Osservatorio Astrofisico di Arcetri

VLT/SPHERE imaged the optical jet of a young star with the best spatial resolution ever

Over the last decade, high-resolution instruments have tried to shoot the planet formation process by imaging circumstellar disks around relatively evolved young stars, namely those stars that have already lost their natal envelope. However, it is possible that most of the planet formation actually occurs earlier in the evolution of newly born stars. At this stage, stars are often still accompanied by a large-scale nebulosity and drive a powerful jet emitting in the optical. The nebulosity is the element that makes these stars more elusive than their evolved counter-parts since they are hardly observed at optical/near-IR wavelengths.

In the recent work "The SPHERE view of the jet and the envelope of RY Tau" published in Astronomy & Astrophysics, a group of researchers led by Antonio Garufi and including Linda Podio, Francesca Bacciotti and Claudio Codella from INAF-Arcetri Astrophysical Observatory, employed the high-contrast imager and polarimeter SPHERE (Spectro-Polarimetric High-Contrast Exoplanet Research) at the Very Large Telescope to characterize one prototypical early-stage star in Taurus, at approximately 140 pc from us. The name of the source is RY Tau. The data collected by the authors include observations from 0.6 to 2.2 micron (visible to near-IR) with the highest spatial resolution ever obtained for an atomic jet-driving star (see Fig. 1). The authors could image the jet of RY Tau in several gaseous lines (hydrogen, helium, sulfur, iron) with an unprecedented level of detail for this type of objects. This allowed to infer that the increasing jet width with the distance from the star is complementary to the cavity visible in the natal envelope, suggesting that these two objects are in strong mutual interaction.

Fig. 1: Composite image of the surrounding of RY Tau. The optical/NIR images from this work of the remnant envelope (in blue) and of the jet (in green and in red) are combined to the ALMA image of the concealed disk by Long et al 2018 (in rainbow colors). (From Fig. 4 in Garufi et al 2019).


Furthermore, the authors could reveal a wiggling structure for the jet constrained in 5 degree every 30 years. This is a possible indirect evidence to the presence of a giant planet or a brown dwarf very close to the central RY Tau. This companion would be slightly misaligned with respect to the outer disk, inducing a precessing inner disk to launch the jet with variable directions over time. Finally, having observations at different epochs it was possible to estimate the jet velocity and trace back the observed sub-structures to the time they were launched from the star (see Fig. 2). Interestingly, these dates correspond to the moments where the stars have been more luminous over the last century. This indicates that processes internal to the disk, like an increased accretion rate, leave their observed imprint on the disk morphology opening a new potential path to the investigation of the planet formation processes.

Fig. 2: Jet morphology from 2015 and 2017 (above). Some sub-structures are recognizable further away from the star in the second epoch. From this apparent motion, it was possible to constrain their tangential velocity. Their launch date would correspond to the time of photospheric maxima experienced by the star, as shown by the panel below. (From Fig. 3 in Garufi et al 2019).

 Edited by A. Garufi and A. Gallazzi