1997A&A...324.1036N

We have observed a sample of 32 Hα selected T Tauri stars in the Lupus 2 and 3 T association with the facility bolometer at the SEST and detected cold dust emission at 1.3mm from 12 of the objects. For the remaining objects we have derived 3σ upper limits (≃40mJy), which suggest that the cold dust masses are less than 5x10^–5M_☉. For stellar masses below 0.7M_☉ the mean disk mass (gas + dust) is approximately 3% of the stellar mass. The face value detection rate (38%) of the Lupus stars is very similar to that of young stars in the Taurus-Auriga association. Thus, the young stellar population in Lupus seems to have an equal incidence of circumstellar disks. In comparison to Taurus-Auriga the low-mass pre-main sequence stars in Lupus show a lack of disk masses below 3.10^–3M_☉ (gas + dust), which in part can be explained by the absence of weak-line T Tauri stars in our sample. Both samples show a strong correlation between relative disk mass and stellar age. Considering the absolute disk mass of 1.3mm detected sources in Lupus we see a tentative decrease with increasing age, too, while there is no correlation for the Taurus-Auriga sample. This effect might be due to different modes of star formation: isolated star formation in Taurus-Auriga versus clustered star formation in Lupus. Considering both samples globally none of the 6 stars older than 3Myr shows dust emission, which could mean that almost no small dust grains are left over in the disk at this age. Furthermore, for both samples we do not see any indication that more massive stars have more massive disks. On the contrary, statistical tests suggest that the absolute disk mass as well as the relative disk mass decreases with increasing stellar mass. Finally, in order to investigate the age-dependent behaviour of the spectral energy distribution, we have determined the IR indices α_IR between λ=2.2µm and λ=12...25µm (IRAS) for 11 sources of our sample. According to the revised IR classification by Andre & Montmerle (1994ApJ...420..837A) these objects belong to the IR class II. Neither a correlation between the infrared indices and the stellar age nor between the infrared indices and the disk mass could have been found.