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

Abstract

Context. Constraining the gas and dust disk structure of transition disks, particularly in the inner dust cavity, is a crucial step toward understanding the link between them and planet formation. HD 135344B is an accreting (pre-)transition disk that displays the CO 4.7 µm emission extending tens of AU inside its 30 AU dust cavity. Aims. We constrain HD 135344B's disk structure from multi-instrument gas and dust observations. Methods. We used the dust radiative transfer code MCFOST and the thermochemical code ProDiMo to derive the disk structure from the simultaneous modeling of the spectral energy distribution (SED), VLT/CRIRES CO P(10) 4.75 µm, Herschel/PACS [O ] 63 µm, Spitzer/IRS, and JCMT 12 CO J = 3−2 spectra, VLTI/PIONIER H-band visibilities, and constraints from (sub-)mm continuum interferometry and near-IR imaging. Results. We found a disk model able to describe the current gas and dust observations simultaneously. This disk has the following structure. (1) To simultaneously reproduce the SED, the near-IR interferometry data, and the CO ro-vibrational emission, refractory grains (we suggest carbon) are present inside the silicate sublimation radius (0.08 < R < 0.2 AU). (2) The dust cavity (R < 30 AU) is filled with gas, the surface density of the gas inside the cavity must increase with radius to fit the CO ro-vibrational line profile, a small gap of a few AU in the gas distribution is compatible with current data, and a large gap of tens of AU in the gas does not appear likely. (4) The gas-to-dust ratio inside the cavity is >100 to account for the 870 µm continuum upper limit and the CO P(10) line flux.(5) The gas-to-dust ratio in the outer disk (30 < R < 200 AU) is <10 to simultaneously describe the [O ] 63 µm line flux and the CO P(10) line profile. (6) In the outer disk, most of the gas and dust mass should be located in the midplane, and a significant fraction of the dust should be in large grains. Conclusions. Simultaneous modeling of the gas and dust is required to break the model degeneracies and constrain the disk structure. An increasing gas surface density with radius in the inner cavity echoes the effect of a migrating jovian planet in the disk structure. The low gas mass (a few Jupiter masses) throughout the HD 135344B disk supports the idea that it is an evolved disk that has already lost a large portion of its mass.

Shadows cast on the transition disk of HD 135344B

“…We aim to find two separate solutions by restricting the radial position of the protoplanets to be either inside or outside the scattered light cavity. The disk aspect ratio is limited to 0.05 < h p < 0.4 such that it is consistent with radiative transfer modeling (Andrews et al 2011;Carmona et al 2014) and the temperatures required are still physical. The rotational profile is assumed to be Keplerian with ζ = 3/2 and we use the flaring index, ψ = ζ − η − 1 = 0.25, from the radiative transfer model in Sect.…”

“…The near-infrared-excess in the SED of HD 135344B is an indicator for dust at small orbital radii (Brown et al 2007;Carmona et al 2014). A dust belt at sub-au distance from the star was confirmed by mid-and near-infrared interferometric observations (Carmona et al 2014;Menu et al 2015) with possibly a misalignment of the inner and outer disk (Fedele et al 2008).…”

“…A dust belt at sub-au distance from the star was confirmed by mid-and near-infrared interferometric observations (Carmona et al 2014;Menu et al 2015) with possibly a misalignment of the inner and outer disk (Fedele et al 2008). Multi-epoch spectroscopic and near-infrared photometric observations show variability over the course of months which might be related to instabilities in or perturbations of the inner disk (Sitko et al 2012).…”

“…Negative valued pixels are identified in the central regions of the unscaled Q φ images, where instrumental effects, PSF smearing effects (Avenhaus et al 2014a), and low-wind effects dominate, and also in the outer disk regions were the scattered light flux becomes comparable to the background noise level. The inner dust disk of HD 135344B is expected to be located at sub-au distance from the star (Carmona et al 2014) and a direct detection is not possible. However, the signal pattern in the inner regions of the ZIMPOL images is expected to contain some of the smeared scattered light from the inner disk but at this point it is not clear how significant this is with respect to the instrumental signal.…”

“…We assume a Keplerian disk, optically thin temperature profile and estimate the disk mass from the optically thin ALMA band 7 (0.85 mm) dust continuum . The disk mass is given by ) with a typical gas-todust ratio of g/d = 100, g/d = 80 (van der Marel et al 2016), and g/d = 4 (Carmona et al 2014). The dashed lines show the 25 au scattered light cavity, the 46 au millimeter cavity from Andrews et al (2011), and the outer radius at which the spiral arms are visible.…”