High Angular Resolution Evidence of Dust Traps in LkCa15
The manuscript by Sierra et al. presents an in-depth analysis of dust trapping phenomena in the protoplanetary disc around LkCa15, using high angular resolution observations obtained with ALMA at various frequencies. The paper leverages novel Band 3 (3 mm wavelength) data, complementing previous high-resolution observations at 0.87 mm and 1.3 mm, to investigate the distribution of dust within this transitional disc — a prime candidate for examining dust trap mechanisms due to its well-resolved ring and gap structures.
The researchers report that the widths of the observed dust rings diminish with increasing frequency, aligning with expectations from dust trapping models, which postulate that pressure maxima can concentrate dust particles and thus promote planetesimal formation. This is particularly evident in the rings located at approximately 42, 69, and 101 au from the star, with these structures exhibiting decreased full-width-half-maximum (FWHM) as the observing wavelength increases. Such differential concentration of dust with wavelength is a haLLMark of dust traps, which inhibit the rapid inward drift of grains, a problem prevalent in planet formation theories.
The paper also explores the spectral properties of the disc, calculating spectral indices which are found to reach local minima at ring locations—indicative of the presence of large dust grains. By integrating multi-wavelength datasets into dust evolution models, the authors constrain the disc's dust surface density and maximum grain size, inferring a total dust mass between 13 and 250 Earth masses, contingent on the assumed opacity model. The models incorporate a viscosity parameter (α = 10-3) and a fragmentation velocity (10 m/s), suggesting that these conditions, alongside DSHARP opacities, best reproduce the observed characteristics of dust distribution in LkCa15.
An intriguing aspect of the paper is the detection of unresolved 3 mm emission in the innermost disc regions, where the spectral index suggests a significant contribution from free-free emission, potentially linked to ionized gas in a jet or disc wind scenario. This is consistent with findings in other transitional discs, where non-thermal emission can dominate in inner disc regions, particularly at long wavelengths.
The absence of significant azimuthal residuals, which would indicate non-axisymmetric structures, is noted at the new band, although detection limits might obscure smaller features. This raises questions about the broader applicability of observing longer wavelengths for detecting such features due to the expected lower optical depth and corresponding emission levels.
The implications of these findings are profound for theories of planet formation. The effective trapping of dust at specific disc radii provides natural laboratories for the paper of planetesimal and protoplanetary core assembly, offering a mechanism to overcome rapid inward drift — a significant hurdle for forming terrestrial planets and the cores of giant planets. Future observations and models will undoubtedly refine these parameters, offering even greater insight into the initial stages of planet formation.
This research highlights several critical advances in the paper of protoplanetary discs, emphasizing the utility of multi-wavelength high-resolution studies in unraveling the complexities of dust evolution and distribution. It paves the way for further investigations that might include additional molecular line data or polarimetric imaging to better understand the interactions between gas dynamics and dust growth in these fascinating systems.