The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

Abstract

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, (3836 +/- 30)d, from the analysis of 56 measurements of the mean magnetic field modulus <B> based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field <B_z> spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of <B> and of <B_z> are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accuractely determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

arXiv comment12 pages, 8 figures. Accepted for publication in A&A. In the journal, at the request of the editor, the abstract will appear in traditional form, as shown in this header. This arXiv preprint features a structured version of the abstract, so that the LaTeX source could be successfully processed by arXiv

References in corpus (12)

• Ap stars with resolved magnetically split lines: Magnetic field determinations from Stokes $I$ and $V$ spectra
• An Investigation of the Rotational Properties of Magnetic Chemically Peculiar Stars
• The FORS1 catalogue of stellar magnetic field measurements
• Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
• HARPS spectropolarimetry of Herbig Ae/Be stars
• On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive
• HD 18078: A very slowly rotating Ap star with an unusual magnetic field structure
• The variation of the magnetic field of the Ap star HD~50169 over its 29 year rotation period
• HD 965: An extremely peculiar A star with an extremely long rotation period
• Magnetic field topology and chemical spot distributions of the Ap star HD119419
• Two new chemically peculiar stars with resolved magnetically split lines
• Project VeSElkA: Abundance analysis of chemical species in HD41076 and HD148330

Cited by in corpus (7)

• Modeling of Magneto-Rotational Stellar Evolution I. Method and first applications
• TESS Cycle 1 observations of roAp stars with 2-min cadence data
• Long period Ap stars discovered with TESS data
• Photometric variability of the LAMOST sample of magnetic chemically peculiar stars as seen by TESS
• The anomalous atmospheric structure of the strongly magnetic Ap star HD166473
• The rapidly oscillating Ap star gamma Equ: linear polarization as an enhanced pulsation diagnostic?
• A new sample of super-slowly rotating Ap (ssrAp) stars from the Zwicky Transient Facility survey