Timescales of Disk Evolution and Planet Formation: [ITAL]HST[/ITAL], Adaptive Optics, and [ITAL]ISO[/ITAL] Observations of Weak-Line and Post–T Tauri Stars

Abstract

We present high-spatial resolution HST and ground-based adaptive optics observations, and highsensitivity 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 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 µm) dust grains relative to larger size (≈5 µm) 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.

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs in wide orbits

“…We summarize here their properties, as the binarity has to be considered to correct photometric distances and temperatures; binarity is also relevant for a proper interpretation of the age indicators. Three close binaries were previously known: HIP 108422 (Chauvin et al 2003, physical association confirmed by our NaCo observations), TYC 7835-2569-1 (Brandner et al 1996), and TYC 6786-0811-1 (Köhler et al 2000). Three of the newly discovered companions are proposed members of the AB Dor moving group (MG) and one belongs to the Cha group.…”

“…However, we can not conclusively demonstrate this without a measurement of the binary's orbital period. TYC 7835-2569-1 = HD 137059 (ASAS 152517−3845.4;1SWASP+J152516.99-384526.0) Close visual binary (1.05 arcsec separation, ΔV = 0.3 mag, ΔH = 0.1 mag) first discovered by Brandner et al (1996) and confirmed by NaCo-LP data. The star was found to be SB2 from CORAVEL spectra.…”

A spectroscopic survey of the youngest field stars in the solar neighbourhood

“…5.4 and 5.5) of our sample, among which five appear as possible members of young stellar kinematical groups in the solar neighbourhood. Brandner et al (1996) have pointed out that ROSAT unresolved binaries are X-ray sources statistically brighter than single stars. Thus, X-ray flux-limited surveys suffer from a detection bias towards binarity.…”

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…They showed that the numbers and sizes of far flung proplyds are consistent with statistical expectations based on the X-ray luminosity function (though the correlation between instantaneous X-ray luminosity and resolvable proplyd structure on a source by source basis is weak, an effect that might be attributed to variability). Similarly there are far flung proplyds detected in Carina (Smith et al 2003), NGC 3603 (Brandner et al 2000) and Cyg OB2 (Wright et al 2012) although some of these are likely to instead be ionised clumps of molecular gas (Sahai et al 2012 a,b).…”