Thermal Radio Emission from Disk‐driven Centrifugal Winds

Abstract

Disk-driven centrifugal winds can produce considerable bremsstrahlung radiation that could account for the radio observations of many low-luminosity young stellar objects. The wind is generated by ions that are flung out centrifugally along magnetic field lines threading the disk. Collisional drag between ions and neutrals heats the gas, giving rise to a hot (T ~ 104 K), partially ionized (nH+/nH ~ 0.1) region in the wind. The thermal radio emission from this hot region is computed for several wind models, and it is shown that the flux in the radio regime (1 v 20 GHz) is consistent with the observed values of, and upper limits on, the flux from classical T Tauri stars. Synthetic radio maps are calculated for different observed inclinations and are found to be compatible with the observed morphologies. Inclinations near 90° (edge-on to the disk) result in elongated radio maps, whereas smaller inclinations yield more circular contours. The models considered in this study have outflow rates of ~10–7 M☉ yr–1 tron densities in the range inferred from observations of forbidden lines. In particular, a general parameter study within the context of the model indicates that electron densities from ~ 107 to ~ 1012 cm–3 at a distance of ~ 1 AU from the central protostar are required to produce the range in observed radio luminosities and spectral shapes. The calculated wind models turn out to be partially optically thick, implying that any peristellar nonthermal emission would in general be unobservable. This could explain why classical T Tauri stars lack the nonthermal emission found in weak-lined T Tauri stars.