Mass‐Loss Histories of Three Carbon‐rich Evolved Stars as Revealed by12CO Emission

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Abstract
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) × 10-6 and (6 ± 4) × 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) × 10-4 to (3 ± 1) × 10-5 M☉ yr-1 and a more recent superwind lasting ~4000 yr when the mass-loss rate increased to (2 ± 1) × 10-4 M☉ yr-1.