The milliarcsecond-scale radio structure of AB Doradus A^⋆

¹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
e-mail: j.bautista.climent@uv.es
² Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Donostia International Physics Center, Paseo de Manuel Lardizabal 4, 20018 Donostia-San Sebastián, Spain
⁵ CSIRO Astronomy and Space Science, Australia Telescope National Facility, Epping, NSW 1710, Australia
⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁷ Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC, 61 Av. de l’Observatoire, 75014 Paris, France

Received: 21 January 2020
Accepted: 2 July 2020

Abstract

Context. The fast rotator, pre-main sequence star AB Dor A is a strong and persistent radio emitter. The extraordinary coronal flaring activity is thought to be the origin of compact radio emission and other associated phenomena, such as large slingshot prominences.

Aims. We aim to investigate the radio emission mechanism and the milliarcsecond radio structure around AB Dor A.

Methods. We performed phase-referenced VLBI observations at 22.3 GHz, 8.4 GHz, and 1.4 GHz over more than one decade using the Australian VLBI array.

Results. Our 8.4 GHz images show a double core-halo morphology, similar at all epochs, with emission extending at heights between 5 and 18 stellar radii. Furthermore, the sequence of the 8.4 GHz maps shows a clear variation of the source structure within the observing time. However, images at 1.4 GHz and 22.3 GHz are compatible with a compact source. The phase-reference position at 8.4 GHz and 1.4 GHz are coincident with those expected from the well-known milliarcsecond-precise astrometry of this star, meanwhile the 22.3 GHz position is 4σ off the prediction in the north-west direction. The origin of this offset is still unclear.

Conclusions. We have considered several models to explain the morphology and evolution of the inner radio structure detected in AB Dor A. These models include emission from the stellar polar caps, a flaring, magnetically-driven loop structure, and the presence of helmet streamers. We also investigated a possible close companion to AB Dor A. Our results confirm the extraordinary coronal magnetic activity of this star, capable of producing compact radio structures at very large heights that have so far only been seen in binary interacting systems.