Evidence for a Developing Gap in a 10 Myr Old Protoplanetary Disk

Abstract

We have developed a physically self-consistent model of the disk around the nearby 10 Myr old star TW Hya which matches the observed spectral energy distribution and 7mm images of the disk. The model requires both significant dust size evolution and a partially-evacuated inner disk region, as predicted by theories of planet formation. The outer disk, which extends to at least 140 AU in radius, is very optically thick at infrared wavelengths and quite massive (∼ 0.06M ⊙ ) for the relatively advanced age of this T Tauri star. This implies long viscous and dust evolution timescales, although dust must have grown to sizes of order ∼ 1 cm to explain the sub-mm and mm spectral slopes. In contrast, the negligible near-infrared excess emission of this system requires that the disk be optically thin inside ∼ < 4 AU. This inner region cannot be completely evacuated; we need ∼ 0.5 lunar mass of ∼ 1 µm particles remaining to produce the observed 10µm silicate emission. Our model requires a distinct transition in disk properties at ∼ 4 AU, separating the inner and outer disk. The inner edge of the optically-thick outer disk must be heated almost frontally by the star to account for the excess flux at mid-infrared wavelengths. We speculate that this truncation of the outer disk may be the signpost of a developing gap due to the effects of a growing protoplanet; the gap is still presumably evolving because material still resides in it, as indicated by the silicate emission, the molecular hydrogen emission, and by the continued accretion onto the central star (albeit at a much lower rate than typical of younger T Tauri stars). TW Hya thus may become the Rosetta stone for our understanding of the evolution and dissipation of protoplanetary disks.

Imaging protoplanetary disks with a square kilometer array

“…A substantial mass fraction of the dust grains in protoplanetary disks has apparently evolved and started to agglomerate. The low values of β derived from observations of TW Hya and a few other stars are robustly interpreted as evidence for particle growth to sizes of order a millimeter or more [2,15].…”

A photoevaporative gap in the closest planet-forming disc

“…These estimates, however, are tailored to a significantly different disc and photoevaporation model. The mass accretion rate (and thus surface density) assumed in the Alexander & Armitage (2007) models are about an order of magnitude lower than TW Hya, and the α viscosity parameter about a factor ten higher than what would be needed to explain the longevity and high mass of the TW Hya disc (∼ 0.05 M at 10 Myr Bergin et al, 2013;Calvet et al 2002 suggest α ∼ 10 −3 for a similar mass estimate of ∼ 0.06 M at 10 Myr) 2 . These two facts alone result in two orders of magnitude difference in the drift timescales, as will be shown below.…”

“…Its nearly pole-on inclination (i ∼ 7 o , Qi et al 2004) further contributes to making TW Hya one of the reference objects for the study of protoplanetary discs. Calvet et al (2002) first reported evidence for a dustdepleted inner hole, which they placed at a radius of 4 au via detailed radiative transfer modelling of its spectral energy distribution (SED). On the basis of the 10 µm silicon emission, Calvet et al (2002) concluded that the inner region cannot be completely devoid of dust, and they estimate that ∼ 0.5 lunar mass of ∼ 1 µm particles must be still present inside the cavity, and interpret this as evidence of a developing gap in the disc.…”

“…Calvet et al (2002) first reported evidence for a dustdepleted inner hole, which they placed at a radius of 4 au via detailed radiative transfer modelling of its spectral energy distribution (SED). On the basis of the 10 µm silicon emission, Calvet et al (2002) concluded that the inner region cannot be completely devoid of dust, and they estimate that ∼ 0.5 lunar mass of ∼ 1 µm particles must be still present inside the cavity, and interpret this as evidence of a developing gap in the disc. The exact size of the gap of TW Hya has been an object of dispute in the literature.…”

NACO polarimetric differential imaging of TW Hya

“…Being an excellent target for planet formation studies, during the past few years several attempts have been made to image its disk as close as possible to the star and to identify possible disk structures (Krist et al 2000;Weinberger et al 2002). In addition, the presence of a giant planet was predicted (Calvet et al 2002) from modeling the spectral energy distribution (SED) (see also Steinacker & Henning 2003). The first application of the PDI to TW Hya by Potter (2003a,b) made use of the 36 element curvature sensing Adaptive Optics (AO) system Hokupa'a (Graves et al 2000), of the Gemini North telescope.…”