Non-LTE line formation for Pr II and Pr III in A and Ap stars

Non-LTE line formation for Pr II and Pr III is considered through a range of effective temperatures between 7250 K and 9500 K. A comprehensive model atom for Pr II/III is based on the measured and the predicted energy levels, in total, 6708 levels of Pr II and Pr III. We describe calculations of the Pr II energy levels and oscillator strengths for the transitions in Pr II and Pr III. The influence of departures from LTE on Pr abundance determinations is evaluated. At Teff >= 8000 K departures from LTE lead to overionization of Pr II and to systematically depleted total absorption in the line and positive abundance corrections. At the lower temperatures, different lines of Pr II may be either weakened or amplified depending on the line strength. The non-LTE effects strengthen the Pr III lines and lead to negative abundance corrections. Non-LTE corrections grow with effective temperature for the Pr II lines, and, in contrast, they decline for the Pr III lines. The Pr II/III model atom is applied to determine the Pr abundance in the atmosphere of the roAp star HD 24712 from the lines of two ionization stages. In the chemically uniform atmosphere with [Pr/H] = 3, the departures from LTE may explain only small part (0.3 dex) of the difference between the LTE abundances derived from the Pr II and Pr III lines (2 dex). We find that the lines of both ionization stages are described for the vertical distribution of the praseodymium where the Pr enriched layer with [Pr/H] > 4 exists in the outer atmosphere at log tau_5000 < -4. The departures from LTE for Pr II/III are strong in the stratified atmosphere and have the opposite sign for the Pr II and Pr III lines. Using the revised partition function of Pr II and experimental transition probabilities, we determine the solar non-LTE abundance of Pr as log (Pr/H) = -11.15\pm0.08.Comment: 17 pages, 4 tables, 11 figures, accepted for publication in A&

Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s) for REE lines and the Halpha core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.Comment: accepted by A&

gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT

Pulsations in pre-main sequence stars have been discovered several times within the last years. But nearly all of these pulsators are of delta Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by theory, but lack observational evidence. We present the investigation of variability caused by rotation and (gammaDoradus-type) pulsation in two pre-main sequence members of the young open cluster NGC2264 using high-precision time series photometry from the CoRoT satellite and dedicated high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC 2264VAS87 was obtained by the CoRoT satellite during the dedicated short run SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run SRa05 in December 2011 and January 2012. Frequency analysis was conducted using Period04 and SigSpec. The spectral analysis was performed using equivalent widths and spectral synthesis. The frequency analysis yielded 10 and 14 intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the range from 0 to 1.5c/d which are attributed to be caused by a combination of rotation and pulsation. The effective temperatures were derived to be 6380$\pm$150K for NGC2264VAS20 and 6220$\pm$150K for NGC2264VAS87. Membership of the two stars to the cluster is confirmed independently using X-ray fluxes, radial velocity measurements and proper motions available in the literature. The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and NGC2264VAS87, respectively, are in agreement with the Li abundance for other stars in NGC2264 of similar Teff reported in the literature. We conclude that the two objects are members of NGC2264 and therefore are in their pre-main sequence evolutionary stage. Assuming that part of their variability is caused by pulsation, these two stars might be the first pre-main sequence gamma Doradus candidates.Comment: 11 pages, 10 figures, A&A accepte

The Core-Wing Anomaly of Cool Ap Stars: Abnormal Balmer Profiles

Paper by Cowley et al. The Core-Wing Anomaly Etc. The profiles of H$\alpha$ in a number of cool Ap stars are anomalous. Broad wings, indicative of temperatures in the range 7000-8000K end abruptly in narrow cores. The widths of these cores are compatible with those of dwarfs with temperatures of 6000K or lower. This profile has been known for Przybylski's star, but it is seen in other cool Ap's. The H$\beta$ profile in several of these stars shows a similar core-wing anomaly (CWA). In Przybylski's star, the CWA is probably present at higher Balmer members. We are unable to account for these profiles within the context of LTE and normal dwarf atmospheres. We conclude that the atmospheres of these stars are not ``normal.'' This is contrary to a notion that has long been held.Comment: 4 Pages 5 Figures. Submitted to Astronomy and Astrophysics 4 Dec. 200