Astron. Astrophys. 325, 725-744 (1997)

6. Comparison with other star forming regions

The star forming region surrounding IRAS 20126+4104 is merely one of a number of high mass star forming regions which have been studied in detail over the past few years. We compare in this section with the results for other regions where the formation of O-B stars is occurring and thus attempt to put the region surrounding IRAS 20126+4104 in context. It is important to realise in the first place that in the vast majority of cases, the formation of O-B stars is accompanied by the formation of a cluster of lower mass stars detectable in NIR surveys (see e.g. Lada et al. 1991a, 1991b) and that such NIR clusters are always closely associated with dense molecular clumps of mass 10-1000  and size 0.1-0.3 pc. Typical masses in the form of young stars are less but not greatly so than the masses of the dense associated clumps.

Our results for IRAS 20126+4104 are consistent with this general picture. Lada et al. (1991b) find for example 105 sources with m (K) 14 mag in the cluster of effective radius 0.6 pc associated with NGC 2071. In our TIRGO survey, we find 60 sources with m (K) 16.2 mag in a box 1 pc². The NGC 2071 infrared cluster is associated with a molecular clump of radius 0.5 pc and mass 450  (see Lada et al. 1991b, Walther et al. 1993). When one considers the difference in distance between the NGC 2071 and IRAS 20126+4104 clusters, (equivalent to roughly 3 mag), the situation seems comparable. The molecular clump seen by Lada et al. (1991a) towards NGC 2071 is somewhat more massive and extended than that observed by us towards IRAS 20126+4104 but this probably mainly reflects the difference in linear resolutions employed in the two sets of observations.

NGC 2071 is also similar to IRAS 20126+4104 in that it is associated with a well collimated outflow (see e.g. Chernin & Masson 1993, Lada 1985) associated with a mechanical luminosity of 175  and 20  of high velocity gas (as compared to 80  and 100  in IRAS 20126+4104). NGC 2071 however has a bolometric luminosity which is an order of magnitude lower (750  ) and we conclude that our estimates for the mass and energetics of the IRAS 20126+4104 outflow are not exceptionally high. The morphology of the outflow in IRAS 20126+4104 and, in particular, the switches in orientation on different size scales are at first sight surprising. We have already noted that this may partly be due to the effects expected when observing an outflow of large lobe opening angle close to the plane of the sky. It should also be noted that many double or multiple outflows from embedded infrared sources have been recently detected (e.g. Bachiller et al. 1995, Ladd & Hodapp 1997). This has been variously interpreted in terms of a "wandering jet" and/or multiple sources clustered within distances of less than 0.1 pc of one another. In the case of IRAS 20126+4104 effects of either kind are likely to be operating and more high angular resolution observations are needed to decide which is most important.

Finally, IRAS 20126+4104 is also unusual in that it contains a "hot core" source of mass roughly 10  , diameter 0.01 pc, and temperature 200 K. These parameters are roughly similar to Orion-KL for example (see Walmsley & Schilke 1993, Wright et al. 1992) where one also detects compact emission in species such as methyl cyanide on a size scale of roughly 0.03 pc. There is a difference between the two cases however in that the bolometric luminosity in Orion-KL seems to be an order of magnitude larger (). Given that one expects the ultimate energy source to be the embedded protostar, it is surprising that an object ten times less luminous is capable of heating a similar mass of gas to similar temperatures. Part of the explanation for this may lie in the uncertain distance to IRAS 20126+4104 mentioned earlier. If IRAS 20126+4104 is in fact at 4.2 kpc instead of 1.7 kpc, the estimates for the bolometric luminosity and disk mass rise by a factor of 6 but the inferred temperature is unaffected. We note however that the water maser source associated with W3(OH) (see Wink et al. 1994) has also associated compact emission in methyl cyanide with rather similar properties to IRAS 20126+4104. The bolometric luminosity in the case of W3(OH)-H₂ O is poorly known but is less than . Thus, the basic parameters of the IRAS 20126+4104 and W3(OH)-H₂ O "hot core" sources may be similar. There is unfortunately at least one difference. In the case of W3(OH), the orientation of the peaks from the CH₃ CN channel maps is parallel to the outflow direction rather than being perpendicular to it as we have found for IRAS 20126+4104!

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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