Red Supergiant Cauldrons let off steam

Dr Anita Richards, who is presenting results at the RAS National Astronomy Meeting in Preston on 17th April, said, “Red supergiants lose more than half their mass before ending their lives as supernovae. Our observations show that this doesn’t happen smoothly, like an onion shedding layers. We see water vapour clouds which are over-dense, over-magnetised and rapidly accelerated away from the star. They are embedded in a cooler, more diffuse gas producing distinctive emission from hydroxyl, a break-down product of water.”

The group studied ‘maser’ emissions from the gas clouds surrounding the star: molecules in the gas amplify and emit beams of microwave radiation in much the same way as a laser produces very narrow, bright beams of light. Water emits at 1.3 cm wavelength, under hot, dense conditions (around 1000 degrees Kelvin). Hydroxyl emission at 18.0 cm can only occur from cooler, less dense gas and it was very unexpected to detect it as close to two of the stars as the water masers. The only explanation seems to be that the water masers come from clumps where the gas density is, typically, 50 times higher than the rest of the wind from the star. Supporting evidence comes from measurements of the magnetic field strength associated with the hydroxyl masers, which is much weaker than that of the adjacent water masers, as is expected if the hydroxyl environment is more diffuse.

The water vapour clouds appear to be very dusty and are accelerating faster than the surrounding gas. Only a few of these steam clouds form each stellar period (several years), filling just a few percent of the volume of the maser shell around the star, but they contain most of the mass lost by the star.

In the study, the maser emissions from the water vapour appeared to show that the clouds had a lifetime of only a few decades, although clouds were observed at distances that would have taken about a century to reach. The puzzle was solved by comparing the MERLIN results with longer-term observations from the Puschino radio telescope in Russia, which revealed individual clouds winking off and back on again due to the fickle nature of maser excitation or beaming. Dr Anita Richards said, “These observations are intriguing because, from the size of the masing shell, we estimate that the water vapour clouds take about 100 years to bubble away into interstellar space, but we can only actually ‘see’ any particular cloud for a few years.”

The group hope to follow this up by using the e-MERLIN, eVLBI and ALMA networks of radio telescopes to trace the mass loss process back to the star to discover whether star-spots, convection cells, dust formation or some other mechanism gives birth to the clumps.