Chemistry of a newly detected circumbinary disk in Ophiuchus

Abstract

Context. Astronomers recently started discovering exoplanets around binary systems. Therefore, understanding the formation and evolution of circumbinary disks and their environment is crucial for a complete scenario of planet formation. Aims. The purpose of this paper is to present the detection of a circumbinary disk around the system Oph-IRS67 and analyse its chemical and physical structure. Methods. We present high-angular-resolution (0.′′4, ~60 AU) observations of C17O, H13CO+, C34S, SO2, C2H and c−C3H2 molecular transitions with the Atacama Large Millimeter/submillimeter Array (ALMA) at wavelengths of 0.8 mm. The spectrally and spatially resolved maps reveal the kinematics of the circumbinary disk as well as its chemistry. Molecular abundances are estimated using the non-local thermodynamic equilibrium (LTE) radiative-transfer tool RADEX. Results. The continuum emission agrees with the position of Oph-IRS67 A and B, and reveals the presence of a circumbinary disk around the two sources. The circumbinary disk has a diameter of ~620 AU and is well traced by C17O and H13CO+ emission. Two further molecular species, C2H and c−C3H2, trace a higher-density region which is spatially offset from the sources (~430 AU). Finally, SO2 shows compact and broad emission around only one of the sources, Oph-IRS67 B. The molecular transitions which trace the circumbinary disk are consistent with a Keplerian profile on smaller disk scales (≲200 AU) and an infalling profile for larger envelope scales (≳200 AU). The Keplerian fit leads to an enclosed mass of 2.2 M⊙. Inferred CO abundances with respect to H2 are comparable to the canonical ISM value of 2.7 × 10−4, reflecting that freeze-out of CO in the disk midplane is not significant. Conclusions. Molecular emission and kinematic studies prove the existence and first detection of the circumbinary disk associated with the system Oph-IRS67. The high-density region shows a different chemistry than the disk, being enriched in carbon chain molecules. The lack of methanol emission agrees with the scenario where the extended disk dominates the mass budget in the innermost regions of the protostellar envelope, generating a flat density profile where less material is exposed to high temperatures, and thus, complex organic molecules would be associated with lower column densities. Finally, Oph-IRS67 is a promising candidate for proper motion studies and the detection of both circumstellar disks with higher-angular-resolution observations.