Ice chemistry that can be unveiled with the JWST: SynthIceSpec, a synthetic spectrum generator to test spectral limits. Solid CO_2 as a dust thermometer and solid CH_3CN detectability in cold cores

Abstract

As the (JWST) pursues its observing journey, several thousands of icy-grain spectra are expected to be measured and analysed. The inventory of ices in particular, via the observations of background sources, is accessible for hundreds of lines of sight (LOSs) per molecular-cloud region, opening the possibility to add strong constraints on the solid phase chemistry in a vast domain of cloud densities. SynthIceSpec is a synthetic infrared (IR) spectrum generator that has been designed as a tool to support observing proposals and to test the outcome of chemical models. It is based on laboratory measurements of pure and mixed ices, where each vibrational component is fitted by a sum of Gaussian profiles. Given an initial ice chemical composition (either set by the user or the outputs of a chemical model), a full JWST spectrum is generated, to which the contribution of silicates; continuum, stellar photospheric absorption bands; and extinction law can be added. For the continuum, stellar photospheric models for a wide range of spectral types can be selected by the program, or, Spectral Energy Distribution (SEDs). We present a few use cases of SynthIceSpec: we probed the impact of dust temperature on CO_2 ice formation using IR data and gas-grain modelling. Next, we used SynthIceSpec to explore the detectability of the main feature of CH_3CN at 4.45 um in a cold core environment with the JWST, which was previously tentatively detected in YSOs. The detection thresholds we derive are reasonably low and observable, but identification is directly impacted by the photosphere absorptions that can greatly hinder identification. For some photostellar types, it could remain feasible. Coupled with the Estimated Time Calculator of the Space Telescope Science Institute, SynthIceSpec can be used to find the optimum observational setup for new observations.

Figures (10)

• Figure 1: Our Gaussian fitting (in orange) of the 3$\mu$m absorption feature of pure H$_2$O from Oberg_2007 (in blue) as a function of the wave numbers. The top right panel focuses on the dangling OH of water. The bottom right panel shows the residuals between our fit and the laboratory spectrum.
• Figure 2: Polynomial fits to the continuum emission from extracted data points from Yang_2022 and Tychoniec_2024 for a class 0 and class I protostar respectively.
• Figure 3: Comparison between JWST observation of NIR38 background star (top plot) in Chameleon I Mcclure_2023 with NIRSpec FS/395H and the synthetic spectrum (middle plot) obtained with the same column densities derived by the observers, with NIRSpec FS/395H parameters. The stellar spectrum applied is from a K7V star, as it was determined in Dartois_2024 as a good match to the observed photospheric lines. The absolute residuals (observations minus the synthetic spectrum) are given in the bottom plot. The pink rectangles indicate where the grain growth effect is notable in the observation.
• Figure 4: Observed column densities for H$_2$O, CO$_2$ and CH$_3$OH and their uncertainties as derived in Boogert_2011 towards the background star 2MASS J18170957-0814136, shown in dark blue. We also plot the column densities predicted by three different models aiming to reproduce the observed CO$_2$ column density. Except the dust temperature, they all share the same physical parameters retrieved from Herschel data of the cold core L429-C (see text). Grey squares correspond to a model with the dust temperature computed with the parametrisation from hocuk_parameterizing_2017 with T$_{dust}$ = 6.6 K; cyan triangles correspond to a model with a fixed temperature of T$_{dust}$ = 10.3 K, and red stars correspond to a dust temperature retrieved from Herschel data, with T$_{dust}$ = 14 K.
• Figure 5: Top: Optical depth derived from observation towards 2MASS J18170957-0814136 from Boogert_2011 (black) and the synthetic optical-depth spectrum using the column densities derived from the latter (orange). Middle: Synthetic optical depth from Nautilus predictions; the 'cold' model where the dust temperature (T$\rm_{dust}$) was computed with the parametrisation from hocuk_parameterizing_2017 (T$\rm_{dust}$ = 6.6 K) is shown in grey; the 'intermediate' model (T$\rm_{dust}$ = 10.3 K) is shown in cyan; the 'warm' model where T$\rm_{dust}$ was obtained from Herschel data (T$\rm_{dust}$ = 13.2 K) is shown in red. Bottom: Absolute residuals from subtraction between observations and synthetic spectra presented above.