Mass-loss histories of three carbon-rich evolved stars as revealed by 12CO emission

We investigate the history and geometry of mass loss in three carbon-rich evolved stars, CIT 6, AFGL 618, and IRAS 21282 + 5050, using observations of the (CO)-C-12 J = 1-0 line emission and a simple radiative transfer code to model these observations. Combining data from the Berkeley-Illinois-Maryland Association millimeter array and the NRAO 12 m, we have constructed full synthesis data cubes of the (CO)-C-12 J = 1-0 line emission from CIT 6, AFGL 618, and IRAS 21282 + 5050 with angular resolutions of approximately 8 ", 3 ", and 4 ", respectively. We find clumpy envelopes with zero-order structures of bright cores, surrounded by lower surface brightness halos in all three sources; however, the contrast between the core and halo is greater for AFGL 618 than for IRAS 21282 + 5050 and CIT 6. The total flux line profile for CIT 6 has a parabolic shape, in contrast to the hat-topped shape found by all previous single-dish observations that have resolved the envelope, which we measure to be at least 100 " in diameter. The 12CO emission in AFGL 618 has a similar east-west bipolar morphology as its optical reflection nebulosity, although on a much larger scale (90 " x 60 "). The bulk of molecular gas in AFGL 618 participates in a symmetric expansion; however, a significant east-west bipolar outflow appears at the heart of the core with detected velocities up to similar to 70 km s(-1). A blueshifted component (similar to -40 km s(-1)) of the this bipolar outflow is observed in absorption against the continuum source in AFGL 618. We resolve a central hole in IRAS 21282 + 5050 with a size, 6 ", slightly larger than its H pi region. A bright ring of (CO)-C-12 emission surrounds this central hole, and a similar to 70 " diameter halo surrounds this ring. The ring appears broken in position-velocity cuts due to a significant blueshifted self-absorption of the gas. We develop a simple radiative transfer code that assumes spherically symmetric expansion to model the zero-order core-halo structures observed in these sources. We assume a temperature power-law profile with respect to radius and fit a power-law index between -0.7 and -0.8 for all three sources. The fitted density profiles with respect to radius reflect the observed differences in the core-halo structures and suggest differences in the mass-loss histories of the three sources. The models of both CIT 6 and IRAS 21282 + 5050 are consistent with constant mass-loss rates of (6 +/- 2) x 10(-6) and (6 +/- 4) x 10(-5) M. yr(-1) respectively. The model of AFGL 618 suggests two phases of mass loss: an older asymptotic giant branch (AGB) wind lasting similar to 8000 yr when the mass-loss rate decreased from (2 +/- 1) x 10(-4) to (3 +/- 1) x 10(-5) M. yr(-1) and a more recent superwind lasting similar to 4000 yr when the mass-loss late increased to (2 +/- 1) x 10(-4) M. yr(-1)