High-Resolution Images of CO J = 2-1 Emission from the Carbon Star V Cygni

This paper presents observations of the CO J = 2–1 emission from the circumstellar envelope of the mass-losing carbon star V Cyg. The observations were made with the Caltech Millimeter Array. A previously published single-dish map was used to construct short-spacing visibilities not sampled by the interferometer data, thereby recovering missing flux in extended low brightness emission. The images have an angular resolution of ∼1.″2 with a velocity resolution of 1 MHz (1.3 km s-1). The channel maps are consistent with an expanding envelope that is roughly spherical, but they also show evidence for asymmetric structure, as well as small-scale clumping. We compare these observations, as well as other published spectra, with statistical equilibrium models for CO in a circumstellar envelope. Models that fit the spherically averaged data must invoke a mass-loss rate, Ṁ, that has decreased with time by a factor of ∼2–3 over the past several hundred years. The model kinetic temperature structure in radius, TK(r), decreases as r-0.8 out to r ≈ 6 × 1015 cm and levels off to a constant value at TK = 23 K beyond. The secular change in Ṁ may be related to changes in the stellar luminosity or temperature, as predicted by recent numerical hydrodynamic models for mass loss. The inferred kinetic temperature structure suggests that heating by the photoelectric effect on dust grains is important in the outer envelope.