On the morphology of the compact dust shell in the symbiotic system HM Sagittae

TL;DR

This study uses mid-infrared interferometry to analyze the dust shell morphology of HM Sagittae, revealing an elongated, optically thick silicate dust envelope whose shape varies with wavelength.

Contribution

It provides a detailed interferometric analysis with expanded uv coverage, confirming the dust shell's elongation and its wavelength-dependent shape, improving understanding of the circumstellar structure.

Findings

The dust shell is elongated perpendicular to the binary axis.

The shell's size increases from 8 to 13 microns.

Elongation level increases with wavelength.

Abstract

The symbiotic system HM Sagittae consists of a Mira star and a secondary White Dwarf component. The dust content of the system was severely affected by the nova outburst in 1975, which is still ongoing. The capabilities of optical interferometry operating in the mid-IR allow us to investigate the current geometry of the dust envelope. We test our previous spectro-interferometric study of this system with new interferometric configurations, increasing the uv coverage and allowing us to ascertain the appearance of the source between 8 and 13micron. We used the MIDI instrument of the VLTI with the unit telescopes (UTs) and auxiliary telescopes (ATs) providing baselines oriented from PA=42degrees to 127 degrees. The data are interpreted by means of an elliptical Gaussian model and the spherical radiative transfer code DUSTY. We demonstrate that the data can be reproduced well by an optically thick dust shell of amorphous silicate, typical of those encountered around Mira stars, whose measured dimension increases from 8 to 13micron. We confirm that the envelope is more extended in a direction perpendicular to the binary axis. The level of elongation increases with wavelength in contrast to our claim in a previous study. The wider uv coverage allows us to deepen our previous investigations of the close circumstellar structure of this object.