Abstract
We have undertaken mapping and spectroscopy of a broad range of type I post-Main-Sequence nebulae in COJ=1→0,J=2→1, andJ=3→2, using the 12 m antenna at Kitt Peak, and the 45 m facility of the Nobeyama Radio Observatory. As a consequence, we find COJ=2→1 emission associated with NGC 3132 and NGC 6445, determine the location of COJ=1→0 emission in the nucleus of NGC 6302, and obtain (for the first time) COJ=3→2 spectroscopy for a substantial cross-section of type I sources. LVG analysis of the results suggests densitiesn(H2) ∼ 104 cm−3, and velocity gradients dv/dr ∼ 2×102 in both NGC 7027 and CRL 618, commensurate with uniform expansion of a constant velocity outflow, whilst for the case of NGC 2346 these values probably exceedn(H2) ∼ 4.0×105 cm−3. dv/dr ∼ 2.6×103 km s−1 andT k ∼102 K, implying appreciable compression (and shock heating?) of the CO excitation zone. Hi masses extend over a typical range 0.01<M(Hi)/M ⊙<1, whilst corresponding estimates of the progenitor mass imply 0.7<M prog/M ⊙<2.3; values significantly in excess of those pertinent for normal PN, although somewhat at the lower end of the type I mass range.
COJ=3→2 profiles for CRL 2688 confirm the presence of an extended plateau with width Δv∼85 km s−1, whilst modestJ=3→2 enhancement is also observed for the high-velocity components in NGC 7027. TheJ=3→2 spectrum for NGC 2346 appears to mimic lower-frequency results reasonably closely, confirming the presence of a double-peaked structure towards the core, and predominantly unitary profiles to the north and south, whilst there is also evidence to suggest appreciableJ=3→2 asymmetry in CRL 618 compared to lower-frequency measures. The status of an extended cloud near HB 5 remains uncertain, although this clearly represents a remarkably complex region with velocity span ΔV∼50 km s−1. Our presentJ=3→2 results appear to track lower frequency measures extremely closely, implying local densitiesn(H2)>3×103 cm−3—although temperatures close to theV lsr of HB 5 are relatively weak, and of orderT MB (J=3→2)≤0.9 K.
Finally, as a result of both this, and previous investigations we find that of type I sources so far observed in CO, some ∼42% appear to possess detectable levels of emissionT * r >0.1 K. Similarly, in cross-correlating this data with other results, we note a closely linear relation betweenJ=2→1 antenna temperaturesT MB, and the surface brightness of H2 S(1) quadrupole emissionS(H2)—a trend which appears also to be reflected betweenS(H2) and corresponding parameters for [Oi], [Oii], [Ni], [Nii], and [Sii].
Such relations almost certainly arise from comparable secular variations in line intensities, although the CO, H2, and optical emission components are likely to derive from disparate line excitation zones. As a consequence, it is clear that whilst H2 S(1) emission is probably enhanced as a result of local shock activity, the evidence for post-shock excitation of the CO and optical forbidden lines is at best marginal. Similarly, although it seems likely that CO emission derives from circum-nebular Hi shells with kinetic temperatureT k ∼ 30 K or greater, the predominant fraction of low-excitation emission arises from a mix of charge exchange reactions, nebular stratification and, probably most importantly, the influence of UV shadow zones and associated neutral inclusions.
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Phillips, J.P., Williams, P.G., Mampaso, A. et al. Molecular and low excitation emission in high mass post-main-sequence nebulae. Astrophys Space Sci 188, 171–204 (1992). https://doi.org/10.1007/BF00644908
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DOI: https://doi.org/10.1007/BF00644908