The morphology of the HD 163296 jet as a window on its planetary system

Abstract

Context. HD 163296 is a Herbig Ae star which drives a bipolar knotty jet with a total length of ~6000 au. Strong evidence exists that the disk of HD 163296 harbors planets. Studies have shown that the presence of companions around jet-driving stars could affect the morphology of the jets. This includes a ‘wiggling’ of the jet axis and a periodicity in the positions of the jet knots. Aims. In this study we investigate the morphology (including the jet width and axis position) and proper motions of the HD 163296 jets, and use our results to better understand the whole system. Methods. This study is based on optical integral-field spectroscopy observations obtained with VLT/MUSE in 2017. Using spectro-images and position velocity diagrams extracted from the MUSE data cube, we investigated the number and positions of the jet knots. A comparison was made to X-shooter data collected in 2012 and the knot proper motions were estimated. The jet width and jet axis position with distance from the star were studied from the extracted spectro-images. This was done using Gaussian fitting to spatial profiles of the jet emission extracted perpendicular to the position angle of the jet. The centroid of the fit is taken as the position of the jet axis. Results. We observe the merging of knots and identify two previously undetected knots. We find proper motions that are broadly in agreement with previous studies. The jet width increases with distance from the source and we measure an opening angle of ~5° and 2.5° for the red and blue lobes, respectively. Measurements of the jet axis position, derived from Gaussian centroids of transverse intensity profiles, reveal a similar pattern of deviation in all forbidden emission lines along the first 20 arcsec of the jets. This result is interpreted as being due to asymmetric shocks and not due to a wiggling of the jet axis. Conclusions. The number of new knots detected and their positions challenge the 16-yr knot ejection periodicity proposed in prior studies, arguing for a more complicated jet system than was previously assumed. We use the non-detection of a jet axis wiggling to rule out companions with a mass >0.1 M⊙ and orbits between 1 au and 35 au. Any object inferred at these distances using other methods must be a brown dwarf or planet, otherwise it would have impacted the jet axis position. Both the precession and orbital motion scenarios are considered. Overall it is concluded that it is difficult to detect planets with orbits >1 au through a study of the jet axis.