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

by Neal Turner (JPL/Caltech and MPIA Heidelberg)

Friday 10 Aug 2012, 10:30 → 11:15 Europe/Stockholm

FA31

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.