Maser mapping of small-scale structure in the circumstellar envelope of S Persei

Water and hydroxyl maser emission from the typical red supergiant star S Pe rsei has been mapped using MERLIN. Water maser components are found grouped in clumps which appear to be discrete clouds with a velocity span of simil ar to 1 km s(-1) and an angular size of similar to 8 milliarcsec (mas). Thi s corresponds to a diameter of (2.5-3) x 10(12) m, and is the first measure ment of the unbeamed size of maser clouds. By comparing this with the full- width half-maximum (FWHM) beamed angular size, the beaming angle of the bri ghtest partially saturated masers is (1.5 +/- 0.8) x 10(-3) sr. Water maser cloud brightness temperatures are in the range similar to 10(6) to > 10(12 ) K. The water maser shell has well-defined inner and outer limits of (8-26 ) x 10(12) m. The quenching density at the inner rim shows that the water m aser clouds are about 30 times denser than the average wind density in this region. This can be explained if 4-6 dense dusty clouds (with a filling fa ctor of similar to 1/70) are formed close to the photosphere during each st ellar pulsation period of 2-3 yr. The water masers show evidence for significant acceleration of the wind, an d this continues with a shallower gradient into the hydroxyl regions at up to 9.3 x 10(13) m. These results are consistent with a wind driven by radia tion pressure on dust, but the ongoing acceleration implies that the dust s urface absorption efficiency increases throughout the maser zones. Dust mom entum is more efficiently coupled to the gas in the denser clouds, and the drift velocity is greater in less dense regions, consistent with conditions (such as a longer velocity resonance length) required by 1612-MHz masers. The circumstellar envelope appears to be spherical but irregularly filled, The mas resolution at 22 GHz allows the radial distribution of maser bright ness to be related to AAVSO light curves. An anomalously dim region in the centre of the shell corresponds to low-amplitude periods in the 1930s; such behaviour is thought to reduce mass-loss and dust formation efficiency.