HARPS spectropolarimetry of Herbig Ae/Be stars

TL;DR

This study uses HARPS spectropolarimetry to detect and analyze magnetic fields in six Herbig Ae/Be stars, revealing weak magnetic fields and magnetic phenomena, with some results challenging previous measurements.

Contribution

First detections of weak magnetic fields in two Herbig Ae/Be stars and confirmation of previous detections, using HARPS spectropolarimetry and the moment technique.

Findings

Detected weak magnetic fields in HD58647 and HD98922.

Confirmed magnetic field in HD104237 and HD190073.

No crossover effect detected in the sample.

Abstract

Our knowledge of the presence and the strength of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. We present new magnetic field measurements in six Herbig Ae/Be stars observed with HARPS in spectropolarimetric mode. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for six Herbig Ae/Be stars. Wavelength shifts between right- and left-hand side circularly polarised spectra were interpreted in terms of a longitudinal magnetic field <B_z>, using the moment technique introduced by Mathys. The application of the moment technique to the HARPS spectra allowed us in addition to study the presence of the crossover effect and quadratic magnetic fields. Our search for longitudinal magnetic fields resulted in first detections of weak magnetic fields in the Herbig Ae/Be stars HD58647 and HD98922. Further, we confirm the previous tentative detection of a weak magnetic field in HD104237 by Donati et al. and confirm the previous detection of a magnetic field in the Herbig Ae star HD190073. Surprisingly, the measured longitudinal magnetic field of HD190073, <B_z>=91+-18G at a significance level of 5sigma is not in agreement with the measurement results of Alecian et al. (2013), <B_z>=-10+-20G, who applied the LSD method to exactly the same data. No crossover effect was detected for any star in the sample. Only for HD98922 the crossover effect was found to be close to 3sigma with a measured value of -4228+-1443 km/s G. A quadratic magnetic field of the order of 10kG was detected in HD98922, and of ~3.5kG in HD104237.