by gravitational collapse rather than competitive accretion , 2005, Nature.

[6]  P. Kroupa,et al.  The maximum stellar mass, star-cluster formation and composite stellar populations , 2005, astro-ph/0511331.

[7]  G. Fuller,et al.  The circumstellar environment of high mass protostellar objects. III. Evidence of infall , 2005, astro-ph/0508098.

[8]  I. Bonnell,et al.  Binary systems and stellar mergers in massive star formation , 2005, astro-ph/0506689.

[9]  S. Bontemps,et al.  The earliest phases of massive star formation within entire molecular cloud complexes , 2005, Proceedings of the International Astronomical Union.

[10]  R. Larson.  Thermal physics, cloud geometry and the stellar initial mass function , 2005 .

[11]  C. Clarke,et al.  Photoionizing feedback in star cluster formation , 2005, astro-ph/0501160.

[12]  R. Cesaroni.  Massive star birth : a crossroads of astrophysics : proceedings of the 227th symposium of the International Astronomical Union, held in Acireale, Italy, May 16-20, 2005 , 2005 .

[13]  M. Felli,et al.  Massive star birth: A crossroads of Astrophysics , 2005 .

[14]  R. Klein,et al.  How Protostellar Outflows Help Massive Stars Form , 2004, astro-ph/0411526.

[15]  I. Bonnell,et al.  The origin of the initial mass function and its dependence on the mean Jeans mass in molecular clouds , 2004, astro-ph/0411084.

[16]  C. Brunt,et al.  The Universality of Turbulence in Galactic Molecular Clouds , 2004, astro-ph/0409420.

[17]  B. Elmegreen.  Variability in the stellar initial mass function at low and high mass: three‐component IMF models , 2004, astro-ph/0408231.

[18]  D. Johnstone,et al.  An Extinction Threshold for Protostellar Cores in Ophiuchus , 2004, astro-ph/0406640.

[19]  I. Bonnell,et al.  Massive star formation: nurture, not nature , 2004, astro-ph/0401059.

[20]  Zhi-Yun Li,et al.  Does Magnetic Levitation or Suspension Define the Masses of Forming Stars? , 2003, astro-ph/0311426.

[21]  P. Padoan,et al.  The “Mysterious” Origin of Brown Dwarfs , 2002, astro-ph/0205019.

[22]  I. Bonnell,et al.  The hierarchical formation of a stellar cluster , 2003, astro-ph/0305082.

[23]  C. Lada,et al.  Embedded Clusters in Molecular Clouds , 2003, astro-ph/0301540.

[24]  Volker Bromm,et al.  The formation of a star cluster: predicting the properties of stars and brown dwarfs , 2002, astro-ph/0212380.

[25]  C. McKee,et al.  The Formation of Massive Stars from Turbulent Cores , 2002, astro-ph/0206037.

[26]  I. Bonnell,et al.  The formation of close binary systems by dynamical interactions and orbital decay , 2002, astro-ph/0212403.

[27]  I. Bonnell,et al.  Accretion in stellar clusters and the collisional formation of massive stars , 2002 .

[28]  I. Bonnell,et al.  The formation mechanism of brown dwarfs , 2002, astro-ph/0206365.

[29]  H. Yorke,et al.  On the Formation of Massive Stars , 2002, astro-ph/0201041.

[30]  V. Debattista,et al.  A fast bar in the post-interaction galaxy NGC 1023 , 2001, astro-ph/0112288.

[31]  K. Menten,et al.  Massive molecular outflows , 2001, astro-ph/0112508.

[32]  J. Bally,et al.  HERBIG-HARO FLOWS: Probes of Early Stellar Evolution , 2001 .

[33]  C. Clarke,et al.  Accretion in stellar clusters and the initial mass function , 2001 .

[34]  Ralf Klessen,et al.  (ACCEPTED FOR PUBLICATION IN APJ) Preprint typeset using L ATEX style emulateapj v. 04/03/99 THE FORMATION OF STELLAR CLUSTERS: MASS SPECTRA FROM TURBULENT MOLECULAR CLOUD , 2001 .

[35]  B. Reipurth,et al.  The Formation of Brown Dwarfs as Ejected Stellar Embryos , 2001, astro-ph/0103019.

[36]  C. Clarke,et al.  Accretion in stellar clusters and the IMF , 2001, astro-ph/0102121.

[37]  C. Clarke,et al.  Competitive accretion in embedded stellar clusters , 2001, astro-ph/0102074.

[38]  D. Johnstone,et al.  Large-Area Mapping at 850 Microns. II. Analysis of the Clump Distribution in the ρ Ophiuchi Molecular Cloud , 2000 .

[39]  P. Padoan,et al.  The Stellar Initial Mass Function from Turbulent Fragmentation , 2000, astro-ph/0011465.

[40]  B. Elmegreen.  Star Formation in a Crossing Time , 1999, astro-ph/9911172.

[41]  R. Klessen,et al.  The Formation of Stellar Clusters: Gaussian Cloud Conditions. I. , 1999, astro-ph/9904090.

[42]  L. Hillenbrand,et al.  A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics , 1998 .

[43]  R. Klessen,et al.  Fragmentation of Molecular Clouds: The Initial Phase of a Stellar Cluster , 1997, astro-ph/9805125.

[44]  Bruce G. Elmegreen,et al.  The Initial Stellar Mass Function from Random Sampling in a Turbulent Fractal Cloud , 1997 .

[45]  C. Clarke,et al.  Accretion and the stellar mass spectrum in small clusters , 1997 .

[46]  F. Adams,et al.  A Theory of the IMF for Star Formation in Molecular Clouds , 1996, astro-ph/9601139.

[47]  I. Bonnell,et al.  Modelling accretion in protobinary systems , 1995, astro-ph/9510149.

[48]  J. Silk.  A Theory for the Initial Mass Function , 1995 .

[49]  B. Elmegreen.  Star Formation at Compressed Interfaces in Turbulent Self-gravitating Clouds , 1993 .

[50]  Richard B. Larson,et al.  Towards understanding the stellar initial mass function , 1992 .

[51]  M. Wolfire,et al.  Conditions for the formation of massive stars , 1987 .

[52]  R. Larson.  Cloud fragmentation and stellar masses , 1985 .

[53]  H. Zinnecker.  Star formation from hierarchical cloud fragmentation - A statistical theory of the log-normal Initial Mass Function , 1984 .

[54]  R. Larson.  Turbulence and star formation in molecular clouds , 1980 .