TDF: Neal Turner (JPL): Shadows round the campfire: what young stars' infrared variability reveals about protostellar disks

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.