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+ towards 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 towards the
individual disk of GV Tau N, and the emission of the HCO+ 3->2 line towards 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 photo-chemical
processes in the warm (>400K) and dense disk surface.
