Segregation of Calcium Isotopes in the Atmospheres of CP Stars as a Consequence of Light-Induced Drift

Abstract

A mechanism for the segregation of calcium isotopes in the atmospheres of chemically peculiar (CP) stars due to light-induced drift (LID) of singly charged ⁴⁸Ca⁺ ions is discussed. One peculiarity of Ca⁺ is that an adequate description of the effect of LID requires taking into account several energy levels of Ca⁺, and thus several pairs of relative differences (ν _i − ν _k )/ν _i for the transport frequencies for collisions of levels i and k with neutral atoms (hydrogen, helium). The known real (calculated ab initio) interaction potentials are used to numerically calculate the factors (ν _i − ν _k )/ν _i for several states of Ca⁺ for collisions with H and He atoms. These computations show that, at the temperatures characteristic of the atmospheres of CP stars, T = 6600−12 000 K, fairly high values are obtained for Ca⁺ ions, (ν _i − ν _k )/ν _i ≈ 0.4−0.6. Simple, transparent computations demonstrate that the LID rates of Ca⁺ ions in the atmospheres of cool CP stars (T_eff = 6600 K) exceed the drift rate due to light pressure by two orders of magnitude. The LID is directed upward in the stellar atmosphere, and the heavy isotope ⁴⁸Ca is pushed into upper layers of the atmosphere. This can explain the observed predominance of the heavy isotope ⁴⁸Ca in the upper atmospheric layers of CP stars; according to the radiative-diffusion theory, the action of light pressure alone (in the absence of LID) would lead to sinking of the isotope ⁴⁸Ca deeper into stellar atmosphere, following the lighter main isotope ⁴⁰Ca. The ⁴⁸Ca⁺ LIDrate decreases and its drift rate due to light pressure increases with growth of the effective temperatures in the atmospheres of CP stars. The manifestations of LID and light pressure are roughly comparable in the atmospheres of CP stars with effective temperatures near T_eff = 9500 K.