A deep search for H2D+ in protoplanetary disks
  The structure in density and temperature of protoplanetary disks surrounding
low-mass stars is not yet well known. The protoplanetary disks mid-planes are
expected to be very cold and thus depleted in molecules in gas phase,
especially CO. Recent observations of molecules at very low apparent
temperature (~ 6 K) challenge this current picture of the protoplanetary disk
structures. We aim at constraining the physical conditions, and in particular
the gas-phase CO abundance in the mid-plane of protoplanetary disks. The light
molecule H2D+,is a tracer of cold and CO-depleted environment. It is therefore
a good candidate to explore the disks mid-planes. We performed a deep search
for H2D+ in the two well-known disks surrounding TW Hya and DM Tau using the
APEX and JCMT telescopes. The analysis of the observations are done with
DISKFIT, a radiative transfer code dedicated to disks. In addition, we used a
chemical model describing deuterium chemistry to infer the implications of our
observations on the level of CO depletion and on the ionization rate in the
disks mid-plane. The ortho-H2D+(1(1,0))-1(1,1)) line at 372 GHz was not
detected. Although our limit is three times better than previous observations,
comparison with the chemical modeling indicates that it remains insufficient to
put valuable constraints on the CO abundance in the disk mid-plane. Even with
ALMA, the detection of H2D+ may not be straightforward, and H2D+ may not be a
sufficiently sensitive tracer of the protoplanetary disks mid-plane
