The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous -2theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of (1-3)×10 −3 M , depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.
Understanding planet formation requires to discern how dust grows in protoplanetary disks. An important parameter to measure in disks is the maximum dust grain size present. This is usually estimated through measurements of the dust opacity at different millimeter wavelengths assuming optically thin emission and dust opacity dominated by absorption. However, ALMA observations have shown that these assumptions might not be correct in the case of protoplanetary disks, leading to overestimation of particle sizes and to underestimation of the disk's mass. Here, we present an analysis of high quality ALMA and VLA images of the HL Tau protoplanetary disk, covering a wide range of wavelengths, from 0.8 mm to 1 cm, and with a physical resolution of ∼7.35 au.We describe a procedure to analyze a set of millimeter images without any assumption about the optical depth of the emission, and including the effects of absorption and scattering in the dust opacity. This procedure allows us to obtain the dust temperature, the dust surface density and the maximum particle size at each radius. In the HL Tau disk, we found that particles have already grown up to a few millimeters in size. We detect differences in the dust properties between dark and bright rings, with dark rings containing low dust density and small dust particles. Different features in the HL Tau disk seem to have different origins. Planet-disk interactions can explain substructure at the external half of the disk, but the internal rings seem to be associated to the presence of snow lines of several molecules.
We present Very Large Array observations at 7 mm that trace the thermal emission of large dust grains in the HD 169142 protoplanetary disk. Our images show a ring of enhanced emission of radius ∼25-30 AU, whose inner region is devoid of detectable 7 mm emission. We interpret this ring as tracing the rim of an inner cavity or gap, possibly created by a planet or a substellar companion. The ring appears asymmetric, with the western part significantly brighter than the eastern one. This azimuthal asymmetry is reminiscent of the lopsided structures that are expected to be produced as a consequence of trapping of large dust grains. Our observations also reveal an outer annular gap at radii from ∼40 to ∼70 AU. Unlike other sources, the radii of the inner cavity, the ring, and the outer gap observed in the 7 mm images, which trace preferentially the distribution of large (millimeter/centimeter sized) dust grains, coincide with those obtained from a previous near-infrared polarimetric image, which traces scattered light from small (micron-sized) dust grains. We model the broadband spectral energy distribution and the 7 mm images to constrain the disk physical structure. From this modeling we infer the presence of a small (radius ∼0.6 AU) residual disk inside the central cavity, indicating that the HD 169142 disk is a pre-transitional disk. The distribution of dust in three annuli with gaps in between them suggests that the disk in HD 169142 is being disrupted by at least two planets or substellar objects.
Context. A key piece of information to understand the origin and role of protoplanetary disk substructures is their dust content. In particular, disk substructures associated with gas pressure bumps can work as dust traps, accumulating grains and reaching the necessary conditions to trigger the streaming instability. Aims. In order to shed some light on the origin and role that disk substructures play in planet formation, we aim to characterize the dust content of substructures in the disk of TW Hya. Methods. We present Atacama Large Millimeter Array (ALMA) observations of TW Hya at 3.1 mm with ~50 milliarcsecond resolution. These new data were combined with archival high angular resolution ALMA observations at 0.87, 1.3, and 2.1 mm. We analyze these multiwavelength data to infer a disk radial profile of the dust surface density, maximum particle size, and slope of the particle size distribution. Results. Most previously known annular substructures in the disk of TW Hya are resolved at the four wavelengths. Inside the inner 3 au cavity, the 2.1 and 3.1 mm images show a compact source of free–free emission, likely associated with an ionized jet. Our multiwavelength analysis of the dust emission shows that the maximum particle size in the disk of TW Hya is >1 mm. The inner 20 au are completely optically thick at all four bands, which results in the data tracing different disk heights at different wavelengths. Coupled with the effects of dust settling, this prevents the derivation of accurate density and grain size estimates in these regions. At r > 20 au, we find evidence of the accumulation of large dust particles at the position of the bright rings, indicating that these are working as dust traps. The total dust mass in the disk is between 250 and 330 M⊕, which represents a gas-to-dust mass ratio between 50 and 70. Our mass measurement is a factor of 4.5–5.9 higher than the mass that one would estimate using the typical assumptions of large demographic surveys. Conclusions. Our results indicate that the ring substructures in TW Hya are ideal locations to trigger the streaming instability and form new generations of planetesimals.
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