Simulations of the Chemistry in the Small Magellanic Cloud

Abstract

The Large (LMC) and Small (SMC) Magellanic Clouds are irregular satellite galaxies of the Milky Way. Both are metal-and dust-poor, although the SMC is significantly poorer in both. We have recently simulated the chemistry in cold dense regions of the LMC and found that a rich chemistry exists in the gas-phase. In this paper, we report a companion study of the chemistry of dense regions of the SMC, confining our attention to cold regions of dense clouds with a variety of densities, visual extinctions, and grain temperatures, and a fixed gas-phase temperature. With a gas-to-dust ratio and elemental abundances based on observations and scaling, we found that for molecules like CO and N 2 , which are predominantly formed in the gas phase, their abundances are consistent with the reduced elemental abundances of their constituent elements above 25 K; however, for species that are produced fully (e.g., CH 3 OH) or partially on the grain surface (e.g., H 2 CO, NH 3 ), the dependence on metallicity can be complex. Most of the major gas-phase species observed in our Galaxy are produced in the SMC although in lower quantities. With our simulations, we are able to explain observed gas-phase abundances reasonably well in the dense sources N27 and LIRS 36. We have also compared our calculated abundances of selected ices with limited observations in dense regions in front of young stellar objects.

ALMA Astrochemical Observations of the Infrared-luminous Merger NGC 3256

“…A possible reason for the difference in the N nu-cleus might be that HNCO and CH 3 OH abundances may act differently to the dust temperature. Methanol abundances decrease at high dust temperature because the hydrogen atom evaporate faster at the high dust temperatures, and the hydrogen atom is less likely to react (Acharyya & Herbst 2016). HNCO may be able to sustain its abundance at higher dust temperatures than CH 3 OH because there is less number of hydrogen necessary for HNCO.…”

Dense Molecular Gas in the Nearby Low-metallicity Dwarf Starburst Galaxy IC 10

“…In addition to the above density-based effects, the lower metallicities of dwarf galaxies may also change their molecular abundance patterns. Chemical modeling of low metallicity systems suggests that the relationship between metallicity and abundances of individual molecules is complex; a lower abundance of a particular element does not necessarily imply that all molecules containing that element have lower abundances (Millar & Herbst 1990;Acharyya & Herbst 2015. Comparing the properties of dense molecular gas tracers in a wide variety of systems as well as multiple dense gas tracers with different chemical pathways can yield insights into the effects of low metallicity on the abundances in the system.…”

Modeling CO, CO2and H2O Ice Abundances in the Envelopes of Young Stellar Objects in the Magellanic Clouds

“…The cosmic ray ionization rate, ζ, was held fixed throughout our model grid at a value of 1.3 ×10 −17 s −1 . Other chemical model work in the Magellanic Clouds have used either the galactic local value or an enhanced value (Chin et al 1998;Acharyya & Herbst 2015. Data on ζ LMC and ζ SMC is scarce; Abdo et al (2010b) analyzed a Fermi-LAT >100 MeV gamma ray map of the LMC and found the globallyaveraged cosmic ray ionization rate to be 20-30% of the local MW value.…”

“…Past work by Acharyya & Herbst (2015 showed that for chemical models with reduced elemental abundances, reproducing observed CO 2 /H 2 O ice abundance ratios towards Magellanic Cloud YSOs requires models with A V = 10 and warm dust temperatures, either 20 K ≤ T d ≤ 25 K or 35 K ≤ T d ≤ 45 K. These works produce CO 2 via mobile CO in CO + OH; below 20 K, immobile CO causes CO 2 /H 2 O to drop below 0.01. They used static cloud models, keeping A V and density fixed while running an array of models across a range of dust temperatures.…”