Stellar magnetism: empirical trends with age and rotation (1404.2733v1)

Abstract: We investigate how the observed large-scale surface magnetic fields of low-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging (ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence objects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field <|Bv|> is related to age as $t^{-0.655 \pm 0.045}$. This relation has a similar dependence to that identified by Skumanich (1972), used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method ("magnetochronology"). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star-disc interaction, rather than being driven by the dynamo.

• The paper establishes that non-accreting stars' average large-scale magnetic field strength decays with age as t^(–0.655 ± 0.045), supporting magnetochronology.
• The paper demonstrates that magnetic field strength scales with rotation period (approximately P_rot^(–1.32 ± 0.14)), indicating a linear dynamo mechanism.
• The paper shows a strong correlation between X-ray activity and magnetic flux, emphasizing the role of stellar magnetism in driving stellar activity.

Stellar Magnetism: Empirical Trends with Age and Rotation

The paper by Vidotto et al. explores the empirical trends in the large-scale surface magnetic fields of low-mass stars, with a focus on how these fields vary with stellar age, rotation, and X-ray emission. Utilizing Zeeman-Doppler imaging (ZDI), the paper investigates a diverse sample of 104 magnetic maps from 73 stars. These stars range from accreting pre-main sequence to main-sequence in age, spanning approximately from 1 Myr to 10 Gyr.

Key Findings

1. Magnetism-Age Relationship: The paper finds that for non-accreting dwarfs, the unsigned average large-scale surface magnetic field strength scales with age as $\langle |B_V| \rangle \propto t^{-0.655 \pm 0.045}$. This relationship mirrors the age-rotation dependence identified by Skumanich (1972), supporting its use in gyrochronology and potentially serving as a basis for "magnetochronology" for age-dating stars.
2. Rotation and Magnetism: The relation between the large-scale fields and rotation is consistent with a linear dynamo mechanism, showing that $$\langle |B_V| \rangle \propto P_{\rm rot}^{-1.32 \pm 0.14}$$. This suggests a coupling of small- and large-scale fields in stars through similar dynamo processes. The results indicate that the large-scale magnetic topology plays a significant role in the stellar wind mechanism and angular momentum loss over the main-sequence lifetime.
3. Accreting Stars: For accreting stars, fewer significant trends were observed. However, a notable correlation was found between magnetic flux and rotation period, which could reflect influences from star-disc interactions rather than the dynamo process.
4. X-ray Correlations: In non-accreting stars, X-ray activity shows a strong correlation with magnetic flux ($\Phi_V$), with an empirical relation of $L_X \propto \Phi_V^{1.80 \pm 0.20}$. This suggests that stellar magnetic fields are a key determinant of X-ray emission levels, with implications for understanding stellar magnetic activity in younger, more active stars.

Implications

The comprehensive analysis provided by Vidotto et al. offers pivotal insights into the evolution and fundamental processes governing stellar magnetism. The magnetism-age relationship enhances our understanding of stellar magnetic evolution and may serve as a valuable tool for estimating stellar ages. Moreover, these findings reinforce the view that both small- and large-scale magnetic field components are driven by similar dynamo mechanisms in stars, particularly in those outside the saturated regimes.

Future Directions

A key area for continued exploration is the detailed comparison between large-scale and small-scale magnetic fields using both Zeeman-Doppler imaging and Zeeman broadening techniques. This may improve our understanding of the magnetic saturation phenomenon observed in rapidly rotating stars. Moreover, extending the dataset to include more stars, particularly those with extreme rotation rates or magnetic field strengths, could further clarify the role of magnetic fields in stellar evolution and angular momentum regulation.

Overall, the research offers a significant contribution to stellar astrophysics, enhancing both theoretical and practical approaches to studying stellar magnetism and its evolutionary implications.