by Sudeshna Boro Saikia
The Sun which is our nearest stellar neighbour is a giant ball of gas and plasma. Observations of the Sun have revealed that it is covered by sunspots and plages and it releases streams of gas and plasma in the form of coronal mass ejections, solar winds and solar flares. Solar activity evolve on timescales of seconds to years, where it is observed to be cyclic with a period of 11 years. The cyclic activity is caused by the internal dynamo processes, where the magnetic field is amplified and regenerated. Detailed theoretical modelling of the solar dynamo has been carried out in the past few decades. It is now well established that the solar dynamo is the interplay between rotation and convection. However, precise details of how it works is still unknown. In order to provide better constraints on dynamo models we need to observe other solar-type stars in different parameter regimes. Observations of solar-type stars with a wide range of mass, convection zone depth and rotation, will hence provide valuable information about how the magnetic field is generated and amplified over long activity time-scales.
Solar-type stars are similar to the Sun as they have the same internal structure with a radiative core and a surrounding convective zone. As a result of which solar-type stars should also exhibit similar magnetic activity as in the Sun. However, one of the major challenges in observing magnetic fields in solar-type stars is the fact that other stars are too far away. Even with our most advanced telescopes we won’t be able to resolve them to Solar levels. Hence, astrophysicists use a tomographic technique similar to medical imaging techniques, known as Zeeman Doppler Imaging (ZDI) to be able to reconstruct the large-scale magnetic geometry of solar-type stars.
As part of CRC 963, we have reconstructed the surface magnetic field of two solar-type stars HN Peg (mass = 1.002 Msun, rotation period = 4.6 days) and 61 Cyg A (mass = 0.66 Msun, rotation period = 34.5 days). HN Peg is comparatively younger than the Sun at 200 Myr, where as 61 Cyg A has an age comparable to the Sun. Our observations of HN Peg spread over 7 years and 61 Cyg A was observed over 8 years. During our observational time span HN Peg exhibits a highly variable magnetic field geometry but no magnetic cycles were
detected as shown in Fig 1. On the other hand 61 Cyg A exhibits solar type magnetic field geometry with a polarity reversal during our observational time span. This indicates a probable magnetic cycle of 7 years for 61 Cyg A. Our recent results indicate that there is a wide variety of magnetic field geometry and variability in solar-type stars. 61 Cyg A is particularly interesting as it is the first star to exhibit magnetic polarity reversals and large-scale field variability similar to the Sun.