Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics

TL;DR

This study uses high-resolution mid-infrared adaptive optics imaging to reveal detailed structures in the dusty envelope of the hypergiant NML Cyg, including asymmetries likely caused by stellar wind interactions with nearby massive stars.

Contribution

First high-resolution mid-IR adaptive optics images of NML Cyg's dusty envelope, revealing complex structures and asymmetries linked to stellar wind disruption by Cyg OB2.

Findings

Resolved the dusty envelope structures (~240 AU) responsible for silicate absorption.

Detected asymmetric excess emission (~520-870 AU) aligned with Cyg OB2.

Linked the asymmetry to wind disruption by UV radiation from nearby massive stars.

Abstract

We present sub-arcsec angular resolution, high-Strehl ratio mid-IR adaptive optics images of the powerful OH/IR source and cool hypergiant NML Cyg at 8.8, 9.8 and 11.7 um. These images reveal once more the complexity in the dusty envelope surrounding this star. We spatially resolve the physical structures (radius ~0.14", ~240 AU adopting a distance of 1.74 kpc) responsible for NML Cyg's deep 10 um silicate dust absorption feature. We also detect an asymmetric excess, at separations of ~0.3" to 0.5" (~520 to 870 AU), NW from the star. The colors of this excess are consistent with thermal emission of hot, optically thin dust. This excess is oriented in the direction of the Cyg OB2 stellar association, and is likely due to the disruption of NML Cyg's dusty wind with the near-UV radiation flux from the massive hot stars within Cyg OB2. This interaction was predicted in our previous paper (Schuster et al. 2006), to explain the geometry of an inverted photo-dissociation region observed at optical wavelengths.