Equatorial Enhancement in the Dustiest OH/IR Stars in the Galactic Bulge

TL;DR

This study detects silicate and crystalline features in 21 dusty, oxygen-rich AGB stars in the Galactic Bulge, revealing their highly-obscured, equatorially-enhanced circumstellar environments and analyzing their pulsation and wind properties.

Contribution

It provides the first detailed spectral and modeling analysis of these stars, suggesting they are likely to have equatorially-enhanced circumstellar envelopes, a novel insight into their structure.

Findings

Detection of 10 um silicate and 11.3 um crystalline features in absorption.

Most sources are likely fundamental mode pulsators with extended envelopes.

Evidence suggests these stars have equatorially-enhanced circumstellar environments.

Abstract

We have detected the 10 um silicate feature and the 11.3 um crystalline forsterite feature in absorption in 21 oxygen-rich AGB stars in the Galactic Bulge. The depths of the 10 um feature indicate highly-obscured circumstellar environments. The additional crystalline features may suggest either an extended envelope or dust formation in a high-density environment. We have also modeled the Spectral Energy Distributions of the sample using radiative transfer models, and compared the results to wind speeds measured using 1612 MHz circumstellar OH masers, as well as previous estimates of circumstellar properties. The sixteen sources with measured pulsation periods appear on sequence D of the mid-infrared Period-Luminosity relation, associated with the Long Secondary Period. We suspect that all of these sources are in fact fundamental mode pulsators. At least two sources appear on the fundamental mode sequence when accounting for the dust content. For the remainder, these sources are also likely fundamental mode pulsators with extended envelopes. Taken as a whole, 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 and OH 30.1-0.7) lead us to believe that these sources are likely to be equatorially-enhanced.