Magnetic field topology, chemical spot distributions and photometric variability of the Ap star phi Draconis

TL;DR

This study provides a comprehensive analysis of phi Draconis, revealing its magnetic field topology, chemical spot distribution, and photometric variability, including the effects of binarity and detailed magnetic mapping.

Contribution

It offers the first detailed magnetic and chemical surface maps of phi Draconis, combining photometry, spectroscopy, and polarimetry to advance understanding of magnetic chemically peculiar stars.

Findings

Magnetic field is dominated by a distorted dipolar component with a 1.4 kG peak.

Chemical spots of Cr, Fe, and Si are located around magnetic and rotational equator intersections.

Photometric variability is modulated by the binary orbit and magnetic surface features.

Abstract

The primary component of the multiple star phi Dra is one of the brightest magnetic chemically peculiar stars in the northern sky. Here we report results of a comprehensive study of the rotational photometric variability, binarity, magnetic field geometry, and surface chemical spot structure for this star. We derived a precise photometric rotational period of 1.71650213(21) d based on one year of TESS nearly continuous space observations and discovered modulation of the stellar light curve with the phase of the 127.9-d binary orbit due to the light time travel effect. We revised parameters of the binary orbit and detected spectroscopic contribution of the secondary. A tomographic mapping technique was applied to the average intensity and circular polarisation profiles derived from Narval high-resolution spectropolarimetric observations. This analysis yielded a detailed map of the global magnetic field topology together with the surface distributions of Si, Cr, and Fe abundances. Magnetic mapping demonstrates that the surface field structure of phi Dra is dominated by a distorted dipolar component with a peak field strength of 1.4 kG and a large asymmetry between the poles. Chemical maps show an enhancement of Cr, Fe and, to a lesser extent, Si in a series of spots encircling intersections of the magnetic and rotational equators. These chemical spot geometries do not directly correlate with either the local field strength or the field inclination.