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

Zapata, Luis A.
Schilke, Peter
Ho, Paul
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Sensitive and subarcsecond resolution ($\sim$ 0.7\arcsec) CH$_3$OH(7$_{-2,6}$ $\to$ 6$_{-2,5}$) line and 890 $\mu$m continuum observations made with the Submillimeter Array (SMA) towards the hot molecular circumbinary ring associated with the young multiple star Ori 139-409 are presented. The CH$_3$OH(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 LTE model of a Keplerian disk 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 V$_{inf}$ $\sim$ 1.5 km s$^{-1}$ toward the binary central compact circumstellar disks reported at a wavelength of 7 mm. {\bf The inner central cavity seems to be formed by the tidal effects of the young stars in the middle of the ring.} The ring appears to be not 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$_\odot$/yr. We found that the dust emission associated with Ori 139-409 appears to be arising from the circumstellar disks with no strong contribution from the molecular gas ring. 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 up to date. Finally, our results confirm that the circumbinary rings are actively funneling fresh gas material to the central compact binary circumstellar disks, i.e. to the protostars in the very early phases of their evolution.
Comment: Accepted by MNRAS
Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics