Dust evolution in protoplanetary disks with SKA and precursors

Among the variety of structures that can be observed at millimeter and sub-millimeter wavelength within disks, some present an inner cavity depleted from gas and dust. Such disks are called transitional disks (TDs). Cavities, are generally attributed to the formation and evolution of one or more giant planets (as it is the case for PDS 70), severing the connection between the outer and inner disks, which is then rapidly depleted by accretion onto the star. At the same time, these planets are modifying the disk gas pressure profile leading to the creation of a pressure trap and, consequently, a ring-like structure in the dust distribution, as large grains are confined as they drift in from the outer disk. This scenario, thus implies that the planets inside the ring greatly stops the exchange  of pebbles between the outer and the inner part of the disk. Even though, observations show that material continues to pass through the planet to the star, the stellar mass accretion in TDs is lower than for full disks of same age or mass. On top of it, since large grains are trapped within the ring, only small grains and gas can filtrate through the cavity, failing to fully explain the large amount of material needed by the central star. As a result, within the inner cavity, no significant dust emission at (sub-)millimeter wavelength should be observe. On the contrary, a compact millimeter emission it observed for about 50% of TDs by ALMA, as it is the case for the TD surrounding CQ Tau, a nearby, intermediate mass pre-main sequence star of spectral type F2. 

As of today, it is not clear if such compact emission is due to the presence of pebbles or whether it is due to non-thermal emission related, e.g., an ionized wind. To quantify this possibility, we performed a detailed multi-wavelength analysis to study the emission in the inner disk of CQ Tau, combining a large set of sensitive and high angular resolution continuum observations from ALMA and VLA, from 0.87 mm to 6 cm (see Figure 1, left panel). Our goal is to try and separate dust and gas emission in the inner region of this system. We present the results of a detailed spectral index analysis (see Figure 1, right panel) in order to characterize the nature of the emission in every part of the disk, extrapolate the free-free emission present and then finally separate between this emission and dust thermal emission for CQ Tau. Finally, we present initial attempts to extend these results to a broader sample of TDs, for which we collected the necessary observations.

 

Figure 1: CQ Tau. Left: Archival data at 1.3 mm (Band 6) of ALMA with in contours the archival data from VLA, at 2 cm (Ku Band). The contours levels are comprise between 10⁻⁵(white contour) and 2 × 10⁻⁵Jy.beam⁻¹ (red contour). A 1 arcsecond scale is shown in the upper part of the plot and the beam is presented on the left bottom corner. Right: Spectral index map using data at centimeter and millimeter wavelength. This computation is done pixels-by-pixels, all data smoothed to the lowest resolution map and each pixel’s color is set to the value of the spectral index. Values within the disk are comprise between 0 and 2.