HD 149277: a rare short-period SB2 system with a subsynchronously rotating magnetic He-rich primary

TL;DR

This paper reports on HD 149277, a rare binary system with a magnetic, helium-rich primary star exhibiting a unique subsynchronous rotation period, with detailed spectroscopic and magnetic field analysis revealing its magnetic variability and rotation characteristics.

Contribution

It presents the first detailed analysis of a magnetic SB2 system with a subsynchronously rotating primary, including magnetic field measurements and rotation period determination.

Findings

Primary star shows strong subsynchronous rotation with P_rot ≈ 25.4 days.

Resolved Zeeman split lines allow magnetic field variability analysis.

Magnetic and photometric periods are in good agreement.

Abstract

HD 149277 is a rare SB2 system with a slowly rotating magnetic He-rich primary with $P_{\rm rot}=25.4$ d. The CFHT/ESPaDOnS archive spectra revealed $P_{\rm orb}=11.5192 \pm 0.0005$ d indicating strong subsynchronous rotation of the primary component. Such a strong subsynchronous rotation was not detected in any other SB2 system with a magnetic chemically peculiar component. Our inspection of the spectra revealed the presence of resolved Zeeman split spectral lines allowing us to determine the variability of the mean magnetic field modulus over the rotation period. The maximum of the magnetic field modulus concides roughly with the positive extremum of the longitudinal field, whereas the minimum of the modulus with the negative extremum of the longitudinal field. No evidence for a longitudinal magnetic field was seen in the circularly polarized spectra of the secondary component. Using archival data from the ASAS3 survey, we find in the frequency spectrum only one significant peak, corresponding to the period $P_{\rm phot}=25.390 \pm 0.014$ d. This value is in good agreements with the previous determination of the rotation period, $P_{\rm rot}=25.380\pm0.007$ d, which was based on longitudinal magnetic field measurements.