Bi-dimensional element stratifications computed for magnetic Ap star atmospheres

Abstract

Context. Theoretical modelling of abundance stratifications and surface distributions of chemical elements in Ap stars constitutes a major challenge. The atomic diffusion model provides the most appropriate framework in which to understand these abundance anomalies.Aims. We present theoretical 2D stratifications of 16 metals in upper main sequence chemically peculiar stars, with and without magnetic fields to provide a reference point for further theoretical and observational studies. Methods. We used our code CaratStrat to compute a large grid of stratifications (equilibrium solutions in LTE) for plane-parallel T eff = 8500, 10 000, 12 000, and 14 000 K stellar atmospheres. By interpolation, we constructed bi-dimensional cuts through these stellar atmospheres, which are permeated by a dipolar magnetic field of strength 20 kG at the magnetic pole. We also provide vertical (1D) stratifications of metals in non-magnetic stars (HgMn).Results. We present a large number of 2D and 1D stratifications, mostly as online material. We discuss in detail the case of Fe for the T eff = 8500 K model in the printed version, and compare it with stratifications derived from observed spectra.

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

“…Thus we may suppose that without these limits, the equilibrium calculations would imply equilibrium abundances of Ti and Cr of log (N el /N H ) similar to those of Fe, in the range of −3 to −4. This is confirmed by the calculations of Alecian & Stift (2010). Although we observe overabundances of these two elements of up to 2 dex when the stars are young, the Cr abundance seems to be generally less than the equilibrium value, and the Ti abundances almost certainly are below equilibrium even at young ages.…”

“…Silicon equilibrium atmospheric abundance has been studied by a number of authors, including Alecian & Vauclair (1981), Leblanc et al (2009), andStift (2010). While detailed results differ, the overall conclusion is that Si is expected to be of order 1 dex underabundant in the atmosphere at T eff ∼ 12 000 K. Instead, as has been found in the past, we observe an overabundance of about 1 dex at all ages.…”

“…This may be compared with the predicted Mg abundance profile for a star of T eff = 12 000 K of Leblanc et al (2009) and of Alecian & Stift (2010). The abundance predicted by the equilibrium calculation (g R = g throughout the atmosphere) is mildly non-uniform through the line-forming region, but is of about this same value.…”

“…Expected abundances for these elements based on equilibrium have recently been computed for stars in our mass range by Leblanc et al (2009), Alecian & Stift (2010), and (for Fe) by Stift & Alecian (2012). The results are most extensive for iron.…”

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

“…This model attributes the observed variability to a modulation of the aspect angle at which an inhomogeneous surface structure is seen by the observer. The magnetic fields present in these stars have significant influence on the energy and mass transport within the atmosphere and are also responsible for the creation of abundance patches and vertical chemical stratification (Khan & Shulyak 2006;Alecian & Stift 2010). Atmospheric processes, such as atomic diffusion (Michaud 1970), magnetically driven stellar winds (Babel & Montmerle 1997), and convection, should play important roles but are still poorly understood.…”