New spectroscopic and polarimetric observations of the A0 supergiant HD92207

TL;DR

This study reports magnetic field measurements of the A0 supergiant HD92207, confirming a weak magnetic field and investigating short-term spectral variability through spectropolarimetric observations over multiple epochs.

Contribution

First detection of a weak magnetic field in HD92207 with detailed spectropolarimetric analysis across different epochs and time scales.

Findings

Magnetic field of approximately 104 G detected in 2014

No significant short-term spectral variability on 3-10 minute scales

Spectral line profiles show slight splitting in polarized light

Abstract

Our recent search for the presence of a magnetic field in the bright early A-type supergiant HD92207 using FORS2 in spectropolarimetric mode revealed the presence of a longitudinal magnetic field of the order of a few hundred Gauss. However, the definite confirmation of the magnetic nature of this object remained pending due to the detection of short-term spectral variability probably affecting the position of line profiles in left- and right-hand polarized spectra. We present new magnetic field measurements of HD92207 obtained on three different epochs in 2013 and 2014 using FORS2 in spectropolarimetric mode. A 3sigma detection of the mean longitudinal magnetic field using the entire spectrum, <B_z>_all=104+-34G, was achieved in observations obtained in 2014 January. At this epoch, the position of the spectral lines appeared stable. Our analysis of spectral line shapes recorded in opposite circularly polarized light, i.e. in light with opposite sense of rotation, reveals that line profiles in the light polarized in a certain direction appear slightly split. The mechanism causing such a behaviour in the circularly polarized light is currently unknown. Trying to settle the issue of short-term variability, we searched for changes in the spectral line profiles on a time scale of 8-10min using HARPS polarimetric spectra and on a time scale of 3-4min using time series obtained with the CORALIE spectrograph. No significant variability was detected on these time scales during the epochs studied.