Abstract <p>Molecules in the interstellar medium form both in the gas phaseand on dust particles. The chemical pathways for molecularformation are not yet well understood in detail, so the questionof which pathway predominates for a particular molecule remainsopen. We analysed broadband emission spectra of a dense molecularclump in the RCW120 region, obtained with the APEX telescope inthe 200–260 GHz range, to investigate molecular formationpathways in regions of massive star formation at an earlyevolutionary stage. The correlations between the derived molecularcolumn densities, obtained under the LTE assumption, wereinvestigated. An excess of methanol was found in the southern partof the dense clump compared to its northern part, while theabundance of other molecules, such as CH<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({}_{3}\)</EquationSource>  </InlineEquation>CN and CH<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({}_{3}\)</EquationSource>  </InlineEquation>CCH,remain comparable. The methanol abundance is also elevatedrelative to other oxygen-bearing molecules, such as OCS and SO. Toidentify possible causes of the enhanced methanol abundance in thesouthern part of the clump, we conducted simulations using theastrochemical model <Emphasis FontCategory="NonProportional">Presta</Emphasis> in a two-phase approximation,accounting for chemical processes in both the gas phase and themantles of dust grains. Modeling showed that the enhanced methanolabundance in the gas phase may be due to its photodesorption fromicy mantles. At an <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(A_{V}\)</EquationSource>  </InlineEquation> value between <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(4^{\textrm{m}}\)</EquationSource>  </InlineEquation> and<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(6^{\textrm{m}}\)</EquationSource>  </InlineEquation>, methanol efficiently desorbs from ice mantles ofdust grains upon interaction with photons, but it is not yetdestroyed by UV radiation in the gas phase. A strong linearcorrelation between molecular column densities indicates thatthese molecules form in the same phase—either in the gas phaseor on dust. However, integrated intensity maps of these moleculesmay differ, as seen for CCH and CH<InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({}_{3}\)</EquationSource>  </InlineEquation>CN. If molecules form indifferent phases—one in the gas phase and the other in dustmantles—no correlation is observed, as in the case of CCH andCH<InlineEquation ID="IEq7"> <EquationSource Format="TEX">\({}_{3}\)</EquationSource>  </InlineEquation>OH. The weak correlation between methanol and oxygen-bearingmolecules that form on dust suggests that only the upper part ofthe dust mantles, rich in CO ice, is being destroyed in thesouthern part of the clump.</p>

Gas-phase and Surface Chemistry in the Massive Star-Forming Region RCW 120

期刊论文
发表日期：

2025年9月1日

查看全文

K. V. Plakitina,
M. S. Kirsanova,
D. S. Wiebe,
O. V. Kochina

摘要

Abstract

Molecules in the interstellar medium form both in the gas phaseand on dust particles. The chemical pathways for molecularformation are not yet well understood in detail, so the questionof which pathway predominates for a particular molecule remainsopen. We analysed broadband emission spectra of a dense molecularclump in the RCW120 region, obtained with the APEX telescope inthe 200–260 GHz range, to investigate molecular formationpathways in regions of massive star formation at an earlyevolutionary stage. The correlations between the derived molecularcolumn densities, obtained under the LTE assumption, wereinvestigated. An excess of methanol was found in the southern partof the dense clump compared to its northern part, while theabundance of other molecules, such as CH \({}_{3}\) CN and CH \({}_{3}\) CCH,remain comparable. The methanol abundance is also elevatedrelative to other oxygen-bearing molecules, such as OCS and SO. Toidentify possible causes of the enhanced methanol abundance in thesouthern part of the clump, we conducted simulations using theastrochemical model Presta in a two-phase approximation,accounting for chemical processes in both the gas phase and themantles of dust grains. Modeling showed that the enhanced methanolabundance in the gas phase may be due to its photodesorption fromicy mantles. At an \(A_{V}\) value between \(4^{\textrm{m}}\) and \(6^{\textrm{m}}\) , methanol efficiently desorbs from ice mantles ofdust grains upon interaction with photons, but it is not yetdestroyed by UV radiation in the gas phase. A strong linearcorrelation between molecular column densities indicates thatthese molecules form in the same phase—either in the gas phaseor on dust. However, integrated intensity maps of these moleculesmay differ, as seen for CCH and CH \({}_{3}\) CN. If molecules form indifferent phases—one in the gas phase and the other in dustmantles—no correlation is observed, as in the case of CCH andCH \({}_{3}\) OH. The weak correlation between methanol and oxygen-bearingmolecules that form on dust suggests that only the upper part ofthe dust mantles, rich in CO ice, is being destroyed in thesouthern part of the clump.