Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21734006l&link_type=abstract
American Astronomical Society, AAS Meeting #217, #340.06; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
The magnetorotational instability (MRI) is the most likely candidate for angular momentum transport and radial flow of mass in protoplanetary disks (PPDs), but demands a minimum ionization fraction for the gas to couple to the magnetic field. It is known that large regions of PPDs are insufficiently ionized for the MRI to act, and are “dead zones.” Only at the disk’s surface will the MRI operate: the disk undergoes “layered accretion.” The ionization fraction and magnetic coupling are sensitive to the disk’s temperature structure. All previous models of PPD temperatures including accretion have assumed a height-independent viscosity. We have constructed the first model of the temperature structure considering accretional heating in the active layers only. Our model self-consistently calculates the temperature and ionization fraction, assuming hydrostatic equilibrium, solving the equations of radiative transfer at each annulus self-consistently with the disk flaring angle, and solving a sophisticated ionization chemistry network. The location of the dead zones and active layers is found using a linear stability criterion accounting for Ohmic dissipation, ambipolar diffusion and Hall terms.
As an example, we consider a “typical” T Tauri star (M=0.5 MSun, R=2.5 RSun, T=4000 K) and disk (MDISK=0.01 MSun, column density Σ α r-3/2). For a magnetic field strength B=0.1 G, we find that inside 1.5 AU the active layers have uniform thickness Σa 25 g cm-2, but Σa rises rapidly with radius between 1.5 and 4 AU until the entire disk is active at larger radii. The midplane temperatures are sensitive to the magnitude and spatial variation of the active layer: they are higher than those of a passive disk, but lower than those of a disk with the same mass accretion rate but a uniform viscosity (no dead zone). Near the boundary of the dead zone, midplane temperatures can actually rise with increasing radius.
Desch Steven J.
Lesniak Michael V.
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