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Astron. Astrophys. 360, L9-L12 (2000)

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4. Discussion and summary

The multiple jet-like features which we have found in Frosty Leo are probably the result of past collimated outflows interacting with its dense AGB CSE, since no emission lines from shocked gas have been observed either in the optical or near-IR (e.g. 2.1µm H2 line, Weintraub et al. 1988). Although several PPNe have been well-resolved by HST [e.g. Roberts 22 (SZBtL99), Hen 401 (Sahai et al. 1999b), OH231.8 (Bieging et al. 2000)], Frosty Leo is only the second PPN after CRL2688 (Cox et al. 2000), and the first oxygen-rich one, which shows direct evidence for well-focussed jets close to the equatorial plane . Such jets were hypothesized by Sahai and Trauger (1998) to explain the morphologies seen in their sample of young PNe. The extended, knotty structure of the ansae suggests that they are the result of a polar jet which varied both its direction and intensity with time. The blobs [FORMULA] & [FORMULA] appear to be distinct, dense entities embedded inside extended tenuous bubbles ([FORMULA] & [FORMULA]), similar to those seen in Roberts 22. The creation of the blobs is thus a puzzle, since any interacting-winds mechanism would have difficulty finding sufficient matter inside the tenuous bubbles for sweeping up into the blob structures.

Our CO spectra show the presence of slow ([FORMULA]10 km s-1) and fast ([FORMULA]50 km s-1) components in the molecular outflow; the slow outflow may help in explaining the low grain velocities (4-15 km s-1) observed by Dougados et al. (1992). We find that Frosty Leo has the largest ratio of the mechanical momentum in the fast outflow ([FORMULA]) to the radiative momentum ([FORMULA]) amongst PPNe (scaling [FORMULA] for all objects to the same 13CO/H2 ratio) which, as a class, show large ratios (Bujarrabal et al. 2000). Only one other PPN, OH231.8+4.2, shows a momentum excess comparable to that of Frosty Leo. Since the CO emission comes from the inner ([FORMULA]10") region of the nebula in Frosty Leo, this very energetic outflow seems to be associated with low-latitude material in the vicinity of the jets seen in the HST image. The very high mechanical momentum excess in this PPN indicates that the jets that disrupted the AGB shell, yielding the complex actual shape of the nebula, were not powered by radiation pressure. The source of this excess is probably associated with binarity, and may derive from the gravitational energy of material accreted by a compact companion. However, the separation of the binary apparently seen in Frosty Leo (190 AU, for a minimum distance of 1 kpc) is much too large for the formation of an accretion disk which could drive a collimated outflow.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000