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, ζ. We estimated ζ CMM3 = 1.6 × 10 −17 s −1 . This value is 1.3 times higher than the standard CR ionisation rate, ζ ISM = 1.3 × 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 (≤ 3×10 4 yrs) of their chemical evolution. Models with variations in the core density between 1 -20 × 10 6 cm −3 are also in good agreement with observations during the early time interval 1 × 10 4 < t (yr) < 5 × 10 4 . In addition, models with higher CR ionisation rates (5 -10) ×ζ ISM are often overestimating the fractional abundances of the species. However, models with ζ CMM3 = 5 ζ ISM may best fit observations at times ∼ 2 × 10 4 yrs. Our results suggest that CMM3 is (1 -5) × 10 4 yrs old. Therefore, the core is chemically young and it may host a Class 0 object as suggested by previous studies.

Deuterium chemistry in the young massive protostellar core NGC 2264 CMM3

“…DCO + and DCN best match observations at times (2 -6) × 10 4 yrs while the time of best fit of DNC is slightly later (∼ 4 × 10 4 yrs) due to the lack of its parent molecules (CHD and CD 2 ) during earlier times. The time of best fit is within the expected age of CMM3 (Awad and Shalabeia 2017).…”

“…The chemical model used in the present work is based on the previously published model by Awad and Shalabeia (2017) for the protostellar core NGC 2264 CMM3 , but with a modified chemical network that includes all the possible forms of deuterated species of a given normal isotope. Briefly, the model used is a single point, time-dependent, gas-grain astrochemical model.…”

“…This value is in agreement with the observed range of visual extinctions in the region NGC 2264 . The core has a temperature of 15 K and is irradiated by the standard interstellar radiation field of 1 Draine, and the cosmic ray ionisation reactions occur at a rate of ζ CMM3 = 1.67 × 10 −17 s −1 (Awad and Shalabeia 2017). The physical conditions of CMM3 and the initial chemical elemental abundances are listed in Table 1.…”

“…We considered the statistical orthoto-para ratio of H 2 of 3 (Coutens et al 2014;Hincelin et al 2014). The surface chemical network is mainly simple hydrogenation and deuteration of the depleted species, in addition to the other surface reactions described in details in Awad and Shalabeia (2017). Mantle species are returned to the gas-phase by non-thermal desorption mechanisms; H 2 formation, CR-photodissociation, and CR induced photodesorption (Roberts et al 2007).…”

“…The most recent observations by Watanabe et al (2015) motivated Awad and Shalabeia (2017) to introduce the first gas-grain astrochemical model of CMM3 that looked into the chemical evolution of a selected set of these species that do not include deuterium. The identification of few deuterated species in CMM3 motivated us to modify this model and update its chemical network to include all possible deuterated species in order to understand their chemical evolution of in CMM3.…”