Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998adil...mm...01m&link_type=abstract
NCSA Astronomy Digital Image Library
Astronomy and Astrophysics
Astronomy
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 12m, 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 zeroth-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 and IRAS 21282+5050 than for 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 which have resolved the envelope that 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 spherically symmetric expansion; however, a significant east-west bipolar outflow appears at the heart of the core with detected velocities upto 70 km/s. A blue-shifted component (-40 km/s) 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 HII 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 blue-shifted self-absorption of the gas. We develop a simple radiative transfer code, that assumes spherically symmetric expansion, to model the zeroth order core halo structures observed in these sources. We assume a temperature power law profile and fit a power law index of -0.7 for all three sources. The fitted density profiles reflect the observed differences in the core halo structures and suggest differences in the mass loss histories of the three sources. CIT 6 has had a constant mass loss rate of rate of 4 X 10^{-6} M_sol/yr. IRAS 21282+5050 has experienced two phases: an older asymptotic giant branch (AGB) wind with a mass loss rate of 2.3X 10^{-5} M_sol/yr lasting 5000 years, and a more recent superwind with a higher mass loss rate of 9.3 X 10^{-5} M_sol/yr lasting 5000 years. AFGL 618 also experienced two distinct phases, but the AGB wind mass loss rate decreased from 7.3 X 10^{-5} to 1.3 X 10^{-5} M_sol/yr over 8000 years followed by a sudden increase to 1.7 X 10^{-4} M_sol/yr during the superwind phase that lasted for 4000 years and caused its departure from the AGB. Our results may indicate that intermediate mass stars end their lives on the AGB with a superwind.
Campbell M. T.
Likkel Lauren
Meixner Matthew
Welch William J.
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