Bondi-Hoyle Accretion in an Isothermal Magnetized Plasma

Journal Article · Wed Feb 12 00:00:00 EST 2014 · The Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637x/783/1/50· OSTI ID:1670549

Lee, Aaron T. ^[1]; Cunningham, Andrew J. ^[2]; McKee, Christopher F. ^[1]; Klein, Richard I. ^[3]

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1. Univ. of California, Berkeley, CA (United States)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
3. Univ. of California, Berkeley, CA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β $$\lesssim$$ 1, so magnetic fields have the potential to play a significant role in the accretion process. Here, we have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up to $${\cal M}\approx 45$$; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and $${\cal M}$$ for the parallel and perpendicular orientations. Using typical molecular cloud values of $${\cal M}\sim 5$$ and β ~ 0.04 from the literature, our fits suggest that a 0.4 M $$\bigodot$$ star accretes ~4 × 10^–9 M $$\bigodot$$ yr^–1, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than predicted by Bondi-Hoyle rates. Our results should find application in numerical codes, enabling accurate sub-grid models of sink particles accreting from magnetized media.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: National Aeronautic and Space Administration (NASA); National Science Foundation (NSF); USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1670549
• Alternate ID(s): OSTI ID: 22351351; OSTI ID: 1244658
• Report Number(s): LLNL-JRNL--638417; 757053
• Journal Information: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 783; ISSN 0004-637X
• Publisher: Institute of Physics (IOP)
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS