Glenn E. Ciolek

Magnetic Fields and Star Formation: A Theory Reaching Adulthood

The correctness of any theory rests with the lack of mathematical or numerical errors, so that its conclusions would follow from its assumptions and known physical laws. A correct theory, however, can be irrelevant unless its assumptions are consistent with observations or experiments. It is therefore essential that the physical properties of star-forming interstellar clouds be understood quantitatively before a theory of star formation becomes both correct and relevant.

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 and its critical dependence on the magnetic tension force 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.

Effect of Ambipolar Diffusion on Ion Abundances in Contracting Protostellar Cores

Numerical simulations and analytical solutions have established that ambipolar diffusion can reduce the dust-to-gas ratio in magnetically and thermally supercritical cores during the epoch of core formation. We study the effect that this has on the ion chemistry in contracting protostellar cores and present a simplified analytical method that allows one to calculate the ion power-law exponent k (≡ d ln ni/d ln nn, where ni and nn are the ion and neutral densities, respectively) as a function of core density. We find that, as in earlier numerical simulations, no single value of k can adequately describe the ion abundance for nn 109 cm-3, a result that is contrary to the canonical value of k =

Magnetic Fields, Interstellar Dust, UV Radiation, and Star Formation

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in The Origin of Stars and Planetary Systems

--> found in previous static equilibrium chemistry calculations and often used to study the effect of ambipolar diffusion in interstellar clouds. For typical cloud and grain parameters, reduction of the abundance of grains results in k >

Ambipolar diffusion, interstellar dust, and the formation of cloud cores and protostars. IV. Effect of ultraviolet ionization and magnetically controlled infall rate

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Ambipolar diffusion, interstellar dust, and the formation of cloud cores and protostars. III. Typical axisymmetric solutions

--> during the core formation epoch (densities 105 cm-3). As a consequence, observations of the degree of ionization in cores could be used, in principle, to determine whether ambipolar diffusion is responsible for core formation in interstellar molecular clouds. For densities 105 cm-3, k is generally

DYNAMICAL COLLAPSE OF NONROTATING MAGNETIC MOLECULAR CLOUD CORES: EVOLUTION THROUGH POINT-MASS FORMATION

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Structure and evolution of magnetically supported molecular clouds: Evidence for ambipolar diffusion in the Barnard 1 cloud

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Effect of ambipolar diffusion on dust-to-gas ratio in protostellar cores

We present calculations which demonstrate the formation (due to ambipolar diffusion) of collapsing, magnetically and thermally supercritical protostellar cores in otherwise magnetically supported model molecular clouds, accounting for the effects of interstellar grains (charged and neutral) and UV radiation. Charged grains couple to magnetic field lines either by direct attachment or by electrostatic attraction to electron-shielded ions (“quasiparticles”), which are themselves attached to the field. How grain-neutral drag increases the core formation timescale is discussed. We also show how ambipolar diffusion affects the grain abundance and the amount of mass and magnetic flux within in a protostellar core. Ionization by the interstellar UV radiation field effectively cuts off the (ambipolar-diffusion controlled) mass infall rate in model cloud envelopes.