Shadows round the campfire: what young stars' infrared variability reveals about protostellar disks
Astronomy and astrophysics
Friday 10 August 2012
from 10:30
to 11:15 at
FA31
Speaker :
Neal Turner (JPL/Caltech and MPIA Heidelberg)
Abstract :
Young stars' near-infrared emission shows several puzzling features,
including variability uncorrelated with visible-light changes,
foreground extinction that recurs erratically on timescales of weeks,
and excesses over the stellar photosphere too large to explain by
reprocessing in a hydrostatic circumstellar disk. I demonstrate that
each of these features can be explained by a time-varying,
magnetically-supported disk atmosphere like those found in MHD
calculations of stratified magneto-rotational turbulence. The
atmosphere resembles Solar prominences in that the magnetic fields
suspend cool plasma above the disk's photosphere. Through Monte Carlo
radiative transfer calculations I show that the suspended plasma
blocks the starlight, casting shadows across the disk. The shadows
move as the magnetic fields wax and wane, yielding near-infrared
variability with a range of amplitudes similar to that observed.
Since the starlight-absorbing surface lies higher than in hydrostatic
models, a greater fraction of the stellar luminosity is reprocessed
into the near-infrared, providing a natural explanation for the large
excesses. Finally the atmosphere intermittently rises high enough to
obscure the star in systems viewed near edge-on, if the dust in the
disk's outer parts has undergone some growth or settling. Thus the
variability is a fresh opportunity to probe the magnetic fields, the
turbulence and the heating processes at work in planet-forming disks.
