Warm gas at 50 AU in the disk around Herbig Be star HD 100546
  The disk atmosphere is one of the fundamental elements of theoretical models
of a protoplanetary disk. However, the direct observation of the warm gas (>>
100 K) at large radius of a disk (>> 10 AU) is challenging, because the line
emission from warm gas in a disk is usually dominated by the emission from an
inner disk. Our goal is to detect the warm gas in the disk atmosphere well
beyond 10 AU from a central star in a nearby disk system of the Herbig Be star
HD 100546. We measured the excitation temperature of the vibrational transition
of CO at incremental radii of the disk from the central star up to 50 AU, using
an adaptive optics system combined with the high-resolution infrared
spectrograph CRIRES at the VLT. The observation successfully resolved the line
emission with 0".1 angular resolution, which is 10 AU at the distance of HD
100546. Population diagrams were constructed at each location of the disk, and
compared with the models calculated taking into account the optical depth
effect in LTE condition. The excitation temperature of CO is 400-500 K or
higher at 50 AU away from the star, where the blackbody temperature in
equilibrium with the stellar radiation drops as low as 90 K. This is
unambiguous evidence of a warm disk atmosphere far away from the central star.
