Measuring stellar magnetic fields with the low resolution spectropolarimeter of the William Herschel Telescope

Abstract

Despite the influence of magnetic fields on the structure and evolution of stars is largely demonstrated from the theoretical point of view, their observational evidence in non-degenerated stars is still rather scanty and mainly circumscribed to bright objects (V<10). Stellar magnetic fields are commonly measured on the basis of circular spectropolarimetry at high/middle resolution across the profile of metal lines. The present sensitivity of telescopes and spectrographs makes this still an almost prohibitive method for faint stars. In principle, stellar magnetic fields can be also measured on the basis of low resolution spectropolarimetry, with very important results obtained at the 8 m ESO telescopes with FORS1. The trade off between S/N and spectral resolution in measuring stellar magnetic fields justify an attempt, here presented, to perform these measurements at the 4.5 m William Herschel Telescope. HD3360, one of the stars with the weakest known magnetic field, and the magnetic chemically peculiar stars HD10783, HD74521 and HD201601 have been observed with the spectropolarimeter ISIS in the 3785-4480 A range. Measured stellar magnetic fields, from Stokes I and V spectra with S/N > 600, show an internal error of <50 G selecting the whole interval and <200 G within a Balmer line. Ripples in the Stokes V spectra of HD3360 result in an instrumental positive magnetic field certainly not larger than 80 G.

7 pages, 4 figures, MNRAS in press

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• The multi-line slope method for the measure of the effective magnetic field of the cool stars: an application to the solar like cycle of ε Eri
• Calibrating and Stabilizing Spectropolarimeters with Charge Shuffling and Daytime Sky Measurements
• Analysis of eight magnetic chemically peculiar stars with rotational modulation