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⊙, 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
