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تسجيل مستخدم جديدThe milliarcsecond-scale radio structure of AB Dor A
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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 as large slingshot prominences. Aim: 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 4$sigma$ 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 which include emission from the stellar polar caps, a flaring, magnetically-driven loop structure, and the presence of helmet streamers. A possible close companion to AB Dor A has been also investigated. Our results confirm the extraordinary coronal magnetic activity of this star, able to produce compact radio structures at very large heights, so far only seen in binary interacting systems.
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Studies of fundamental parameters of very low-mass objects are indispensable to provide tests of stellar evolution models that are used to derive theoretical masses of brown dwarfs and planets. However, only objects with dynamically determined masses
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Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. The aims of this article are to infer the radial and vertical structure of these disks at scales of order o
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Although chromospheric activity cycles have been studied in a larger number of late-type stars for quite some time, very little is known about coronal activity-cycles in other stars and their similarities or dissimilarities with the solar activity cy
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