Tracing the inner regions of circumstellar envelopes via high-excitation water transitions

Water is a ubiquitous molecule in circumstellar envelopes (CSEs). Its emission has been detected at a wide range of distances from the central oxygen-rich evolved star. In particular, the water maser transition at 22 GHz, typically extending from about 5–20 stellar radii to as far as several hundred stellar radii from the star, has been commonly used to probe the structure and dynamics of the intermediate regions of the CSE where dust is condensing and the inner wind is being accelerated. The advent of ALMA has opened the door to high-angular resolution mapping of much higher excitation transitions of water, probing the inner regions of the CSEs, some of which are anticipated to exhibit maser action. The ALMA ATOMIUM large program observed many such transitions towards a sample of AGB stars & red supergiants. The preliminary results show that while some transitions depart only slightly from LTE, others clearly show signs of maser action. The Gaussian fitting of the non-diffuse/compact part of some of the (quasi) thermal & maser transitions reveal interesting velocity gradients, signatures of outflowing and infalling motions hence providing important constraints for stellar wind models.