Evolved star water maser cloud size determined by star size

Abstract

Context. Cool, evolved stars undergo copious mass loss but the detailed mechanisms and the form in which the matter is returned to the ISM are still under debate. Aims. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from asymptotic giant branch and red supergiant stars. Methods. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs, covering several stellar periods. We compared our results with long-term single dish monitoring provided by the Pushchino radio telescope. Results. The 22-GHz emission is located in approximately spherical, thick, unevenly filled shells. The outflow velocity increases twofold or more between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by less than two years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In five sources, most of the matched maser features brighten or dim in concert from one epoch to the next. A number of individual maser features show idiosyncratic behaviour, including one cloud in W Hya caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are one or two orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are about ten times bigger than those round the AGB stars. Conclusions. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds presumably survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Only radiative effects can explain changes in flux density throughout the maser shells on short timescales. The size of the clouds is proportional to that of the parent star, being of a similar radius to the star once the clumps reach the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.

The physics of water masers observable with ALMA and SOFIA: model predictions for evolved stars

“…Although an explanation in terms of instabilities, with an associated physical scale corresponding to an optimum growth rate, may at first appear to be at odds with the observed scaling with star size (Richards et al 2012), the observation and clumping theory may still be compatible. Recent work (Gray, Khamis and Amos, in preparation) on instabilities in a spherical geometry generates perturbations in terms of spherical harmonics that have a natural radial scaling as a multiple of the stellar radius.…”

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

“…5), and could therefore be related to, e.g., an H 2 O or SiO maser clump (i.e. Richards et al 2011Richards et al , 2012.…”

H2O maser emission in circumstellar envelopes around AGB stars: Physical conditions in gas-dust clouds

“…The estimates of the mass loss rate for a red giantṀ lie within the range from 4 × 10 −6 (Neri et al 1998) to 3 × 10 −5 M yr −1 (Gonzalez Delgado et al 2003). In our model, the geometrical sizes of the gas-dust cloud and the distance from the cloud to the star correspond to the mean values of these parameters for the maser sources in the envelope of IK Tau (Bains et al 2003;Richards et al 2011Richards et al , 2012. The turbulent velocity in the gas-dust cloud is taken to be 1.5 km s −1 (Decin 2012).…”

“…The turbulent velocity in the gas-dust cloud is taken to be 1.5 km s −1 (Decin 2012). The gas velocity gradient in the cloud is assumed to be 0.22 km s −1 AU −1 -the mean stellar wind velocity gradient in the inner region of the gas-dust envelope around IK Tau, where the H 2 O maser emission is observed (Richards et al 2012). In the inner regions of the gas-dust envelopes around cool late-type stars, hydrogen is contained mainly in the form of H 2 molecules (Glassgold and Huggins 1983).…”

“…To estimate the mean velocity and mean velocity gradient of the gas, we used observational data on the velocity distribution of H 2 O maser sources in the circumstellar envelope of IK Tau (Richards et al 2011(Richards et al , 2012. In our calculations of the gas temperature, we consider the gas heating and cooling rates averaged over the cloud, q i = 1 H H 0 dz q i (z).…”