Circumstellar dust shells around long-period variables: V. A consistent time-dependent model for the extreme carbon star AFGL 3068

J.M. Winters, A.J. Fleischer, T. Le Bertre and E. Sedlmayr

Astronomy & Astrophysics, 326, 305-317 (1997)

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Abstract:

We present a consistent time-dependent model for the extreme carbon star AFGL 3068. The model includes a consistent treatment of the interactions among hydrodynamics, thermodynamics, dust formation, and chemistry. It is determined by the stellar parameters of the initial hydrostatic model: stellar mass M*=1Msun, stellar luminosity L*=1.3*104Lsun, stellar temperature T*=2200K, and carbon-to-oxygen abundance ratio εC/εO=1.38. The pulsation of the star is simulated by a piston approximation at the inner boundary where the velocity varies sinusoidally with a period of P=696d and an amplitude of Δu=8km s-1. This model yields a time averaged outflow velocity of 14.7km s-1 in good agreement with the rather well determined observed
value and an average mass loss rate of 1.2*10-4Msun yr-1 which is about a factor of 4 larger than the value usually derived from the fitting of CO rotational lines profiles. Based on the atmospheric structure resulting from this hydrodynamic calculation, we have performed angle- and frequency-dependent continuum radiative transfer calculations, which yield the spectral energy distributions at different phases of the pulsation cycle, synthetic light curves at different wavelengths and synthetic brightness profiles of the dust shell model. The computed energy distributions and the synthetic light curves are in good agreement with the observations of AFGL 3068. We derive a distance to AFGL 3068 of 1200 parsec. Furthermore, properties of the resulting grain size distribution function are discussed. The dominant size of the grains formed in the circumstellar shell is found to be <a>sim 0.1µm but grains
with radii up to 0.7µm are also formed in such a quantity that they may contribute significantly to light scattering in the near-infrared.

We propose to perform high-resolution spatial observations of the circumstellar dust shell around AFGL 3068 which could be compared with the predictions of the model calculation.

Keywords:

hydrodynamics - radiative transfer - stars: carbon - circumstellar matter - stars: late-type - stars: individual: AFGL 3068