V. G. Elkin

Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines ⋆

Chemically peculiar A stars (Ap) are extreme examples of the interaction of atomic element diffusion processes with magnetic fields in stellar atmospheres. The rapidly oscillating Ap stars provide a means for studying these processes in three dimensions and are at the same time important for studying the pulsation excitation mechanism in A stars. As part of the first comprehensive, uniform, high-resolution spectroscopic survey of Ap stars, which we are conducting in the Southern hemisphere with the Michigan Spectral Catalogues as the basis of target selection, we report here the discovery of 17 new magnetic Ap stars having spectroscopically resolved Zeeman components from which we derive magnetic field moduli in the range 3–30 kG. Among these are (1) the current second strongest known magnetic A star, (2) a double-lined Ap binary with a magnetic component and (3) an A star with particularly peculiar and variable abundances. Polarimetry of these stars is needed to constrain their field geometries and to determine their rotation periods. We have also obtained an additional measurement of the magnetic field of the Ap star HD 92499.

The fundamental parameters of the roAp star α Circini

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙ . We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K . This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of α Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of α Cir.

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite the importance of magnetic fields to a full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied until now over the rotation cycle. One reason for the paucity of these observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD 101412 with a strong surface magnetic field has become in the past few years one of the most well-studied targets among the Herbig Ae/Be stars. We present our multi-epoch polarimetric spectra of this star acquired with FORS 2 to search for a rotation period and constrain the geometry of the magnetic field. Methods: We measured longitudinal magnetic fields for 13 different epochs distributed over 62 days. These new measurements and our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. Results: We find the rotation period to be P = 42.076+/-0.017 d.

Discovery of pulsational line profile variations in the δ Scuti star HD 21190 and in the Ap Sr star HD 218994

Asteroseismology has the potential to provide new insights into the physics of stellar interiors. We have obtained UVES high time resolution observations of theScuti star HD21190 and of the Ap Sr star HD218994 to search for pulsational line profile variations. We report the discovery of a new roAp star, HD218994, with a pulsation period of 14.2min. This is one of the most evolved roAp stars. No rapid pulsations have been found in the spectra of the cool Ap star - � Scuti star HD21190. However, we detect with unprecedented clarity for aSct star moving peaks in the cores of spectral lines that indicate the presence of high degree non-radial pulsations in this star.

Line identification in high-resolution, near-infrared CRIRES spectra of chemically peculiar and Herbig Ae stars

Context. Contrary to the late-type stars, our knowledge of atomic transitions in intermediate-mass stars is still very poor. The recent availability of ESO’s high-resolution spectrograph CRIRES offers now the opportunity to study numerous spectral features in the near-IR in intermediate-mass main-sequence and pre-main-sequence stars. Aims. The aim of the study is to explore the diagnostic potential of near-IR spectral regions. We carry out the first line identification in a few spectral regions for the two strongly magnetic Ap stars γ Equ and HD 154708, and their potential precursors two pre-main sequence Herbig Ae/Be stars HD 101412 and 51 Oph. Methods. High-resolution CRIRES spectra were obtained in three spectral regions, two regions around 1 μm and one region around 1.57 μm containing magnetically sensitive Fe i lines. To study the spectral line variability in the Herbig Ae star HD 101412, the observations were collected on six different rotation phases. All currently available atomic line lists were involved to properly identify the detected spectral features. Results. The largest number of near-IR spectral features was detected and identified in the well-studied magnetic Ap star γ Equ. Nearly 30% of the spectral lines in the Ap star HD 154708 with one of the strongest magnetic fields known among the Ap stars of the order of 25 kG, remain unidentified due to a lack of atomic data. Only very few lines belonging to the rare earth element group have been identified in both Ap stars. A number of spectral lines including the Ce iii and Dy ii lines appear magnetically split due to the presence of a strong magnetic field in their atmospheres. The content of the spectra of the Herbig Ae/Be stars HD 101412 and 51 Oph is rather unexciting. Variable behaviour of lines of the elements He, N, Mg, Si, and Fe over the rotation period in the spectra of HD 101412 confirm our previous finding of variability in the optical region. Due to the very fast rotation of 51 Oph, only a few spectral lines have been identified with certainty.

