We present high-spatial resolution HST and ground-based adaptive optics observations , and high-sensitivity ISO ( ISOCAM & ISOPHOT ) observations of a sample of X-ray selected weak-line ( WTTS ) and post ( PTTS ) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations .
HST/NICMOS and adaptive optics observations aimed at identifying substellar companions ( young brown dwarfs ) at separations \geq 30 A.U .
from the primary stars .
No such objects were found within 300 A.U .
of any of the target stars , and a number of faint objects at larger separations can very likely be attributed to a population of field ( background ) stars .
ISOCAM observations of 5 to 15 Myr old WTTS and PTTS in ScoCen reveal infrared excesses which are clearly above photospheric levels , and which have a spectral index intermediate between that of younger ( 1 to 5 Myr ) T Tauri stars in Chamaeleon and that of pure stellar photospheres .
The difference in the spectral index of the older PTTS in ScoCen compared to the younger classical and weak-line TTS in Cha can be attributed to a deficiency of smaller size ( 0.1 to 1 \mu m ) dust grains relative to larger size ( \approx 5 \mu m ) dust grains in the disks of the PTTS .
The lack of small dust grains is either due to the environment ( effect of nearby O stars and supernova explosions ) or due to disk evolution .
If the latter is the case , it would hint that circumstellar disks start to get dust depleted at an age between 5 to 15 Myr .
Dust depletion is very likely related to the build-up of larger particles ( ultimately rocks and planetesimals ) and thus an indicator for the onset of the period of planet formation .