Mixing length and opacity effects - Deep convective envelopes on the asymptotic giant branch

Details Mixing length and opacity effects - Deep convective envelopes on the asymptotic giant branch Mixing length and opacity effects - Deep convective envelopes on the asymptotic giant branch

Jan 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991apj...366..529s&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 366, Jan. 10, 1991, p. 529-534. Research supported by the University of Tor

Astronomy and Astrophysics

Astrophysics

29

Asymptotic Giant Branch Stars, Late Stars, Opacity, Stellar Envelopes, Stellar Interiors, Chemical Composition, Convection, Mixing Length Flow Theory, Stellar Luminosity, Stellar Structure, Stellar Temperature, Opacities, Stars: Interiors, Stars: Late-Type

Scientific paper

We constructed detailed envelopes appropriate to stars on the asymptotic giant branch (AGB) for a wide range of mixing length parameter values alpha, considering different opacities, stellar masses, luminosities, and chemical compositions. Due to the molecular opacities present in the new LAOL opacity tables, a given AGB effective temperature Te requires a mixing length parameter α more than twice as large as that necessary with the older Cox and Stewart opacities. For any set of molecular opacity tables, authors should normalize their value for α by matching observations of the Sun (radius and luminosity at the solar age) or by matching the observed positions of the lower main sequence and red giant branch in the H-R diagram. (For our LAOL opacities, our normalized value turned out to be α ≍ 2.1.) Although this normalization of the mixing length results in a solar convective envelope essentially independent of molecular opacities, this is not true on the AGB: different opacities result in different conditions at the base of the convective envelope on the AGB, even after this normalization. Reliable molecular opacities are imperative for stellar models on the AGB.
The temperature TCE at the base of the convective envelope is strongly dependent on the envelope mass and on the luminosity, ranging between 105 K and 107 K for our normalized value of α ≍ 2.1: hot-bottom envelope burning seems not to be totally out of reach. The effect of the chemical composition on TCE is surprisingly small: changes in helium abundance Y have a completely negligible effect, and even a large change in metallicity from Z = 0.02 to 0.001 increases TCE by only a factor of 2.
On the AGB, we find that Te ∝ α1/2and R ∝ 1/α.

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