The 2014 KIDA network for interstellar chemistry (1503.01594v1)

Abstract: Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reactions and rate coefficients are partially compiled from data in the literature, when available. We present in this paper kida.uva.2014, a new updated version of the kida.uva public gas-phase network first released in 2012. In addition to a description of the many specific updates, we illustrate changes in the predicted abundances of molecules for cold dense cloud conditions as compared with the results of the previous version of our network, kida.uva.2011.

• The paper introduces a major update to the KIDA network by incorporating 1038 new reactions and modifying 446 rate coefficients for astrochemical modeling.
• It quantitatively compares molecular abundances, revealing a shift in the optimal chemical age of dense clouds from 2.5×10^5 to 1.3×10^6 years.
• It integrates enhanced photo-rates, OCS chemistry, and high-temperature reaction data, broadening its applicability to diverse interstellar environments.

The 2014 KIDA Network for Interstellar Chemistry

The paper by V. Wakelam et al., titled "The 2014 KIDA Network for Interstellar Chemistry," presents an update to the KInetic Database for Astrochemistry (KIDA), focusing on the subset kida.uva.2014. This database serves as a critical resource for modeling the chemical composition of the interstellar medium (ISM), particularly for understanding gas-phase chemistry in cold dense cloud conditions ranging from 10 K to 300 K.

Overview of the KIDA Update

The 2014 update builds upon the kida.uva.2011 network by incorporating numerous modifications derived from recent literature and new experimental data collected between 2011 and 2014. The updated network accounts for:

• Incorporation of reactions from high-temperature studies, such as those by Harada et al.
• Inclusion of updated photo-rates affecting reactions with UV radiation.
• Enhanced data on OCS chemistry and branching ratios for the formation of various carbon and hydrogen species.

These updates have resulted in the addition of 1038 new reactions and modifications to 446 rate coefficients, thereby expanding the network to include 7509 reactions and 489 species, which now also consider grain and electron interactions.

Numerical Results and Impacts

The paper provides a quantitative comparison of molecular abundances predicted by kida.uva.2014 against its predecessor under typical dense cloud conditions. This comparative analysis highlights chemical changes brought on by new reaction pathways and updated rate coefficients. Significant changes include reductions in the abundance of species such as HC$_2$N$^+$ and CCS, attributed to the inclusion of new destructive reaction paths.

Interestingly, the changes in the network imply a shift in the "chemical age" of modeled interstellar environments, pushing the optimal agreement between model predictions and observational data from TMC-1 from $2.5 \times 10^5$ years to $1.3 \times 10^6$ years.

Broader Implications and Future Directions

From a theoretical standpoint, this work underscores the importance of accurate rate coefficients and comprehensive reaction networks in astrochemical modeling. The robust updates provided by the kida.uva.2014 network offer researchers more precise tools to simulate ISM conditions, ultimately enhancing our understanding of complex interstellar chemistry.

Practically, these changes pave the way for investigating higher-temperature ISM environments, although the paper calls for caution in extrapolating data beyond the temperature ranges validated by laboratory and theoretical studies.

In the future, further enhancement of KIDA will likely involve detailed studies into low-temperature reactions, particularly those involving quantum mechanical tunneling, which can significantly affect rate coefficients. Additionally, expansion into grain-surface reactions and their coupling with gas-phase processes remains a critical frontier for creating more comprehensive models of the ISM.

In conclusion, the 2014 KIDA network represents a substantial step forward for astrochemical modeling, providing a more refined and expanded foundation for future research in this field. The meticulous compilation of reactions and rate coefficients offers astrochemists a valuable resource for exploring both fundamental chemical processes and their astronomical implications.