RINGED SUBSTRUCTURE AND A GAP AT 1 au IN THE NEAREST PROTOPLANETARY DISK

Abstract

We present long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870 µm continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace millimeter-sized particles down to spatial scales as small as 1 AU (20 mas). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1-6 AU) with modest contrasts (5-30%). We associate these features with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 AU from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ∼AU scales can be common, fundamental factors in disk evolution, and that high resolution microwave imaging can help characterize them during the epoch of planet formation.

Disk Masses for Embedded Class I Protostars in the Taurus Molecular Cloud

“…As our models assume p=3.5, an increase in the maximum dust grain size by a factor of ∼75, say, from 1 mm to 10 cm, could explain this drop in flux. This is borne out by a number of studies that have found cavities, gaps, spiral arms, and other asymmetries in Class II disks that may indicate the presence of planets (Isella et al 2010Andrews et al 2011Andrews et al , 2016Casassus et al 2013; Weidenschilling 1977). We find that our Class I disks, on average, are more massive than the Taurus Class II disks, likely due to dust grain processing hiding matter in larger bodies in the older Class II disks.…”

A photoevaporative gap in the closest planet-forming disc

“…see their Figure 11). Hence, the dark annulus detected by Andrews et al (2016) may indeed be the first stage of the dispersal phase of the TW Hya disc. Direct evidence of currently ongoing photoevaporation is provided by the detection of a few km/s blueshift in the profile of the [NeII] 12.8 µm line (Herczeg et al 2007;Pascucci & Sterzik 2009), which is well fit by both EUV and X-ray photoevapoaration models (Alexander 2008.…”

“…The ALMA observations together with the dip in the SED at mid-infrared wavelengths (Calvet et al 2002) suggest that the gap must be depleted of grains from µm to cm size. On that basis, Andrews et al (2016) argue against the dip being a consequence of changing solid properties at around the H2O snow-line. As an alternative, they suggest that a more detailed modeling of the ALMA data may show whether a young super-Earth may be responsible for dynamically carving the observed gap (e.g., Picogna & Kley 2015, Rosotti et al 2016.…”

“…Andrews et al (2016) presented long-baseline ALMA observations of the 870 µm continuum from TW Hya, which are sensitive to millimeter-sized dust grains down to a spatial scale of 1 au. The data show a series of concentric dark and bright shallow ring-like structures in the outer disc, while the inner disk is dominated by a dark annulus centred at 1 au, separating a bright ring that peaks at 2.4 au and an unresolved continuum source less than 0.5 au in radius and emitting ∼ 1 mJy in flux.…”

“…However, the direct observation of a disc undergoing gap formation via photoevaporation is still lacking. In this Letter we propose that the gap at 1 au in the nearest protoplanetary disc, TW Hya, observed in the 870 µm continuum by Andrews et al (2016), using the long-baseline Atacama Large Millimeter/submillimeter array (ALMA), may indeed be the first directly imaged photoevaporative gap in a solar-type star to date. Surprisingly, this possibility has not yet been properly explored in the recent literature.…”