Molecules in the shadows of the protoplanetary disk HD 143006. Evidence of C/O,≳,1 from simultaneously modeling fourteen ALMA lines

The inner disk of the T,Tauri star HD,143006 is inclined and casts a broad shadow over a large portion of the outer disk. We use this extraordinary geometry to study the impact of stellar ultraviolet (UV) irradiation on disk chemistry, temperature, and line formation. We present new ALMA observations of this disk, covering CO, 13 CO, C^18O, CN, HCN, HCO^+$, CS, and C_3H_2. Combined with previously published C_2H, N_2H^+ and H_2CO observations, these data constitute an extensive dataset of fourteen ALMA lines from nine different molecules. We subdivide these data into two disk halves: the illuminated part and the shadowed part. We develop ProDiMo models for these two disk halves by optionally including an inclined inner disk that blocks the stellar illumination of the upper layers of the outer disk without hindering the stellar illumination of the outer disk along the midplane. The observed line flux contrasts between the illuminated and the shadowed disk sides are not very pronounced. We explain this phenomenon by introducing ``equilibrated models'' that consider a slow heating and cooling relaxation of gas parcels orbiting in and out of the shadow. Our results fit the radial line intensity profiles of all ALMA lines, both in flux and radial shape, with an overall reduced ̧hi of about 2.3, both from the illuminated and the shadowed disk sides, along with two ALMA continuum images, where we see evidence of radial drift, and the spectral energy distribution (SED) beyond 20,μm, all with one model. In most cases, the line intensities only increase slightly on the illuminated side because the gas warms up slowly, although fast UV photochemistry significantly reduces the observable molecular concentrations. The dust temperatures increase even more rapidly, but this has little effect on the line formation. However, for the lines of N_2H^+ and CS, which probe deeper layers where the gas and dust temperatures are coupled, slow UV desorption of ices such as NH_3 and H_2S plays a crucial role. This can result in a brightening of these lines on the shadowed side. Our analysis suggests with high confidence that the carbon-to-oxygen ratio in the outer disk of HD,143006 is slightly greater than one (∼,1.05). The disk mass is fit to be as low as 0.0012,M_⊙. However, almost equally well-fitting models with a fixed disk mass of $0.021 M_⊙ can be obtained after readjusting the disk shape, dust, and gas heating and cooling parameters.