Magnetic Field of the Eclipsing M-dwarf Binary YY Gem

Abstract

YY Gem is a short-period eclipsing binary system containing two nearly identical, rapidly rotating, very active early-M dwarfs. This binary represents an important benchmark system for calibrating empirical relations between fundamental properties of low-mass stars and for testing theories of interior structure and evolution of these objects. Both components of YY Gem exhibit inflated radii, which has been attributed to poorly understood magnetic activity effects. Despite a long history of magnetic activity studies of this system no direct magnetic field measurements have been made for it. Here we present a comprehensive characterisation of the surface magnetic field in both components of YY Gem. We reconstructed the global field topologies with the help of a tomographic inversion technique applied to high-resolution spectropolarimetric data. This analysis revealed moderately complex global fields with a typical strength of 200-300 G and anti-aligned dipolar components. A complementary Zeeman intensification analysis of the disentangled intensity spectra showed that the total mean field strength reaches 3.2-3.4 kG in both components of YY Gem. We used these results together with other recent magnetic field measurements of M dwarfs to investigate the relation between the global and small-scale fields in these stars. We also assessed predictions of competing magnetoconvection interior structure models developed for YY Gem, finding that only one of them anticipated the surface field strength compatible with our observations. Results of our star spot mapping of YY Gem do not support the alternative family of theoretical stellar models which attempts to explain the radii inflation by postulating a large spot filling factor.

Hidden magnetic fields of young suns

“…Cranmer & Saar 2011). The presence of such fields is now definitively established for active M dwarfs (Shulyak et al 2017;Kochukhov & Shulyak 2019) and T Tauri stars (Sokal et al 2020). This suggests that local field strength is not limited by the confinement of magnetic flux tubes by the photospheric gas pressure as was repeatedly assumed in the past (Saar & Linsky 1986b;Cranmer 2017;See et al 2019).…”

Magnetic fields in M dwarfs from the CARMENES survey

“…The first evidence of such variability was reported in Kochukhov & Lavail (2017) for the B component of the binary system Gl 65. At the same time, no line profile variability was recently detected by Kochukhov & Shulyak (2019) in another fast rotating binary YY Gem, possibly implying that the variability is very weak due to a different distribution of small scale magnetic fields over the surface of these stars compared to the case of Gl 65 B. Unfortunately, the SNR of individual spectra for our stars is much lower than those used in both mentioned above studies and we could not detect any significant variability above the photon noise limit.…”

“…We additionally plot measurements from Sh2017 and other literature sources for stars that are not in our sample. We also include recent measurements of the magnetic field in the eclipsing binary system YY Gem (Kochukhov & Shulyak 2019). The rotation periods are from Diez Alonso et al (2018); Morin et al (2010, and references therein).…”

Measuring stellar magnetic helicity density

“…One of the limitations of the ZDI technique is that it is insensitive to small-scale magnetic flux elements whose polarities cancel out (Reiners & Basri 2009;Morin et al 2010;Kochukhov & Shulyak 2019;See et al 2019). We are therefore unable to explore the evolution of helicity across a large range of length scales in the way that is possible for the Sun.…”