Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.275..983e&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 275, Issue 4, pp. 983-1002.
Astronomy and Astrophysics
Astronomy
21
Convection, Stars: Evolution, Hertzsprung-Russell (Hr) Diagram
Scientific paper
Evolutionary calculations are presented for stars in the mass range 1.5 to 20 M_ that have solar-like initial chemical composition. The stellar models were evolved beyond the core He-burning phase, with the primary aim being to investigate the influence of convective mixing on the properties of the red-blue loops in the Hertzsprung-Russell diagram. We performed various test computations based on the Schwarzschild criterion and the Ledoux criterion for convection. We also studied the effects of semi-convective mixing and core overshooting on the properties of the blue loops. We found that semiconvective mixing promotes the formation of the loops, but core overshooting strongly inhibits their extensions. We investigated the correlation between the extension of the loops and the efficiency of the hydrogen-shell burning, which crucially depends on the shape of the hydrogen profile created in a model star at the beginning of core helium-burning. In the mass range 1-13 M_, similar loops are obtained with the Schwarzschild criterion, or with the Ledoux criterion if combined with efficient semiconvective mixing. Above this mass, the loops disappear abruptly when the Schwarzschild criterion is used. However, they are recovered if semiconvective mixing is included with the Ledoux criterion. Our investigations show, in agreement with previous works quoted in the text (Section 4), that the properties of blue loops of stars of masses above 11 M_ are very sensitive to the treatment of mixing in the semiconvective layers, where hydrogen shell-burning occurs. We present a set of evolutionary sequences which exhibit extended blue loops up to a stellar mass of 19 M_.
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