Evidence for a massive dust-trapping vortex connected to spirals

Abstract

Context. Spiral arms, rings and large scale asymmetries are structures observed in high resolution observations of protoplanetary disks, and it appears that some of the disks showing spiral arms in scattered light also show asymmetries in millimeter-sized dust. HD 135344B is one such disk. Planets are invoked as the origin of these structures, but no planet has been observed so far and upper limits are becoming more stringent with time. Aims. We want to investigate the nature of the asymmetric structure in the HD 135344B disk in order to understand the origin of the spirals and of the asymmetry seen in this disk. Ultimately, we aim to understand whether or not one or more planets are needed to explain such structures. Methods. We present new ALMA sub-0.1′′ resolution observations at optically thin wavelengths (λ = 2.8 and 1.9 mm) of the HD 135344B disk. The high spatial resolution allows us to unambiguously characterize the mm-dust morphology of the disk. The low optical depth of continuum emission probes the bulk of the dust content of the vortex. Moreover, we have combined the new observations with archival data at shorter wavelengths to perform a multi-wavelength analysis and to obtain information about the dust distribution and properties inside the observed asymmetry. Results. We resolve the asymmetric disk into a symmetric ring + asymmetric crescent, and observe that (1) the spectral index strongly decreases at the centre of the vortex, consistent with the presence of large grains; (2) for the first time, an azimuthal shift of the peak of the vortex with wavelength is observed; (3) the azimuthal width of the vortex decreases at longer wavelengths, as expected for dust traps. These features allow confirming the nature of the asymmetry as a vortex. Finally, under the assumption of optically thin emission, a lower limit to the total mass of the vortex is 0.3MJupiter. Considering the uncertainties involved in this estimate, it is possible that the actual mass of the vortex is higher and possibly within the required values (~4 MJupiter) to launch spiral arms similar to those observed in scattered light. If this is the case, then explaining the morphology does not require an outer planet.

Constraining the masses of planets in protoplanetary discs from the presence or absence of vortices – comparison with ALMA observations

“…There are many observational examples of discs containing vortices that could potentially be the result of planet disc interactions (e.g. van der Marel et al 2013;Isella et al 2013;Fukagawa et al 2013;Pérez et al 2014;Marino et al 2015;Kraus et al 2017;Dong et al 2018;Cazzoletti et al 2018). These are also particularly interesting cases to us as, similarly to the HL Tau and TW Hydrae cases, allow us to make tentative predictions about the planets in these systems.…”

“…The disc surrounding the Herbig Ae/Be star HD135344B (Pérez et al 2014;Cazzoletti et al 2018) shows interesting phenomena. This disc contains a cavity with a radius of ≈ 50AU at the edge of which is a bright ring of material.…”

“…Additionally, recent investigation into the shape of planet induced vortices have accounted for the timescale over which the planet grows and have found that vortices can become elongated (Hammer et al 2017). As a result elongated vortices have their own specific observational features (Hammer et al 2019) that have potentially been observed (Pérez et al 2014;Cazzoletti et al 2018). However, the majority of observational results do not show the features that imply elongated vortices, although this may be due to the resolution of the observations In this paper we are investigating the required planet growth timescales for which no vortices are formed in a protoplanetary disc.…”

Planet gap opening across stellar masses

“…The most striking result of these observations is the ubiquity of annular structures, colloquially described as gaps and rings. While some discs do show alternative structures like spirals or crescents (e.g., van der Marel et al 2013;Casassus et al 2015;Pérez et al 2016;Boehler et al 2018;Cazzoletti et al 2018), most of the discs observed at high resolution are characterised by axisymmetric structures. The prevalence of axisymmetric structures was already clear from the publication of several high-resolution observations targeting individual sources (ALMA Partnership et al 2015;Isella et al 2016;Andrews et al 2016;van der Plas et al 2017;Fedele et al 2017;Loomis et al 2017;Fedele et al E-mail: cas213@cam.ac.uk 2018; Dipierro et al 2018;Clarke et al 2018) and from the survey in Taurus (Long et al 2018), but recently it was made even clearer by the publication of DSHARP (Andrews et al 2018b;Huang et al 2018), a homogeneous high resolution survey of 20 discs.…”

Catastrophic Events in Protoplanetary Disks and Their Observational Manifestations

“…One of the most interesting results obtained using ALMA and VLT is the detection of substructures on images of protoplanetary disks observed with high angular resolution (see, e.g. Avenhaus et al, 2018;Cazzoletti et al, 2018;van Boekel et al, 2017). Statistical analysis shows that the observed substructures often have ring-like form, less often they look like wrapped spiral arms and very rarely exhibit crescent-like azimuthal asymmetries.…”