High-Resolution Near Infrared Spectroscopy of HD 100546: II. Analysis of
  variable rovibrational CO emission lines
  We present observations of rovibrational CO in HD 100546 from four epochs
spanning January 2003 through December 2010. We show that the equivalent widths
of the CO lines vary during this time period with the v=1-0 CO lines
brightening more than the UV fluoresced lines from the higher vibrational
states. While the spectroastrometric signal of the hot band lines remains
constant during this period, the spectroastrometric signal of the v=1--0 lines
varies substantially. At all epochs, the spectroastrometric signals of the UV
fluoresced lines are consistent with the signal one would expect from gas in an
axisymmetric disk. In 2003, the spectroastrometric signal of the v=1-0 P26 line
was symmetric and consistent with emission from an axisymmetric disk. However,
in 2006, there was no spatial offset of the signal detected on the red side of
the profile, and in 2010, the spectroastrometric offset was yet more strongly
reduced toward zero velocity. A model is presented that can explain the
evolution of the equivalent width of the v=1-0 P26 line and its
spectroastrometric signal by adding to the system a compact source of CO
emission that orbits the star near the inner edge of the disk. We hypothesize
that such emission may arise from a circumplanetary disk orbiting a gas giant
planet near the inner edge of the circumstellar disk. We discuss how this idea
can be tested observationally and be distinguished from an alternative
interpretation of random fluctuations in the disk emission.
