大质量恒星形成区 RCW 120 中的气相与表面化学
Gas-phase and Surface Chemistry in the Massive Star-Forming Region RCW\,120
AI总结：
本研究通过 APEX 望远镜观测与 Presta 模型模拟，探究 RCW 120 致密团块中甲醇丰度升高的原因，发现其源于尘埃冰幔的光解吸作用，且分子形成相决定其柱密度相关性。
AI中文摘要：
我们分析了使用 APEX 望远镜在 200-260 GHz 波段获取的 RCW 120 中一个致密分子团块的宽带发射光谱，以研究早期演化阶段大质量恒星形成区域的分子形成路径。我们检验了在 LTE 假设下推导的分子柱密度之间的相关性。在该致密团块的南部，相对于北部，甲醇（methanol）出现过量，而其他分子如 CH₃CN 和 CH₃CCH 的丰度仍具有可比性。甲醇的丰度相对于其他含氧分子如 OCS 和 SO 也有所升高。为了确定团块南部甲醇丰度升高的可能原因，我们使用天体化学模型 Presta 进行了两阶段近似模拟，该模拟考虑了气相和尘埃颗粒幔层中的化学过程。模拟结果表明，气相甲醇丰度升高可能源于冰幔的光解吸作用。在消光值 Aᵥ 介于 4ᵐ 至 6ᵐ 之间时，甲醇会在与光子相互作用时从尘埃颗粒的冰幔中高效解吸，但尚未被气相中的紫外辐射破坏。分子柱密度之间的强线性相关性表明，这些分子在同一相中形成——要么在气相中，要么在尘埃上。不过，它们的积分强度图可能存在差异，例如 CCH 和 CH₃CN 的情况。如果两种分子在不同相中形成——一种在气相中，另一种在尘埃幔层中——则不会观察到相关性，例如 CCH 和 CH₃OH 的情况。甲醇与在尘埃上形成的含氧分子之间的弱相关性表明，团块南部仅富含 CO 冰的尘埃幔层上部被破坏。
英文摘要：
We analysed broadband emission spectra of a dense molecular clump in RCW 120, obtained with the APEX telescope in the 200--260 GHz range, in order to investigate molecular formation pathways in regions of massive star formation at an early evolutionary stage. We examined the correlations between the molecular column densities derived under the LTE assumption. An excess of methanol was found in the southern part of the dense clump relative to its northern part, while the abundances of other molecules, such as CH$_3$CN and CH$_3$CCH, remain comparable. The methanol abundance is also elevated relative to that of other oxygen-bearing molecules, such as OCS and SO. To identify possible causes of the enhanced methanol abundance in the southern part of the clump, we carried out simulations with the astrochemical model Presta in a two-phase approximation, accounting for chemical processes both in the gas phase and in the mantles of dust grains. The modelling shows that the enhanced gas-phase methanol abundance may be due to photodesorption from icy mantles. At Av values between $4^{m}$ and $6^{m}$, methanol desorbs efficiently from the ice mantles of dust grains upon interaction with photons, but is not yet destroyed by UV radiation in the gas phase. A strong linear correlation between molecular column densities indicates that the molecules form in the same phase --- either in the gas phase or on dust. Their integrated intensity maps may nevertheless differ, as is the case for CCH and CH$_3$CN. If two molecules form in different phases --- one in the gas phase and the other in dust mantles --- no correlation is observed, as for CCH and CH$_3$OH. The weak correlation between methanol and the oxygen-bearing molecules that form on dust suggests that only the upper part of the dust mantles, rich in CO ice, is destroyed in the southern part of the clump.
