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The nature of the peculiar emission properties of the T Tauri stars is examined in the light of contributions from regions designated as photospheres, chromosphere coronae, and extended envelopes which are often characterized by mass loss. Although some manifestations of T Tauri activity can be linked to vigorous solar-stellar phenomena characteristic of cool stars of low mass, other characteristics...
The status and implications of recent X-ray studies of pre-main sequence (PMS) stars are reviewed in detail. Images of nearby star formation regions including the Orion, Taurus-Auriga, ρ Ophiuchi and Chamaeleon clouds obtained with the Einstein X-ray Observatory have revealed more than 150 X-ray luminous PMS stars to date. Some are classical strong emission line T Tauri stars while others are late-type...
Current ideas on the early stages of star formation suggest strongly that proto-stars enter the pre-main sequence phase rotating rapidly. The subsequent rotational evolution of both single and binary stars is largely determined by angular momentum transport by a magnetically-controlled stellar wind. A massive star with a cool corona but with a modest relic magnetic field may be prevented from fragmenting...
We consider models of the processes by which a molecular cloud acquires dense cores, a (magnetized) rotating core collapses to give a protostar plus nebular disk, and a powerful stellar wind sets in to reverse the accretion flow and reveal the central object as a pre-main-sequence star. At each stage, we rely on a combination of theory and observation to fix the basic parameters of the model. We show...
A review of the classical and modern measurements of axial rotation and their implications for the evolution of rotation of stars on the lower main sequence is given. From stars of main-sequence spectral type A through early K, the dependence of rotation on mass and age is investigated, from results for stars in open clusters and in the field. The high-mass, single, normal dwarf stars of spectral...
Stars spend most of their lifetime and show the least amount of nuclear evolution on the main sequence. However, the x-ray luminosities of cool star coronas change by orders of magnitude as a function of main sequence age. Such coronal evolution will be discussed in relation to our knowledge of the solar corona, solar and stellar flares, stellar rotation and binarity. The relevance of X-ray observations...
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