The Origin of Weakened Magnetic Braking in Old Solar Analogs

Metcalfe, T. S.; Finley, Adam J.; Kochukhov, O.; See, Victor; Ayres, Thomas R.; Stassun, Keivan G.; Saders, Jennifer L. van; Clark, Catherine A.; Godoy-Rivera, D.; Ilyin, I.; Pinsonneault, Marc H.; Strassmeier, K. G.; Petit, P.

doi:10.3847/2041-8213/ac794d

Cited by 31 publications

(25 citation statements)

References 50 publications

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“…Stars akin to β Hydri are characterized by reduced outer atmospheric heating as well as a noticeably decreased level of chromospheric emission, with the latter confirmed by observations (e.g., Dravins et al 1993b;Buccino & Mauas 2008). Clearly, when stars evolve away from the main-sequence approaching the subgiant stage, the magnitude of magnetic activity is considerably reduced as a consequence of the restructuring of the stellar interior, associated with the redistribution of the angular momentum, and the loss of angular momentum due to the stellar wind (e.g., Schrijver & Pols 1993;Charbonneau et al 1997;Johnstone et al 2015); see also Metcalfe et al (2022) for updated analyses. Preliminary results for β Hyi based on theoretical twocomponent chromosphere models (acoustic and magnetic) have been given by Cuntz & Fawzy (2022), which also include some comparisons with observations.…”

mentioning

confidence: 82%

Evolution of solar-type activity: acoustic and magnetic energy generation and propagation in β Hydri (G2 IV)

Fawzy¹,

Cuntz²

2022

Preprint

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We examine the acoustic and magnetic energy generation and propagation in β Hydri (G2 IV). The underlying motivation for this work is based on the solar, stellar, and galactic relevance of β Hyi (a star in the Southern hemisphere), which is readily understood as a prime example and proxy of the future Sun --- thus allowing assessments and analyses of the secular decay of solar activity. Regarding the magnetic energy generation, we consider longitudinal flux tube waves. We also assess acoustic waves. For the acoustic wave energy flux, the difference between the results obtained for β Hyi and the Sun is significantly smaller than typically attained for main-sequence stars, which is largely due to the gravity-dependence of the acoustic energy generation. Furthermore, we study the height-dependent behavior of the magnetic energy flux for different magnetic filling factors corresponding to different flux tube spreadings. Finally, we comment on possible directions of future research.

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mentioning

confidence: 82%

Evolution of solar-type activity: acoustic and magnetic energy generation and propagation in β Hydri (G2 IV)

Fawzy¹,

Cuntz²

2022

Preprint

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“…Preliminary support for this interpretation was identified (Metcalfe et al 2016) in measurements of the large-scale magnetic field strength and morphology from Zeeman Doppler imaging (Petit et al 2008) and was reinforced by the associated changes in stellar activity cycles (Metcalfe & van Saders 2017). Targeted observations for an evolutionary sequence of two stars slightly hotter than the Sun (Metcalfe et al 2019(Metcalfe et al , 2021, followed by a sequence of several solar analogs (Metcalfe et al 2022), provided the first direct evidence of a magnetic morphology shift in older solar-type stars and placed new constraints on the relative importance of various contributions to the overall reduction in the wind braking torque. Such constraints may ultimately help to identify a corresponding transition in the stellar dynamo.…”

Section: Introductionmentioning

confidence: 89%

Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and τ Cet

Metcalfe¹,

Strassmeier

Ilyin

et al. 2023

ApJL

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During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking have previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf τ Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Lyα measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4–9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.

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“…This would challenge the current hypothesis of weakened magnetic braking (van Saders et al 2016), with angular momentum being more efficiently lost due to the favourable configuration of the stellar magnetic field and differential rotation pattern. It seems more likely that something prevents Sun-like stars from entering this configuration, or that a decreasing mass-loss rate could counteract this effect, producing the expected weakening of the wind-braking torque (discussed in Metcalfe et al 2022).…”

Section: Potential Impacts For Other Sun-like Starsmentioning

confidence: 99%

“…It has also been suggested that the wind-braking of Sunlike stars begins to decrease significantly around the age of the Sun (van Saders et al 2016;Metcalfe et al 2016;Booth et al 2017). Evidence for this has begun to grow thanks to new asteroseismic observations (Hall et al 2021), and models of the wind-braking torque for stars crossing the so called transition (Metcalfe & Egeland 2019;Metcalfe et al 2022).…”

Section: Introductionmentioning

confidence: 99%

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Finley¹,

Brun²

2023

A&A

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Context. Sun-like stars shed angular momentum due to the presence of magnetised stellar winds. Magnetohydrodynamic models have been successful in exploring the dependence of this "wind-braking torque" on various stellar properties, however the influence of surface differential rotation is largely unexplored. As the wind-braking torque depends on the rotation rate of the escaping wind, the inclusion of differential rotation should effectively modulate the angular momentum-loss rate based on the latitudinal variation of wind source regions.Aims. Here we aim to quantify the influence of surface differential rotation on the angular momentum-loss rate of the Sun, in comparison to the typical assumption of solid-body rotation. Methods. To do this, we exploit the dependence of the wind-braking torque on the effective rotation rate of the coronal magnetic field, which is known to be vitally important in magnetohydrodynamic models. This quantity is evaluated by tracing field lines through a Potential Field Source Surface (PFSS) model, driven by ADAPT-GONG magnetograms. The surface rotation rates of the open magnetic field lines are then used to construct an open-flux weighted rotation rate, from which the influence on the wind-braking torque can be estimated. Results. During solar minima, the rotation rate of the corona decreases with respect to the typical solid-body rate (the Carrington rotation period is 25.4 days), as the sources of the solar wind are confined towards the slowly-rotating poles. With increasing activity, more solar wind emerges from the Sun's active latitudes which enforces a Carrington-like rotation. Coronal rotation often displays a north-south asymmetry driven by differences in active region emergence rates (and consequently latitudinal connectivity) in each hemisphere.Conclusions. The effect of differential rotation on the Sun's current wind-braking torque is limited. The solar wind-braking torque is ∼ 10 − 15% lower during solar minimum, (compared with the typical solid body rate), and a few percent larger during solar maximum (as some field lines connect to more rapidly rotating equatorial latitudes). For more rapidly-rotating Sun-like stars, differential rotation may play a more significant role, depending on the configuration of the large-scale magnetic field.

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The Origin of Weakened Magnetic Braking in Old Solar Analogs

Cited by 31 publications

References 50 publications

Evolution of solar-type activity: acoustic and magnetic energy generation and propagation in β Hydri (G2 IV)

Evolution of solar-type activity: acoustic and magnetic energy generation and propagation in β Hydri (G2 IV)

Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and τ Cet

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

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