Large gaps and high accretion rates in photoevaporative transition disks with a dead zone

Abstract

Context. Observations of young stars hosting transition disks show that several of them have high accretion rates, despite their disks presenting extended cavities in their dust component. This represents a challenge for theoretical models, which struggle to reproduce both features simultaneously. Aims. We aim to explore if a disk evolution model, including a dead zone and disk dispersal by X-ray photoevaporation, can explain the high accretion rates and large gaps (or cavities) measured in transition disks. Methods. We implemented a dead zone turbulence profile and a photoevaporative mass-loss profile into numerical simulations of gas and dust. We performed a population synthesis study of the gas component and obtained synthetic images and SEDs of the dust component through radiative transfer calculations. Results. This model results in long-lived inner disks and fast dispersing outer disks that can reproduce both the accretion rates and gap sizes observed in transition disks. For a dead zone of turbulence αdz = 10−4 and an extent rdz = 10 AU, our population synthesis study shows that 63% of our transition disks are still accreting with Ṁg ≥ 10−11 M⊙ yr−1 after opening a gap. Among those accreting transition disks, half display accretion rates higher than 5.0 × 10−10 M⊙ yr−1. The dust component in these disks is distributed in two regions: in a compact inner disk inside the dead zone, and in a ring at the outer edge of the photoevaporative gap, which can be located between 20 and 100 AU. Our radiative transfer calculations show that the disk displays an inner disk and an outer ring in the millimeter continuum, a feature that resembles some of the observed transition disks. Conclusions. A disk model considering X-ray photoevaporative dispersal in combination with dead zones can explain several of the observed properties in transition disks, including the high accretion rates, the large gaps, and a long-lived inner disk at millimeter emission.

High-resolution ALMA observations of transition disk candidates in Lupus

“…However, the latter study was entirely based on SED based transition disk candidates rather than confirmed cavities from millimeter continuum imaging. Furthermore, several studies with more advanced photoevaporation models have shown that larger cavity sizes with higher accretion rates could be produced as well with photoevaporative clearing (Ercolano et al 2018;Picogna et al 2019;Gárate et al 2021). The new Lupus transition disk sample allows a comparison over a large range of cavity sizes and the latest photoevaporation models.…”

“…The disks without resolvable inner cavity are included as upper limits based on the analysis in Section 5.1. The photoevaporation regimes from a number of modeling studies (Owen et al 2011;Ercolano et al 2018;Picogna et al 2019;Gárate et al 2021) overlaid. These are the regimes with a >0.1% probability of finding a disk with the given cavity and accretion rate as the result of X-ray photoevaporation for different assumptions.…”

“…Picogna et al (2019) derived new prescriptions for the mass-loss rate based on 2D hydrodynamical simulations with an improved temperature structure in the wind region and found a somewhat larger regime than Owen et al (2011) as well. Finally, Gárate et al (2021) developed photoevaporation models of disks including a dead zone in the inner part of the disk, with the explicit goal of understanding large inner cavities with high accretion rates.…”

“…Bergin et al 2016;Miotello et al 2019;Krijt et al 2020;Bosman et al 2021), the timescale of the gas clearing with respect to the dust evolution time scales and whether the observed dust cavity radius is actually representative for the model cavity radius (see e.g. discussion in Gárate et al 2021), and the impact of the stellar mass on the mass loss rate (Picogna et al 2021). In particular, all models were run for a 0.7 M star whereas our sample contains a range of stellar masses.…”

“…It has been suggested that two types of transition disks exist (Owen & Clarke 2012), caused by either photoevaporation or by other mechanisms such as clearing by a wide-orbit companion (Lin & Papaloizou 1979), where the latter is the most likely explanation for the large cavity transition disks with high accretion rates. More advanced photoevaporation models including carbon depletion (Ercolano et al 2018;Wölfer et al 2019), updated models of X-ray radiation (Picogna et al 2019) and dead zones (Gárate et al 2021) can reproduce a larger range of observed transition disks, including the ones with high accretion rates and large cavities, but not yet at the observed fraction. However, these comparisons still mostly rely on transition disk candidates with small <10 au cavities identified through SED modeling (e.g.…”