(Proposal ID) S13A-036

(PI) Gandhi, Poshak

(Proposal Title)
The composition of freshly-formed dust in recent (post-)AGB thermal pulses

(Abstract)
We recently discovered a candidate Asymptotic Giant Branch (AGB) star 
undergoing a thermal pulse (TP). WISE J1810--3305 is one of only two sources 
in the WISE sky survey which show very red WISE colors but a very blue 2MASS 
[K] vs. WISE [W1 (3.4 mu m)] color, and drastic brightening at 12 mu m since 
IRAS observation. This favours a scenario in which we have caught a massive 
dust ejection event during a TP that began only ~15 years ago. The other 
source is Sakurai's object, which also underwent a massive dust expulsion 
around the same time, but is in a later evolutionary (post-AGB) phase. Few 
firm constraints exist on the TP stage because of its brevity. These objects 
provide a unique opportunity for understanding TP evolution and dust production
 in real-time. Here we propose COMICS spectroscopy of WISE J1810--3305 in order
 to study the composition of the circumstellar dust. We will search for 
molecular bands, and identify whether the central object is an Oxygen or Carbon
 rich AGB star. We also propose identical spectroscopy of Sakurai's object in 
order to compare AGB with post-AGB evolution. These objects are presently 
brightest in the mid-IR, and COMICS is the only ground-based mid-IR camera 
with the requisite capability for observation.