New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star-forming dwarf galaxies

• Published in: Monthly notices of the Royal Astronomical Society Vol. 445; no. 4; pp. 4287 - 4305
• Main Authors: Tang, Jing, Bressan, Alessandro, Rosenfield, Philip, Slemer, Alessandra, Marigo, Paola, Girardi, Léo, Bianchi, Luciana
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21.12.2014
• Subjects: Astronomical models; Astronomy; Astrophysics; Envelopes; Evolutionary; Galaxies; Hydrogen; Luminosity; Massive stars; Mathematical models; Metallicity; Star & galaxy formation; Star formation; Hertzsprung–Russell and colour-magnitude diagrams; stars: massive; stars: interiors; stars: evolution
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu2029

We extend the PARSEC library of stellar evolutionary tracks by computing new models of massive stars, from 14 to 350 M⊙. The input physics is the same used in the PARSEC V1.1 version, but for the mass-loss rate from considering the most recent updates in the literature. We focus on low metallicity, Z = 0.001 and Z = 0.004, for which the metal-poor dwarf irregular star-forming galaxies, Sextans A, the Wolf–Lundmark–Melotte galaxy and NGC 6822, provide simple but powerful workbenches. The models reproduce fairly well the observed colour-magnitude diagrams (CMDs) but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with a canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-up. The mixing scales required to reproduce the observed loops, EO = 2H P or EO = 4H P, are definitely larger than those derived from, e.g. the observed location of the red-giant-branch bump in low mass stars. This effect, if confirmed, would imply a strong dependence of the mixing scale below the formal Schwarzschild border, on the stellar mass or luminosity. Reproducing the features of the observed CMDs with standard values of envelope overshooting would require a metallicity significantly lower than the values measured in these galaxies. Other quantities, such as the star formation rate and the initial mass function, are only slightly sensitive to this effect. Future investigations will consider other metallicities and different mixing schemes.