OH 1612 MHz masers have been detected in about 67% of the sources searched
in our main sample. This confirms what is expected from previous systematic
OH surveys (e.g.
Sivagnanam et al. 1990;
Eder et al. 1988) for this range
of IRAS colours.
OH/IR stars are indeed very numerous in regions IIIa1 and IIIa2.
This high detectability increases up to 
if we consider only
objects with low 
(< 100) values, as shown in Table 5 (click here).
It is only slightly smaller (
) in the objects which display a
relatively low CO emission (
).
| Group 1 | Group 2 | |
 100 |  120 | |
| OH detections | 31 | 20 (of which 6 SG and 6 giants) |
| OH non-detections | 11 | 14 (of which 11 SG and 2 giants) |
|
|
and
OH maser detectabilitySince OH is more sensitive to photodissociation than CO, if one admits that a weak CO emission reflects photodestruction of CO, one could expect that OH masers are rare in sources with weak CO. Since this is not verified, either CO photodestruction is negligible or there are other factors favouring OH masers in these sources. An obvious explanation of the relatively high OH detection rate in group 2 can be simply an efficient OH pumping resulting from the high infrared luminosity, characterisitc of these objects. It should be also stressed that the relation of OH masers to photodestruction is complex, since OH is both destroyed and formed (from H2O) by photodissociation.
| IRAS name | spectral type | Comment |
| 00428+6854 | S/M8+ III | no published spectrum |
| 04575+1251 | M8+ III | rather good quality spectrum |
| 07180-1314 | good quality spectrum | |
| 08357-1013 | M7e | one peak spectrum |
| 17239-2812 | good quality spectrum | |
| 17313-1531 | good quality spectrum | |
| 17376-3021 | M8+ III ? | noisy spectrum; only 1 peak |
| 17482-2824 | no published spectrum | |
| 18025-2113 | M3-4 I | blue peak at  |
| 18079-1810 | K0-2 III | no published spectrum |
| 18304-0728 | M7: | no published spectrum |
| 18551+0323 | 3 peak spectrum | |
| 18556+0811 | M8+ III | one peak spectrum |
| 18585+0900 | rather good quality spectrum | |
| 19043+1009 | no published spectrum | |
| 19422+3506 | M8+ III | good quality spectrum |
| 20000+4954 | M5e | no published spectrum |
| 21245+6221 | M3.5 I | no published spectrum |
| 22345+5809 | K0 Ia | good quality spectrum |
| 22525+6033 | M6-7 Iab | noisy spectrum; only 1 peak |
| 23000+5932 | M2-3 I | good quality spectrum |
| 23416+6130 | M2-4 Ia | good quality spectrum |
|
|
flux to CO intensity ratio
In fact, a more precise study of the OH spectra of such
objects tends to reduce the number of those with significant
circumstellar OH.
It is possible that a few of the OH detections may be interstellar,
as only one peak is
clearly visible. Since these objects are mainly at low galactic
latitude, such contamination is not surprising. Also, some
spectra are unusual, such as those of 18551+0323 (
, OH data
from Le Squeren et al. 1992), having 3 peaks, or 22525+6033
(MY Cep, 
, OH data from Sivagnanam et al. 1990),
where the 1612
and 1667 MHz emission are not at the same velocity (but the signal is
very faint: 
).
Furthermore, because of the large beam of centimeter telescopes,
contamination by other sources is possible.
The remaining stars with reliable OH data
are: 04575+1251 (
), 07180-1314 (
),
17239-2812 (
), 17313-1531 (
),
18585+0900 (
)
19422+3506 (
), 22345+5809 (W Cep, 
),
23000+5932 (AS Cep, 
), and 23416+6130 (PZ Cas, well-known
supergiant only observed in CO(2-1)).
Of these, we have spectral types for 04575+1251 (M8 + III),
19422+3506 (M8 + III), 22345+5809 (K I), 23000+5932 (M3 I),
and 23416+6130 
. For the two giants,
while the first case is not really puzzling and
may just be a very luminous AGB star, the second one is slightly
more problematic.
If one considers the hypotheses considered in Sect. 4, the explanation of weak CO emission based on a chromosphere may not fit here, since OH, formed closer to the star than CO, would also be affected. However, the clumpy structure of envelopes, especially around supergiants could contribute to protect OH inside blobs or clumps against photodestruction by UV radiation. Finally a change in mass-loss remains a possible explanation both for AGB stars and supergiants. The mass-loss is presently large for the OH masering region, but in the past the mass loss rate was small at large radii where CO arises.
In summary, the quality of the OH data does not allow precise conclusions in many cases. However, in a significant number of objects, CO lines are weak, not only with respect to the infrared emission, but also to the OH maser. The main reason is probably again the large luminosity which enhances the far-infrared pumping of OH masers. Additional effects of time variations of the mass-loss are also possible. An enhanced photodestruction of CO with respect to that of OH could be surprising. However, it could be favoured by a clumpy structure of the circumstellar envelope and by the fact that OH is not only destroyed by photodissociation but also produced by photodissociation of H2O.
Ten sources show strong CO emission but were not detected in OH. They
are mainly at high galactic latitude and have spectral types typical of red
giants or AGB stars. Two of them seem to have slightly self-absorbed
silicates 
.
Then their envelopes should be rather optically
thick and should produce masers.
Lewis (1994) has proposed possible explanations for the lack of OH 1612 MHz
emission, and in particular the possible combined effect of a superwind phase
(to increase the opacity) and a hot companion that emits UV radiation and
destroys OH. The result is what he calls a "symbiotic nova'',
but their reality is by no means proved.
For statistical reasons, binarity could be a good scenario.