Dust Shells Around Carbon Mira Variables |
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Authors: | M A T Groenewegen P A Whitelock C H Smith F Kerschbaum |
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Institution: | 1. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-straβe 1, D-85748, Garching, Germany 2. South African Astronomical Obsevatory, P.O. Box 9, 7935, Observatory, South Africa 3. Dept. of Physics, University College, ADFA, Canberra, ACT, 2600, Australia 4. Institut für Astronomie der Universit?t Wien, Türkenschanzstraβe 17, A-1180, Wien, Austria
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Abstract: | The spectral energy distributions and mid-infrared spectra of 44 carbon Mira variables are fitted using a dust radiative transfer
model. The periods cover the entire range observed for carbon Miras. The luminosities are calculated from a period-luminosity
relation. Parameters derived are the distance, the dust mass loss rate and the ratio of silicon carbide to amorphous carbon
dust. The total mass loss rate is derived from a modified relation between the photon momentum (L/c) and the momentum in the
wind (M υ∞). Mass loss rates between 1 × 10−8 and 4 × 10−5 M⊙ yr−1 are found. We find good correlations between mass loss rate and pulsation period, and mass loss rate and luminosity. The
dust-to-gas ratio appears to be almost constant up to periods of about 500 days, corresponding to about 7900 L⊙, and then to increase by a factor of 5 towards longer periods and higher luminosities. A comparison is made with radiation-hydrodynamical
calculations including dust formation. The mass loss rates predicted by these models are consistent with those derived in
this paper. The main discrepancy is in the predicted expansion velocities for models with luminosities below ∼5000 L⊙. The radiation-hydrodynamical calculations predict expansion velocities which are significantly too large. This is related
to the fact that these models need to be calculated with a large C/O ratio to get an outflow in the first place. This is contrary
to observations. It indicates that a principle physical ingredient in these radiation-hydrodynamical calculations is still
missing. Possibly the winds are ‘clumpy’ which may lead to dust formation on a local scale, or there is an additional outwards
directed force, possibly radiation pressure on molecules.
This revised version was published online in September 2006 with corrections to the Cover Date. |
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