Hidden magnetic fields of young suns

Abstract

Global magnetic fields of active solar-like stars are nowadays routinely detected with spectropolarimetric measurements and are mapped with Zeeman-Doppler imaging (ZDI). However, due to the cancellation of opposite field polarities, polarimetry captures only a tiny fraction of the magnetic flux and cannot assess the overall stellar surface magnetic field if it is dominated by a smallscale component. Analysis of Zeeman broadening in high-resolution intensity spectra can reveal these hidden complex magnetic fields. Historically, there were very few attempts to obtain such measurements for G dwarf stars due to the difficulty of disentangling Zeeman effect from other broadening mechanisms affecting spectral lines. Here we developed a new magnetic field diagnostic method based on relative Zeeman intensification of optical atomic lines with different magnetic sensitivity. Using this technique we obtained 78 field strength measurements for 15 Sun-like stars, including some of the best-studied young solar twins. We find that the average magnetic field strength B f drops from 1.3-2.0 kG in stars younger than about 120 Myr to 0.2-0.8 kG in older stars. The mean field strength shows a clear correlation with the Rossby number and with the coronal and chromospheric emission indicators. Our results suggest that magnetic regions have roughly the same local field strength B ≈ 3.2 kG in all stars, with the filling factor f of these regions systematically increasing with stellar activity. Comparing our results with the spectropolarimetric analyses of global magnetic fields in the same stars, we find that ZDI recovers about 1% of the total magnetic field energy in the most active stars. This figure drops to just 0.01% for the least active targets.

Stellar coronal X-ray emission and surface magnetic flux

“…A similar conclusion can be drawn from a study of stars with different spectral types and activity levels (See et al 2019). In that study, the estimated filling factor f using the large-scale surface magnetic field and total surface magnetic flux follows a similar power-law relation as in the work of Kochukhov et al (2020) but with δ = 0.78. In conclusion, these results of stellar observations provide further support for using Eq.…”

“…( 7) is also roughly valid for other stars. In a study of solar-like stars, Kochukhov et al (2020) found a similar power-law relation based on the filling factor f and the averaged surface magnetic field B as f ∝ B δ with δ = 0.86 (see their Fig. 8).…”

“…Different studies found different power-law relations, depending on the structures and stars that were investigated. For example, studying the X-ray emission and the surface magnetic field of solar-like stars, Kochukhov et al (2020) found a relation of L X ∝ Φ 2.68 . Observations of different solar magnetic structures, such as active regions, bright points, or microflares, and of stars with various levels of activity, reveal power-law relations between X-ray emission and the magnetic field.…”

The impact of unresolved magnetic spots on high-precision radial velocity measurements

“…For low-mass stars, the averaged surface magnetic field measured from Stokes I is at least 10 times stronger than that measured from Stokes V (Wade et al 2000;Reiners 2012;Lehmann et al 2018;Kochukhov et al 2020), because it is not cancelled out like Stokes V. If there are magnetic spots (even of opposite polarity) on the surface, they add together to result in the Stokes I field. For an active star we can assume that the Stokes I field is entirely coming from star spots.…”

Solar and Stellar Superflares

“…Пока не ясно, как в таких условиях будет работать динамо и совместимо ли это с генерацией сильных полей. Однако не исключено, что недавние наблюдения магнитных полей на звёздах-близнецах молодого Солнца [11] можно понимать как указание на возможность именно такой работы звёздного динамо. По-видимому, такое динамо должно работать непосредственно на поверхности звезды, так что гипотетическое пятно, занимающее практически всю поверхность звезды, должно непрерывно регенерироваться механизмом динамо.…”