On massive dust clumps in the envelope of the red supergiant VY Canis Majoris

Abstract

The envelope of the red supergiant VY CMa has long been considered an extreme example of episodic mass loss that is possibly taking place in other cool and massive evolved stars. Recent submm observations of the envelope revealed massive dusty clumps within 800 mas from the star which reinforce the picture of drastic mass-loss phenomena in VY CMa. We present new ALMA observations at an angular resolution of 0.1" and at an unprecedented sensitivity that reveal further details about the dusty clumps. We resolve more discrete features and identify a submm counterpart of a more distant Clump SW known from visual observations. The brightest clump, named C, is resolved in the observations. Gas seen against the resolved continuum emission of clump C produces a molecular spectrum in absorption. Except for SW Clump, no molecular emission is found to be associated with the dusty clumps and we propose that the dusty structures have an atypically low gas content. We attempt to reproduce the properties of the dusty clumps through three-dimensional radiative-transfer modeling. Although a clump configuration explaining the observations is found, it is not unique. A very high optical depth of all clumps to the stellar radiation make the modeling very challenging and requires unrealistically high dust masses. It is suggested that the dusty features have substructures, e.g. porosity, that allows deeper penetration of stellar photons within the clumps than in a homogeneous configuration. A comparison of the estimated clumps ages to variations in the stellar visual flux for over a century suggests that the mechanism responsible for their formation is not uniquely manifested by enhanced or strongly diminished visual light. The study demonstrates that the dusty mass-loss episodes of VY CMa are indeed unparalleled among all known non-explosive stars. The origin of these episodes remains an unsolved problem.

accepted to A&A

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