A search for rapid pulsations in the magnetic cool chemically peculiar star HD 3980

The Ap star HD 3980 appears to be a promising roAp candidate based on its fundamental parameters, leading us to search for rapid pulsations with the VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcos parallax and estimated temperature of 8100 K place HD 3980 in the middle of the theoretical instability strip for rapidly oscillating Ap stars, about halfway through its main-sequence evolution stage. The star has a strong, variable magnetic field, as is typical of the cool magnetic Ap stars. Dipole model parameters were determined from VLT observations using Focal Reducer and low Dispersion Spectrograph (FORS)1. From Doppler shift measurements for individual spectral lines of rare-earth elements and the Hα line core, we find no pulsations above 20–30 m s −1 . This result is corroborated by the inspection of lines of several other chemical elements, as well as with cross-correlation for long spectral regions with the average spectrum as a template. Abundances of chemical elements were determined and show larger than solar abundances of rare-earth elements. Further, ionization disequilibria for the first two ionized states of Nd and Pr are detected. We also find that the star has a strong overabundance of manganese, which is typical for much hotter HgMn and other Bp stars. Line profile variability with the rotation period was detected for the majority of chemical species.

On the spectroscopic nature of the cool evolved Am star HD 151878

Recently, Tiwari, Chaubey & Pandey detected the bright component of the visual binary HD 151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude rapidly oscillating Ap (roAp) star known. As a new roAp star, HD 151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star – the other being HD 116114. We used the FIbre-fed Echelle Spectrograph at the Nordic Optical Telescope to obtain high time-resolution spectra at high dispersion to attempt to verify the rapid oscillations. We show here that the star at this epoch is spectroscopically stable to rapid oscillations of no more than a few tens of m s −1 . The high-resolution spectra furthermore show the star to be of type Am rather than Ap and we show the star lacks most of the known characteristics for roAp stars. We conclude that this is an Am star that does not pulsate with a 6-min period. The original discovery of pulsation is likely to be an instrumental artefact.

Some recent discoveries in roAp stars

Research in roAp stars is being vigorously pursued, both theoretically and observationally by many groups. We report the discovery of a 21-min period, luminous roAp star, HD116114. Longer periods for more luminous stars have been predicted theoretically and this is the first discovery of such a star. We discuss a model for the blue-to-red line profile variability observed in some roAp stars involving a shock wave high in the atmosphere of roAp stars, yet show that the Hα line in 33 Lib has the blue-to-red-to-blue line profile variability expected for subsonic dipolar pulsation concentrated towards the pulsation pole. Further we report for 33 Lib unprecedented observations of the amplitudes and phases of its principal mode at 2.015mHz and its first harmonic of that at 4.030mHz.

Discoveries in the Atmospheres of roAp Stars

We have obtained a large amount of data on over 40 roAp stars and potential roAp stars with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT with time resolution typically around 1 min and radial velocity precision as high as 1 m s−1. Abundance stratification caused by atomic diffusion in the presence of strong global magnetic fields gives promise of three‐dimensional maps of the pulsation amplitude and phase, and of the abundance distributions of many ions that may provide the strongest observational tests of atomic diffusion theory. Studies of individual spectral lines and of line profile variability sample the observable atmospheres of roAp stars from continuum optical depth τ5000∼1 to as high as τ5000∼10−5, revealing fascinating new pulsational behaviour not observed in other types of pulsating stars, including, inter alia, line profile variability in rare earth elements lines interpreted by as evidence for shock waves in the high atmosphere of these stars, an intriguing range of line...

Rapid radial velocity variations in the Ap star HD 965

We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine exact radial velocities using spectra obtained with the UVES spectrograph on the ESO VLT. Special attention is given to the spectral lines of the Rare Earth Elements which in roAp stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of