Shock-type inference of L1157 B2 using methanol desorption

TL;DR

This work tackles the challenge of diagnosing shock type (C-type vs J-type) in non-irradiated protostellar outflows by exploiting methanol desorption from icy mantles as a diagnostic. Using the Paris-Durham shock code to simulate a grid of non-irradiated shocks ($V_s$ from $5$ to $19$ km s$^{-1}$, $n_H$ from $10^2$ to $10^5$ cm$^{-3}$) and tracking the methanol desorption percentage $\%_{deso}$, the authors demonstrate that CH$_3$OH desorption behaves differently for C-type versus J-type shocks, especially at $V_s \gtrsim 12$ km s$^{-1}$. Benchmarking against shocks with known types shows the method reliably identifies shock type, and applying it to L1157 B2 indicates a non-irradiated C-type shock with $V_s \approx 18$ km s$^{-1}$ and $n_H \approx 10^5$ cm$^{-3}$, consistent with prior constraints. The approach provides a practical, observation-driven diagnostic that can be extended to other COMs and non-irradiated environments, while acknowledging limitations related to grain-surface physics and binding-energy distributions that warrant future refinement.

Abstract

Shock types of low-velocity molecular outflows are not always well constrained. Astrochemical comparisons are often made between low-velocity and high-velocity outflows, but without considering the question of the shock type. We investigated molecular abundances of post-shock regions to determine whether strong differences between non-irradiated C-type and J-type shocks can be highlighted. One of the main application goals is to diagnose the shock type of the protostellar object L1157 B2 through the use of molecular tracers. We simulated grid sets of shock models with the Paris-Durham Shock code with velocities ranging from 5 to 19 km/s and low densities from $10^2$ to $10^5$ cm$^{-3}$. We computed the desorption percentage of methanol in these simulations and estimated it at higher velocities. We compared our results to observational measurements of L1157 B2 and with a benchmark of four already identified shocks. L1157 B2 has been diagnosed as a non-irradiated C-type shock, and the method showed a good applicability through the benchmark. Methanol formed in the icy mantle of grains can serve to trace the differences between shock types, at least in non-irradiated conditions. A requirement for the applicability of a species as a shock-type tracer is that it does not undergo significant enhancement or destruction, but is mainly impacted by desorption processes under shocked conditions. The desorption percentage of methanol is a good criterion in characterizing the shock type of L1157 B2 and should be investigated as a general method to diagnose the shock type in non-irradiated regions. We identify L1157 B2 as a non-irradiated C-type shock with velocities and densities fitting with previous studies.

Figures (8)

• Figure 1: Desorption percentage of methanol in C-type (top) and J-type shocks (bottom). The x-axis represents the shock velocity ($V_s$) and the y-axis the pre-shock density ($nH$).
• Figure 2: Comparison factor for methanol. The x-axis represents the shock velocity ($V_s$) and the y-axis the pre-shock density ($nH$).
• Figure 3: Reduced chemical network where gas-phase formation routes of methanol are neglected. CH$_3$OH represents the molecule in the gas phase while CH$_3$OH* is for the icy mantle phase. "..." represents the products of methanol destruction.
• Figure 4: Geometric deviation between simulations and L1157 B2 for C-type shocks (top figure) and J-type shocks (bottom figure). The x-axis represents the shock velocity ($V_s$) and the y-axis the pre-shock density ($nH$).
• Figure 5: Geometric deviation between simulations and L1157 B1 for C-type shocks (top) and J-type shocks (bottom). The x-axis represents the shock velocity ($V_s$) and the y-axis the pre-shock density ($nH$).