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Jets from Young Stellar Objects Frank H. Shu National Tsing Hua University Hsinchu, Taiwan Abstract This talk reviews the modern view that the sun and planets both formed from an accretion disk that is built up by continuing infall from a surrounding molecular cloud core. A difficulty with this idea is that pre-main-sequence sunlike stars rotate much more slowly than naive expectations predict. A solution to the difficulty is possible if young stellar objects (YSOs) are strongly magnetized and truncate the inwardly drifting material of their accretion disks before the disks can reach the stellar surfaces. The resulting interactions drive powerful bipolar outflows that collimate slowly into protostellar jets. A prediction of the model is that the highly linear appearances of the outflows at their base, as imaged by the Hubble Space Telescope, are optical illusions. Using quantitative calculations of the collimation and heating and ionization processes, we compare the theoretical models with images and spectroscopic observations of YSO jets |