A magnetic study of the δ Scuti variable HD 21190 and the close solar-type background star CPD -83°64B

Abstract

HD 21190 is a known δ Scuti star showing Ap star characteristics and a variability period of 3.6 h discovered by the Hipparcos mission. Using Gaia DR1 data for an astrometric analysis, it was recently suggested that HD 21190 forms a physical binary system with the companion CPD -83° 64B. An atmospheric chemical analysis based on High Accuracy Radial velocity Planet Searcher (HARPS) observations revealed the presence of overabundances of heavy and rare-earth elements, which are typically observed in chemically peculiar stars with large-scale organized magnetic fields. Previous observations of HD 21190 indicated a magnetic field strength of a few hundred Gauss. The presence of a magnetic field in CPD -83° 64B remained unexplored. In this work, we reanalyse this system using Gaia DR2 data and present our search for the magnetic field in both stars based on multi-epoch HARPSpol high-resolution and FORS 2 low-resolution spectropolarimetric observations. The Gaia DR2 results clearly indicate that the two stars are not physically associated. A magnetic field detection at a significance level of more than 6σ (<B_z>_all = 230 ± 38 G) was achieved for the δ Scuti variable HD 21190 in FORS 2 observations using the entire spectrum for the measurements. The magnetic field appears to be stronger in CPD -83° 64B. The highest value for the longitudinal magnetic field in CPD -83° 64B, <B_z>_all = 509 ± 104 G, is measured at a significance level of 4.9σ. Furthermore, the high-resolution HARPSpol observations of this component indicate the presence of pulsational variability on a time scale of tens of minutes.