| We present a search for 18 cm OH maser emission from all IRAS sources north of -38° declination, within -0.4 < [25- 12] < 0.0 and -1.2 < [60-25] < -0.8 colors, and for which an LRS classification has been published. Such sources are presumably Asymptotic Giant Branch stars with high mass loss rates. This survey results in 74 detections, almost all at 1612 MHz several sources have a main-line counterpart. The observed OH spectra are typical of thick circumstellar shells. The average detection rate is about 50% its value is found to be strongly dependent on the LRS class of the objects, and on their galactic location. The OH pump efficiency is shown to be an increasing function of the mass loss rate and a decreasing function of the envelope expansion velocity. The nature of the objects is investigated on the basis of their colors, LRS and OH spectra, and galactic latitude. This allows the splitting of the LRS class 2 objects into two subgroups of different envelope thicknesses in the ""thick"" group, the 10 mum silicate emission is depressed. The LRS class 4 is shown to be constituted of three subgroups. The first one contains the true carbon-rich stars. In the second one, of equal importance, the objects are oxygen-rich they show OH masers, and self-absorption within the 10 m silicate emission band. Two objects form the last class 4 subgroup they seem to be carbon-rich stars with an oxygen-rich outer envelope their presumed origin -binary system or genuine carbon enrichment- is briefly discussed. The nature of the LRS class 1 objects is not fully clear they have thick envelopes, and most of them are unlikely to be oxygen-rich. On the contrary, the LRS class 3 appears to comprise quite an homogeneous population of very thick envelopes showing a high OH detection rate. These subgroups describe a collection of characteristic domains on the color-color sequence of circumstellar envelopes. For blue colors, we confirm the envelope expansion velocity to be an increasing function of the [25-12] color. Consequently, it is argued that in the hypothesis of a time dependence of the AGB mass loss rate, very likely, only relatively massive objects can reach high mass loss phases characterized by very red IRAS colors. We observe that on the average, the velocity is stationary for the reddest part of our [25-12] color coverage. |
Comments:
Velocity resolution: 0.58 and 1.16 km/s
Upper limit for non-detections: 0.2 Jy (3σ)
IRAS 18554+0231: Blue peak at v=-8.5 km/s requires confirmation
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