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Дата индексирования: Mon Jun 28 04:04:18 2010
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Поисковые слова: nebula


A systematic study of IRAS selected proto-planetary nebula
   candidates}

J.Y. Hu, P. te Lintel Hekkert, S. Slijkhuis, F. Baas, R. Sahai, P.R. Wood

Abstract

We present OH maser observations of a sample of 62 IRAS-selected proto-planetary nebula (PPN) candidates, and CO observations of the 29 sources in this sample which are brighter than 25\,Jy in the IRAS 25\,$\mu$m band. Sixteen sources have been detected in \co\ or \coto. We observed and detected two sources in \cotr. We have detected sixteen sources (four of which are CO sources) in the 1612\,MHz transition of OH. Half of these are also detected at 1667 and/or 1665\,MHz. The CO and OH spectra are characteristic of emission from an expanding \cs\ shell, with an average expansion velocity of $\sim 15\,\kms $. The OH maser sources are not variable over a period of one to five years, with only a few exceptions. Several OH sources show multiple components in their spectra, sometimes strongly circularly polarized. One double-peak OH maser source, IRAS\,20406+2953, shows an enigmatic variability in circular polarization.

Keywords

circumstellar matter -- stars: asymptotic, post-asymptotic giant branch (AGB, post-AGB) -- radio lines: molecular: circumstellar