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

Abstract

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.

Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049

“…Another B-type star, HD 137509, exhibits Zeeman splitting consistent with the field modulus of 29 kG (Kochukhov 2006). Finally, a much cooler Ap star HD 75049 with a field modulus variation of between 24.5 and 29.5 kG over its ≈4 d rotation period was discovered by Freyhammer et al (2008) and subsequently analysed by Elkin et al (2010). Apart from the modelling of Zeeman-resolved Stokes I metal line profiles in HD 215441 by Landstreet et al (1989), none of these extremely magnetic objects were studied with high-resolution time-series spectroscopy or polarimetry.…”

“…The broad-band photometric variation of HD 75049 has an amplitude of only 10 mmag in Johnson V (Freyhammer et al 2008). As we show below, the surface gradients of chemical elements (Si, Cr, Fe) that are most important for atmospheric structure are relatively mild.…”

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

“…A magnetic field with B ∼ 30 kG was discovered in the star HD 75049 by Freyhammer et al (2008). The star has been studied in detail by Elkin et al (2010), who obtained 13 B z magnetic field measurements using FORS1 with the 600 B grism during a period of about 3 months.…”

Measuring cosmic magnetic fields with very large telescopes

“…But as the Zeeman splitting increases and eventually exceeds the other line broadening mechanisms (thermal and rotational Doppler broadening), one can securely measure the mean field strength in a simple and direct way. The optical spectra of slowly rotating strongly magnetic Ap stars provide such a possibility (Mathys, Hubrig, Landstreet, et al 1997;Freyhammer, Elkin, Kurtz, et al 2008). For low-mass stars, especially the mid-and late-M active dwarfs, the optical and near-infrared spectra yield evidence of kG-strength fields (Johns-Krull & Valenti 1996; Reiners & Basri 2007; Reiners, this conference).…”