A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations★

Lavail, Alexis; Kochukhov, O.; Wade, G. A.

doi:10.1093/mnras/sty1825

Cited by 32 publications

(39 citation statements)

References 28 publications

(48 reference statements)

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“…Carleo et al (2020) suggest that, instead of originating from dark starspots, the RV variations for AD Leo might be tied to inhibition of convection from the global magnetic field. In this hypothesis, the RV rotation signal will decay as the global field evolves, as observed by Lavail et al (2018), rather than with individual starspots. Further evidence that the RV rotation signal may not be entirely due to starspots is the 8 m s −1 NIR amplitude we observe with HPF.…”

Section: Discussionmentioning

confidence: 94%

“…At just under 5 parsecs from Earth, AD Leo is one of the closest and most well-studied M dwarfs. Its high levels of activity and evolving magnetic field have been well-studied, including its flares (e.g., Hawley et al 2003;van den Besselaar et al 2003), starspots (Hunt-Walker et al 2012), and global magnetic field (Morin et al 2008;Lavail et al 2018).…”

Section: Ad Leomentioning

confidence: 99%

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Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES

Robertson

Stefánsson

Mahadevan

et al. 2020

ApJ

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Young, rapidly rotating M dwarfs exhibit prominent starspots, which create quasiperiodic signals in their photometric and Doppler spectroscopic measurements. The periodic Doppler signals can mimic radial velocity (RV) changes expected from orbiting exoplanets. Exoplanets can be distinguished from activity-induced false positives by the chromaticity and long-term incoherence of starspot signals, but these qualities are poorly constrained for fully convective M stars. Coherent photometric starspot signals on M dwarfs may persist for hundreds of rotations, and the wavelength dependence of starspot RV signals may not be consistent between stars due to differences in their magnetic fields and active regions. We obtained precise multiwavelength RVs of four rapidly rotating M dwarfs (AD Leo, G227-22, GJ 1245B, GJ 3959) using the near-infrared (NIR) Habitable-zone Planet Finder and the optical Keck/HIRES spectrometer. Our RVs are complemented by photometry from Kepler, TESS, and the Las Cumbres Observatory network of telescopes. We found that all four stars exhibit large spotinduced Doppler signals at their rotation periods, and investigated the longevity and optical-to-NIR chromaticity for these signals. The phase curves remain coherent much longer than is typical for Sunlike stars. Their chromaticity varies, and one star (GJ 3959) exhibits optical and NIR RV modulation consistent in both phase and amplitude. In general, though, we find that the NIR amplitudes are lower than their optical counterparts. We conclude that starspot modulation for rapidly rotating M stars frequently remains coherent for hundreds of stellar rotations and gives rise to Doppler signals that, due to this coherence, may be mistaken for exoplanets.

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Section: Discussionmentioning

confidence: 94%

Section: Ad Leomentioning

confidence: 99%

Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES

Robertson

Stefánsson

Mahadevan

et al. 2020

ApJ

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“…Since both spot and flare events are connected to the magnetic field (Lavail et al 2018), we search for an empirical correlation between the measured RVs and the longitudinal magnetic field B , in an attempt to mitigate the effects of stellar activity on our RV data. The longitudinal magnetic field B is calculated for each AU Mic polarized spectrum using the following equation from Donati et al (1997):…”

Section: Magnetic Activitymentioning

confidence: 99%

Spin-orbit alignment and magnetic activity in the young planetary system AU Mic

Martioli

Hébrard

Moutou

et al. 2020

A&A

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We present high-resolution near-infrared spectropolarimetric observations using the SPIRou instrument at Canada-France-Hawaii Telescope (CFHT) during a transit of the recently detected young planet AU Mic b, with supporting spectroscopic data from iSHELL at NASA InfraRed Telescope Facility. We detect Zeeman signatures in the Stokes V profiles and measure a mean longitudinal magnetic field of ¯Bℓ = 46.3 ± 0.7 G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of dBℓ/dt = −108.7 ± 7.7 G d−1. We apply the cross-correlation technique to measure line profiles and obtain radial velocities through CCF template matching. We find an empirical linear relationship between radial velocity and Bℓ, which allows us to estimate the radial-velocity induced by stellar activity through rotational modulation of spots for the five hours of continuous monitoring of AU Mic with SPIRou. We model the corrected radial velocities for the classical Rossiter-McLaughlin effect, using MCMC to sample the posterior distribution of the model parameters. This analysis shows that the orbit of AU Mic b is prograde and aligned with the stellar rotation axis with a sky-projected spin-orbit obliquity of λ = 0°−15°+18°. The aligned orbit of AU Mic b indicates that it formed in the protoplanetary disk that evolved into the current debris disk around AU Mic.

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“…The magnetic fields of stars that have been mapped with ZDI over multiple epochs are also known to evolve over time (e.g. Jeffers et al 2014;Boro Saikia et al 2016;Lavail et al 2018;Boro Saikia et al 2018). Correspondingly, their estimated torques also change as a function of time.…”

Section: Stellar Variabilitymentioning

confidence: 99%

Do Non-dipolar Magnetic Fields Contribute to Spin-down Torques?

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Matt

Finley

et al. 2019

ApJ

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Main sequence low-mass stars are known to spin-down as a consequence of their magnetised stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum-loss rate and the magnetic field properties of low-mass stars are fundamentally linked making this a challenging task. Of particular interest is the stellar magnetic field geometry. In this work, we consider whether non-dipolar field modes contribute significantly to the spin-down of low-mass stars. We do this using a sample of stars that have all been previously mapped with Zeeman-Doppler imaging. For a given star, as long as its mass-loss rate is below some critical mass-loss rate, only the dipolar fields contribute to its spin-down torque. However, if it has a larger mass-loss rate, higher order modes need to be considered. For each star, we calculate this critical mass-loss rate, which is a simple function of the field geometry. Additionally, we use two methods of estimating mass-loss rates for our sample of stars. In the majority of cases, we find that the estimated mass-loss rates do not exceed the critical mass-loss rate and hence, the dipolar magnetic field alone is sufficient to determine the spin-down torque. However, we find some evidence that, at large Rossby numbers, non-dipolar modes may start to contribute.

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A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations★

Cited by 32 publications

References 28 publications

Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES

Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES

Spin-orbit alignment and magnetic activity in the young planetary system AU Mic

Do Non-dipolar Magnetic Fields Contribute to Spin-down Torques?

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