Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...509..392m&link_type=abstract
The Astrophysical Journal, Volume 509, Issue 1, pp. 392-414.
Astronomy and Astrophysics
Astronomy
38
Stars: Circumstellar Matter, Stars: Carbon, Stars: Individual: Alphanumeric: Cit 6, Stars: Individual: Alphanumeric: Gl 618, Stars: Individual: Alphanumeric: Iras 21282+5050, Stars: Mass Loss
Scientific paper
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" 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 ~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) x 10^-6 and (6 +/- 4) x 10^-5 M_&sun; 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) x 10^-4 to (3 +/- 1) x 10^-5 M_&sun; yr^-1 and a more recent superwind lasting ~4000 yr when the mass-loss rate increased to (2 +/- 1) x 10^-4 M_&sun; yr^-1.
Campbell M. T.
Likkel Lauren
Meixner Matthew
Welch William J.
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