McCarthy Michael C, Gottlieb Carl A, Cernicharo Jose
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, U.S.A.
Group of Molecular Astrophysics, IFF-CSIC. C/Serrano 121, 28006 Madrid, Spain.
J Mol Spectrosc. 2019 Feb;356:7-20.
This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon rich Asymptotic Giant Branch (AGB) stars. The increased sensitivity and angular resolution of high altitude ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra - both in the ground and vibrationally excited states of the normal and rare isotopic species - of the principal molecules in the inner region which appear to contain only two or three heavy atoms Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeters-wave rotational lines that are observed in the inner envelope of carbon rich AGB stars. As an illustration we refer to the example of an initial laboratory approach for establishing whether vibrationally excited SiC and HCN are the carriers of some of the unassigned features observed in the prototypical carbon rich AGB star IRC+10216 with ALMA. Also highlighted are ongoing laboratory studies of the silicon carbides SiC and SiCSi in their ground and excited vibrational states, and SiC in the ground vibrational state. Following the initial detection of SiC and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with ≤ 48, ≤ 3 and upper level energies ≤ 484 K in the range from 178 - 391 GHz, and 35 new transitions of SiC with ≤ 38, ≤ 20 and ≤ 875 K between 315 and 440 GHz were measured in the laboratory. In addition five to six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi, and the two lowest rotational transitions in the previously unexplored C-C stretching mode ( ) of SiCC were measured in the normal and doubly substituted C isotopic species.
本文概述了近期的天文学研究以及一项紧密协调的实验室计划，该计划致力于研究富含碳的渐近巨星分支（AGB）恒星的物理和化学性质。在过去几年中，高海拔地基毫米波干涉仪的灵敏度和角分辨率有所提高，这使得分子天文学家能够确定在中央恒星的几个恒星半径范围内分子的激发和空间分布，而尘埃的分子种子正是在这个范围内形成的，并且能够严格评估尘埃形成和生长的物理化学机制。然而，天文学研究在很大程度上依赖于对正常和稀有同位素物种的基态和振动激发态下主要分子的转动光谱进行精确的实验室测量，这些主要分子位于内部区域，似乎只包含两三个重原子。实验室光谱学家还有很多工作要做，这从在富含碳的AGB恒星内包层中观察到的大量未归属的毫米波转动谱线就可以看出来。作为一个例子，我们提及一种初步的实验室方法，该方法用于确定振动激发的碳化硅（SiC）和氰化氢（HCN）是否是在利用阿塔卡马大型毫米/亚毫米波阵列（ALMA）对典型的富含碳的AGB恒星IRC +10216进行观测时所观察到的一些未归属特征的载体。还重点介绍了正在进行的关于碳化硅（SiC）和硅碳化硅（SiCSi）的基态和激发振动态以及基态振动的SiC的实验室研究。在首次在IRC +10216的外层分子包层中检测到SiC和SiCSi之后，实验室光谱学扩展到了更高频率，以支持最近的干涉测量。在实验室中测量了178 - 391吉赫兹范围内32条新的硅碳化硅（SiCSi）毫米波转动跃迁，其J≤48、K≤3且上能级能量≤484开尔文，以及在315至440吉赫兹之间35条新的碳化硅（SiC）跃迁，其J≤38、K≤20且≤875开尔文。此外，还测量了硅碳化硅（SiCSi）三种基本振动模式中每种模式一个量子的五到六个转动跃迁，以及在硅碳碳（SiCC）先前未探索的C - C伸缩模式（ ）中的两个最低转动跃迁，测量使用的是正常和双取代碳同位素物种。
