# # Program information file # PROGRAM_ID 2022A039 PROGRAM_TITLE What darkens Chi Cyg? Investigating the dynamics of the variable S-type star via TiO and SiO high-resolution mid-IR observations PROGRAM_INV1 Guido Fuchs PROGRAM_INV2 Eileen Doering PROGRAM_INV3 Thomas Giesen PROGRAM_INV4 Thomas Greathouse PROGRAM_INV5 John Lacy PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG The star Chi Cyg is one of the best examples of an S-type Mira variable and is only 169 pc away from us. It is an asymptotic giant branch star [AGB] with a low surface temperature, which appears in the sky as a bright infrared object. In a recent observational run a first-time detection of TiO succeeded at mid-IR wavelengths in the inner envelope of this star at high spectral resolution. This molecule is believed to be a major driver for the optical darkening of the variable star which is most notable at phase 0.5. The previous data have shown a material infall for SiO and TiO during the darkening phase. What happens at phases between 0.5 and 1.0 and at the extreme points [0, and 1] needs to be found out. For these observations a high frequency resolution at mid-IR wavelengths is mandatory because only with a resolved line profile the ongoing dynamics can be understood. Although SiO does not have the same strong effect at optical wavelength than TiO it is more clearly coupled to dust. Furthermore, in the IR region SiO plays a key role as it is much more intense than TiO and is needed to define the IR line shape that guides the identification of TiO. The TEXES spectrograph on IRTF with a resolution better than 65,000 is the most appropriate instrument for this comparative study. PROGRAM_ABSTRACT_END