Chemical templates of the Central Molecular Zone. Shock and protostellar object signatures under Galactic Center conditions

TL;DR

This work builds CMZ-tailored chemical templates by coupling gas-grain chemistry to CMZ-like physical conditions using UCLCHEM, exploring protostellar-warm-up and C-type shock scenarios for 24 species, including COMs. A two-stage approach (collapse then Stage 2 protostellar/shock environments) across a broad parameter grid reveals that cosmic-ray ionization rate and temperature dominate chemical trends, with 300 K acting as a turning point distinguishing shocks from protostellar chemistry. The authors identify robust tracers of ionization (HCO$^+$, H$_2$CO, CH$_3$SH) and molecules that differentiate energetic processes (e.g., CH$_3$OH, CH$_3$NCO, HCOOCH$_3$, HCO) and propose diagnostic abundance ratios (e.g., CH$_3$SH/HCO$^+$, H$_2$CO/HCO$^+$, NS$^+$/HCO$^+$) suitable for CMZ-like conditions. Comparisons with Galactic Center observations support recurring shocks and enhanced ionization in regions like Sgr B2(N2), while highlighting underpredicted COM abundances in shocks and pointing to future work on shock recurrence and metallicity effects to refine the templates.

Abstract

(Abridged) The Central Molecular Zone (CMZ) of the Milky Way exhibits extreme conditions, including high gas densities, elevated temperatures, enhanced cosmic-ray ionization rates, and large-scale dynamics. Large-scale molecular surveys reveal increasing chemical and physical complexity in the CMZ. A key step to interpreting the molecular richness found in the CMZ is to build chemical templates tailored to its diverse conditions. The combined impact of high ionization, elevated temperatures, and dense gas remains insufficiently explored for observable tracers. In this study, we utilized UCLCHEM, a gas-grain time-dependent chemical model, to link physical conditions with their corresponding molecular signatures and identify key tracers of temperature, density, ionization, and shock activity. We ran a grid of models of shocks and protostellar objects representative of typical CMZ conditions, focusing on twenty-four species, including complex organic molecules. Shocked and protostellar environments show distinct evolutionary timescales ($\lesssim 10^4$ vs. $\gtrsim 10^4$ years), with 300 K emerging as a key temperature threshold for chemical differentiation. We find that cosmic-ray ionization and temperature are the main drivers of chemical trends. HCO$^+$, H$_2$CO, and CH$_3$SH trace ionization, while HCO, HCO$^+$, CH$_3$SH, CH$_3$NCO, and HCOOCH$_3$ show consistent abundance contrasts between shocks and protostellar regions over similar temperature ranges. While our models underpredict some complex organics in shocks, they reproduce observed trends for most species, supporting scenarios involving recurring shocks in Galactic Center clouds and enhanced ionization towards Sgr B2(N2). Future work should assess the role of shock recurrence and metallicity in shaping chemistry.

Figures (12)

• Figure 1: Stage 2 time-averaged temperature profiles for all models of shocks and protostellar objects defined in Tab. \ref{['tab:UCLCHEM']}. "Time-averaged" refers to averaging the temperature across all models of a given type at fixed logarithmic time steps. The solid black line represents the mean coupled gas and dust temperature at each logarithmic time step, while shaded regions indicate the 1$\sigma$ confidence interval. Dashed lines mark the upper and lower temperature limits. Since these profiles are averaged across all models, multiple shock events appear, though each individual model experiences only one. The timing of each shock event is dictated by the pre-shock medium density, with higher densities leading to shorter timescales, while the temperature is primarily determined by shock velocity. The densities associated with each shock event are annotated, with color-matched rectangular patches at the top of the figure indicating the full possible timing of those events. In contrast, protostellar heating operates on significantly longer timescales. Once the heating stage concludes, the temperature stabilizes at a plateau, set by the maximum protostellar object temperature. Since more massive cores warm up faster, influencing the onset of the temperature plateau, we mark with vertical lines at the top of the figure the earliest plateau onset times: pink for 10 M$_\odot$ objects and green for 25 M$_\odot$.
• Figure 2: Time-averaged chemical evolution of the studied species across all second stage models, as defined in Tab. \ref{['tab:UCLCHEM']}. Line styles and shading follow those in Fig. \ref{['fig:temp_evolution']}. Only gas-phase abundances above the detectable threshold ($X > 10^{-14}$) are considered. Changes in abundance, including both enhancement and depletion, generally follow the timescales seen in the average temperature profiles (see Fig. \ref{['fig:temp_evolution']}). In models of protostellar objects, the species that most frequently dominates over the others is HCN, whereas in shocks, CH$_3$OH is the most dominant during the shock phase, while SiO takes over in the post-shock phase.
• Figure 3: Box plots of the fractional abundances of each species in the protostellar objects (fully warmed-up phase), as a function of the cosmic-ray ionization rate (x-axis) and initial temperature of gas and dust (indicated by different colors). The interquartile ranges (IQRs) represent the middle 50% of the abundance distribution, while the whiskers extend to the minimum and maximum values, covering the full range of the data. Additionally, the evolution of mean abundances for different object masses is overplotted, with circles denoting the mean values for each mass. If no distribution is observed at a given $\zeta/\zeta_0$, it suggests the species remained below the observational threshold throughout.
• Figure 4: As in Fig. \ref{['fig:CRIR_evolution_PO']}, but for shock models. Here, the mean abundance evolution is calculated for different shock sub-stages, i.e., shock and post-shock.
• Figure 5: As in Fig. \ref{['fig:CRIR_evolution_PO']}, but with the mean abundance evolution as a function of final temperature and object density.