Shadows round the campfire: what young stars' infrared variability
reveals about protostellar disks

Neal Turner (JPL-Caltech)

Young stars' near-infrared emission shows several puzzling features,
including time variations 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 will discuss how
each of these features can be explained by a time-varying,
magnetically-supported disk atmosphere like those suggested by MHD
calculations of magneto-rotational turbulence.  Through Monte Carlo
radiative transfer calculations I will show that such support yields
near-infrared variations spanning the observed range of amplitudes.
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 larger
excesses.  The atmosphere rises high enough to obscure the star in
systems viewed near edge-on, if the dust in the outer parts of the
disk has undergone some growth or settling.