High - resolution observations at λ = 3 mm of the OH 231 . 8 + 4 . 2 molecular outflow

We present high spatial resolution observations of HCO+ (J=1–0), SO (J=22–11), H13CN (J=1–0), SiO (v = 1, J = 2–1), and the continuum at 3 mm from OH 231.8+4.2, taken with the IRAM interferometer at Plateau de Bure. We also report the first detection of NS in circumstellar envelopes. The overall distribution of the emission of all molecules (except for HCO+ and the SiO maser) is similar to that of CO. The most intense emission arises from a compact, slowly-expanding component around the central star. The rest of the emission comes from gas distributed in a narrow region along the symmetry axis, that flows outwards following a velocity gradient also similar to that found in CO. Our observations show with high accuracy the distribution of the HCO+ intensity, that is found to be very clumpy and strongly enhanced in the shock-accelerated lobes. We argue that such a distribution is due to the efficient formation of this molecule by shock-induced reactions. An expanding disk or ring around the central star is detected from the SO emission. The characteristic radius and expansion velocity of this structure are 2 1016 cm and 6–7 km s−1, respectively. The SiO maser emission could arise from the innermost parts of such a disk. The 3 mm continuum emission seems to be due to cold dust (∼ 20 K) distributed in the lobes of OH 231.8+4.2 as well as from warmer (∼ 55 K) dust located in a compact region surrounding the central star.