Mass-loss histories of three carbon-rich evolved stars as revealed by¹²CO 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 ¹²CO 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 ¹²CO 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 flat-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″ × 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 ∼70 km s^-1. A blueshifted component (∼-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 II region. A bright ring of ¹²CO emission surrounds this central hole, and a ∼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) times; 10^-6 and (6 ± 4) times; 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 ∼8000 yr when the mass-loss rate decreased from (2 ± 1) times; 10^-4 to (3 ± 1) times; 10^-5 M_⊙ yr^-1 and a more recent superwind lasting ∼4000 yr when the mass-loss rate increased to (2 ± 1) times; 10^-4 M_⊙ yr^-1.