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Title: What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

Abstract

As star-forming clouds collapse, the gas within them fragments to ever-smaller masses. Naively one might expect this process to continue down to the smallest mass that is able to radiate away its binding energy on a dynamical time-scale, the opacity limit for fragmentation, at ~0.01M⊙. However, the observed peak of the initial mass function (IMF) lies a factor of 20-30 higher in mass, suggesting that some other mechanism halts fragmentation before the opacity limit is reached. Here, we analyse radiation-magnetohydrodynamic simulations of star cluster formation in typical Milky Way environments in order to determine what physical process limits fragmentation in them. We examine the regions in the vicinity of stars that form in the simulations to determine the amounts of mass that are prevented from fragmenting by thermal and magnetic pressure. We show that, on small scales, thermal pressure enhanced by stellar radiation heating is the dominant mechanism limiting the ability of the gas to further fragment. In the brown dwarf mass regime, ~0.01M⊙, the typical object that forms in the simulations is surrounded by gas whose mass is several times its own that is unable to escape or fragment, and instead is likely to accrete. This mechanism explains whymore » ~0.01M⊙ objects are rare: unless an outside agent intervenes (e.g. a shock strips away the gas around them), they will grow by accreting the warmed gas around them. In contrast, by the time stars grow to masses of ~0.2M⊙, the mass of heated gas is only tens of percent of the central star mass, too small to alter its final mass by a large factor. This naturally explains why the IMF peak is at ~0.2M⊙.« less

Authors:
 [1];  [2];  [3];  [4]
  1. Australian National Univ., Canberra, ACT (Australia). Research School of Astronomy and Astrophysics
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of California, Berkeley, CA (United States). Dept. of Astronomy
  4. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy and Dept. of Physics
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR) (SC-21); Australian Research Council (ARC); National Aeronautic and Space Administration (NASA); National Science Foundation (NSF)
OSTI Identifier:
1456926
Grant/Contract Number:  
AC02-05CH11231; DP160100695; NNX-14AB52G; NNX-13AB84G; AST-1211729; AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Volume: 460; Journal Issue: 3; Related Information: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.; Journal ID: ISSN 0035-8711
Publisher:
Royal Astronomical Society
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; radiative transfer; stars: formation; stars: luminosity function; mass function; ISM: clouds

Citation Formats

Krumholz, Mark R., Myers, Andrew T., Klein, Richard I., and McKee, Christopher F. What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations. United States: N. p., 2016. Web. doi:10.1093/mnras/stw1236.
Krumholz, Mark R., Myers, Andrew T., Klein, Richard I., & McKee, Christopher F. What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations. United States. doi:10.1093/mnras/stw1236.
Krumholz, Mark R., Myers, Andrew T., Klein, Richard I., and McKee, Christopher F. Tue . "What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations". United States. doi:10.1093/mnras/stw1236. https://www.osti.gov/servlets/purl/1456926.
@article{osti_1456926,
title = {What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations},
author = {Krumholz, Mark R. and Myers, Andrew T. and Klein, Richard I. and McKee, Christopher F.},
abstractNote = {As star-forming clouds collapse, the gas within them fragments to ever-smaller masses. Naively one might expect this process to continue down to the smallest mass that is able to radiate away its binding energy on a dynamical time-scale, the opacity limit for fragmentation, at ~0.01M⊙. However, the observed peak of the initial mass function (IMF) lies a factor of 20-30 higher in mass, suggesting that some other mechanism halts fragmentation before the opacity limit is reached. Here, we analyse radiation-magnetohydrodynamic simulations of star cluster formation in typical Milky Way environments in order to determine what physical process limits fragmentation in them. We examine the regions in the vicinity of stars that form in the simulations to determine the amounts of mass that are prevented from fragmenting by thermal and magnetic pressure. We show that, on small scales, thermal pressure enhanced by stellar radiation heating is the dominant mechanism limiting the ability of the gas to further fragment. In the brown dwarf mass regime, ~0.01M⊙, the typical object that forms in the simulations is surrounded by gas whose mass is several times its own that is unable to escape or fragment, and instead is likely to accrete. This mechanism explains why ~0.01M⊙ objects are rare: unless an outside agent intervenes (e.g. a shock strips away the gas around them), they will grow by accreting the warmed gas around them. In contrast, by the time stars grow to masses of ~0.2M⊙, the mass of heated gas is only tens of percent of the central star mass, too small to alter its final mass by a large factor. This naturally explains why the IMF peak is at ~0.2M⊙.},
doi = {10.1093/mnras/stw1236},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 3,
volume = 460,
place = {United States},
year = {2016},
month = {5}
}

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