Astron. Astrophys. 357, 651-660 (2000)

4. Conclusions

We have obtained high spatial resolution (2"-4") maps of OH 231.8+4.2 in several molecular lines - HCO⁺ (J=1-0), SO (J=2₂-1₁), H¹³CN (J=1-0), SiO (v=1, J=2-1) and NS (, J=5/2-3/2, parity-e) - and the continuum at 3 mm. The main conclusions reached in this work are:

• All the molecules are distributed in a narrow region along the symmetry axis, and flow outwards following a velocity gradient similar to that found in CO (Alcolea et al., in preparation).

• Except for HCO⁺, the most intense emission from all the molecules arises from the nebula center, from a relatively compact, slowly expanding component that could be the non-accelerated remnant of the envelope ejected by the star during the AGB.

• In the nebula center, HCO⁺ is distributed in an expanding, hollow cylinder. In the equatorial direction, the HCO⁺ emission is slightly more extended than that of ¹²CO and other molecules.

• The HCO⁺ emission is found to be very clumpy and strongly enhanced in the shock-accelerated lobes. This intensity enhancement can only be explained by an increase of the HCO⁺ abundance in these regions. In our opinion, the efficient formation of this radical in the outer, fast flowing clumps is probably induced by shocks.

• The SO maps have revealed the presence of an expanding, equatorial disk or ring around the central star. The characteristic radius and the expansion velocity of this structure are 2 10¹⁶ cm and 6-7 km s^-1, respectively.

• The spectral distribution and the small extent of the SiO maser emission suggest that it arises from the innermost parts of the expanding disk. The intensity and profile of the different spectral features of the line have been found to strongly vary with time. The good agreement between the SiO and n-IR light curves suggests radiative maser pumping.

• We report the first detection of NS in circumstellar envelopes with a relative abundance of 2 10^-8.

• The large variety of S- and N-containing molecules indicates that an active chemistry, that (like in the case of HCO⁺) could involve shock-induced reactions, takes place in OH 231.8+4.2.

• The 3 mm continuum emission seems to arise from cold ( 20 K) dust distributed along the nebular lobes as well as from a warmer ( 55 K) dust component arising from a compact region surrounding the central star.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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