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Astron. Astrophys. 322, 159-166 (1997)

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[FORMULA] maser emission from irregular variables *

M. Szymczak 1 and D. Engels 2

1 Toru Radio Astronomy Observatory, Nicolaus Copernicus University, ul. Gagarina 11, PL-87100 Toru, Poland
2 Sternwarte der Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany

Received 31 July 1996 / Accepted 13 November 1996

Abstract

We have performed a search for the 22 GHz water maser line among 72 optically identified irregular and semiregular red variables. New detections were made of five stars, while only four of nine objects previously known as maser sources were redetected. The probability for the detection of H2 O maser emission increases with V light amplitude, and with [FORMULA] and [FORMULA] colours just as in regular Mira and semiregular variables of SRa- and SRb-types. The detection rate of water masers is about 25% for nearby Lb objects ([FORMULA] 400 pc) in the sample, comparable to that observed in the SRa and SRb stars. No masers were detected in objects with mass loss rates [FORMULA] yr-1. Maser luminosities are [FORMULA]  photons s-1 similar to that of the bluest Miras and typical SRa and SRb stars showing water maser emission. A comparison of our data on irregular stars with those previously obtained on SRa and SRb variables suggests that most radio and infrared properties are indistinguishable among both classes of objects.

Key words: masers – stars: circumstellar matter – stars: mass-loss – stars: AGB, post-AGB – radio lines: stars

* Table 2 is only available electronically at CDS via anonymous ftp 130.79.128.5

Send offprint requests to: M. Szymczak

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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