8 - The disks of pre-main-sequence stars

Summary

The idea that the early Sun was surrounded by a rotating flattened nebula or disk out of which the planets formed has had a long history. However, a detailed application of the disk model to pre-main-sequence stars was not made until the seminal work of Lynden-Bell and Pringle (1974). These authors suggested that the excess emission of the low-mass, pre-main-sequence T Tauri stars could be powered by disk accretion; the extended dusty disk accounts for the excess infrared emission of T Tauri stars, while the hot gas predicted at the boundary layer between the star and disk produces the observed ultraviolet continuum emission. Lynden-Bell and Pringle further suggested that T Tauri disks could be quite massive, and might even outshine the central star in their early stages.

In retrospect, researchers in the field were not ready for these insights, partly due to the observational limitations of the time. Complicating the situation, ultraviolet and X-ray observations with the IUE and Einstein satellites in the late 1970s showed that young stars exhibit high-temperature chromospheric and coronal emission at much higher levels than observed on the Sun (e.g., Gahm et al. 1979; Cram et al. 1980; Walter & Kuhi 1981), undoubtedly as a result of solar-type magnetic activity. Thus, it was natural to assume that this excess optical and ultraviolet emission represented the extreme youthful limit of solar magnetic activity (Herbig 1970; Cram 1979; Calvet et al. 1983).