[论文解读] The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust

S. Massalkhi, M. Agúndez+7 more|arXiv (Cornell University)|Oct 31, 2017

Astrophysics and Star Formation Studies参考文献 78被引用 13

一句话总结

本研究利用IRAM 30米望远镜的观测数据，在大多数富碳的渐近巨星支（AGB）星中探测到SiC2，发现其丰度随包层密度增加而降低。结果强烈表明，SiC2是SiC尘埃的关键气相前体，因其在高密度区域能高效地被吸附到尘埃颗粒上，类似于富氧化物星中SiO的作用。

ABSTRACT

Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si--C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes around carbon stars.

研究动机与目标

• 确定富碳AGB星包层中SiC2、SiC和Si2C的丰度与分布。
• 鉴于其Si–C键结构，探讨SiC2是否在SiC尘埃形成过程中充当气相前体。
• 通过分子丰度与包层密度的相关性，推断尘埃形成过程。
• 将SiC2的行为与富氧化物星中已知前体（如SiO）进行比较，评估其在尘埃成核中的作用。
• 建立SiC2与SiC之间的化学联系，通过光致离解和形成路径，揭示恒星喷流中的化学演化过程。

提出的方法

• 利用IRAM 30米望远镜对25颗富碳AGB星进行灵敏的2毫米波段发射线观测。
• 采用局部非局域热动平衡（LVG）方法进行非-LTE激发与辐射转移建模，以解释观测到的SiC2谱线。
• 基于辐射转移解和谱线强度，计算SiC2的相对丰度。
• 分析SiC2丰度与包层密度的相关性，推断尘埃颗粒的结合效率。
• 利用光致离解模型关联SiC2与SiC的线发射，支持从SiC2到SiC的化学转化路径。
• 将结果与富氧化物星中已知的尘埃形成过程（特别是SiO的耗竭趋势）进行比较。

实验结果

研究问题

• RQ1 SiC2、SiC和Si2C在富碳AGB星包层中有多普遍、多丰富？
• RQ2 SiC2丰度与包层密度之间是否存在可检测的相关性？其对尘埃形成意味着什么？
• RQ3 SiC2与SiC之间存在何种化学关系？SiC2是否在包层外层通过光致离解形成SiC？
• RQ4 SiC2丰度随密度变化的观测趋势是否支持其作为SiC尘埃气相前体的角色？
• RQ5 SiC2的行为与富氧化物星中SiO相比如何，特别是在因尘埃形成导致的耗竭方面？

主要发现

• 在25颗富碳AGB星中的22颗中探测到SiC2，表明其在碳星包层中广泛存在。
• 约一半的观测源中探测到SiC，且SiC与SiC2的线发射呈正相关。
• 除IRC +10216外，所有源均未显著检测到Si2C，表明其在这些环境中稀少或不稳定。
• SiC2相对丰度与包层密度之间存在明显的负相关关系，即在高密度区域SiC2丰度降低。
• 在高密度区域观察到的SiC2耗竭现象被解释为高效地结合到尘埃颗粒上，其驱动力是碰撞频率的增加。
• 结果强烈支持SiC2作为富碳AGB星中SiC尘埃的主要气相前体，其作用类似于富氧化物星系统中的SiO。