Survival of molecular gas in cavities of transition disks

Abstract

Context. Planet formation is closely related to the structure and dispersal of protoplanetary disks. A certain class of disks, called transition disks, exhibit cavities in dust images at scales of up to a few 10s of AU. The formation mechanism of the cavities is still unclear. The gas content of such cavities can be spatially resolved for the first time using the Atacama Large Millimeter/submillimeter Array (ALMA). Aims. We develop a new series of models to simulate the physical conditions and chemical abundances of the gas in cavities to address the question whether the gas is primarily atomic or molecular inside the dust free cavities exposed to intense UV radiation. Molecular/atomic line emission by carbon monoxide (CO), its isotopologues ( Methods. We use a thermo-chemical model, which calculates the radiative transfer both in lines and the continuum, and solves for the chemical abundances and gas temperature. The model is based on our previous work, but includes several improvements. We study the dependence of CO abundances and lines on several parameters such as gas mass in the cavity, disk mass and luminosity of the star.Results. The gas can remain in molecular form down to very low amounts of gas in the cavity (∼1% of M Earth ). Shielding of the stellar radiation by a dusty inner disk ("pre-transition disk") allows CO to survive down to lower gas masses in the cavity. The column densities of H 2 and CO in the cavity scale almost linearly with the amount of gas in the cavity down to the mass where photodissociation becomes important. The main parameter for the CO emission from cavity is the gas mass. Other parameters such as the outer disk mass, bolometric luminosity, shape of the stellar spectrum or PAH abundance are less important. Since the CO pure rotational lines readily become optically thick, the CO isotopologues need to be observed in order to quantitatively determine the amount of gas in the cavity. Determining gas masses in the cavity from atomic lines (Conclusions. A wide range of gas masses in the cavity of transition disks (∼4 orders of magnitude) can be probed using combined observations of CO isotopologue lines with ALMA. Measuring the gas mass in the cavity will ultimately help to distinguish between different cavity formation theories.

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

“…In order to constrain changes of the gas surface density profile in particular the drop in the gas surface density inside the dust cavity and the inner gas hole, we use the combined physicalchemical model DALI (Dust And LInes) by Bruderer (2013), based on Bruderer et al (2012). The models solve for the dust radiative transfer, the chemical abundance, the molecular excitation, and the thermal balance to obtain the gas temperature in a self-consistent way.…”

“…The main focus of our modeling are the regions inside the constraints from optically thin isotopologue emission (Bruderer 2013) which are not yet available.…”

“…The far-infrared continuum (60−180 μm), observed by Herschel (Fedele et al 2013), is not spatially resolved and it is unknown whether it is ring-like or lopsided. For our modeling, we adopt the physical structure suggested by Andrews et al (2011), as implemented by Bruderer (2013). The surface density profile is assumed to be a power-law and the vertical structure follows a Gaussian distribution (vertically isothermal structure).…”

“…The CO rovibrational emission line at 4.7 μm tracing the several 100 K warm gas from the inner regions of the disk has been detected for several transition disks (Goto et al 2006;Pontoppidan et al 2008;van der Plas et al 2009;Salyk et al 2009;Brown et al 2012Brown et al , 2013. In contrast, submillimeter observations of the rotational gas lines trace the bulk of the colder gas and are more suitable for constraining the gas mass and distribution inside the cavity (Bruderer 2013). So far, submillimeter observations have suffered from the low angular resolution and sensitivity, have barely detected the gas in the outer disk, and did not allow the study of properties of the gas inside the cavity (Dutrey et al 2008;Lyo et al 2011).…”

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

“…Bruderer (2013) showed that ALMA has the sensitivity to detect through observations of the CO 3-2 transition in Band 7 up to a few Earth masses of gas in the cavity. Measuring how much mass is present involves more complicated modelling using different isotopologues of the molecule.…”