Gas structure inside dust cavities of transition disks: Ophiuchus IRS 48 observed by ALMA

Abstract

Context. Transition disks are recognized by the absence of emission of small dust grains inside a radius of up to several 10s of AUs. Owing to the lack of angular resolution and sensitivity, the gas content of these dust holes has not yet been determined, but is of importance for constraining the mechanism leading to the dust holes. It is thought that transition disks are currently undergoing the process of dispersal, setting an end to the giant planet formation process. Aims. We present new high-resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of gas lines towards the transition disk Oph IRS 48 previously shown to host a large dust trap. The ALMA telescope has detected the J = 6−5 line of 12 CO and C 17 O around 690 GHz (434 μm) at a resolution of ∼0.25 corresponding to ∼30 AU (FWHM). The observed gas lines are used to set constraints on the gas surface density profile. Methods. New models of the physical-chemical structure of gas and dust in Oph IRS 48 are developed to reproduce the CO line emission together with the spectral energy distribution and the VLT-VISIR 18.7 μm dust continuum images. Integrated intensity cuts and the total spectrum from models having different trial gas surface density profiles are compared to observations. The main parameters varied are the drop in gas surface density inside the dust-free cavity with a radius of 60 AU and inside the gas-depleted innermost 20 AU. Using the derived surface density profiles, predictions for other CO isotopologues are made, which can be tested by future ALMA observations of the object. Results. From the ALMA data we find a total gas mass of the disk of 1.4 × 10 −4 M . This gas mass yields a gas-to-dust ratio of ∼10, but with considerable uncertainty. Inside 60 AU, the gas surface density drops by a factor of ∼12 for an assumed surface density slope of γ = 1 (Σ ∝ r −γ ). Inside 20 AU, the gas surface density drops by a factor of at least 110. The drops are measured relative to the extrapolation to small radii of the surface density law at radii >60 AU. The inner radius of the gas disk at 20 AU can be constrained to better than ±5 AU. Conclusions. The derived gas surface density profile points to the clearing of the cavity by one or more massive planets/companions rather than just photoevaporation or grain-growth.

Gas density drops inside dust cavities of transitional disks around young stars observed with ALMA

“…The derived physical models in this study can be used as a starting point for future ALMA observations of CO isotopologs, whose emission is optically thin and which therefore directly trace the density structure. Our analysis of the gas density structure is similar to that used for Oph IRS 48 through 12 CO 6-5 and C 17 O 6-5 line observations as presented in Bruderer et al (2014). The analysis of HD 142527, which has been observed in 12 CO, 13 CO and C 18 O 6-5, 3-2 and 2-1, is left for future studies.…”

“…In the fitting procedure, we used an approach of manual fitting by eye and varied the parameters in a logical order so that they converged to a model that fits the data, similar to the approach in Bruderer et al (2014). We did not use a χ 2 or Markov chain Monte Carlo (MCMC) method because the computational time of the models is too long and the number of parameters too large.…”

“…With spatially resolved CO observations it is possible to infer emission from different lines of sight, which was not possible with previous single-dish spectral analysis. Bruderer et al (2014) demonstrated for 12 CO 6-5 lines in Oph IRS 48 that the emission in the optically thin wings can be used to constrain the gas density. In this case, the observations are found to be consistent with a physical disk model with two gas density drops inside the millimeter dust cavity.…”

Constraining the structure of the transition disk HD 135344B (SAO 206462) by simultaneous modeling of multiwavelength gas and dust observations

“…Near-IR interferometry provides evidence of dust emission (and inhomogeneities) inside the dust cavity of transition disks (e.g., Kraus et al 2013). Furthermore, recent ALMA observations have spatially resolved gas inside the cavity of transition disks (Casassus et al 2013;Bruderer et al 2013).…”

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

“…However, here we are concerned with the discs that cannot be explained by photo-evaporation (those with huge cavities and high accretion rates), and that is why we explore the consequences of the presence of a planet. While some unsuccessful attempt to detect gas in the cavity was done with the previous generation of interferometers (Dutrey et al 2008;Lyo et al 2011), ALMA has now the sensitivity and the spatial resolution necessary to accomplish this goal (Rosenfeld et al 2012;Bruderer et al 2014;Casassus et al 2013;Zhang et al 2014). As measurements of this kind are becoming routinely available, it is of primary importance for models of transition discs to provide predictions for the gas content.…”