A study of the effect of rotational mixing on massive stars evolution: surface abundances of Galactic O7-8 giant stars

MARTINS, F.; SIMÓN-DÍAZ, S.; BARBÁ, R.; GAMEN, R. C.; ELKSTROM, S.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2017 vol. 599 p. 30 - 44

0004-6361

Context. Massive star evolution remains only partly constrained. Inparticular, the exact role of rotation has been questioned by puzzlingproperties of OB stars in the Magellanic Clouds. Aims: Our goalis to study the relation between surface chemical composition androtational velocity, and to test predictions of evolutionary modelsincluding rotation. Methods: We have performed a spectroscopicanalysis of a sample of fifteen Galactic O7-8 giant stars. This sampleis homogeneous in terms of mass, metallicity and evolutionary state. Itis made of stars with a wide range of projected rotational velocities. Results: We show that the sample stars are located on the secondhalf of the main sequence, in a relatively narrow mass range (25-40M⊙). Almost all stars with projected rotationalvelocities above 100 km s-1 have N/C ratios about ten timesthe initial value. Below 100 km s-1 a wide range of N/Cvalues is observed. The relation between N/C and surface gravity is wellreproduced by various sets of models. Some evolutionary models includingrotation are also able to consistently explain slowly rotating, highlyenriched stars. This is due to differential rotation which efficientlytransports nucleosynthesis products and allows the surface to rotateslower than the core. In addition, angular momentum removal by windsamplifies surface braking on the main sequence. Comparison of thesurface composition of O7-8 giant stars with a sample of B stars withinitial masses about four times smaller reveal that chemical enrichmentscales with initial mass, as expected from theory. Conclusions:Although evolutionary models that include rotation face difficulties inexplaining the chemical properties of O- and B-type stars at lowmetallicity, some of them can consistently account for the properties ofmain-sequence Galactic O stars in the mass range 25-40M⊙.