The magnetic flux problem and ambipolar diffusion during star formation - One-dimensional collapse. II - Results

Mouschovias, Telemachos Ch.; Paleologou, E. V.; Fiedler, Robert

doi:10.1086/163114

Cited by 44 publications

(20 citation statements)

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“…The small degree of ionization in cloud interiors also allows ambipolar diffusion to dump the hydromagnetic waves or magnetic turbulence (thus rendering them irrelevant to star formation) over a characteristic lengthscale comparable to the critical thermal lengthscale (%Jean's length), thereby also leading to the thermalization of linewidths in agreement with observations by and . This point of view is substantiated by multifluid MHD numerical calculations in rectilinear (Paleologou & Mouschovias 1983;Mouschovias et al 1985), cylindrical (Mouschovias & Morton 1991, 1992a, 1992b, and axisymmetric geometries (Fiedler & Mouschovias 1992Ciolek & Mouschovias 1993Morton et al 1994;Desch & Mouschovias 2001; C. Eng & T. Mouschovias 2005, in preparation). For a cloud as a whole and for a core during its ambipolardiffusion-controlled, quasistatic (or subcritical) phase of contraction, magnetic braking is very effective and keeps the core and the cloud essentially corotating with the background (Mouschovias 1977(Mouschovias , 1979aMouschovias & Paleologou 1979Mouschovias & Morton 1985a, 1985bBasu & Mouschovias 1994, 1995a, 1995b.…”

Section: à3mentioning

confidence: 85%

Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

Tassis¹,

Mouschovias²

2005

ApJ

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We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 10 8 cm À3 . The ''central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 10 11 cm À3 . In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star are calculated, as are their effects on the structure & evolution of the surrounding disk. The numerical method of solution first uses an adaptive grid and later, after a central region a few AU in radius becomes opaque, switches to a stationary but nonuniform grid with a central sink cell. It also involves an implicit time integrator, an advective difference scheme that possesses the transportive property, a second-order difference approximation of forces inside a cell, an integral approximation of the gravitational and magnetic fields, and tensor artificial viscosity that permits an accurate investigation of the formation and evolution of shocks in the neutral fluid.

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Section: à3mentioning

confidence: 85%

Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

Tassis¹,

Mouschovias²

2005

ApJ

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“…In all of the processes discussed so far, AD has not played a role. This is of course due to the well‐known fact that AD is not dynamically relevant until densities as high as n AD ∼ 10 5 cm −3 are reached (Mouschovias, Paleologou & Fiedler 1985). Such densities are only reached in the dense cores of MCs and therefore AD is not expected to cause any important effects on the global evolution of MCs.…”

Section: Evolution Of the M2fr In Molecular Clouds: From Sub‐ To Sumentioning

confidence: 99%

Molecular cloud evolution - IV. Magnetic fields, ambipolar diffusion and the star formation efficiency

Vázquez‐Semadeni

Banerjee

Gómez

et al. 2011

Monthly Notices of the Royal Astronomical Society

148

141

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We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate and efficiency (SFE), as well as of the mass‐to‐magnetic‐flux ratio (M2FR) in the forming clouds. We find that (1) clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re‐expand, oscillating on the scale of tens of Myr; (2) our suite of simulations with the initial magnetic field strength of 2, 3 and 4 μ G show that only supercritical or marginal critical streams lead to reasonable star forming rates. This result is not altered by the inclusion of AD; (3) the GMC’s M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM; (4) the M2FR is a highly fluctuating function of position in the clouds. This implies that a significant fraction of a cloud’s mass may remain magnetically supported, while SF occurs in the supercritical regions that are not supported; (5) in our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate to which it converts mass to stars. In such an approximately stationary regime, we find that the SFE provides a proxy of the supercritical mass fraction in the cloud; and (6) the low‐M2FR regions exhibit buoyancy within the gravitationally contracting GMCs, so that the latter naturally segregate into a high‐density, high‐M2FR ‘core’ and a low‐density, low‐M2FR ‘envelope’, without the intervention of AD.

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“…Such a blob of matter would therefore have to get rid of most of its flux before becoming a star. Ambipolar diffusion has long been suggested as a means by which the magnetic flux problem could be resolved (e.g., Mestel & Spitzer 1956;Mouschovias 1978;Paleologou & Mouschovias 1983;Nakano 1984;Mouschovias, Paleologou, & Fiedler 1985). In general, these earlier studies focused primarily on the role of ambipolar diffusion and the magnetic flux problem for the pre-PMF phase of protostellar evolution.…”

Section: Implications To the Magnetic Flux Problemmentioning

confidence: 99%

Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution through Point‐Mass Formation

Ciolek

Königl

1998

ApJ

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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 M ⊙ 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 of this type was predicted by Li & McKee (1996), and we find that the basic shock structure given by their analytic 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.The passage of the shock leads to a substantial redistribution, by ambipolar diffusion but possibly also by magnetic interchange, of the mass contained within the magnetic flux tubes in the inner core. In particular, ambipolar diffusion reduces the flux initially threading a collapsing ∼ 1 M ⊙ core by a factor ∼ > 10 3 by the time this mass accumulates within the inner radius (≃ 7.3 AU) of our computational grid. This reduction, which occurs primarily during the post-PMF phase of the collapse, represents a significant step towards the resolution of the protostellar magnetic flux problem.Our calculations indicate that a 1 M ⊙ protostar forms in ∼ 1.5 × 10 5 yr for typical cloud parameters. The mass accretion rate is time dependent, in part because of the C-shock that decelerates the infalling matter as it propagates outward: the accretion rate rises to ≃ 9.4 M ⊙ Myr −1 early on and decreases to ≃ 5.6 M ⊙ Myr −1 by the time a solar-mass protostar is formed. The infalling gas disk surrounding the protostar has a mass ∼ 10 −2 M ⊙ at radii r ∼ > 500 AU. A distinguishing prediction of our model is that the rapid ambipolar diffusion after the formation of a protostar should give rise to large ( ∼ > 1 km s −1 ), and potentially measurable, ion-neutral drift speeds on scales r ∼ < 200 AU.The main features of our simulation, including the C-shock formation after PMF, are captured by a similarity solution that incorporates the effects of ambipolar diffusion (Contopoulos, Ciolek, & ...

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The magnetic flux problem and ambipolar diffusion during star formation - One-dimensional collapse. II - Results

Cited by 44 publications

References 0 publications

Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

Molecular cloud evolution - IV. Magnetic fields, ambipolar diffusion and the star formation efficiency

Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution through Point‐Mass Formation

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