A systematic search at 1612 MHz for OH maser sources.
Abstract
The results of three systematic 1612 MHz OH surveys are combined to study the properties of Type II OH/IR sources on a galactic scale. The sample contains 114 sources found at 10° ≧ l ≧ 150°, |b| ≲ 4°. Most have no optical identification but we assume that a well evolved red giant or supergiant is at the centre of each source. We estimate the incompleteness of the sample.
l From a comparison of the observed radial velocities with those predicted by galactic rotation it follows that the sources can be separated into at least two kinematically different groups. A statistically useful separator is ΔV, the velocity difference between the two emission peaks. Type II OH/IR sources with ΔV > 29 km s-1 show only small (≲10 km s-1) random motions with respect to galactic rotation, whereas sources with ΔV < 29 km s-1 show much larger (≈ 30-40 km s-1) random motions. The first group of sources contains predominantly young stars (M supergiants, ≲107 yr), and the second group much older stars (Mira variables, ≈ 109 yr). The galactic density distributions of both groups show a pronounced maximum at a distance R =4.5 kpc from the Galactic Centre, in common with several other Population I species. The star formation rate at R <4.5 kpc has therefore been low for the last 109 yr. Type II OH/IR sources are not the precursors of the majority of the planetary nebulae. In contrast to previous authors we find that the absolute 1612 MHz flux densities span a range of a factor 100-1000, and their numbers appear to decrease strongly with increasing flux density. This explains why nearby, optically identified sources are intrinsically weak and the unidentified sources are strong and distant.- Publication:
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Astronomy and Astrophysics
- Pub Date:
- February 1981
- Bibcode:
- 1981A&A....95..156B
- Keywords:
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- Galactic Structure;
- Hydroxyl Emission;
- Infrared Astronomy;
- Interstellar Masers;
- Supergiant Stars;
- Variable Stars;
- Angular Distribution;
- Galactic Nuclei;
- Galactic Rotation;
- M Stars;
- Radial Velocity;
- Stellar Rotation;
- Type 2 Bursts;
- Astrophysics