Mid-infrared spectra of T Tauri disks: Modeling the effects of a small inner cavity on CO2 and H2O emission

Abstract

[Abridged] The inner few AU of disks around young stars are best probed in the infrared. The James Webb Space Telescope (JWST) is now starting to characterize the chemistry of these regions in unprecedented detail. One peculiar subset of sources are the so-called ``CO2-only sources'', in which only a strong 15 $\mu$m CO2 feature was detected in the spectrum. One scenario that could explain the weak emission from H2O is the presence of a small, inner cavity in the disk. If this cavity were to extend past the H2O snowline, but not past the CO2 snowline, this could strongly suppress the H2O line flux w.r.t. that of CO2. In this work, we aim to test the validity of this statement. Using the thermo-chemical code Dust And LInes (DALI), we created a grid of T Tauri disk models with an inner cavity, meaning we fully depleted the inner region of the disk in gas and dust starting from the dust sublimation radius and ranging until a certain cavity radius. We present the evolution of the CO2 and H2O spectra of a disk with inner cavity size, showing that, when a large-enough cavity is introduced, a spectrum that was initially dominated by H2O lines can become CO2-dominated instead. However, the cavity size needed for this is around 4-5 AU, exceeding the nominal position of the CO2 snowline in a full disk. The cause of this is most likely the alteration of the thermal structure by the cavity, which pushes the snowlines outward. Alternative explanations for bright CO2 emission are also briefly discussed. Our modeling work shows that it is possible for the presence of a small inner cavity to explain strong CO2 emission in a spectrum. However, the cavity needed to do so is larger than what was initially expected. As such, this scenario will be easier to test with sufficiently high angular resolution (millimeter) observations.