Astron. J., 120, 950-962 (2000/August-0)
Timescales of disk evolution and planet formation: HST, adaptive optics, and ISO observations of weak-line and post-T Tauri stars.
BRANDNER W., ZINNECKER H., ALCALA J.M., ALLARD F., COVINO E., FRINK S., KOEHLER R., KUNKEL M., MONETI A. and SCHWEITZER A.
Abstract (from CDS):
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 ≥30 AU from the primary stars. No such objects were found within 300 AU 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 WTTSs and PTTSs 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 PTTSs in ScoCen compared with the younger classical and weak-line TTSs in Cha can be attributed to a deficiency of smaller size (0.1 to 1 µm) dust grains relative to larger size (~5 µm) dust grains in the disks of the PTTSs. 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 become 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.
Stars: Circumstellar Matter - open clusters and associations: individual (Scorpius-Centaurus, Chamaeleon) - Stars: Planetary Systems - Stars: Low-Mass, Brown Dwarfs - Stars: Pre-Main-Sequence
Table 4: [BZA2000] RX JHHMM.m+DDMM a, [BZA2000] RX JHHMM.m+DDMMA a, N=12+5.
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