Some circumstellar disks are observed to show prominent spiral arms in infrared scattered light or (sub-)millimeter dust continuum. The spirals might be formed from self-gravity, shadows, or planet-disk interactions. Recently, it was hypothesized that massive vortices can drive spiral arms in protoplanetary disks in a way analogous to planets. In this paper, we study the basic properties of vortex-driven spirals by the Rossby Wave Instability in 2D hydrodynamics simulations. We study how the surface density contrast, the number, and the shape of vortex-driven spirals depend on the properties of the vortex. We also compare vortex-driven spirals with those induced by planets. The surface density contrast of vortex-driven spirals in our simulations are comparable to those driven by a sub-thermal mass planet, typically a few to a few tens of Earth masses. In addition, different from the latter, the former is not sensitive to the mass of the vortex. Vortex-driven spiral arms are not expected to be detectable in current scattered light observations, and the prominent spirals observed in scattered light in a few protoplanetary disks, such as SAO 206462 (HD 135344B), MWC 758, and LkHα 330, are unlikely to be induced by the candidate vortices in them.

Several nearby protoplanetary disks have been observed to display large scale crescents in the (sub)millimeter dust continuum emission. One interpretation is that these structures correspond to anticyclonic vortices generated by the Rossby wave instability within the gaseous disk. Such vortices have local gas over-densities and are expected to concentrate dust particles with Stokes number around unity. This process might catalyze the formation of planetesimals. Whereas recent observations showed that dust crescent are indeed regions where millimeter-size particles have abnormally high concentration relative to the gas and smaller grains, no observations have yet shown that the gas within the crescent region counter-rotates with respect to the protoplanetary disk. Here we investigate the detectability of anticyclonic features through measurement of the line-of-sight component of the gas velocity obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative transfer calculation of a protoplanetary disk characterized by a vortex created by the tidal interaction with a massive planet. As a case study, the disk parameters are chosen to mimic the IRS 48 system, which has the most prominent crescent observed to date. We generate synthetic ALMA observations of both the dust continuum and 12 CO emission around the frequency of 345 GHz. We find that the anticyclonic features of vortex are weak but can be detected if both the source and the observational setup are properly chosen. We provide a recipe for maximizing the probability to detect such vortex features and present an analysis procedure to infer their kinematic properties.

We search a large parameter space of the LkCa 15's disk density profile to fit its observed radial intensity profile of 12 CO (J = 3-2) obtained from ALMA. The best-fit model within the parameter space has a disk mass of 0.1 M (using an abundance ratio of 12 CO/H 2 = 1.4 ×10 −4 in mass), an inner cavity of 45 AU in radius, an outer edge at ∼ 600 AU, and a disk surface density profile follows a power-law of the form ρ r ∝ r −4 . For the disk density profiles that can lead to a small reduced χ 2 of goodness-of-fit, we find that there is a clear linear correlation between the disk mass and the power-law index γ in the equation of disk density profile. This suggests that the 12 CO disk of LkCa 15 is optically thick and we can fit its 12 CO radial intensity profile using either a lower disk mass with a smaller γ or a higher disk mass with a bigger γ. By comparing the 12 CO channel maps of the best-fit model with disk models with higher or lower masses, we find that a disk mass of ∼ 0.1 M can best reproduce the observed morphology of the 12 CO channel maps. The dust continuum map at 0.87 mm of the LkCa 15 disk shows an inner cavity of the similar size of the best-fit gas model, but its out edge is at ∼ 200 AU, much smaller than the fitted gas disk. Such a discrepancy between the outer edges of the gas and dust disks is consistent with dust drifting and trapping models.