Multi-epoch scattered-light analysis of HD 135344B: New evidence for a spiral-driving protoplanet

Context . The HD 135344B (SAO 206462) disk exhibits strong signposts of planet formation both in scattered light and submillimeter continuum images. ALMA images in the submillimeter revealed a gap-crossing dust filament whose position coincides with a twist detected in the scattered-light spiral structure. Analysis of the spiral dynamics in polarized light also hints at a spiral-driving protoplanet in the submillimeter gap. Aims . We aim to study the overall dynamics of the three spirals in the disk, as well as the motion of the twist over a 10-year baseline, at different IR wavelengths. We also seek to assess the authenticity of a candidate protoplanet recently claimed in the disk. Methods . We used high-fidelity postprocessing algorithms such as iterative principal component analysis to minimize the biases induced by angular differential imaging on extended sources and conduct a thorough analysis of archival VLT/NACO, VLT/SPHERE, and VLT/ERIS datasets in order to obtain the spiral traces and measure their orbital motion in multiple wavelength bands in scattered light. We also reprocessed archival JWST/NIRCam datasets with these algorithms. Results . We measured an average spiral orbital motion of 0°.81 ± 0°.05 yr −1 , in agreement with the literature value of about 0°.85 yr −1 at all wavelengths. With simple modeling of the twist morphology, we confirm that it is indeed co-moving with the spiral in which it is embedded. While the position angle of the twist coincides with the dust filament, it is located at a smaller angular separation from the star, which we attribute to the fact that the spiral trace moves away from the central star with increasing wavelength. We find that a previously claimed protoplanet candidate in the disk can be adequately explained as a postprocessing artifact. Conclusions . Our confirmation that the motion of the scattered light twist is consistent with the orbital velocity of a planet at 69 ± 4 au over a 10-year baseline suggests that the spirals, the gap, and the dust filament in the submillimeter continuum, as well as the twist in scattered light, could indeed all be attributed to the same hypothetical protoplanet deeply embedded within the spiral. A perplexing trend for a wavelength-dependence of the angular distance of the spiral traces to the central star still remains to be explained.