Submillimetre mapping and photometry of bipolar flows — Evidence for compact disks

Abstract

The mm/sub-mm dust emission from bipolar outflow sources is discussed. In nearby outflows the stars driving the flows appear point like and unresolved, i.e. apparent angular sizes ≤ 7″ (i.e. 700AU at 100pc). The spectra are surprisingly flat, suggesting that the dust emission is affected by temperature and density gradients or that the dust surrounding these stars differs from that of the general interstellar medium. If the spectra are characterized with a conventional dust emissivity index, our sample gives an average value of 1.1, rather than the figure of 2, which is usually claimed at these wavelengths for dust in molecular clouds. The total dust and gas masses associated with these stars are a few tenths of a solar mass for nearby low luminosity sources and the gas densities are >10⁷ cm⁻³, if the gas is in a shell, and the densities could be much higher if the gas densities fall off as a function of radius, or if the mass is distributed in a disk. These masses correspond to extremely high extinctions, which are not observed, and we therefore argue that the mass distribution must be anisotropic, most likely as an inclined disk surrounding the star. Hence, stars driving outflows are surrounded by a dense disk, possibly an accretion disks and/or collimating disk. If these stars are compared to the more evolved T Tauri stars, we find that the difference in disk mass could account for the mass loss observed in these stars. Because of the high gas densities in the disk, most commonly used molecular lines will be optically thick, and molecular observations should be done in rare isotopes and high density tracers in order to yield reliable results of gas densities and gas dynamics.

The unresolved continuum emission is often found to be surrounded by weaker extended emission, which is typically orthogonal to the outflow direction. There may also be extended emission in the the direction of the outflow, either due to hot dust in the beginning of the flow (jet) or dust compressed by the flow.