49 research outputs found
Thermal instability in ionized plasma
We study magnetothermal instability in the ionized plasmas including the
effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single
fluid approximation does not allow transverse thermal condensations, however,
non-ideal effects highly diminish the stabilizing role of the magnetic field in
thermally unstable plasmas. Therefore, enhanced growth rate of thermal
condensation modes in the presence of the diffusion mechanisms speed up the
rate of structure formation.Comment: Accepted for publication in Astrophysics & Space Scienc
Turbulent Control of the Star Formation Efficiency
Supersonic turbulence plays a dual role in molecular clouds: On one hand, it
contributes to the global support of the clouds, while on the other it promotes
the formation of small-scale density fluctuations, identifiable with clumps and
cores. Within these, the local Jeans length \Ljc is reduced, and collapse
ensues if \Ljc becomes smaller than the clump size and the magnetic support
is insufficient (i.e., the core is ``magnetically supercritical''); otherwise,
the clumps do not collapse and are expected to re-expand and disperse on a few
free-fall times. This case may correspond to a fraction of the observed
starless cores. The star formation efficiency (SFE, the fraction of the cloud's
mass that ends up in collapsed objects) is smaller than unity because the mass
contained in collapsing clumps is smaller than the total cloud mass. However,
in non-magnetic numerical simulations with realistic Mach numbers and
turbulence driving scales, the SFE is still larger than observational
estimates. The presence of a magnetic field, even if magnetically
supercritical, appears to further reduce the SFE, but by reducing the
probability of core formation rather than by delaying the collapse of
individual cores, as was formerly thought. Precise quantification of these
effects as a function of global cloud parameters is still needed.Comment: Invited review for the conference "IMF@50: the Initial Mass Function
50 Years Later", to be published by Kluwer Academic Publishers, eds. E.
Corbelli, F. Palla, and H. Zinnecke
Star Formation in Cold, Spherical, Magnetized Molecular Clouds
We present an idealized, spherical model of the evolution of a magnetized
molecular cloud due to ambipolar diffusion. This model allows us to follow the
quasi-static evolution of the cloud's core prior to collapse and the subsequent
evolution of the remaining envelope. By neglecting the thermal pressure
gradients in comparison with magnetic stresses and by assuming that the ion
velocity is small compared with the neutral velocity, we are able to find exact
analytic solutions to the MHD equations. We show that, in the case of a
centrally condensed cloud, a core of finite mass collapses into the origin
leaving behind a quasi-static envelope, whereas initially homogeneous clouds
never develop any structure in the absence of thermal stresses, and collapse as
a whole. Prior to the collapse of the core, the cloud's evolution is
characterized by two phases: a long, quasi-static phase where the relevant
timescale is the ambipolar diffusion time (treated in this paper), and a short,
dynamical phase where the characteristic timescale is the free-fall time. The
collapse of the core is an "outside-in" collapse. The quasi-static evolution
terminates when the cloud becomes magnetically supercritical; thereafter its
evolution is dynamical, and a singularity develops at the origin-a protostar.
After the initial formation of the protostar, the outer envelope continues to
evolve quasi-statically, while the region of dynamical infall grows with
time-an "inside-out" collapse. We use our solution to estimate the magnetic
flux trapped in the collapsing core and the mass accretion rate onto the newly
formed protostar. Our results agree, within factors of order unity, with the
numerical results of Fiedler & Mouschovias (1992) for the physical quantities
in the midplane ofComment: 18 postscript figures Accepted by The Astrophysical Journa
Submillimeter Studies of Prestellar Cores and Protostars: Probing the Initial Conditions for Protostellar Collapse
Improving our understanding of the initial conditions and earliest stages of
protostellar collapse is crucial to gain insight into the origin of stellar
masses, multiple systems, and protoplanetary disks. Observationally, there are
two complementary approaches to this problem: (1) studying the structure and
kinematics of prestellar cores observed prior to protostar formation, and (2)
studying the structure of young (e.g. Class 0) accreting protostars observed
soon after point mass formation. We discuss recent advances made in this area
thanks to (sub)millimeter mapping observations with large single-dish
telescopes and interferometers. In particular, we argue that the beginning of
protostellar collapse is much more violent in cluster-forming clouds than in
regions of distributed star formation. Major breakthroughs are expected in this
field from future large submillimeter instruments such as Herschel and ALMA.Comment: 12 pages, 9 figures, to appear in the proceedings of the conference
"Chemistry as a Diagnostic of Star Formation" (C.L. Curry & M. Fich eds.
Self-Similar Collapse of Nonrotating Magnetic Molecular Cloud Cores
We obtain self-similar solutions that describe the gravitational collapse of
nonrotating, isothermal, magnetic molecular cloud cores. We use simplifying
assumptions but explicitly include the induction equation, and the semianalytic
solutions we derive are the first to account for the effects of ambipolar
diffusion following the formation of a central point mass. Our results
demonstrate that, after the protostar first forms, ambipolar diffusion causes
the magnetic flux to decouple in a growing region around the center. The
decoupled field lines remain approximately stationary and drive a hydromagnetic
C-shock that moves outward at a fraction of the speed of sound (typically a few
tenths of a kilometer per second), reaching a distance of a few thousand AU at
the end of the main accretion phase for a solar-mass star. We also show that,
in the absence of field diffusivity, a contracting core will not give rise to a
shock if, as is likely to be the case, the inflow speed near the origin is
nonzero at the time of point-mass formation. Although the evolution of
realistic molecular cloud cores will not be exactly self-similar, our results
reproduce the main qualitative features found in detailed core-collapse
simulations (Ciolek & Konigl 1998)Comment: 25 pages, 3 figures, AASTeXv4.0 Accepted for publication in The
Astrophysical Journa
Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution Through Point-Mass Formation
We present a numerical simulation of the dynamical collapse of a nonrotating
magnetic molecular cloud core and follow the core's evolution through the
formation of a central point mass and its subsequent growth to a 1 solar-mass
protostar. The epoch of point-mass formation (PMF) is investigated by a self-
consistent extension of previously presented models of core formation and
contraction in axisymmetric, self-gravitating, isothermal, magnetically
supported interstellar molecular clouds. Prior to PMF, the core is dynamically
contracting and is not well approximated by a quasistatic equilibrium model.
Ambipolar diffusion, which plays a key role in the early evolution of the core,
is unimportant during the dynamical pre-PMF collapse phase. However, the
appearance of a central mass, through its effect on the gravitational field in
the inner core regions, leads to a "revitalization" of ambipolar diffusion in
the weakly ionized gas surrounding the central protostar. This process is so
efficient that it leads to a decoupling of the field from the matter and
results in an outward propagating hydromagnetic C-type shock. The existence of
an ambipolar diffusion-mediated shock was predicted by Li & McKee (1996), and
we find that the basic shock structure given by their analytical model is well
reproduced by our more accurate numerical results. Our calculation also
demonstrates that ambipolar diffusion, rather than Ohmic diffusivity operating
in the innermost core region, is the main field decoupling mechanism
responsible for driving the shock after PMF.Comment: 59 pages, 10 figures, AASTeX4.0 accepted for publication in The
Astrophysical Journa
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
The Dynamical Structure and Evolution of Giant Molecular Clouds
Giant molecular clouds (GMCs) are the sites of star formation in the Galaxy. Many of their properties can be understood in terms of a model in which the GMCs and the star-forming clumps within them are in approximate pressure equilibrium, with turbulent motions treated as a separate pressure component