arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 2005.06784astro-ph.SRastro-ph.GA
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太阳系尺度上低质量原恒星中的复杂有机分子-- I。含氧物种

Complex organic molecules in low-mass protostars on solar system scales -- I. Oxygen-bearing species

M. L. van Gelder, B. Tabone, Ł. Tychoniec, E. F. van Dishoeck, H. Beuther, A. C. A. Boogert, A. Caratti o Garatti, P. D. Klaassen, H. Linnartz, H. S. P. Müller, V. Taquet

摘要:

复杂的有机分子(COMs)被认为是在恒星形成的最早阶段形成于冰冷的尘埃颗粒上。这些COM从最年轻的0/I类原恒星阶段向更进化的II类阶段的演变仍然没有完全了解。由于行星形成似乎很早就开始了,成熟的盘对于特征COM发射线来说太冷了,因此在0/I级阶段研究太阳系尺度上的COM清单是相关的。ALMA波段3(3毫米)和波段6(1毫米)的观测获得了七个0级原恒星的英仙座和巨蛇座恒星形成区。通过用“LTE”模型模拟原恒星内部区域,确定了几个含氧COM的激发温度和柱密度。B1-c、B1-bS和Serpens S68 N显示COM发射,即七个源中的三个。没有明确的相关性似乎存在之间的发生COM和源光度。对于三个富含COM的来源,以及IRAS 16293- 2422 B和HH 212,几种COM相对于CH 3OH的丰度非常相似。对于其他COM的丰度差异高达一个数量级,表明当地的源条件的情况下确定。B1-c包含COM发射的冷($T_{ex}\approx60$ K)、更扩展的分量,其柱密度通常为暖/热($T_{ex}\sim 200$ K)、中心分量的几个%。基于CH 2DOH/CH 3OH比率推导出1- 3%的D/H比率,表明甲醇形成期间的温度为$\sim$ 15 K。这个比例与其他低质量原恒星一致。未来的中红外设施,如JWST/MIRI,将是直接观测彗星冰的必要条件。将其与更大的富含COM的源样本与ALMA相结合,将允许直接将冰和气相丰度联系起来,以限制在恒星形成过程中产生和维持化学复杂性的路线。

英文摘要:

Complex organic molecules (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on solar system scales in the Class 0/I stage is relevant. ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained of seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using 'LTE' models, the excitation temperature and column densities are determined for several O-bearing COMs. B1-c, B1-bS, and Serpens S68N show COM emission, i.e, three out of the seven sources. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs with respect to CH3OH are remarkably similar for the three COM-rich sources, and to IRAS 16293-2422B and HH 212. For other COMs the abundances differ by up to an order of magnitude, indicating that local source conditions are case determining. B1-c hosts a cold ($T_{ex}\approx60$ K), more extended component of COM emission with a column density of typically a few % of the warm/hot ($T_{ex}\sim 200$ K), central component. A D/H ratio of 1-3 % is derived based on the CH2DOH/CH3OH ratio suggesting a temperature of $\sim$15~K during the formation of methanol. This ratio is consistent with other low-mass protostars. Future mid-infrared facilities such as JWST/MIRI will be essential to directly observe COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow for directly linking ice and gas-phase abundances in order to constrain the routes that produce and maintain chemical complexity during the star formation process.

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