MINDS. The DR Tau disk II: probing the hot and cold H₂O reservoirs in the JWST-MIRI spectrum

The MRS mode of the JWST-MIRI instrument gives insights into the chemical richness and complexity of the inner regions of planet-forming disks. Here, we analyse the H₂O-rich spectrum of the compact disk DR Tau. We probe the excitation conditions of the H₂O transitions observed in different wavelength regions across the entire spectrum using LTE slab models, probing both the rovibrational and rotational transitions. These regions suggest a radial temperature gradient, as the excitation temperature (emitting radius) decreases (increases) with increasing wavelength. To explain the derived emitting radii, we require a larger inclination for the inner disk (i~20-23 degrees) compared to the outer disk (i~5 degrees), agreeing with our previous analysis on CO. We also analyse the pure rotational spectrum (<10 micron) using a large, structured disk (CI Tau) as a template, confirming the presence of the radial gradient, and by fitting multiple components to further characterise the radial and vertical temperature gradients present in the spectrum. At least three temperature components (T~180-800 K) are required to reproduce the rotational spectrum of H₂O arising from the inner ~0.3-8 au. These components describe a radial temperature gradient that scales roughly as ~R-0.5 in the emitting layers. As the H₂O is mainly optically thick, we derive a lower limit on the abundance ratio of H₂O/CO~0.17, suggesting a potential depletion of H₂O. Similarly to previous work, we detect a cold H₂O component (T~180 K) originating from near the snowline. We cannot conclude if an enhancement of the H₂O reservoir is observed following radial drift. A consistent analysis of a larger sample of compact disks is necessary to study the importance of drift in enhancing the H₂O abundances.