The effects of surface fossil magnetic fields on massive star evolution: I. Magnetic field evolution, mass-loss quenching, and magnetic braking

Monthly Notices of the Royal Astronomical Society

et al. 2020

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The time evolution of angular momentum and surface rotation of massive stars is strongly influenced by fossil magnetic fields via magnetic braking. We present a new module containing a simple, comprehensive implementation of such a field at the surface of a massive star within the Modules for Experiments in Stellar Astrophysics (mesa) software instrument. We test two limiting scenarios for magnetic braking: distributing the angular momentum loss throughout the star in the first case, and restricting the angular momentum loss to a surface reservoir in the second case. We perform a systematic investigation of the rotational evolution using a grid of OB star models with surface magnetic fields (M = 5 − 60 M , Ω/Ω crit = 0.2 − 1.0, B p = 1 − 20 kG). We then employ a representative grid of B-type star models (M = 5, 10, 15 M , Ω/Ω crit = 0.2, 0.5, 0.8, B p = 1, 3, 10, 30 kG) to compare to the results of a recent selfconsistent analysis of the sample of known magnetic B-type stars. We infer that magnetic massive stars arrive at the zero age main sequence with a range of rotation rates, rather than with one common value. In particular, some stars are required to have close-to-critical rotation at the ZAMS. However, magnetic braking yields surface rotation rates converging to a common low value, making it difficult to infer the initial rotation rates of evolved, slowly-rotating stars.

Section: Discussionmentioning

confidence: 99%

The effects of surface fossil magnetic fields on massive star evolution – II. Implementation of magnetic braking in mesa and implications for the evolution of surface rotation in OB stars

Keszthelyi

Meynet

Monthly Notices of the Royal Astronomical Society

et al. 2020

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“…The possibiity that its core might still be contracting should be explored, once grids of evolutionary models for OB stars with surface fossil magnetic fields become available (e.g. Keszthelyi et al 2019). Mikulášek et al (2018) suggested that vertically stratified differential rotation, due to episodic magnetic coupling and decoupling of the upper and lower layers of the photosphere, may explain the phenomenon for CU Vir and HD 37776.…”

Section: Discussionmentioning

confidence: 99%

The accelerating rotation of the magnetic He-weak star HD 142990

Monthly Notices of the Royal Astronomical Society

Rivinius

Das

et al. 2019

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HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period (P rot ∼ 1 d). While it is clearly a rapid rotator, recent determinations of P rot are in formal disagreement. In this paper we collect magnetic and photometric data with a combined 40-year baseline in order to re-evaluate P rot and examine its stability. Both period analysis of individual datasets and O − C analysis of the photometric data demonstrate that P rot has decreased over the past 30 years, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rateṖ of approximately −0.6 s/yr, although there is some indication thatṖ may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.

“…Magnetic models incorporating magnetic braking and tidal effects on stellar evolution were developed by Song et al (2018), but they concern very close magnetic binaries that are, again, more massive than any stars in the present sample (the majority of which are furthermore single). An appropriate grid of models utilizing fossil magnetic fields and self-consistently accounting for all effects is still in development (Keszthelyi et al 2017a(Keszthelyi et al , 2018(Keszthelyi et al , 2019.…”

Section: Evolutionary Modelsmentioning

confidence: 99%

The magnetic early B-type stars – III. A main-sequence magnetic, rotational, and magnetospheric biography

Monthly Notices of the Royal Astronomical Society

Wade

Rivinius

et al. 2019

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138

Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into Centrifugal Magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in Hα -the principal diagnostic of these structures -are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A-and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with Hα emission originating in a CM are 1) rapid rotators, 2) strongly magnetic, and 3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.