The mass loss mechanism in red supergiants (RSG) is presently not identified. These stars show obvious similarities with AGB stars undergoing heavy mass loss, for which the cause results from the combination of strong atmospheric pulsations and radiation pressure on solid condensates. However, although RSGs show luminosity variations, they do not exhibit the large amplitude pulsations typical of Miras, which current models require to be an essential ingredient for heavy mass loss (≥10-6 Mʘ/yr).

The identification of the processes accompanying the acceleration of matter from the stellar atmosphere to the region where the wind becomes supersonic should help to describe the mass loss mechanism and thereby to understand its cause. Several observing programmes are presently focused on the description of the atmosphere and the inner circumstellar envelope of Betelgeuse (α Ori), the nearest RSG, together with Antares (α Sco). They make use of the most powerful facilities presently available at IRAM, ESO (VLTI, SPHERE, VISIR,...) and ALMA. The aims of these observations is to constrain the morphology of the stars and their dust envelopes (through high angular resolution observations), as well as the kinematics of the gas (through high spectral resolution, especially in the radio range).

We propose a meeting for getting together the investigators involved in our different observing programmes. The main objectives are to combine the results already available, to extract transverse information on the morpho-kinematics, to prepare the interpretation in terms of the mass loss process, and to evaluate the needs for future observational studies. In particular, the potential of GRAVITY for addressing our topic of interest will be considered.

Although concentrated on Betelgeuse, we will also discuss Antares and μ Cep, two other nearby red supergiants, which happen to be observed with the same facilities, in order to place our favorite object in the context of its family. The identification of the cause for mass loss in red supergiants may also help to understand the mass loss origin in low pulsation AGB stars (i.e. non-Mira AGB stars, for which another mechanism for mass loss needs to be identified).