uGMRT probes stellar magnetospheres through study of stars with rare emission

Approximately 10% of massive stars are known to host large scale dipolar surface magnetic fields, the simplest form of magnetic field, which emanate from two opposite poles, as in the north and south poles of a magnet. Their origin is unclear; it could be fossil fields (interstellar magnetic fields that get locked into stars at the time of their formation) or magnetic fields generated in stellar mergers. New observations with uGMRT, an SKA pathfinder facility, probe these stellar magnetospheres.

Understanding their origin has deep implications in stellar evolution theory. These magnetic massive stars are probably the best laboratory to study plasma-magnetic field interactions, which lead to the formation of a magnetosphere around the star that traps wind plasma up to a certain distance away from the star. The magnetosphere is the host of a variety of phenomena that produce radiation over a wide range of the electromagnetic spectrum.

A magnetosphere of a massive star and the trapped plasma around a magnetic massive star

Credit: R. Townsend.

Using the upgraded Giant Metrewave Radio Telescope (uGMRT), a team of researchers are studying a unique subset within this 10% of massive stars, called main-sequence radio pulse-emitters (MRPs), which produce very bright emission for a very short duration of time, roughly twice during the full rotation of the star. Such emission is called Electron Cyclotron Maser Emission (ECME). By the very nature of ECME pulses, the higher frequencies originate closer to the star and the lower frequencies are produced farther away from the star. Thus observation over a wide frequency range allows one to study the magnetised plasma at different heights from the stellar surface and reconstruct a 3D picture of the star’s magnetosphere. This uGMRT study has led to numerous academic papers in recent years, with two more due to be submitted in the coming weeks.

There was only one known MRP (called CU Vir) for nearly 15 years, until the team discovered a second candidate with the uGMRT, given the name HD 133880. Since then, there is a renewed interest in this field and five more MRPs have been discovered, bringing the total number to seven, four of which were discovered/confirmed using the uGMRT via the team’s survey and follow-up programme.

The study also aims to characterise the properties of the emission so it can be used to probe the otherwise impenetrable magnetosphere of the star. Combining uGMRT data with the Karl G. Jansky Very Large Array (VLA), the team has conducted the first ultrawideband (0.3-4 GHz) study of these stars. It is the first that explores their properties at frequencies below 1 GHz, revealing that the plasma distribution in the stellar magnetosphere is not symmetrical around the magnetic dipole axis.

Overall, this study clearly shows how important it will be to conduct sub-GHz to GHz studies of MRPs using the SKA, since they might show very different behaviour at lower frequencies which, once explained, can give valuable information about plasma magnetic interplay in these objects. Read more in this paper and references therein: https://ui.adsabs.harvard.edu/abs/2020pase.conf...66D/abstract

By Barnali Das and Dr Poonam Chandra (National Centre for Radio Astrophysics, Tata Institute for Fundamental Research)

A hot magnetic star HD 133880, emitting Electron Cyclotron Maser Emission (ECME). HD 133880 is the second star showing ECME pulses with the GMRT at 600 MHz.