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 0.3 M _(Jupiter). 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 M _(Jupiter)) 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.
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机译：上下文。螺旋臂，环和大规模不对称是在原行星盘的高分辨率观测中观察到的结构，并且似乎某些在散射光中显示出螺旋臂的盘在毫米大小的尘埃中也显示出不对称性。 HD 135344B就是这样一种磁盘。行星被认为是这些结构的起源，但是到目前为止还没有观测到行星，并且上限随着时间的推移变得越来越严格。目的我们想研究HD 135344B磁盘中不对称结构的性质，以便了解螺旋的起源和在此磁盘中看到的不对称性。最终，我们旨在了解是否需要一个或多个行星来解释这种结构。方法。我们在HD 135344B磁盘的光学薄波长（λ= 2.8和1.9 mm）上提出了新的ALMA sub-0.1''分辨率观测结果。高空间分辨率使我们能够清楚地表征磁盘的mm灰尘形态。连续发射的低光学深度探测了旋涡中大部分的灰尘。此外，我们将新的观测值与较短波长的档案数据相结合，以进行多波长分析，并获得有关观测到的不对称内部的尘埃分布和特性的信息。结果。我们将不对称圆盘解析为对称环+不对称新月形，并观察到（1）光谱指数在涡旋中心大大降低，这与大晶粒的存在一致; （2）首次观察到旋涡峰随波长的方位角偏移; （3）如尘埃捕集器所预期的，涡旋的方位角宽度在更长的波长处减小。这些特征允许确认不对称性为涡旋的性质。最后，在光学上薄发射的假设下，涡旋总质量的下限为0.3 M _（Jupiter）。考虑到此估计中涉及的不确定性，涡旋的实际质量可能更高，并且可能在所需值（〜4 M _（Jupiter））之内，以发射类似于在散射光中观察到的螺旋臂。如果是这种情况，那么解释形态就不需要外行星。
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