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Observations of different molecular lines in protoplanetary disks providevaluable information on the gas kinematics, as well as constraints on theradial density and temperature structure of the gas. With ALMA we have detectedH13CO+ (J=4-3) and HC15N (J=4-3) in the HD97048 protoplanetary disk for thefirst time. We compare these new detections to the ringed continuum mm-dustemission and the spatially resolved CO (J=3-2) and HCO+ (J=4-3) emission. Theradial distributions of the H13CO+ and HC15N emission show hints of ringedsub-structure whereas, the optically thick tracers, CO and HCO+, do not. Wecalculate the HCO+/H13CO+ intensity ratio across the disk and find that it isradially constant (within our uncertainties). We use a physio-chemicalparametric disk structure of the HD97048 disk with an analytical prescriptionfor the HCO+ abundance distribution to generate synthetic observations of theHCO+ and H13CO+ disk emission assuming LTE. The best by-eye fit models requireradial variations in the HCO+/H13CO+ abundance ratio and an overall enhancementin H13CO+ relative to HCO+. This highlights the need to consider isotopeselective chemistry and in particular low temperature carbon isotope exchangereactions. This also points to the presence of a reservoir of cold moleculargas in the outer disk (T < 10K, R > 200au). Chemical models are required toconfirm that isotope-selective chemistry alone can explain the observationspresented here. With these data, we cannot rule out that the known dustsubstructure in the HD97048 disk is responsible for the observed trends inmolecular line emission, and higher spatial resolution observations arerequired to fully explore the potential of optically thin tracers to probeplanet-carved dust gaps. We also report non-detections of H13CO+ and HC15N inthe HD100546 protoplanetary disk.

First detections of H13CO+ and HC15N in the disk around HD 97048: Evidence for a cold gas reservoir in the outer disk