A photo-evaporative gap in the closest planet forming disc

TL;DR

This paper presents evidence that the nearby TW Hya protoplanetary disc exhibits a photoevaporative gap consistent with X-ray driven models, marking possibly the first direct imaging of such a gap during its formation.

Contribution

It provides the first imaging evidence of a photoevaporative gap in a protoplanetary disc, supporting models of disc dispersal without requiring planets.

Findings

The observed gap in TW Hya matches X-ray photoevaporation predictions.

The gap's properties are consistent with disc dispersal models.

The unresolved central emission may originate from residual gas or dust.

Abstract

The dispersal of the circumstellar discs of dust and gas surrounding young low- mass stars has important implications for the formation of planetary systems. Photo- evaporation from energetic radiation from the central object is thought to drive the dispersal in the majority of discs, by creating a gap which disconnects the outer from the inner regions of the disc and then disperses the outer disc from the inside-out, while the inner disc keeps draining viscously onto the star. In this Letter we show that the disc around TW Hya, the closest protoplanetary disc to Earth, may be the first object where a photoevaporative gap has been imaged around the time at which it is being created. Indeed the detected gap in the ALMA images is consistent with the expectations of X-ray photoevaporation models, thus not requiring the presence of a planet. The photoevaporation model is also consistent with a broad range of properties of the TW Hya system, e.g. accretion rate and the location of the gap at the onset of dispersal. We show that the central, unresolved 870 {\mu}m continuum source might be produced by free free emission from the gas and/or residual dust inside the gap.