The importance of non-photon noise in stellar spectropolarimetry. The spurious detection of a non-existing magnetic field in the A0 supergiant HD 92207

Abstract

The low-resolution, Cassegrain mounted, FORS spectropolarimeter of the ESO Very Large Telescope is being extensively used for magnetic field surveys. Some of the new discoveries suggest that relatively strong magnetic fields may play an important role in numerous physical phenomena observed in the atmospheres as well as in the circumstellar environments of certain kinds of stars. We show in detail how small instabilities or data-reduction inaccuracies represent an alternative explanation for the origin of certain signals of circular polarisation published in recent years. With the help of analytical calculations we simulate the observation of a spectral line in spectropolarimetric mode, adding very small spurious wavelength shifts, which may mimic the effects of seeing variations, rapid variations of the stellar radial velocity, or instrument instabilities. As a case study, we then re-visit the FORS2 measurements that have been used to claim the discovery of a magnetic field in the A0 supergiant HD 92207. In addition, we present new observations of this star obtained with the HARPSpol instrument. Both calibration and science data show compelling evidence that photon-noise is not the only source of error in magnetic field measurements, especially in sharp spectral lines. Non-photon noise may be kept under control by accurate data reduction and quality controls. Our re-analysis of FORS2 observations of HD 92207 shows no evidence of a magnetic field, and we are able to reproduce the previous FORS detection only by degrading the quality of our wavelength calibration. Our HARPSpol spectropolarimetric measurements show no evidence of a magnetic field at the level of 10 G.

Accepted for publication by A&A. This new version replaces the original one to accommodate a request from Markus Schoeller, who wants his name removed from the acknowledgments. We have also implemented some minor changes to Figs. 4, 5, 6, 8

References in corpus (8)

• A high-resolution spectropolarimetric survey of Herbig Ae/Be stars - I. Observations and measurements
• Magnetic field measurements and their uncertainties: the FORS1 legacy
• Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
• A search for strong, ordered magnetic fields in Herbig Ae/Be stars
• The magnetic field of the pre-main sequence Herbig Ae star HD 190073
• The magnetic fields of hot subdwarf stars
• Magnetic fields in central stars of planetary nebulae?
• Spectropolarimetric Variability and Co-Rotating Structure in HD 92207

Cited by in corpus (23)

• B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars
• The FORS1 catalogue of stellar magnetic field measurements
• On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive
• Discovery of six new strongly magnetic white dwarfs in the 20 pc local population
• Large Interstellar Polarisation Survey. LIPS I: FORS2 spectropolarimetry in the Southern Hemisphere
• Measuring surface magnetic fields of red supergiant stars
• Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems
• Searching for the weakest detectable magnetic fields in white dwarfs. Highly-sensitive measurements from first VLT and WHT surveys
• Short time scale spectral variability in the A0 supergiant HD92207 and the importance of line profile variations for the interpretation of FORS2 spectropolarimetric observations
• The enigmatic He-sdB pulsator LS IV$-$14${}^{\circ }$116: new insights from the VLT
• Evidence for radio and X-ray auroral emissions from the magnetic B-type star rho Oph A
• Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
• B fields in OB stars (BOB): FORS2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD23478 and HD345439
• Discovery of weak magnetic fields in four DZ white dwarfs in the local 20pc volume. Implications for the frequency of magnetic fields with cooling age
• Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD2047+372
• Dark Dust and single-cloud sightlines in the ISM
• A search for strong magnetic fields in massive and very massive stars in the Magellanic Clouds
• First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds
• Cold gas in hot star clusters: the wind from the red supergiant W26 in Westerlund 1
• HD144941: The most extreme helium-strong star
• Detection of magnetic field in the B2 star $\rho$ Oph A with ESO FORS2
• New spectroscopic and polarimetric observations of the A0 supergiant HD92207
• A magnetic field detection in the massive O-type bright giant 63 Oph