The attempted polarity reversal and evolving magnetic environment of AD Leo

Abstract

In the past two decades, the observed large-scale magnetic field of the active M dwarf star AD Leo has evolved from strongly to mildly negative, raising a suspicion that it might switch polarity. Although magnetic field reversals are observed every 11 years for the Sun, such reversals are poorly understood for M dwarfs. Further, no reversals have been observed for fast-rotating M dwarfs. We examine the properties of AD Leo's large-scale magnetic field and investigate how its evolution affects the space weather environment. We analysed spectropolarimetric data collected by ESPaDOnS and SPIRou in late-2022 and early-2023. With the optical and near-infrared data we computed the longitudinal magnetic field, and with the near-infrared data reconstructed the large-scale magnetic field using Zeeman-Doppler imaging. Using five magnetograms, from 2019 to 2023, we simulated three-dimensional Alfven wave-driven stellar winds using the space weather code SWMF. Although we see an evolution of the large-scale magnetic field of AD Leo, we find no polarity reversal. Rather, we see a restoration of the field to a simpler configuration with consistently negative values for the longitudinal magnetic field strength. Our new large-scale field reconstruction for AD Leo is characterised by a highly axisymmetric, poloidal-dipolar field with an increased mean large-scale field strength. SWMF simulations find the stellar mass loss rates to be, on average, an order of magnitude greater than that of the Sun. Additionally, we find that the habitable zone resides beyond the Alfven surface. Hypothetical magnetised habitable zone planets (with planetary field strengths greater than 0.34 G) would likely be shielded from the incident wind and atmospheric erosion would be negligible. Further, we find variable conditions across each epoch due to the evolving axisymmetry of the stellar large-scale magnetic field.

Figures (10)

• Figure 1: Normalised Stokes LSD profiles from a SPIRou observation of AD Leo on 13 January 2023. The top panel displays the Stokes $V$ (circularly polarised) profile; the middle panel displays the Null profile (a measurement of spurious polarisation signals); the bottom panel displays the Stokes $I$ (unpolarised) profile.
• Figure 2: Temporal evolution of the longitudinal magnetic field ($B_\ell$) from 2006 to 2023, using observations from ESPaDOnS, Narval, and SPIRou. $B_\ell$ and HJD measurements prior to 2023 were sourced from Morin2008, Lavail2018, and Bellotti2023b. The blue circles represent observations made in the optical domain, meanwhile the pink pluses represent near-infrared observations. The text below the data points roughly indicates the year of the observations.
• Figure 3: Full SPIRou 2022.9 time series of Stokes $V$ profiles for AD Leo, normalised by the unpolarised continuum intensity. The observed profiles are presented in black, meanwhile the Unno-Rachkovsky fitted models are shown in red. The profiles are sorted with respect to their rotation cycle (see Eq. \ref{['eq: ephemeris']}).
• Figure 4: ZDI reconstruction of the large-scale magnetic field of AD Leo from 2022.9 SPIRou observations, represented in a flattened polar view. We present the radial (left), azimuthal (middle), and meridional (right) components of the large-scale magnetic field vector. The radial ticks indicate the rotational phases at which observations were made, meanwhile the concentric circles indicate stellar latitudes: +60$^\circ$ (innermost), +30$^\circ$, equator, -30$^\circ$ (outermost). The range of the colour bar is determined by the maximum (in absolute value) strength of the large-scale magnetic field, with the negative and positive polarities represents by blue and red, respectively.
• Figure 5: Three-dimensional Alfvén surface for 2022.9 shown in purple, with the position and approximate size of AD Leo represented by the central red star.