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Issue A&A Volume 433, Number 2, April II 2005 Page(s) L29 - L32 Section Letters DOI 10.1051/0004-6361:200500090

A&A 433, L29-L32 (2005)
DOI: 10.1051/0004-6361:200500090

Letter

Burst of strongly polarized maser emission at 1612 MHz in the proto-planetary nebula OH17.7-2.0

M. Szymczak¹ and E. Gérard<sup>2

1</sup>  Torun Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Torun, Poland
    e-mail: msz@astro.uni.torun.pl
²  GEPI, UMR 8111, Observatoire de Paris, 5 place J. Janssen, 92195 Meudon Cedex, France

(Received 20 January 2005 / Accepted 16 February 2005 )

Abstract
We report the discovery of a burst of highly polarized OH maser emission at 1612 MHz in the proto-planetary nebula candidate OH17.7-2.0. The total flux density of two red-shifted features at 72.8 and 73.3 km s^-1 increased nearly threefold over a period of ~430 days. This burst was not associated with any systematic changes of the other 1612 MHz maser features as well as of the 1665 and 1667 MHz masers. The burst was characterized by a linear growth of left-hand circularly polarized emission accompanied by considerable linearly polarized emission. During the burst, the degrees of circular and linear polarization increased up to -80% and 15% respectively. A relation found between the line width and the peak flux density of the two bursting features implies a saturated amplification. Among several possible causes of the burst, a local propagation effect due to Zeeman overlap initiated by evolutionary changes in the shell appears most plausible.

Key words: polarization - masers - stars: AGB and post-AGB - circumstellar matter - stars: individual: OH17.7-2.0 - radio lines: stars

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