A Multiwavelength Study of the Symbiotic Mira HM Sge with SOFIA and HST

Abstract

We have targeted the dusty symbiotic Mira system HM Sge with four instruments from the IR to the UV. We have used these observations along with archival observations to study how the system has been evolving after its 1975 nova-like outburst. We have detected rovibrational water emission in a symbiotic system for the first time using new EXES high-spectral-resolution infrared spectroscopy. The features, detected in emission, have velocities consistent with the systemic velocity but do not show any clear evidence of high-velocity outflows. Mid-infrared photometry and grism spectroscopy show that the oxygen-rich asymptotic giant branch dust and dust output have shown little to no change over the past 39 years. In the optical/UV, we detect three main [N ii ] nebular features that were detected 22 years ago. Two of these features show a small amount of movement, corresponding to average outflows speeds of 38 and 78 km s ^−1 since they were previously observed; some previously detected [N ii ] features are no longer visible. New UV spectroscopy has shown that the nebular environment continues to steadily relax after the system’s 1975 outburst. The data suggest, however, that the temperature of the hot component has increased from 200,000 K in 1989 to greater than 250,000 K now. Our new and archival observations suggest that the evolution of the system after its outburst is swift with little to no major changes after a period of a couple of years.

Keywords

• Symbiotic binary stars
• Symbiotic novae
• Asymptotic giant branch stars
• Mira variable stars
• Circumstellar dust
• Stellar mass loss