On the Chemistry of the Young Massive Protostellar core NGC 2264 CMM3

Abstract

We present the first gas-grain astrochemical model of the NGC 2264 CMM3 protostellar core. The chemical evolution of the core is affected by changing its physical parameters such as the total density and the amount of gas-depletion onto grain surfaces as well as the cosmic ray ionisation rate, $\zeta$. We estimated $\zeta_{\text {CMM3}}$ = 1.6 $\times$ 10$^{-17}$ s$^{-1}$. This value is 1.3 times higher than the standard CR ionisation rate, $\zeta_{\text {ISM}}$ = 1.3 $\times$ 10$^{-17}$ s$^{-1}$. Species response differently to changes into the core physical conditions, but they are more sensitive to changes in the depletion percentage and CR ionisation rate than to variations in the core density. Gas-phase models highlighted the importance of surface reactions as factories of large molecules and showed that for sulphur bearing species depletion is important to reproduce observations. Comparing the results of the reference model with the most recent millimeter observations of the NGC 2264 CMM3 core showed that our model is capable of reproducing the observed abundances of most of the species during early stages ($\le$ 3$\times$10$^4$ yrs) of their chemical evolution. Models with variations in the core density between 1 - 20 $\times$ 10$^6$ cm$^{-3}$ are also in good agreement with observations during the early time interval 1 $\times$ 10$^4 <$ t (yr) $<$ 5 $\times$ 10$^4$. In addition, models with higher CR ionisation rates (5 - 10) $\times \zeta_{\text {ISM}}$ are often overestimating the fractional abundances of the species. However, models with $\zeta_{\text {CMM3}}$ = 5 $\zeta_{\text {ISM}}$ may best fit observations at times $\sim$ 2 $\times$ 10$^4$ yrs. Our results suggest that CMM3 is (1 - 5) $\times$ 10$^4$ yrs old. Therefore, the core is chemically young and it may host a Class 0 object as suggested by previous studies.