Upper Main Sequence Stars of Pop I

Abstract

In main-sequence stars with an effective temperature above 10,000 K, surface convection zones are very thin or even vanish if the superficial helium abundance decreases due to gravitational settling. Atomic diffusion often occurs in the atmospheric layers of these stars; its effects, together with observational constrains, are examined in this chapter for the various groups of chemically peculiar stars. The time scales are determined to be of the order of years or even less in, and above, the line forming region of overabundant elements such as mercury, but much larger for other metals such as iron. It is possible to define a very simple equilibrium model for HgMn stars which reproduces the main trends of the observed abundance anomalies. However the detailed reproduction of individual stars is challenging, in particular because the abundance stratification build-up is a non-linear time dependent process. Mass loss plays an important role in limiting the size of anomalies: it determines an upper effective temperature limit of about 16,000 K for the HgMn phenomenon and of about 25,000 K for helium stars. Magnetic fields can stabilize the outer convection zones leading to Ap stars with effective temperatures as low as around 6,500 K. The role of magnetic fields in reducing the rotation rate, modifying the structure of the atmosphere and guiding atomic diffusion is emphasized. It is in particular described how it may lead to the formation of spots or rings of anomalies depending on the field’s geometry. Abundance structures are actually observed in these atmospheres but the detailed modeling of individual stars remains very challenging.