Can magnetized turbulence set the mass scale of stars?. (arXiv:2002.01421v4 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Guszejnov_D/0/1/0/all/0/1">David Guszejnov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grudic_M/0/1/0/all/0/1">Michael Y. Grudić</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hopkins_P/0/1/0/all/0/1">Philip F. Hopkins</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Offner_S/0/1/0/all/0/1">Stella S. R. Offner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Faucher_Giguere_C/0/1/0/all/0/1">Claude-André Faucher-Giguère</a>
Understanding the evolution of self-gravitating, isothermal, magnetized gas
is crucial for star formation, as these physical processes have been postulated
to set the initial mass function (IMF). We present a suite of isothermal
magnetohydrodynamic (MHD) simulations using the GIZMO code, that resolve the
formation of individual stars in giant molecular clouds (GMCs), spanning a
range of Mach numbers found in observed GMCs. As in past works, the mean and
median stellar masses are sensitive to numerical resolution, because they are
sensitive to low-mass stars that contribute a vanishing fraction of the overall
stellar mass. The {em mass-weighted} median stellar mass $M_mathrm{50}$
becomes insensitive to resolution once turbulent fragmentation is
well-resolved. Without imposing Larson-like scaling laws, our simulations find
$M_mathrm{50} propto M_mathrm{0} mathcal{M}^{-3} alpha_mathrm{turb}
mathrm{SFE}^{1/3}$ for GMC mass $M_mathrm{0}$, sonic Mach number
$mathcal{M}$, virial parameter $alpha_mathrm{turb}$, and star formation
efficiency $mathrm{SFE}=M_mathrm{star}/M_mathrm{0}$. This fit agrees well
with previous IMF results from the RAMSES, ORION2, and SphNG codes. Although
$M_mathrm{50}$ has no significant dependence on the magnetic field strength at
the cloud scale, MHD is necessary to prevent a fragmentation cascade that
results in non-convergent stellar masses. For initial conditions and SFE
similar to star-forming GMCs in our Galaxy, we predict $M_mathrm{50}$ to be
$>20 M_{odot}$, an order of magnitude larger than observed ($sim 2 M_odot$),
together with an excess of brown dwarfs. Moreover, $M_mathrm{50}$ is sensitive
to initial cloud properties and evolves strongly in time within a given cloud,
predicting much larger IMF variations than are observationally allowed. We
conclude that physics beyond MHD turbulence and gravity are necessary
ingredients for the IMF.
Understanding the evolution of self-gravitating, isothermal, magnetized gas
is crucial for star formation, as these physical processes have been postulated
to set the initial mass function (IMF). We present a suite of isothermal
magnetohydrodynamic (MHD) simulations using the GIZMO code, that resolve the
formation of individual stars in giant molecular clouds (GMCs), spanning a
range of Mach numbers found in observed GMCs. As in past works, the mean and
median stellar masses are sensitive to numerical resolution, because they are
sensitive to low-mass stars that contribute a vanishing fraction of the overall
stellar mass. The {em mass-weighted} median stellar mass $M_mathrm{50}$
becomes insensitive to resolution once turbulent fragmentation is
well-resolved. Without imposing Larson-like scaling laws, our simulations find
$M_mathrm{50} propto M_mathrm{0} mathcal{M}^{-3} alpha_mathrm{turb}
mathrm{SFE}^{1/3}$ for GMC mass $M_mathrm{0}$, sonic Mach number
$mathcal{M}$, virial parameter $alpha_mathrm{turb}$, and star formation
efficiency $mathrm{SFE}=M_mathrm{star}/M_mathrm{0}$. This fit agrees well
with previous IMF results from the RAMSES, ORION2, and SphNG codes. Although
$M_mathrm{50}$ has no significant dependence on the magnetic field strength at
the cloud scale, MHD is necessary to prevent a fragmentation cascade that
results in non-convergent stellar masses. For initial conditions and SFE
similar to star-forming GMCs in our Galaxy, we predict $M_mathrm{50}$ to be
$>20 M_{odot}$, an order of magnitude larger than observed ($sim 2 M_odot$),
together with an excess of brown dwarfs. Moreover, $M_mathrm{50}$ is sensitive
to initial cloud properties and evolves strongly in time within a given cloud,
predicting much larger IMF variations than are observationally allowed. We
conclude that physics beyond MHD turbulence and gravity are necessary
ingredients for the IMF.
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