The circumstellar environment around the extreme Galactic red supergiant NML Cygni: Dense, dusty, and asymmetric

¹ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
² Department of Molecular Astrophysics, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 123, 28006 Madrid, Spain

^★ Corresponding author: elvire.debeck@chalmers.se

Received: 25 March 2025
Accepted: 29 April 2025

Abstract

Context. Supernova observations imply the presence of a dense and asymmetric circumstellar environment around SN Type II progenitors, whereas the mass loss from these progenitors, namely, red supergiants, is still poorly constrained.

Aims. We aim to characterise the dust and gas in the circumstellar environment of the extreme Galactic red supergiant NML Cyg in terms of mass, morphology, and kinematics.

Methods. Based on interferometric observations with NOEMA at 230 GHz, we estimated the dust masses and temperatures, and measured the extent and morphological complexity of the circumstellar environment.

Results. We detected two strong continuum components, amounting to an estimated total dust mass of ∼2 × 10⁻³ M_⊙ located out to ∼2000 AU from the star, largely beyond the dust detected at optical/infrared wavelengths. The extent of the detected CO emission supports the notion that the outflow is formed by a mass-loss rate of several 10⁻⁴ M_⊙ yr⁻¹ and that it is not primarily shaped by extreme irradiation from the Cyg OB2 cluster the star has been associated with. We have detected, but not resolved, previously unseen highvelocity components close to the star. The observations reveal a very complex circumstellar morphology and we propose that some of the detected components could be the imprint of a hitherto unknown binary companion.