Ap stars with resolved magnetically split lines: Magnetic field determinations from StokesIandVspectra

Abstract

Aims. We present the results of a systematic study of the magnetic fields and other properties of the Ap stars with resolved magnetically split lines. Methods. This study is based on new measurements of the mean magnetic field modulus, the mean longitudinal magnetic field, the crossover, the mean quadratic magnetic field, and the radial velocity of 43 stars, complemented by magnetic data from the literature for 41 additional stars. Results. Stars with resolved magnetically split lines represent a significant fraction, of the order of several percent, of the whole population of Ap stars. Most of them are genuine slow rotators, whose consideration provides new insight into the long-period tail of the distribution of the periods of the Ap stars. Emerging correlations between rotation periods and magnetic properties provide important clues for the understanding of the braking mechanisms that have been at play in the early stages of stellar evolution. The geometrical structures of the magnetic fields of Ap stars with magnetically resolved lines appear in general to depart slightly, but not extremely, from centred dipoles. However, there are a few remarkable exceptions, which deserve further consideration. We suggest that pulsational crossover can be observed in some stars; if confirmed, this would open the door to the study of non-radial pulsation modes of degree ℓ too high for photometric or spectroscopic observations. How the lack of short orbital periods among binaries containing an Ap component with magnetically resolved lines is related with their (extremely) slow rotation remains to be fully understood, but the very existence of a correlation between the two periods lends support to the merger scenario for the origin of Ap stars.

A search for spectral variability in the highly magnetized O9.7 V star HD 54879

“…The crossover effect was detected in the HARPSpol spectra at more than 3 σ significance at two observational epochs. As the star very likely rotates too slowly to produce Doppler shifts in the line profiles (our radial velocity measurements for three different observations yield almost identical radial velocity), it is possible that the presence of the crossover effect is due to the impact of some macroscopic velocity field variable across the stellar surface (see, e.g., Mathys ). Notably, since HD 54789 is already slightly evolved on the main sequence of the H–R diagram (Castro et al ), a plausible explanation for the detection of the crossover effect would be the presence of short‐term variability caused by some kind of pulsations.…”

HD 42659: the only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT

“…The spectroscopic binary fraction of Ap stars is of the order 45 per cent (Carrier et al 2002;Mathys 2017). However, the short period (P < 20 d) spectroscopic binary fraction amongst the Ap stars is very low, with only about 10 known (Landstreet et al 2017).…”

New and improved rotational periods of magnetic CP stars from ASAS-3, KELT, and MASCARA data

“…Renson & Manfroid 2009;Netopil et al 2017) and shows the typical peak distribution in the 0.0 < log(P[d]) < 0.5 bin. We note that because of the time span and characteristics of the here employed photometric time-series data, we will have missed very slowly rotating ACV variables with periods of several years or more (Mathys 2017).…”