Evolved Stars

We present a millimetre molecular line survey of seven high mass loss rate carbon stars in both the northern and southern skies. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12C/13C-ratio of about an order of magnitude between the sample stars, mainly reflecting the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on the average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models.

We have obtained high-resolution wide-band, narrowband, and polarimetric images of the bipolar protoÈplanetary nebula Roberts 22 with the Wide Field Planetary Camera 2 on the Hubble Space T elescope. OH maser-line emission has also been observed using the Australia Telescope Compact Array. The wide-band images at 0.6 km show bright bipolar lobes shaped like a butterÑyÏs " " wings,ÏÏ separated by a dark equatorial " " body ÏÏ of dense dust that completely obscures the central star. The material within each lobe appears to be organized into an amazingly complex pattern of a very large number of intersecting loops and Ðlaments. The bright lobes are surrounded by a fainter halo whose inner region contains multiple thin shell structures, reminiscent of those seen in the prototype protoÈplanetary nebula, CRL 2688 (Egg Nebula). The halo can be traced to a distance of about 25A from the nebular center. The radial surface brightness of the halo is inconsistent with that expected for time-invariant mass loss at a constant expansion velocity. A simple model of the radial scattered light distribution supports the hypothesis that the bright lobes are cavities with thin dense walls that are optically thick in scattered (\0A .5), light. The amount of mass in the cavity walls is quite large (0.3 and the dynamical timescale for M _) their formation is short yr). The bright lobes show high fractional polarization (40%È50%) and a ([440 centrosymmetric polarization pattern, typical of a singly scattering reÑection nebula. In contrast, the polarization in the dark equatorial region is small, and the polarization vectors do not show a simple orientation pattern. The polarization data have been compared with published models to set rough constraints on the dust albedo and scattering phase function and the nebular optical depths. The OH maser-line emission at 1665 and 1667 MHz is found to be concentrated in the dark equatorial region ; the kinematics of the OH features indicates that they arise in low-latitude outÑows. The nebular morphology in Roberts 22 is characterized by a high degree of point symmetry, which may result from the interaction of collimated bipolar outÑows with the progenitor AGB envelope. Key words : circumstellar matter È planetary nebulae : general È stars : AGB and post-AGB È stars : mass loss

Using the Hubble Space Telescope's Wide Field Planetary Camera 2, we have discovered that the proto–planetary nebula (PPN) IRAS 220365306 (I22036) is an extended () bipolar nebula. A dark, 5 .7 # 1 .3 (partial) ringlike band defines the nebular waist region and probably results from a tilted, geometrically thick disk that obscures the central star. Knotty, linear structures are seen in each lobe. Very Large Array A configuration observations of OH maser emission in I22036 show the 1667 MHz emission features aligned in a roughly 1 .8 long linear structure along the nebular axis. The source's far-infrared fluxes imply the presence of a massive [∼5(D/2 kpc) 2 ], dusty, cool (K), circumstellar envelope. Strong 3.08 and 11 mm absorption M T ∼ 35–67 , dust features in the Infrared Space Observatory spectra show the presence of icy silicate grains, which probably reside in the disk's shielded outer regions. A low-resolution optical spectrum indicates an F5 (or earlier) spectral type for the post–asymptotic giant branch (AGB) central star and shows Ha and Hb emission lines. The elongated and highly structured lobes of I22036 are likely to have been produced as a result of the interaction of a collimated, fast wind (CFW) with the AGB progenitor's dense, slow wind. Highly collimated jetlike components in the CFW have probably operated at some time during the formation of this PPN. Subject headings: circumstellar matter — planetary nebulae: general — reflection nebulae — stars: AGB and post-AGB — stars: individual (IRAS 220365306) — stars: mass loss

A millimetre molecular line survey of seven high mass-loss rate carbon stars in both the northern and southern skies is presented. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12C/13C-ratio of about an order of magnitude between the sample stars, which mainly shows the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over-abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models. Figures A.1-A.13 are only available in electronic form at http://www.edpsciences.org