[Paper Review] High-resolution spectropolarimetric observations of hot subdwarfs

P. Petit, V. Van Grootel+4 more|arXiv (Cornell University)|Oct 24, 2011

Stellar, planetary, and galactic studies3 references3 citations

TL;DR

This study reanalyzes high-resolution spectropolarimetric data of hot subdwarfs HD 76431 and Feige 66 using ESPaDOnS at CFHT, applying least-squares deconvolution (LSD) to enhance signal-to-noise. Despite prior claims of kilo-gauss magnetic fields, no Zeeman signatures are detected, yielding upper limits of 60 G and 200 G for HD 76431 and Feige 66, respectively, suggesting earlier detections were likely artifacts of data reduction pipelines.

ABSTRACT

We report on high-resolution spectropolarimetric observations of the hot subdwarf stars HD 76431 and Feige 66, using the ESPaDOnS echelle spectropolarimeter at CFHT. We compute cross-correlation Stokes I and V line profiles to enhance the signal-to-noise ratio. We then average all available cross-correlation profiles of each star to further decrease the noise level. Although both targets were previously reported to host kilo-gauss magnetic fields, we do not derive any evidence of large-scale photospheric fields from our sets of observations, in spite of tight error bars on the longitudinal field of the order of 60 gauss for HD 76431 and 200 gauss for Feige 66. A new analysis of FORS1 observations of HD 76431, which provided the basis for the original claim of field detection, confirms the absence of any detectable Zeeman signature, with an error bar of about 100 gauss on the longitudinal magnetic field.

Motivation & Objective

• To reassess the presence of strong magnetic fields in hot subdwarf stars HD 76431 and Feige 66, which were previously reported to host kilo-gauss surface fields.
• To investigate whether the previously reported magnetic field detections were spurious by re-analyzing archival and new spectropolarimetric data with improved techniques.
• To determine the true longitudinal magnetic field strengths in these stars using high-sensitivity cross-correlation methods and temporal averaging to reduce noise.
• To evaluate the reliability of earlier magnetic field detections, particularly those based on FORS1 data, by reprocessing the original data with standardized reduction protocols.
• To assess the broader implications for the prevalence of strong magnetism in hot subdwarf stars and to call for systematic follow-up observations of a larger sample.

Proposed method

• Acquired high-resolution spectropolarimetric data using ESPaDOnS at the Canada-France-Hawaii Telescope, covering 370–1000 nm at R ≈ 65,000 with simultaneous Stokes I and V measurements.
• Applied the Least-Squares Deconvolution (LSD) technique to multiple spectral lines (91 for HD 76431, 39 for Feige 66) to co-add line profiles and enhance signal-to-noise ratio by factors of 2.5–6.
• Computed phase-averaged LSD profiles by stacking all available observations (17 for HD 76431, 13 for Feige 66), achieving signal-to-noise ratios of 7,000 and 1,400, respectively.
• Used the standard Zeeman effect relation: $ V/I = -4.67 \times 10^{-13} \lambda^2 (1/I) (dI/d\lambda) B_l $, to derive the longitudinal magnetic field $ B_l $ from the Stokes V profile.
• Performed a null-horizon test by computing a control (null) profile from the same data to verify that no spurious signals arise from instrumental or reduction artifacts.
• Reprocessed archival FORS1 data from 2004 using identical reduction standards to assess the validity of the original kG field claim for HD 76431.

Experimental results

Research questions

• RQ1 Do high-resolution spectropolarimetric observations of HD 76431 and Feige 66 provide evidence for kilo-gauss magnetic fields at their surfaces?
• RQ2 Could the previously reported magnetic field detections in these stars be artifacts of data reduction pipelines rather than real physical signals?
• RQ3 What are the true upper limits on the longitudinal magnetic field components of HD 76431 and Feige 66 after applying advanced noise-reduction techniques?
• RQ4 How consistent are the longitudinal field measurements derived from different spectral regions (e.g., Balmer lines, He lines, full spectrum) in the same star?
• RQ5 What is the reliability of earlier magnetic field detections in hot subdwarfs when re-evaluated with modern, standardized reduction methods?

Key findings

• No significant Zeeman signatures were detected in the averaged LSD profiles of HD 76431 and Feige 66, with false-alarm probabilities of 0.2 and 0.3, respectively.
• The phase-averaged longitudinal magnetic field for HD 76431 is $ 12 \pm 55 $ G, placing a tight upper limit of 60 G at 3σ confidence.
• For Feige 66, the longitudinal field is $ 240 \pm 200 $ G, yielding an upper limit of 200 G at 3σ confidence.
• Re-analysis of the original FORS1 data for HD 76431 yields a longitudinal field of $ -54 \pm 110 $ G from Balmer lines, $ 252 \pm 117 $ G from He and metal lines, and $ 44 \pm 88 $ G from the full spectrum, all consistent with zero magnetic field.
• Null profile analysis confirms that no spurious signal is introduced by the reduction pipeline, supporting the absence of real magnetic fields.
• The study challenges the validity of prior kG field detections in hot subdwarfs, suggesting they may stem from systematic errors in data reduction rather than intrinsic stellar magnetism.