DUST TRANSPORT IN PROTOSTELLAR DISKS THROUGH TURBULENCE AND SETTLING

Journal Article · Fri Jan 01 00:00:00 EST 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/708/1/188· OSTI ID:21392493

Turner, N J; Carballido, A ^[1]; Sano, T ^[2]

---

1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
2. Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871 (Japan)

We apply ionization balance and magnetohydrodynamical (MHD) calculations to investigate whether magnetic activity moderated by recombination on dust grains can account for the mass accretion rates and the mid-infrared spectra and variability of protostellar disks. The MHD calculations use the stratified shearing-box approach and include grain settling and the feedback from the changing dust abundance on the resistivity of the gas. The two-decade spread in accretion rates among solar-mass T Tauri stars is too large to result solely from variations in the grain size and stellar X-ray luminosity, but can plausibly be produced by varying these parameters together with the disk magnetic flux. The diverse shapes and strengths of the mid-infrared silicate bands can come from the coupling of grain settling to the distribution of the magnetorotational turbulence, through the following three effects. First, recombination on grains 1 mum or smaller yields a magnetically inactive dead zone extending more than two scale heights from the midplane, while turbulent motions in the magnetically active disk atmosphere overshoot the dead zone boundary by only about one scale height. Second, grains deep in the dead zone oscillate vertically in wave motions driven by the turbulent layer above, but on average settle at the rates found in laminar flow, so that the interior of the dead zone is a particle sink and the disk atmosphere will become dust-depleted unless resupplied from elsewhere. Third, with sufficient depletion, the dead zone is thinner and mixing dredges grains off the midplane. The last of these processes enables evolutionary signatures such as the degree of settling to sometimes decrease with age. The MHD results also show that the magnetic activity intermittently lifts clouds of small grains into the atmosphere. Consequently the photosphere height changes by up to one-third over timescales of a few orbits, while the extinction along lines of sight grazing the disk surface varies by factors of 2 over times down to a tenth of an orbit. We suggest that the changing shadows cast by the dust clouds on the outer disk are a cause of the daily to monthly mid-infrared variability found in many young stars.

• OSTI ID: 21392493
• Journal Information: Astrophysical Journal, Vol. 708, Issue 1; Other Information: DOI: 10.1088/0004-637X/708/1/188; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABUNDANCE
DUSTS
GRAIN SIZE
INFRARED SPECTRA
IONIZATION
LAMINAR FLOW
LUMINOSITY
MAGNETIC DISKS
MAGNETOHYDRODYNAMICS
MATTER
PHOTOSPHERE
RECOMBINATION
SCALE HEIGHT
SILICATES
SOLAR SYSTEM
T TAURI STARS
TURBULENCE
X RADIATION
ATMOSPHERES
BINARY STARS
DIMENSIONS
ELECTROMAGNETIC RADIATION
ERUPTIVE VARIABLE STARS
FLUID FLOW
FLUID MECHANICS
HEIGHT
HYDRODYNAMICS
IONIZING RADIATIONS
MAGNETIC STORAGE DEVICES
MECHANICS
MEMORY DEVICES
MICROSTRUCTURE
OPTICAL PROPERTIES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
RADIATIONS
SILICON COMPOUNDS
SIZE
SOLAR ATMOSPHERE
SPECTRA
STARS
STELLAR ATMOSPHERES
VARIABLE STARS