WARM HCN IN THE PLANET FORMATION ZONE OF GV TAU N

The Plateau de Bure Interferometer has been used to map the continuum emission at 3.4 mm and 1.1 mm together with the J = 1→0 and J = 3→2 lines of HCN and HCO+ toward the binary star GV Tau. The 3.4 mm observations did not resolve the binary components, and the HCN J = 1→0 and HCO+ J = 1→0 line emissions trace the circumbinary disk and the flattened envelope. However, the 1.1 mm observations resolved the individual disks of GV Tau N and GV Tau S and allowed us to study their chemistry. We detected the HCN 3→2 line only toward the individual disk of GV Tau N, and the emission of the HCO+ 3→2 line toward GV Tau S. Simple calculations indicate that the 3→2 line of HCN is formed in the inner R < 12 AU of the disk around GV Tau N where the HCN/HCO+ abundance ratio is >300. On the contrary, this ratio is <1.6 in the disk around GV Tau S. The high HCN abundance measured in GV Tau N is well explained by photochemical processes in the warm (>400 K) and dense (n > 107 cm−3) disk surface.