Astron. Astrophys. 353, L5-L8 (2000)

4. Origin of the bullets

Our detection of molecular bullets in BD+30^o3639 is of special interest because they have not previously been seen in any other PNe. Their high velocity and high degree of collimation with respect to the center of the nebula strongly suggest that they originate in underlying bipolar jets emanating from the central star. The location of the bullets just outside the ionized nebula (see Fig. 1) implies that they have formed by interaction of the jets with the surrounding neutral envelope. The emission would then likely come from gas that has been swept up in bow shocks.

The relation of the CO bullets to the surrounding envelope seen in H₂ emission is shown in Fig. 3. The different distributions are striking. The kinematics of CO and H₂ are also quite different (Shupe et al. 1998), but this is not surprising given their very different distributions. A detailed discussion will be given elsewhere (Cox et al. 1999b). The absence of CO emission in the extended envelope can be explained by photo-dissociation, and this is supported by the importance of atomic gas in the envelope (Taylor et al. 1990; Huggins et al. 1996). The presence of CO in the bullets means that either the gas was compressed non-dissociatively at an early stage and the CO has survived, or the CO has reformed in the bullets in post-shock gas. Both possibilities are plausible in view of the relatively low shock velocities ( km s^-1) involved (e.g., Hartquist & Caselli 1998). The absence of strong H₂ emission in the bullets, which might be expected from shocks or the UV radiation that excites the extended H₂ envelope (Shupe et al. 1998), is an interesting issue that requires further study.

Fig. 3. Comparison of the CO 2-1 line emission (Fig. 1) with the H2 1-0 S(1) emission (grey scale) from Cox et al. (1999b).

The bullets and jets are not prominent in other observations of BD+30^o3639, but certain features in optical HST images (Harrington et al. 1997; Sahai & Trauger 1998) do correspond. Gaps are seen in the ionized rims along the CO bipolar axis, suggesting that jets have punctured the main nebula. Other emission line features in the HST images may also be related. In addition, the absorption map made by Harrington et al. (1997, their Fig. 6) reveals a dust clump of  mag that corresponds within 1" to the NE (blue shifted) bullet. The clump is larger than the bullet, but may represent some of the material excavated by the jets. Note that a possible counterpart to the SW (red shifted) bullet would not be seen in absorption because it would lie on the far side of the nebula. The kinematics of the nebula recently reported by Bryce & Mellema (1999) may also be related. The main kinematic axis in the ionized gas is found to lie roughly NE(blue)-SW(red), in the same sense as the bullet axis, and a high velocity ( km s^-1) ionized feature points to localized gas flows outside the main nebula.

The most conspicuous effects of the bullets and their underlying jets are seen in the surrounding neutral envelope. In Fig. 3, the bullets correspond to distinct breaks and disturbed structure in the H₂; in fact the centers of the breaks are slightly offset in position angle from the bullets, which may indicate that the jets have changed direction with time. In any event, the idea that the jets have punctured the nebula is strongly reinforced.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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