Context :

Aims : We have carried out an accurate investigation of the Ca isotopic composition and stratification in the atmospheres of 23 magnetic chemically peculiar ( Ap ) stars of different temperature and magnetic field strength .

Methods : With the UVES spectrograph at the 8 m ESO VLT , we have obtained high-resolution spectra of Ap stars in the wavelength range 3000–10000 Å .
Using a detailed spectrum synthesis calculations , we have reproduced a variety of Ca lines in the optical and ultraviolet spectral regions , inferring the overall vertical distribution of Ca abundance , then we have deduced the relative isotopic composition and its dependence on height using the profile of the the IR-triplet Ca ii line at \lambda 8498 Å .

Results : In 22 out of 23 studied stars , we found that Ca is strongly stratified , being usually overabundant by 1.0–1.5 dex below \log \tau _ { 5000 } \approx - 1 , and strongly depleted above \log \tau _ { 5000 } = -1.5 .
The IR-triplet Ca ii line at \lambda 8498 Å reveals a significant contribution of the heavy isotopes ^ { 46 } Ca and ^ { 48 } Ca , which represent less than 1 % of the terrestrial Ca isotopic mixture .
We confirm our previous finding that the presence of heavy Ca isotopes is generally anticorrelated with the magnetic field strength .
Moreover , we discover that in Ap stars with relatively small surface magnetic fields ( \leq 4–5 kG ) , the light isotope ^ { 40 } Ca is concentrated close to the photosphere , while the heavy isotopes are dominant in the outer atmospheric layers .
This vertical isotopic separation , observed for the first time for any metal in a stellar atmosphere , disappears in stars with magnetic field strength above 6–7 kG .

Conclusions : We suggest that the overall Ca stratification and depth-dependent isotopic anomaly observed in Ap stars may be attributed to a combined action of the radiatively-driven diffusion and light-induced drift .