Energetic Protons, Radionuclides and Magnetic Activity in Protostellar Disks

📝 Abstract

We calculate the location of the magnetically-inactive dead zone in the minimum-mass protosolar disk, under ionization scenarios including stellar X-rays, long- or short-lived radionuclide decay, and energetic protons arriving from the general interstellar medium, from a nearby supernova explosion, from the disk corona, or from the corona of the young star. The disk contains a dead zone in all scenarios except those with small dust grains removed and a fraction of the short-lived radionuclides remaining in the gas. All the cases without exception have an “undead zone” where intermediate resistivities prevent magneto-rotational turbulence while allowing shear-generated large-scale magnetic fields. The mass column in the undead zone is typically greater than the column in the turbulent surface layers. The results support the idea that the dead and undead zones are robust consequences of cold, dusty gas with mass columns exceeding 1000 g/cm^2.

💡 Analysis

We calculate the location of the magnetically-inactive dead zone in the minimum-mass protosolar disk, under ionization scenarios including stellar X-rays, long- or short-lived radionuclide decay, and energetic protons arriving from the general interstellar medium, from a nearby supernova explosion, from the disk corona, or from the corona of the young star. The disk contains a dead zone in all scenarios except those with small dust grains removed and a fraction of the short-lived radionuclides remaining in the gas. All the cases without exception have an “undead zone” where intermediate resistivities prevent magneto-rotational turbulence while allowing shear-generated large-scale magnetic fields. The mass column in the undead zone is typically greater than the column in the turbulent surface layers. The results support the idea that the dead and undead zones are robust consequences of cold, dusty gas with mass columns exceeding 1000 g/cm^2.

📄 Content

arXiv:0908.3874v1 [astro-ph.SR] 26 Aug 2009 Accepted for the Astrophysical Journal Energetic Protons, Radionuclides and Magnetic Activity in Protostellar Disks N. J. Turner Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109; neal.turner@jpl.nasa.gov and J. F. Drake Department of Physics, Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742 ABSTRACT We calculate the location of the magnetically-inactive dead zone in the minimum-mass protosolar disk, under ionization scenarios including stellar X- rays, long- or short-lived radionuclide decay, and energetic protons arriving from the general interstellar medium, from a nearby supernova explosion, from the disk corona, or from the corona of the young star. The disk contains a dead zone in all scenarios except those with small dust grains removed and a fraction of the short- lived radionuclides remaining in the gas. All the cases without exception have an “undead zone” where intermediate resistivities prevent magneto-rotational turbulence while allowing shear-generated large-scale magnetic fields. The mass column in the undead zone is typically greater than the column in the turbulent surface layers. The results support the idea that the dead and undead zones are robust consequences of cold, dusty gas with mass columns exceeding 1000 g cm−2. Subject headings: circumstellar matter — solar system: formation — stars: for- mation — instabilities — MHD 1. INTRODUCTION Angular momentum transport is a key to the evolution of protostellar disks and the origins of the planets, as it governs the flow of raw materials toward the star (Bodenheimer – 2 – 1995). Transport processes likely to be important include the turbulence resulting from the magneto-rotational instability (MRI; Balbus & Hawley 1991, 1998) and the stresses due to large-scale magnetic fields driving an outflow (Blandford & Payne 1982; Wardle & K¨onigl 1993) or shearing within the disk (Turner & Sano 2008). All of the magnetic angular momentum transport processes require the gas is sufficiently ionized to couple to the fields. Collisional ionization leads to good coupling only at the tem- peratures above 1000 K found very close to the star (Pneuman & Mitchell 1965; Umebayashi 1983; Nakano & Umebayashi 1988). In the overwhelming majority of the disk volume, the main ionization processes are non-thermal. Among the important non-thermal processes are the X-rays emitted by the young star (Glassgold et al. 1997), the cosmic rays arriving from interstellar space (Umebayashi 1983; Umebayashi & Nakano 1988), and the decay of radionuclides within the gas (Umebayashi & Nakano 1981). The basic difficulties in reaching high enough levels of ionization through these non-thermal processes are the large column of absorbing material and the high rate of recombination on the surfaces of dust particles. The X-rays and cosmic rays generally penetrate and ionize only the surface layers of the disk, leaving a region near the midplane where magnetic activity is suppressed (Gammie 1996; Sano et al. 2000; Ilgner & Nelson 2006a). The absorption of X-rays and cosmic rays in the atmosphere means that the disk can be divided into three zones. In the upper layers, and in outer annuli where the mass column is low enough for the ionizing radiation to reach the midplane, the magnetic fields couple thoroughly to the gas, and magnetic forces drive turbulence through the MRI. In a “dead zone” near the midplane, the fields decouple from the gas, and magnetic forces are largely irrelevant. At resistivities between these extremes, magnetic fields decouple over scales com- parable to the disk thickness, shutting offthe MRI, but remain coupled over the disk radius. The radial gradient in orbital frequency can then shear out any weak radial magnetic field to generate toroidal fields, enabling angular momentum transport to continue (Turner & Sano 2008). The turbulent surface layers are thus separated from the midplane dead zone by an “undead zone” that becomes magnetically active when supplied with radial fields. In this paper we explore whether three less well-studied additional sources of ionization can reduce the sizes of the dead and undead zones or make protostellar disks magnetically active throughout. The three ionization sources are high-energy particles from (1) a nearby supernova explosion (Fatuzzo et al. 2006), (2) the corona of the protostellar disk itself, and (3) the young star. We estimate the ionization rates to order of magnitude (§2) and calculate the resulting resistivities (§3) in the minimum-mass model of the protosolar disk (§4), finding the undead and dead zones shown in §5. A summary and conclusions are in §6. – 3 – 2. IONIZATION The base ionization rate in our calculations is set by the X-rays emitted from the vicinity of the young star (§2.1). Some of the models include also the decay of radioactive isotopes within the disk (§2.2). Our purpose is to expl

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