A contracting circumbinary molecular ring around Ori 139-409 with an inner cavity of about 140 au

Abstract

We present sensitive and subarcsecond resolution (∼0.7 arcsec) CH3OH(7−2,6–6−2,5) line and 890-μm continuum observations, made with the Submillimeter Array (SMA), towards the hot molecular circumbinary ring associated with the young multiple star Ori 139–409. The CH3OH(7−2,6–6−2,5) emission from the ring is well resolved at this angular resolution, revealing an inner cavity with a size of about 140 au. A local thermodynamic equilibrium model of a Keplerian disc with an inner cavity of the same size confirms the presence of this cavity. Additionally, this model suggests that the circumbinary ring is contracting with a velocity of Vinf∼ 1.5 km s−1 towards the binary central compact circumstellar discs reported at a wavelength of 7 mm. The inner central cavity seems to be formed by the tidal effects of the young stars in the middle of the ring. The ring does not appear to be a stationary object. Furthermore, the infall velocity we determine is about a factor of 3 slower than the free-fall velocity corresponding to the dynamical mass. This would correspond to a mass accretion rate of about 10−5 M⊙ yr−1. We have found that the dust emission associated with Ori 139–409 appears to be arising from the circumstellar discs, with no strong contribution from the molecular gas ring. Furthermore, a simple comparison with other classical molecular dusty rings (e.g. GG Tau, UZ Tau and UY Aur) suggests that Ori 139–409 could be one of the youngest circumbinary rings reported to date. Finally, our results confirm that the circumbinary rings are actively funnelling fresh gas material to the central compact binary circumstellar discs (i.e. to the protostars in the very early phases of their evolution).