Properties of OH/IR stars with IRAS LRS spectra.
Chen P.S., Szczerba R., Kwok S., Volk K.
A&A, 2001 (in press)
Abstract:
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1858 OH satellite line (1612 MHz) maser sources associated with the IRAS
counterparts were assembled from the literature. Cross-correlation
between collected OH/IR - IRAS sources and the recent catalogue of the
IRAS LRS spectra (Kwok et al. 1997, ApJS 112, 557) allows us to subdivide
1024 of them (after excluding OH sources with uncertain IRAS associations)
into 10 groups according to the Volk & Cohen 1989, AJ 98, 931) classification
scheme. We have found that sources with silicate emission form the largest
group (about 57%), followed by the group with silicate absorption (only
about 16 %). For more sensitive individual OH maser surveys these numbers
differentiate even more and the ratio between silicate emission
and absorption sources reaches a value close to 6. Surprisingly, we found
an association of OH maser emission with a small number of stars with carbon-rich
atmospheres. The distribution of these subgroups of OH/IR stars in
the IRAS colour-colour and the period-colour diagrams as well as histograms
of some physical properties are presented. The galactic distribution of
important parameters (periods, IRAS colours, expansion and stellar velocities)
for different classes of OH/IR sources is analyzed from the point
of view of the stars' evolutionary status. The correlations among the periods,
colours, expansion and star velocities and the pumping efficiencies for
selected subgroups of OH/IR stars are also investigated. These analyses
allow us to demonstrate that a significant fraction of silicate emission
group sources do not form an evolutionary sequence with the silicate absorption
objects - as is suggested e.g. by the IRAS colour-colour diagram, but rather
form a lower initial mass sequence, i.e. these sources will never develop
sufficient mass loss to manifest silicate absorption.
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