The submillimetre colour of young stellar objects

A survey is presented of the submillimetre continuum spectra from dust arou nd 73 young stellar objects (YSOs) associated with Herbig-Haro objects and molecular outflows. Spectra at millimetre to submillimetre wavelengths take n with similar beamsizes have been obtained for 42 of the objects, and the remainder either have been detected or have upper limits at 800 mu m. The d ata are combined with far-infrared IRAS fluxes, and then compared with publ ished submillimetre data from other types of YSO; this results in a data se t of similar to 150 submillimetre spectra. We have then employed colour-col our diagrams to investigate the differences in submillimetre continuum spec tra between classes of YSOs. The majority of targets are optically obscured YSOs (generally Lada Class I ); all are found to have a submillimetre spectral index (alpha(mm)) of simi lar to 3.5. None of these objects has alpha(mm) less than or equal to 3.0. This is significantly different from published optically visible T Tauri st ars (Class II-III), many of which have alpha(mm) similar to 2.5. The few ma in-sequence stars so far observed in the submillimetre generally also have a relatively low value of alpha(mm). We fmd that objects which might be cla ssified as T Tauri stars, but which are not directly visible, have higher v alues of alpha(mm) Assuming isothermal optically thin dust emission, this implies a dust emiss ivity index beta of similar to 1.5 for embedded objects, and similar to 0.5 for visible objects. The reduced alpha(mm) in low optical extinction sourc es can be attributed to either grain growth or the formation of a compact o ptically thick circumstellar disc, and we discuss these two options in the light of the new data. If, as the results suggest, large fractal grains are present in the more evolved systems, then these will have higher millimetr e mass opacity, implying that the determination of the circumstellar mass e volution from submillimetre continuum fluxes must take into account the evo lution of the dust itself. High-mass embedded YSOs also have spectra very similar to their low-mass co unterparts, but with higher dust temperature (again, under the isothermal o ptically thin assumption). Also, most Class 0 YSOs have high values of alph a(mm), although three have flatter spectra more typical of fractal dust, wh ich is difficult to explain under simple grain coagulation models. The submillimetre continuum flux is also compared with the intensity of two molecular lines: (CO)-O-18 J = 2-1 and H2CO 3(03) - 2(02). The integrated emission from the higher density tracer, H2CO, is well correlated with cont inuum flux. The (CO)-O-18 line shows a less significant correlation, partic ularly at low masses, suggesting that this may not be a good molecule to se arch for low-mass YSOs. By comparing T Tauri stars and younger YSOs with th e same continuum flux, we find evidence that T Tauri stars are unusually we ak in (CO)-O-18 emission, and we give some possible explanations.