Magnetic field activity near the PMS star-disk boundary: the role of radio
Abstract
The period 1989-1993 has seen rapid developments in the application of radio Very Long Baseline Interferometry (VLBI) to single radio stars in the Pre-Main Sequence phase of evolution. VLBI complements observations with the Very Large Array (VLA), and can only successfully be employed if the radio radiation is nonthermal and of very high brightness temperature (Tb >= 10(8) K). Since (1) such nonthermal processes can only proceed in the presence of significant magnetic fields and (2) VLBI sees activity at radii between 25R_⋆ and the stellar photosphere at typical source distances, VLBI studies offer opportunities to chart the scale sizes (and model-dependent strengths) of magnetic fields around a variety of PMS stars and Young Stellar Objects (YSOs). Initial VLBI detections of magnetic structures involved Weak-lined T Tauri stars (WTTS) believed to be diskless. Later, at least one such object was found to have a remnant disk and other indicators that would place it in a transition category between WTT status and Classical T Tauri star (CTTS). In a parallel development, strong indications of nonthermal radio activity from an organized magnetic field structure was seen on one occasion from a PMS object which clearly has a disk. Finally, some of the magnetic nonthermal radio star sources may not yet be on convective PMS tracks, a development which contradicts the view that magnetic activity in cool stars, from starspots to radio flares, derives from convective dynamos. The implications of recent VLBI and VLA radio results will be placed in a context of potential for studying disk-star interactions and otherwise undetectable magnetic fields at inner radii. The sizes for magnetic activity implied by radio measurements is circumstantial support for theories in which magnetic coupling between star and disk plays a key role in distributing angular momentum and determining Zero-Age Main Sequence (ZAMS) stellar rotation.
- Publication:
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American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1992
- Bibcode:
- 1992AAS...181.1404P