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Astron. Astrophys. 322, 291-295 (1997)

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Mode switching of the water maser in OH 39.7+1.5

D. Engels 1, A. Winnberg 2, C.M. Walmsley 3 and J. Brand 4

1 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
2 Onsala Space Observatory, S-43992 Onsala, Sweden
3 I. Physikalisches Institut, Universität zu Köln, Zülpicherstr. 77, D-50937 Köln, Germany
4 Istituto di Radioastronomia, C.N.R., Via Gobetti 101, I-40129 Bologna, Italy

Received 16 October 1996 / Accepted 22 November 1996


Monitoring of the H2 O maser in OH 39.7+1.5 as well as spatial interferometry show clear evidence that the maser switches occasionally its beaming direction between radial and tangential paths in step with the stellar pulsation cycle. Outside the minimum, several radially beamed maser features are present, which are absent ([FORMULA] mJy) during the minimum. Competition between gain paths causes tangentially beamed maser emission only to occur, when the temperature in the maser shell is too low to excite radially beamed masers.

The radius of the H2 O maser shell extends between 6 [FORMULA] cm (40 AU) and at least 3 [FORMULA] cm (200 AU), with the inner radius probably set by quenching of the maser due to high gas density close to the star. Within the H2 O maser shell, the circumstellar outflow accelerates from v [FORMULA] 9 km s-1 up to the terminal expansion velocity of [FORMULA] 18 km s-1. The expansion velocity of the circumstellar shell is constant between the outer radius of the water maser shell and the location of the 1612 MHz OH maser shell at 3 [FORMULA] cm, showing the absence of strong velocity gradients at large radial distances.

Key words: stars: AGB and post-AGB – masers – stars: individual: OH 39.7+1.5 – radio lines: stars

Send offprint requests to: D. Engels (dengels@hs.uni-hamburg.de)

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998