Magnetized Accretion and Dead Zones in Protostellar Disks

The edges of magnetically-dead zones in protostellar disks have been proposed as locations where density bumps may arise, trapping planetesimals and helping form planets. Magneto-rotational turbulence in magnetically-active zones provides both accretion of gas on the star and transport of mass to the dead zone. We investigate the location of the magnetically-active regions in a protostellar disk around a solar-type star, varying the disk temperature, surface density profile, and dust-to-gas ratio. We also consider stellar masses between 0.4 and 2 $M_\odot$, with corresponding adjustments in the disk mass and temperature. The dead zone's size and shape are found using the Elsasser number criterion with conductivities including the contributions from ions, electrons, and charged fractal dust aggregates. The charged species' abundances are found using the approach proposed by S. Okuzumi. The dead zone is in most cases defined by the ambipolar diffusion. In our maps, the dead zone takes a variety of shapes, including a fish-tail pointing away from the star and islands located on and off the midplane. The corresponding accretion rates vary with radius, indicating locations where the surface density will increase over time, and others where it will decrease. We show that density bumps do not readily grow near the dead zone's outer edge, independently of the disk parameters and the dust properties. Instead, the accretion rate peaks at the radius where the gas-phase metals freeze out. This could lead to clearing a valley in the surface density, and to a trap for pebbles located just outside the metal freeze-out line.Comment: 58 pages, 25 figures, 2 tables, accepted to Ap

Management of mental illness by the British Army

Background: The Ministry of Defence has its own hospital for soldiers requiring admission for mental health problems. Aims: To assess the efficiency of the army psychiatric hospital at restoring patients to full active duty. To assess whether a new military training and rehabilitation unit (MTRU) that emphasises military-skills training, improves outcome. Method: A 2-year, inception-cohort outcome study of hospital in-patients. A 12-month, case-matched, ‘before and after’ outcome study compared MTRU patients with hospital in-patients. Results: I (hospital in-patients, n=309): at 2-year follow-up 67 (22%) were fully fit for active duty. Military psychiatrists' success rate at predicting recovery to active duty was 27%. 2: the odds of a soldier in the MTRU cohort (n=35) returning to active duty were 14 times greater than for the hospital cohort (n=35). The odds of remaining in the army while unfit for active duty were 20 times less for the MTRU than for the hospital cohort. Conclusions: The army hospital is inefficient at rehabilitation to active duty. The MTRU significantly increased the odds of returning to active duty and reduced the odds of remaining in the army while still unfit. These findings may be applicable to the emergency services

Protostellar Disk Evolution Over Million-Year Timescales with a Prescription for Magnetized Turbulence

Magnetorotational instability (MRI) is the most promising mechanism behind accretion in low-mass protostellar disks. Here we present the first analysis of the global structure and evolution of non-ideal MRI-driven T-Tauri disks on million-year timescales. We accomplish this in a 1+1D simulation by calculating magnetic diffusivities and utilizing turbulence activity criteria to determine thermal structure and accretion rate without resorting to a 3-D magnetohydrodynamical (MHD) simulation. Our major findings are as follows. First, even for modest surface densities of just a few times the minimum-mass solar nebula, the dead zone encompasses the giant planet-forming region, preserving any compositional gradients. Second, the surface density of the active layer is nearly constant in time at roughly 10 g/cm2, which we use to derive a simple prescription for viscous heating in MRI-active disks for those who wish to avoid detailed MHD computations. Furthermore, unlike a standard disk with constant-alpha viscosity, the disk midplane does not cool off over time, though the surface cools as the star evolves along the Hayashi track. The ice line is firmly in the terrestrial planet-forming region throughout disk evolution and can move either inward or outward with time, depending on whether pileups form near the star. Finally, steady-state mass transport is a poor description of flow through an MRI-active disk. We caution that MRI activity is sensitive to many parameters, including stellar X-ray flux, grain size, gas/small grain mass ratio and magnetic field strength, and we have not performed an exhaustive parameter study here.Comment: Accepted for publication in Astrophysical Journal. 19 pages, including 8 figure