The dispersal of protoplanetary discs – I. A new generation of X-ray photoevaporation models

Abstract

Photoevaporation of planet-forming discs by high energy radiation from the central star is potentially a crucial mechanism for disc evolution and it may play an important role in the formation and evolution of planetary systems. We present here a new generation of X-ray photoevaporation models for solar-type stars, based on hydrodynamical simulations, which account for stellar irradiation via a significantly improved parameterisation of gas temperatures, based on detailed photoionisation and radiation transfer calculations. This is the first of a series of papers aiming at providing a library of models which cover the observed parameter space in stellar and disc mass, metallicity and stellar X-ray properties. We focus here on solar-type stars (0.7 M ) with relatively low-mass discs (1% of the stellar mass) and explore the dependence of the wind mass-loss rates on stellar X-ray luminosity. We model primordial discs and transition discs at various stages of evolution. Our 2D hydrodynamical models are then used to derive simple recipes for the mass-loss rates that are suitable for one-dimensional disc evolution and/or planet formation models typically employed for population synthesis studies. Line profiles from typical wind diagnostics ([OI] 6300Å and [NeII] 12.8 µm) are also calculated for our models and found to be roughly in agreement with previous studies. Finally, we perform a population study of transition discs by means of one-dimensional viscous evolution models including our new photoevaporation prescription and find that roughly a half of observed transition discs cavities and accretion rates could be reproduced by our models.

Effects of photoevaporation on protoplanetary disc ‘isochrones’

“…As a consequence, the accretion rate dramatically decreases (the so-called UV switch) and the inner disc is accreted on its own (shorter) timescale, producing a transition disc in ∼ 10 5 yr, in agreement with observational results (Ercolano et al 2011a). In recent years, attention has been paid also to other heating radiation fields such as the far ultra-violet (FUV) (Gorti & Hollenbach 2008) and soft X-rays (Alexander et al 2005;Ercolano et al 2008;Owen et al 2010;Picogna et al 2019), leading to the development of multiple photoevaporation models (see Alexander et al 2014 for a review) that differ in the mass-loss rates, the shape of the mass-loss profile and the link between the properties of the wind and the ones of the central star (such as the stellar UV or X-ray flux). In general, however, the more recent models tend to predict significantly higher mass-loss rates compared to the original EUV model.…”

Dust entrainment in photoevaporative winds: The impact of X-rays

“…11 With increasing R, max(a 0 ) slowly decreases out to R max 140 AU, where it quickly drops to max(a 0 ) = 0. This coincides with the maximum radius at which XEUV photoevaporation is effective for the gas component of the disk; Picogna et al (2019) show that the surface mass-loss rate (Σ gas ) drops to negligible values at R ≈ 140 AU. So both gas and dust residing at the disk surface at R 140 AU are very unlikely to be thermally unbound from there.…”

“…Note that omitting disk gravity should not have a strong effect on the strength of the flaring seen here, see Appendix A.15 Picogna et al (2019) provide a more in-depth explanation of the differences between their model and that ofOwen et al (2011b).Article number, page 9 of 18 A&A proofs: manuscript no. Draft…”

“…The set-up of the hydrodynamical calculations for the gas disk has been described in detail in Picogna et al (2019), so here we limit ourselves to summarising the basic parameters of the specific run employed in this work. We studied a protoplanetary disk of M disk 0.01 M * around a M * = 0.7 M T-Tauri star.…”

A Swift view of X-ray and UV radiation in the planet-forming T Tauri system PDS 70

“…By contrast, the effects of the observed stellar XUV flux on the disc are expected to be significant. Models of X-ray photoevaporation show that an X-ray luminosity of L X = 5 × 10 29 erg s −1 produces a wind mass-loss rate ofṀ w ≃ 10 −8 M ⊙ yr −1 Owen & Jackson 2012;Picogna, et al 2019). The mass of gas in the PDS70 disc is not well-constrained, but for standard assumptions (i.e., a gas-to-dust ratio of 100) the disc models used to fit the dust observations imply a total disc mass M d 10 −2 -10 −3 M ⊙ (e.g.…”