# # Program information file # PROGRAM_ID 2022A084 PROGRAM_TITLE Variation of SiO Rovibrational Lines in Asymptotic Giant Branch Stars PROGRAM_INV1 Phillip Stancil PROGRAM_INV2 Matt Richtner PROGRAM_INV3 Ziwei Zhang PROGRAM_INV4 John Lacy PROGRAM_INV5 PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG AGB star outflows are a dominant source of interstellar dust, while the dust formation mechanism is not yet fully understood. It has been proposed that silicate formation could largely be attributed to shocks in C-rich stars, which are believed to occur with periods of 1-2 years. The shocks dissociate CO, freeing-up atomic C and O for an efficient and rich non-equilibrium chemistry. In such shocks, SiO is predicted to form with abundances 10-100 times larger than at equilibrium in C-rich stars. Since SiO is believed to seed silicate grain growth, its production in C-rich AGBs breaks the C/O-dichotomy of grain formation. To gain insight into the shock mechanism, and to place constraints on alternative processes, we propose to use the TEXES instrument on IRTF to make two temporally separated observations of the 8 micron SiO fundamental rovibrational band for the C-rich AGBs IRC +10216 and CIT6, and the O-rich giant IK Tau, to search for temporal variations due to proposed shocks. IRTF/TEXES observations are available for two other epochs. Concurrently with IRTF observations, we have proposed SiO rotational transition observations at the Arizona Radio Observatory [ARO]; observations of SiO on these sources are also available for two other epochs. Spectral modeling of the AGB outflows, from the UV to the mm, will be performed with Cloudy, as well as RADEX, but with updated SiO-H2 collision rates and molecular photochemistry. PROGRAM_ABSTRACT_END