Low-mass premain sequence stars, i.e., T Tauri stars, begin their evolution along the Hayashi tracks with fully convective interiors. As for ultracool dwarfs this should prevent solar-type aQ -dynamo action, which presumably depends on the presence of a transition between a radiative and a convective zone. However, magnetic fields may be generated by alternative types of dynamos operating in the convection zone. The most popular versions of nonsolar-like dynamos are a2-dynamos and turbulent dynamos [6,39]. Different dynamos may coexist in all types of magnetically active stars, and the dominating mechanism may depend on the interior structure and thus on mass. Turbulent dynamos create small-scale fields and are thought to be independent of rotation. Therefore, a study of the rotation-activity connection on the pre-MS might give insight into the nature of the operating dynamo(s).
HAeBe stars are fully radiative and should not drive any dynamo. Similarly to low-mass protostars, their magnetic fields, as evidenced by their outflows, may be fossil in nature, i.e., remnants of the magnetic fields of the parent molecular cloud.
In cTTS accretion is believed to occur along the magnetic field lines onto distinct regions on the stellar surface, forming so-called hot accretion spots . In approaching the star, the accreting material reaches nearly free-fall velocities. Upon hitting the photospheric material, the infalling gas is shocked and heated. Temperatures in the postshock region reach a few Mega Kelvin with ensuing production of X-rays. Recent high resolution spectroscopic observations of a few cTTS with XMM-Newton provide in fact evidence for shocked X-ray emitting material, but it is unclear at present how much accretion processes contribute to the overall energy budget of cTTS. Activity phenomena on accreting stars (protostars, cTTS and HAeBe stars) may also include magnetic reconnection of field lines connecting star and disk.
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