The importance of non-photon noise in stellar spectropolarimetry

Bagnulo, S.; Fossati, L.; Kochukhov, O.; Landstreet, J. D.

doi:10.1051/0004-6361/201322319

Cited by 38 publications

(81 citation statements)

References 28 publications

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“…seeing variations during very short exposures (Bagnulo et al , 2013. In general we found that our error estimates were higher than those published in previous works.…”

Section: Error Estimationcontrasting

confidence: 59%

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

Landstreet

Bagnulo

Fossati³

2014

A&A

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Context. The ESO archive of FORS1 spectropolarimetric observations may be used to create a homogeneous database of magnetic field measurements. However, no systematic comparison of FORS field measurements to those obtained with other instruments has been undertaken so far. Aims. We exploit the FORS archive of circular spectropolarimetric data to examine in a general way how reliable and accurate field detections obtained with FORS are. Methods. We examine the observations of Ap and Bp stars, on the grounds that almost all of the unambiguous detections of magnetic fields in the FORS1 archive are in these kinds of stars. We assess the overall quality of the FORS1 magnetic data by examining the consistency of field detections with what is known from previous measurements obtained with other instruments, and we look at patterns of internal consistency. Results. FORS1 magnetic measurements are fully consistent with those made with other instruments, and the internal consistency of the data is excellent. However, it is important to recognise that each choice of grism and wavelength window constitutes a distinct instrumental measuring system, and that simultaneous field measurements in different instrumental systems may produce field strength values that differ up to 20%, or more. Furthermore, we found that field measurements using hydrogen lines only yield results that meaningfully reflect the field strength as sampled specifically by lines of hydrogen for stars with effective temperatures above about 9000 K. Conclusions. In general the magnetic field measurements of Ap and Bp stars obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1 provides an extremely useful example that offers valuable lessons for field measurements with other low-resolution Cassegrain spectropolarimeters.

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“…seeing variations during very short exposures (Bagnulo et al , 2013. In general we found that our error estimates were higher than those published in previous works.…”

Section: Error Estimationcontrasting

confidence: 59%

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

Landstreet

Bagnulo

Fossati³

2014

A&A

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“…These problems have been thoroughly discussed by Bagnulo et al (2012) and Bagnulo et al (2013), and a discussion on the quality of the non-controversial FORS1 measurements of magnetic Ap stars was presented by Landstreet et al (2014).…”

Section: Introductionmentioning

confidence: 99%

The FORS1 catalogue of stellar magnetic field measurements

Bagnulo

Fossati

Landstreet

et al. 2015

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Context. The FORS1 instrument on the ESO Very Large Telescope was used to obtain low-resolution circular polarised spectra of nearly a thousand different stars, with the aim of measuring their mean longitudinal magnetic fields. Magnetic fields were measured by different authors, and using different methods and software tools. Aims. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Our understanding of the instrument and our data reduction techniques have both improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements. Methods. Based on the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive. Results. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for ∼1400 observations of ∼850 different objects. The spectral type of each object has been approximately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation. Conclusions. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.

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“…For the first reduction, we used a suite of IRAF (Tody 1993) 1 and IDL routines that follow the technique, recipes, and recommendations by Bagnulo et al (2002Bagnulo et al ( , 2012Bagnulo et al ( , 2013 2 . The determination of the mean longitudinal magnetic field using low-resolution FORS spectropolarimetry with the second software package developed in Potsdam is described by Hubrig et al (2014Hubrig et al ( , 2015 and by Schöller et al (in prep.).…”

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confidence: 99%

B fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439

Hubrig

Schöller

Fossati

et al. 2015

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Aims. Massive B-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. Only two such stars, called σ Ori E analogues, were known until recently. A team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD 23478 and HD 345439. The magnetic fields in these newly discovered σ Ori E analogues have not been investigated so far. Methods. In the framework of our ESO Large Programme and one normal ESO programme, we carried out low-resolution FORS 2 spectropolarimetric observations of HD 23478 and HD 345439. Results. In the measurements of hydrogen lines, we discover a rather strong longitudinal magnetic field of up to 1.5 kG in HD 23478 and up to 1.3 kG using the entire spectrum. The analysis of HD 345439 using four subsequent spectropolarimetric subexposures does not reveal a magnetic field at a significance level of 3σ. On the other hand, individual subexposures indicate that HD 345439 may host a strong magnetic field that rapidly varies over 88 min. The fast rotation of HD 345439 is also indicated by the behaviour of several metallic and He i lines in the low-resolution FORS 2 spectra that show profile variations already on this short time-scale.

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The importance of non-photon noise in stellar spectropolarimetry

Cited by 38 publications

References 28 publications

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

The FORS1 catalogue of stellar magnetic field measurements

B fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439

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