Understanding JWST water spectra: what can thermochemical models tell us about the (cold) water in protoplanetary disks?

TL;DR

This work tests the fidelity of LTE slab retrievals on JWST-style H2O spectra by comparing them to two sets of 2D DALI thermochemical disk models (full chemistry and parameterized abundances). It finds that single-temperature fits mainly capture the warm Component ($T\approx$ $500\,\mathrm{K}$), while a three-component MCMC fit more accurately traces the full temperature gradient; non-LTE effects typically cause modest underestimation of the true temperature. The strength of the cold H2O emission is directly linked to the H2O abundance above the snow surface at radii $>1$ au, requiring high outer-layer abundances (\gtrsim10^{-5}) that are not produced in the fiducial chemistry, suggesting that dust transport and vertical mixing, or accretion-driven heating, play a key role. These results underscore the need for dynamical treatments in disk models and motivate future far-IR observations to probe the cold H2O reservoir and snowline vicinity.

Abstract

(Abridged) Rotational H$_2$O spectra as observed with JWST/MIRI provide a good probe of the temperature and column density structure of the inner disk. H$_2$O emission can also be influenced by dynamical processes, such as dust grains drifting inwards and their icy mantles sublimating once they cross the snowlines, thus enriching the inner regions in H$_2$O vapor. Recent work has found that this process may leave an imprint in the H$_2$O spectrum in the form of excess flux in the cold H$_2$O lines. In this work, we aim to test the accuracy of several common retrieval techniques on full 2D thermochemical disk models. Moreover, we investigate the cold H$_2$O emission that has been proposed as a signature of drift, to gain further insights into the underlying radial and vertical distribution of H$_2$O. We present two sets of Dust And LInes (DALI) thermochemical models and run several retrieval techniques to investigate how the retrieved temperature and column density compare to our models. Single-temperature slab retrievals mainly trace the warm ($\sim$500 K) H$_2$O reservoir, whereas a three-component fit is able to better trace the full temperature gradient in the IR emitting region. Retrieved temperatures tend to underestimate the true temperature of the emitting layer due to non-LTE effects. The retrieved column density traces close to the mid-IR dust $τ=1$ surface. We find that the strength of the cold H$_2$O emission is directly linked to the H$_2$O abundance above the snow surface at large radii (>1 au). This implies that sources with excess cold H$_2$O flux likely have a high H$_2$O abundance in this region ($\gtrsim10^{-5}$), higher than predicted by the chemical network. This discrepancy is most likely caused by the absence of dust transport processes in our models, further strengthening the theory that this emission may be a signature of radial drift and vertical mixing.

Figures (20)

• Figure 1: Abundance maps of H2O (left & middle panels) and temperature map (right panel) for the fiducial models with $f_\ell=0.9$. The left panel depicts the model using the chemical network and the middle depicts the model with parameterized abundances (PA). In all panels, the dust $\tau=1$ surface at 15 $\mu$m and the $A_V = 1.5$ surface are indicated with a blue dashed line and a black dotted line, respectively. The red, pink, and blue contours represent the 70% emitting regions of the H2O 17$_{7,10}$ -- 16$_{4,13}$ ($E_{\rm up}$ = 6371 K), 11$_{3,9}$ -- 10$_{0,10}$ ($E_{\rm up}$ = 2438 K), and 8$_{3,6}$ -- 7$_{0,7}$ ($E_{\rm up}$ = 1447 K) lines, respectively, representing hot, warm, and cold H2O. The warm H2O emitting region for both models is also depicted in the right panel in black. The light blue line in the left and middle panels represents the H2O snow surface.
• Figure 2: Synthetic H2O (blue), ^12CO2 (green) and ^13CO2 (purple) spectra. The top row depicts the model using the chemical network and the bottom row depicts the model with parameterized abundances. In the right panels, blue triangles indicate H2O lines with $E_{\rm up} < 2500$ K.
• Figure 3: Temperature as a function of radius within the 70% emitting region of the H2O 17$_{7,10}$ -- 16$_{4,13}$ (6371 K), 11$_{3,9}$ -- 10$_{0,10}$ (2438 K), and 8$_{3,6}$ -- 7$_{0,7}$ (1447 K) lines (red, pink, and blue solid lines) for the model using the chemical network (left panel) and the model with parameterized abundances (right panel). The shaded regions represent the minimum and maximum temperature within the emitting region. The red, pink, and blue crosses represent the retrieved $T$ and $R_{\rm eq}$ from the single-temperature slab fits in the 10-14, 13.5-17.5, and 21-24 $\mu$m region. The white plus symbols represent the retrieved $T$ and $R_{\rm eq}$ values from the 3-temperature-component MCMC routine for H2O.
• Figure 4: Vertically integrated H2O column density as a function of radius. The solid lines show the total model column density and the dashed lines show the model column density integrated up to the dust $\tau=1$ surface at 15 $\mu$m. The red, pink, and blue crosses represent the retrieved $N$ and $R_{\rm eq}$ from the single-temperature slab fits in the 10-14, 13.5-17.5, and 21-24 $\mu$m region. The plus symbols in the left column represent the retrieved $N$ and $R_{\rm eq}$ values from the 3-temperature-component MCMC routine. The left panel depicts the model using the chemical network and the right panel depicts the model with parameterized abundances.
• Figure 5: Synthetic H2O spectra of the fiducial model using the chemical network (black) and three models with parameterized abundances (blue, pink, and red) between 23.7 and 24 $\mu$m. The flux is normalized to the 9$_{8,1}$ -- 8$_{7,2}$ line at 23.87 $\mu$m.