The magnetic field of the stripped primary in the upsilon Sgr system, a member of the rare class of hydrogen-deficient binaries

TL;DR

This study reports the detection of a weak magnetic field in the primary star of the rare hydrogen-deficient binary upsilon Sgr, providing insights into magnetic influences on the evolution of massive binary stars.

Contribution

First high-resolution spectropolarimetric detection of a magnetic field in the primary of upsilon Sgr, a rare hydrogen-deficient binary system, highlighting its potential role in stellar evolution.

Findings

Primary star has a magnetic field of a few tens of Gauss.

The star is spectrum variable, indicating complex stellar activity.

Magnetic field detection supports theories of magnetic influence in massive binary evolution.

Abstract

We present the results of high-resolution spectropolarimetric observations of the optically dominant component in the rare hydrogen-deficient binary system upsilon Sgr. Only a small number of such systems in a very late phase of helium shell burning are currently known. The mass transfer from the donor star in binary systems usually leads to the stripping of its hydrogen envelope. Consequently, since the mass of the secondary increases, it appears rejuvenated. Using a few ESO FORS1 low-resolution spectropolarimetric observations of this system, Hubrig et al. announced in 2009 the presence of a magnetic field of the order of -70 - -80G. Here we report on more recent high-resolution ESO HARPS spectropolarimetric observations showing that the primary in upsilon Sgr is a spectrum variable star and possesses a weak magnetic field of the order of a few tens of Gauss. The detection of a magnetic field in this rare hydrogen-deficient binary is of particular interest, as such systems are frequently discussed as probable progenitors of core-collapse supernovae and gravitational-wave sources. Future magnetic studies of such systems will be worthwhile to gain deeper insights into the role of magnetic fields in the evolution of massive stars in binary systems.