The chemistry of episodic accretion in embedded objects. 2D radiation
  thermo-chemical models of the post-burst phase
  Episodic accretion is an important process in the evolution of young stars
and their environment. The observed strong luminosity bursts of young stellar
objects likely have a long lasting impact on the chemical evolution of the disk
and envelope structure. We want to investigate observational signatures of the
chemical evolution in the post-burst phase for embedded sources. With such
signatures it is possible to identify targets that experienced a recent
luminosity burst. We present a new model for episodic accretion chemistry based
on the 2D, radiation thermo-chemical disk code ProDiMo. We have extended
ProDiMo with a proper treatment for envelope structures. For a representative
Class I model, we calculated the chemical abundances in the post-burst phase
and produced synthetic observables like intensity maps and radial profiles.
During a burst many chemical species, like CO, sublimate from the dust
surfaces. As the burst ends they freeze out again (post-burst phase). This
freeze-out happens from inside-out due to the radial density gradient in the
disk and envelope structure. This inside-out freeze-out produces clear
observational signatures in spectral line emission, like rings and distinct
features in the slope of radial intensity profiles. We fitted synthetic C18O
J=2-1 observations with single and two component fits and find that post-burst
images are much better matched by the latter. Comparing the quality of such
fits allows identification of post-burst targets in a model-independent way.
Our models confirm that it is possible to identify post-burst objects from
spatially resolved CO observations. However, to derive proper statistics, like
frequencies of bursts, from observations it is important to consider aspects
like the inclination and structure of the target and also dust properties as
those have a significant impact on the freeze-out timescale.
