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5. SiO masers in proto-planetary nebulae

5.1. OH 231.8+4.2

OH 231.8 is a bipolar nebula (e.g. Reipurth 1987) with a high-velocity outflow seen in OH and CO, and it has a rich molecular spectrum (Morris et al. 1982, 1987). SiO maser emission has been detected by Barvainis & Clemens (1984), Morris et al. (1987), Jewell et al. (1991), and in this work. The central star is a long-period variable of spectral class M with a period of about 680 days (Feast et al. 1983; Cohen et al. 1985). Cohen et al. demonstrated the presence of a blue companion, and the bipolar outflow is therefore most likely caused by mass transfer between the central stars. The SiO maser is probably associated with the long-period variable.

5.2. OH 42.3-0.1

OH 42.3 belongs to a class of objects where the velocity width of the tex2html_wrap_inline1844 maser emission is larger than that of the OH emission and it has been suggested that they are in the early stages of becoming high-velocity outflow sources (Gomez et al. 1994). SiO maser emission was detected by Jewell et al. (1991), and in this work. The OH maser emission is variable with a period of more than 2000 days.

Other objects where the velocity width of the tex2html_wrap_inline1844 maser emission is larger than that of the OH emission are tex2html_wrap_inline3042 and tex2html_wrap_inline3044 (Gomez et al. 1994). OH 37.1 is a star with non-variable OH emission (Herman & Habing 1985). It was tentatively detected in SiO by Jewell et al. (1991), but not by Gomez et al. (1990), Nyman et al. (1993a), or in this work, and we therefore regard it as a non-detection. No SiO maser has been found toward OH 12.8 (Jewell et al. 1991).

5.3. OH 19.2-1.0

OH 19.2 was not included in our sample, but it has been detected in SiO emission by Jewell et al. (1991). It has a peculiar OH maser spectrum with at least four peaks, and maps of the OH emission has been interpreted in terms of a bipolar outflow (Chapman 1988). The OH emission is variable with a period of about 600 days (Chapman 1988).

5.4. OH 15.7+0.8

OH 15.7 is a non-variable OH/IR star (Herman & Habing 1985) with a double peaked energy distribution. It was tentatively detected by Jewell et al. (1991) and in this work, and regarded as detected by Nyman et al. (1993a) but with a low signal to noise ratio. The possible maser components cover a similar velocity range in the different observations. No SiO masers have been found towards other objects of this type, e.g. tex2html_wrap_inline3048 and tex2html_wrap_inline3050 (this work; Jewell et al. 1991).

5.5. SiO maser properties of the detected PPNe

The SiO masers in OH 231.8, OH 42.3, OH 19.2, and the tentatively detected OH 15.7 all have properties similar to those of SiO masers seen towards Mira variables and OH/IR stars: line widths of tex2html_wrap_inline3052 and center velocities close to the mean velocity of the OH emission.

OH 19.2 is observed to be bipolar and the OH and tex2html_wrap_inline1844 maser properties of OH 42.3 can be modeled in terms of a bipolar outflow (Gomez et al. 1994). Their SiO maser emission can therefore be explained in the same way as for OH 231.8 (Sect. 5.1): the central objects are binary systems where one of the components is a variable red giant presently undergoing mass loss. The bipolar nebula is shaped by the mass transfer between the two stars and the SiO maser is associated with the variable star. Schwarz et al. (1995) made a survey of SiO masers in dusty symbiotic systems, and emission was found only in systems with wide orbits. Their observations show that SiO masers may exist near variable stars also in binary systems.

OH 15.7 has probably very recently left the AGB. Its tex2html_wrap_inline1844 maser emission has been very irregular over the past years and recently disappeared (Engels 1997), indicating that the mass loss rate is irregular and decreasing.

5.6. Detection rates and comparison to OH/IR stars

Only two SiO masers were clearly detected in the objects classified as PPNe in our sample, and one, OH 15.7+0.8, was tentatively detected. This is considerably fewer than towards the OH/IR stars. The low detection rate of SiO emission towards PPNe could be explained if these sources are situated at larger distances than OH/IR objects. However, distance estimates for the various types of objects indicate that they, on the average, are located at similar distances. It is also possible to estimate whether the non-detections are significant by comparing the SiO maser fluxes with the total energy fluxes, since it is likely that the luminosities of OH/IR stars are similar to those of PPNe. In Fig. 2 (click here) we have plotted the SiO (v=2, J=1-0) energy flux versus the far infrared energy flux (calculated from the IRAS intensities). For the OH/IR stars the far infrared energy flux should be a good approximation to the total energy flux, since they mainly radiate in the far infrared, but for the PPNe and PNe it is only a lower limit. Many PPNe have far infrared energy fluxes comparable to those of the detected OH/IR stars, indicating that the absence of SiO emission is real and not caused by a distance effect.

  figure606
Figure 2: SiO (v=2, J=1-0) integrated flux densities plotted as function of the far infrared energy flux. Detections, as well as upper limits, are included. The upper limits to the SiO integrated flux densities were calculated in the same way as in Nyman et al. (1993a). The objects are divided into OH/IR stars, PPNe, PNe, and unclassified sources ("?") according to Table 2 (click here). Note that the FIR energy flux is only an upper limit to the total energy flux of the PPNe and PNe. OH 15.7 was not detected in the v=2 transition and only tentatively detected in the v=1 transition. (Nyman et al. 1993a, reported an integrated flux density of tex2html_wrap_inline3070 in the v=1 transition)


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