Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509

TL;DR

This study investigates the magnetic field geometry and surface chemical abundance distribution of the He-strong star CPD -57 3509, revealing magnetic spots, chemical inhomogeneities, and possible pulsations through spectropolarimetric and photometric observations.

Contribution

First detailed magnetic and chemical surface mapping of CPD -57 3509, combining spectropolarimetry, Doppler imaging, and photometry to understand its magnetic topology and abundance distribution.

Findings

Magnetic field period of 6.36 days with a dipole strength of 3.9-4.5 kG.

He I spots concentrated at magnetic poles, Si III near the equator.

Detection of possible beta Cep-like pulsations affecting spectral lines.

Abstract

The magnetic field of CPD -57 3509 was recently detected in the framework of the BOB (B fields in OB stars) collaboration. We acquired low-resolution spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity, to study the magnetic field geometry, and to determine the surface distribution of silicon and helium. We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations. A period of 6.36d was detected in the measurements of the mean longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed the presence of He I spots located around the magnetic poles, with a strong concentration at the positive pole and a weaker one around the negative pole. In contrast, high concentration Si III spots are located close to the magnetic equator. Further, our analysis of the spectral variability of CPD -57 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of beta Cep-like pulsations. A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data.