Speaker
Description
AGB stars play a crucial role in enriching the ISM, yet the highest mass-loss phase remains poorly understood. We present mid-IR spectroscopic observations from VISIR on the VLT for 21 of the most luminous and reddened oxygen-rich AGB stars in the Galactic Bulge. All targets exhibit the 10 μm silicate and 11.3 μm crystalline forsterite features in absorption, indicating dusty circumstellar environments. The presence of crystalline features suggests either extended envelopes or high-density dust formation regions.
Using radiative transfer modeling of the SEDs combined with 1612 MHz OH maser wind speeds, we find significant discrepancies between observed and modeled outflow properties. The 16 sources with measured pulsation periods also appear on sequence D of the mid-IR period-luminosity relation, associated with the long secondary period..
Taken together, the high optical depths, crystalline features, discrepancies between observed and modeled wind speeds, pulsation periods longer than other fundamental-mode pulsators, and SED and pulsation properties similar to those with known equatorially enhanced circumstellar envelopes (e.g. OH 26.5+0.6) lead us to believe that all of these sources are likely to be equatorially enhanced. We propose that previous estimates of mass-loss rates in these types of stars have likely been overestimated due to inappropriate assumptions of geometry in modeling.