First detections of H13CO+ and HC15N in the disk around HD 97048:
  Evidence for a cold gas reservoir in the outer disk
  Observations of different molecular lines in protoplanetary disks provide
valuable information on the gas kinematics, as well as constraints on the
radial density and temperature structure of the gas. With ALMA we have detected
H13CO+ (J=4-3) and HC15N (J=4-3) in the HD97048 protoplanetary disk for the
first time. We compare these new detections to the ringed continuum mm-dust
emission and the spatially resolved CO (J=3-2) and HCO+ (J=4-3) emission. The
radial distributions of the H13CO+ and HC15N emission show hints of ringed
sub-structure whereas, the optically thick tracers, CO and HCO+, do not. We
calculate the HCO+/H13CO+ intensity ratio across the disk and find that it is
radially constant (within our uncertainties). We use a physio-chemical
parametric disk structure of the HD97048 disk with an analytical prescription
for the HCO+ abundance distribution to generate synthetic observations of the
HCO+ and H13CO+ disk emission assuming LTE. The best by-eye fit models require
radial variations in the HCO+/H13CO+ abundance ratio and an overall enhancement
in H13CO+ relative to HCO+. This highlights the need to consider isotope
selective chemistry and in particular low temperature carbon isotope exchange
reactions. This also points to the presence of a reservoir of cold molecular
gas in the outer disk (T < 10K, R > 200au). Chemical models are required to
confirm that isotope-selective chemistry alone can explain the observations
presented here. With these data, we cannot rule out that the known dust
substructure in the HD97048 disk is responsible for the observed trends in
molecular line emission, and higher spatial resolution observations are
required to fully explore the potential of optically thin tracers to probe
planet-carved dust gaps. We also report non-detections of H13CO+ and HC15N in
the HD100546 protoplanetary disk.
