Complex magnetic field detected on surface of Vega
Astronomers have detected a large-scale fossil field coexisting with a variable small-scale field, likely to be generated by a dynamo mechanism on the star Vega. The magnetic field was revealed by a decade of observations by ground-based observatories.
An international team of astronomers have studied more than 13,000 high-resolution spectra of the star vega collected over a period of more than a decade, and unraveled the complex magnetic field on the surface of the intermediate-mass star. Vega contains about twice the mass of the Sun and is rotating at an equatorial velocity of nearly 200 km/s. A weak magnetic field on the surface was first discovered in 2009, surprising the scientific community that did not expect such a star to generate a magnetic field according to conventional models of stellar evolution, because of the lack of a convective envelope near the surface, as seen on cooler stars.
Since then, a number of intermediate-mass stars have been discovered hosting magnetic fields, but their origins remain a mystery. They could be fossil fields created during earlier stages of the life of the star, or the result of an active dynamo mechanism. Observations in 2015 revealed bright as well as dark spots coorotating with Vega, indicating the presence of a dynamo mechanism. An unprecedented large-scale observation campaign was launched to better understand the mystery. The observations have revealed unexpected complexity in the magnetic field, for the first time.
The complex magnetic field around Vega
For the first time, astronomers have discovered a fossil magnetic field in a prototype intermediate-mass star, in the form of an inclined dipole. The magnetic poles of this fossil mangetic field is not aligned with the rotational axis of Vega, which was detected in 2022. There is also a variable small-scale magnetic field, that is likely to be generated by a dynamo effect. It is likely that the large-scale fossil magnetic field exists on other similar stars as well, and this discovery has a major impact on understanding the stars driving the chemical evolution of the universe. Further research is necessary to understand the weak dynamo in such stars. A paper describing the findings has been published in Astronomy & Astrophysics.