On the theory of disc photoevaporation

2014

Astron Nachr

Protoplanetary discs are a natural consequence of the star formation process and as such are ubiquitous around low-mass stars. They are fundamental to planet formation as they hold the reservoir of material from which planets form. Their evolution and final dispersal and the timescales that regulate these process are therefore of particular interest. In this contribution I will review the observational evidence for the dispersal of discs being dominated by two timescales and for the final dispersal to occur quickly and from the inside out. I will discuss the current theoretical models, including X-ray photoevaporation, showing that the latter provides a natural explanation to the observed behaviour and review supporting and contrasting evidence. I will finally introduce a new mechanism based on the interaction between planet formation and photoevaporation that may explain a particular class of transition discs with large inner holes and high accretion rates that are problematic for photoevaporation models and planet formation models alone.

Section: Introductionmentioning

confidence: 73%

The dispersal of protoplanetary discs

2014

Astron Nachr

“…Following Equation 9 of Owen, Clarke & Ercolano (2012), the corresponding mass loss rate due to X-ray photoevaporation is ∼ 2 × 10 −8 M /yr, which is roughly a factor of 10 higher than the measured accretion rate for TW Hya. Owen et al (2010) predict that for a 0.7 M star and X-ray luminosity of 2 × 10 30 erg/sec X-ray photoevaporation will create a gap at ∼ 1 au, when the accretion rate falls to about a tenth of the wind rate.…”

Section: Evidence Of X-ray Photoevaporation In Tw Hyamentioning

confidence: 99%

A photoevaporative gap in the closest planet-forming disc

Monthly Notices of the Royal Astronomical Society: Letters

Rosotti

Picogna

et al. 2016

Self Cite

The dispersal of the circumstellar discs of dust and gas surrounding young lowmass stars has important implications for the formation of planetary systems. Photoevaporation 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 µm continuum source might be produced by free free emission from the gas and/or residual dust inside the gap.

“…This will change the density vertical structure of the disc; the density at the launching point of the photo-evaporative wind, which sets the mass-loss rate, might be affected. Although X-ray photo-evaporation has been shown to be quite resistant to changes in the underlying disc structure (Owen et al 2012b), it remains to be seen whether this applies also to such extreme differences. In particular, even if this effect is unlikely to change the total mass-loss rate, it will probably change the flow topology (Owen et al 2012b), which might have implications for the inner disc.…”

Section: Discussionmentioning

confidence: 99%

“…The details of the photoevaporation profile assumed can be found in the appendix of Owen et al (2012a). In our simulations we do not allow the planet to migrate, as Rosotti et al (2013) found migration to have little impact.…”

Section: Numerical Modelmentioning

confidence: 99%

See 1 more Smart Citation

The long-term evolution of photoevaporating transition discs with giant planets

Rosotti

Mon. Not. R. Astron. Soc.

Owen

2015

Self Cite

Photo-evaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2d simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work we explored how the formation of a giant planet triggers the dispersal of the inner disc by photo-evaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photo-evaporation alone. In this work we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc the disc is a non accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to understand if it is long lived enough to be found observationally. Depending on the parameters, especially on the X-ray luminosity of the star, we find that the fraction of time spent as a non-accretor greatly varies. We build a population synthesis model to compare with observations and find that in general thermal sweeping is not effective enough to destroy the outer disc, leaving many transition discs in a relatively long lived phase with a gas free hole, at odds with observations. We discuss the implications for transition disc evolution. In particular, we highlight the current lack of explanation for the missing non-accreting transition discs with large holes, which is a serious issue in the planet hypothesis.