A&A 388, 252-267 (2002)
DOI: 10.1051/0004-6361:20020483
Water vapor masers in stars departing from the AGB
D. EngelsHamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
(Received 17 December 2001 / Accepted 25 March 2002)
Abstract
The 22 GHz water vapor masers of four "non-variable"
OH/IR stars considered to be young Proto-Planetary Nebulae (PPNs) were
monitored for more than 10 years. In two of them,
OH 15.7+0.8
and
OH 17.7-2.0
having Mira-like
line profiles, the maser flux decreased and the maser finally
disappeared. The decrease of the maser brightness is spectacular for
OH 17.7-2.0 in particular. The maser had a flux density of several
hundred Jansky in the eighties and is now absent since 1990. The
masers of OH 15.7+0.8 and OH 17.7-2.0 probably emerged in the
"dying AGB wind" during the mass loss reduction process accompanying
the departure from the AGB. The other two,
OH 12.8-0.9
and
OH 37.1-0.8
, have double-peaked profiles with a
considerably larger velocity spread than the OH masers in these
sources. The velocities are incompatible with the spherical symmetric
mass loss process on the AGB and give evidence for the presence of
probably bipolar outflows with a projected velocity of
km s
-1 during the very early PPN phase. The gross
structure of their profiles was rather stable during the monitor
period while individual maser lines appeared and disappeared. An
increase of the velocity range was found in both sources implying that
the outflow is accelerating. Analysis of the profile variations of all
four objects suggests a lifetime of individual maser components of
1-3 years. There is no evidence for ordered motion traced by the
masers. The H
2O maser properties of all these objects are
strongly different from those of ordinary OH/IR stars, allowing to
find new young PPNs among optically still hidden objects by looking
for irregular maser profiles. In contrast to current definitions in
the literature "non-variable" OH/IR stars are proposed to be
classified as (young) PPNs. It is argued that beyond the end of the
AGB, maser properties cannot be used to infer the evolutionary stage
of the objects in transition to Planetary Nebulae.
Key words: stars: AGB and Post-AGB -- masers -- circumstellar matter
SIMBAD Objects
© ESO 2002