Activity correlation and temporal variation of small-scale magnetic fields on young Sun-like stars

TL;DR

This study investigates how small-scale magnetic fields on young Sun-like stars vary over time and how their disc-averaged unsigned field $B_I$ relates to traditional activity indicators and large-scale fields from ZDI. Using time-series, high-resolution spectroscopy from ESPaDOnS and NARVAL, the authors infer $B_I$ via Zeeman broadening in magnetically sensitive Ti I and Fe I lines with a multi-component surface-field model fitted by MCMC; model complexity is chosen with the Bayesian information criterion. They detect clear rotational modulation of $B_I$ on HIP 76768 and tentative modulation for Mel 25-5, while other targets show no robust modulation within the observed windows; $B_I$ correlates positively with activity proxies, though the absolute field strength does not map linearly to these indicators, and cross-star comparisons reveal significant scatter. The results imply that small-scale magnetism can be traced in active stars and that rotational periods can be recovered from $B_I$ time series, but the precision of Zeeman-broadening measurements remains the central limitation for extending this approach to less active stars. Overall, the work highlights the connection between small-scale surface fields and chromospheric activity while clarifying the limits of using activity proxies to compare magnetic fields across stars.

Abstract

We aim to evaluate how well the variation of small-scale magnetic fields on the stellar surface can be monitored with time-series observations. Further, we aim to establish to what extent the measured total unsigned magnetic field traces other activity indicators. We measured the total unsigned magnetic field on four young, stars using Zeeman splitting of magnetically sensitive spectral lines from high-resolution spectra obtained with the spectropolarimeters ESPaDOnS at CFHT and NARVAL at TBL. We then characterised the magnetic field variations using both sinusoidal variation and Lomb-Scargle periodograms. We evaluated how the rotational variation of the total unsigned magnetic field strength correlates with the activity indicators S-index, H$α$-index, Ca IRT-index, and the large-scale magnetic field obtained from ZDI maps obtained in earlier studies. We find clear signals of rotational modulation of the total magnetic field on HIP 76768 and tentative detection on Mel 25-5. This is supported both by the sinusoidal fitting and the periodogram. For the other stars, we find no modulation signals of the total magnetic field. We find positive correlations between the total magnetic field and activity indices on all four stars, indicating that indirect magnetic activity indicators trace the underlying magnetic field variability. However, comparing the activity-magnetic field relationship between the stars in our sample shows a significant deviation between activity level and measured magnetic field strength. Small-scale magnetic field variability can be traced using the Zeeman effect on magnetically sensitive lines, provided that the star is sufficiently active. It is also possible to self-consistently recover rotational periods from such measurements. The primary limit for the detection of magnetic field variations is the precision of Zeeman broadening and intensification measurements.

Figures (11)

• Figure 1: Magnetic field periodicity of HIP 76768. Left: Total unsigned magnetic field variation of HIP 76768 phase-folded using the rotational period from messina:2010. The dashed red line represents the best-fit sinusoidal function to the data with the fixed rotational period from Table \ref{['tab:stellar-parameters']}. The dashed grey line represents the average of the individual measurements, while the dotted line represents the total unsigned magnetic field obtained from the time-averaged spectrum. Right: Lomb-Scargle periodogram obtained from the magnetic field measurements of HIP 76768. The dashed lines indicate false alarm probabilities. Also shown is the expected period from Table \ref{['tab:stellar-parameters']}. Similar plots for all stars can be seen in Fig. \ref{['fig:magnetic_variability']}.
• Figure 2: Total unsigned magnetic field measurements of HH Leo during 2017. The dashed red line marks the field strength obtained from the time-averaged spectra. The horizontal bar represents the rotational period.
• Figure 3: Posterior distribution of the total unsigned magnetic field strengths obtained from the time-averaged spectra of HH Leo from the two epochs. The distribution from the 2015 data is shown in red, and the shaded region represents the 2017 data. The black and blue lines represent median and 68% credence regions of the 2015 and 2017 datasets, respectively.
• Figure 4: Comparison between the variation of total magnetic fields (left y-axis) and other activity indicators (right y-axis). From top to bottom rows display the results for the four targets studied with increasing age. From left to right columns show the results for the S index, H$\alpha$ index, and Ca IRT index.
• Figure 5: Correlations between the total unsigned magnetic field and other activity indicators. Top-left: S index. Top-right: H$\alpha$ index. Bottom-left: Ca IRT index. Symbol colours identify the four targets analysed in this study (see legend).