The very slow rotation of the magnetic O9.7V star HD54879 (1910.13209v1)

Abstract: The first FORS2 spectropolarimetric observation of the longitudinal magnetic field of HD54879 of the order of -600G with a lower limit of the dipole strength of ~2kG dates back to 2014. Since then observations showed a gradual decrease of the absolute value of the mean longitudinal magnetic field. In the course of the most recent monitoring of HD54879 using FORS2 spectropolarimetric observations from 2017 October to 2018 February, a longitudinal magnetic field strength change from about -300G down to about -90G was reported. A sudden increase of the absolute value of the mean longitudinal magnetic field and an accompanying spectral variability was detected on 2018 February 17. New FORS2 spectropolarimetric data obtained from 2018 December to 2019 February confirm the very slow magnetic field variability, with the field decreasing from about 150G to -100G over two months. Such a slow magnetic field variability, related to the extremely slow rotation of HD54879, is also confirmed using high-resolution HARPSpol and ESPaDOnS spectropolarimetry. The re-analysis of the FORS2 polarimetric spectra from 2018 February indicates that the previously reported field increase and the change of the spectral appearance was caused by improper spectra extraction and wavelength calibration using observations obtained at an insufficient signal-to-noise ratio. The presented properties of HD54879 are discussed in the context of the Of?p spectral classification.

• The paper presents a detailed spectropolarimetric analysis of the magnetic O9.7V star HD 54879, investigating its exceptionally slow rotation and magnetic field variability.
• Observational data revealed long-term magnetic field changes consistent with a rotation period likely spanning several years, correcting for initial anomalous readings attributed to data extraction errors.
• The study's findings suggest that HD 54879, despite not fitting classical criteria, shares characteristics with long-period magnetic stars like Of?p stars, prompting reconsideration of O-type magnetic star classification.

Analysis of the Very Slow Rotation of the Magnetic O9.7 V Star HD 54879

Introduction

The research conducted by Hubrig and colleagues delves deeply into the characteristics of the magnetic O9.7 V star HD 54879, with a primary focus on its exceptionally slow rotation and the implications therein. The analysis spans multiple observational periods, scrutinizing both the star's magnetic field variability and spectral properties. Magnetic O-type stars, particularly of such late subclassifications as HD 54879, are rare and enigmatic, characterized by their strong, organized magnetic fields. This study addresses important facets of magnetic and rotational behavior and offers comparisons with known Of?p stars.

Observations and Results

A pivotal aspect of the study is the extensive use of spectropolarimetry to assess the mean longitudinal magnetic field ($B_{\rm z}$). Initial data recorded in 2014 reported field strengths of around $-600$ G, with more recent observations showing a marked decline to values between $-300$ G and $-90$ G by early 2018. An anomalous increment to $-833$ G was observed on February 17, 2018, initially suggesting rapid magnetic variability.

Subsequent data acquisition from December 2018 to February 2019 revealed further gradual changes, with field measurements ranging from 150 G to $-100$ G. Remarkably, these variations, as confirmed by both low and high-resolution spectropolarimetric data, indicate a very slow rotation period that is likely on the order of several years.

A critical discovery is that the aberrant magnetic field spike initially reported was a result of improper spectra extraction and calibration at deficient signal-to-noise ratios (S/N), a factor rectified in later observations.

Implications and Theoretical Insights

The documented slow rotation and the long-term variability of the magnetic field place HD 54879 into discussions surrounding anomalous massive magnetic stars like the Of?p stars. The lack of variable emission at the C III spectral region, due to the star’s cooler temperature compared to classical Of?p stars, precludes a definitive Of?p classification under traditional criteria. However, the variability patterns in the H-alpha line, reminiscent of circumstellar interaction dynamics typical of Of?p stars, underscore potential structural similarities.

Interestingly, HD 54879 demonstrates behavior consistent with long-period magnetic stars, such as HD 108, raising the possibility of a broadening classification criterion for O-type magnetic stars that accounts for temperature and magnetic morphology beyond the Of?p framework.

Future Directions

This research outlines several key areas for continued exploration. Extended high-resolution spectropolarimetric and photometric monitoring will be essential to accurately determine HD 54879's rotational period, magnetic topology, and potential binarity. The observed radial velocity trends, while subtle, suggest binary interactions demanding further investigation.

Theoretical models accounting for magnetic field origins must consider such stars’ evolutionary paths, including binary-mass interactions and potential mergers. Given the scarcity of late O-type magnetic stars, HD 54879 serves as a critical case to study the interface between magnetic field generation mechanisms and stellar evolution in massive stars.

In conclusion, Hubrig et al.'s comprehensive assessment of HD 54879's magnetic and rotational properties offers a substantial contribution to the understanding of magnetic massive stars, setting the stage for broader theoretical models and observational campaigns. The paper underlines the necessity for precision in data acquisition and analysis to unravel the complexities of stellar magnetism.