Molecules in the disk orbiting the twin young suns of V4046 Sagittarii

Context. Direct information concerning the physical conditions and chemistry within the Jovian planet-building zones of circumbinary disks surrounding pre-main sequence stars is essentially nonexistent, especially for the more evolved pre-MS systems in which planets may already be forming or may have formed. Aims. We searched for a gaseous component within the dusty circumbinary disk around the nearby (D similar to 70 pc), 12 Myr-old system V4046 Sgr - a tight (9 R-circle dot separation), short-period (P similar to 2.42 day) binary with nearly equal component masses of similar to 0.9 M-circle dot - so as to assess the mass, chemistry, and kinematics of this gaseous disk. Methods. We conducted a mm-wave molecular line survey of V4046 Sgr with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We use these data to investigate the kinematics, gas mass, and chemical constituents of the V4046 Sgr disk. Results. We detected rotational transitions of (CO)-C-12 (CO)-C-13, HCN, CN, and HCO+. The double-peaked CO line profiles of V4046 Sgr are well fit by a model invoking a Keplerian disk with outer radius of similar to 250 AU that is viewed at an inclination i = 35 degrees. We infer minimum disk gas and dust masses of similar to 13 and similar to 20 Earth masses from the V4046 Sgr CO line and submm continuum fluxes, respectively. The actual disk gas mass could be much larger if the gas-phase CO is highly depleted and/or (CO)-C-13 is very optically thick. Conclusions. The overall similarity of the circumbinary disk of V4046 Sgr to the disk orbiting the single, similar to 8 Myr-old star TW Hya - a star/disk system often regarded as representative of the early solar nebula - indicates that gas giant planets are likely commonplace among close binary star systems. Given the relatively advanced age and proximity of V4046 Sgr, these results provide strong motivation for future high-resolution imaging designed to ascertain whether a planetary system now orbits its twin suns.