On the nature of high reddening of Cygnus OB2 #12 hypergiant

TL;DR

This study investigates the high reddening of the luminous star Cygnus OB2 #12 by mapping interstellar extinction and exploring circumstellar material, revealing a possible shell and multiple components of the star.

Contribution

It provides the first detailed extinction map around MT304 and suggests circumstellar shell ejection as the cause of high reddening, including discovery of multiple star components.

Findings

Reddening increases towards MT304, reaching about 10 mag.

Nearby OB stars also show high reddening (~9 mag).

Detection of multiple components of MT304 via speckle interferometry.

Abstract

To explain the nature of the high reddening (Av~10 mag) towards one of the most luminous stars in the Galaxy - Cyg OB2 #12 (B5 Ia-0), also known as MT304, we carried out spectro-photometric observations of 24 stars located in its vicinity. We included five new B-stars among the members of Cygnus OB2, and for five more photometrically selected stars we spectroscopically confirmed their membership. We constructed the map of interstellar extinction within 2.5 arcmin radius and found that interstellar extinction increases towards MT304. According to our results the most reddened OB-stars in the association after MT304 are J203240.35+411420.1 and J203239.90+411436.2, located about 15 arcsec away from it. Interstellar extinction towards these stars is about 9 mag. The increase of reddening towards MT304 suggests that the reddening excess may be caused by the circumstellar shell ejected by the star during its evolution. This shell absorbs 1 mag, but its chemical composition and temperature are unclear. We also report the detection of a second component of MT304, and discovery of an even fainter third component, based on data of speckle interferometric observations taken with the Russian 6-m telescope.