Temporal Variation of the Warm Molecular Layers around the Mira Variable RR Sco Detected with the VLTI/MIDI Instrument

Infrared spectroscopic observations of asymptotic giant branch (AGB) stars with the Infrared Space Observatory (ISO) have revealed the existence of a quasi-static, warm, dense molecular envelope close to the star (e.g., Tsuji et al. [4], Yamamura et al. [5], Cami et al. [1], Matsuura et al. [2]). Although this warm molecular envelope is most likely to play an important role in mass loss, its formation mechanism is not yet understood. The first midinfrared spectro-interferometric observations of the Mira variable RR Sco (period 281 days, distance 320 pc) using MIDI have revealed that optically thick emission from dense, warm molecular layers (H2O and SiO) as well as dust thermal emission can make the N -band angular size more than twice as large as that in the K band (Ohnaka et al. [3]). The physical properties of the warm molecular layers derived from the MIDI observations (Tmol ∼ 1400 K, Rmol ∼ 2.3 R , H2O column density 3 × 10 cm−2, SiO column density 10 cm−2) have turned out to be consistent with those derived for other Mira variables from ISO observations. Here we present the results of MIDI observations of RR Sco at the second epoch.