Enhanced dust emission in the HL Tau disc: a low-mass companion in formation?

We have imaged the disc of the young star HL Tau using the Very Large Array (VLA) at 1.3 cm, with 0.08-arcsec resolution (as small as the orbit of Jupiter). The disc is around half the stellar mass, assuming a canonical gas mass conversion from the measured mass in large dust grains. A simulation shows that such discs are gravitationally unstable, and can fragment at radii of a few tens of au to form planets. The VLA image shows a compact feature in the disc at 65 au radius (confirming the `nebulosity' of Welch et al.), which is interpreted as a localized surface density enhancement representing a candidate protoplanet in its earliest accretion phase. If correct, this is the first image of a low-mass companion object seen together with the parent disc material out of which it is forming. The object has an inferred gas plus dust mass of ~14M_Jupiter, similar to the mass of a protoplanet formed in the simulation. The disc instability may have been enhanced by a stellar flyby: the proper motion of the nearby star XZ Tau shows it could have recently passed the HL Tau disc as close as ~600 au.