A slowly expanding torus associated with the candidate LBV MGE 042.0787+00.5084

Abstract

The luminous blue variable (LBV) phase is a poorly understood stage in the evolution of high mass stars, characterized for its brevity and instability. The surroundings of LBV stars are excellent test beds to study their dense stellar winds and eruptive mass-loss events. Aiming to improve our knowledge of the LBV phase, we observed the J=1-0 and J=2-1 lines of CO and $^{13}$CO in a field of 1.5'x1.5' around the recently identified candidate LBV MGE 042.0787+00.5084, using the IRAM 30-m radio telescope. We report the first detection of molecular emission associated with this source, tracing a structure with an evident circumstellar distribution. Morphology and kinematics of the gas can be explained by an expanding torus, a structure that may have originated from stellar ejecta or the action of stellar winds onto the parent molecular cloud. We derive the physical properties of the gas by means of LTE and non-LTE line modelling, obtaining densities of H$_2$ in the order of 10$^3$ cm$^{-3}$ and kinetic temperatures below 100 K. In addition, we build a kinematic model to reproduce the structure and observed velocity fields of the gas, which is in good agreement with the observations. We estimate a total molecular gas mass of 0.6$\pm$0.1 Msun and a dynamical age of 6$\times$10$^4$ years, leading to an average mass-loss rate of 0.8-1.2$\times$10$^{-5}$ Msun yr$^{-1}$.