A magnetic study of the $\delta$ Scuti variable HD 21190 and the close solar-type background star CPD -83$^{\circ}$ 64B

TL;DR

This study investigates the magnetic properties and pulsational behavior of the $ ext{δ}$ Scuti star HD 21190 and its background star CPD -83° 64B, revealing magnetic fields and disproving their physical association using Gaia data.

Contribution

It provides the first magnetic field measurements for CPD -83° 64B and clarifies the non-physical association between the two stars through Gaia DR2 data analysis.

Findings

HD 21190 has a magnetic field of 230±38 G.

CPD -83° 64B has a stronger magnetic field of 509±104 G.

HD 21190 and CPD -83° 64B are not physically associated.

Abstract

HD 21190 is a known $\delta$ 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$^{\circ}$ 64B. An atmospheric chemical analysis based on 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$^{\circ}$ 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 star are not physically associated. A magnetic field detection at a significance level of more than 6$\sigma$ ($\left< B_{\rm z}\right>_{\rm all}=230\pm38$ G) was achieved for the $\delta$ 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$^{\circ}$ 64B. The highest value for the longitudinal magnetic field in CPD -83$^{\circ}$ 64B, $\left< B_{\rm z}\right>_{\rm all}=509\pm104$ G, is measured at a significance level of 4.9$\sigma$. Furthermore, the high-resolution HARPS\-pol observations of this component indicate the presence of pulsational variability on a time scale of tens of minutes.