JWST探测到一颗年轻褐矮星中的甲烷发射

中文摘要 AI 辅助

在最近的研究中，我们使用詹姆斯·韦伯太空望远镜（JWST）的低分辨率光谱（R~100）在年轻星团IC 348中发现了低质量褐矮星（2-10 MJup），它们表现出所谓的来自脂肪烃的3.4微米吸收带。大气模型并未预测褐矮星在任何年龄都会出现该吸收带。相反，人们预期在最冷的初生褐矮星中会出现甲烷吸收，但此前并未探测到。我们对IC 348中的一颗褐矮星LRL 11001（约6 MJup）进行了更高分辨率（R~1000）的JWST光谱观测。对3.4微米吸收带的新测量结果可以用四个高斯函数很好地拟合，这些高斯函数代表CH2和CH3基团的CH伸缩振动模式，表明其载体是大型烷烃（CnH2n+2）。此外，光谱中包含3.324和3.333微米的发射线，我们将其归因于甲烷（CH4）热带的Q支，位于八度区与二度区之间（主要是nu3+nu4->nu4）以及十四度区与五度区之间。这是首次探测到来自年轻褐矮星的3.33微米CH4发射。该发射可能产生于光球层而非星周盘，因为已知恒星或褐矮星周围的盘在这些跃迁中均未表现出发射。需要对发射进行建模以约束激发机制，该机制可能是热激发（例如，吸积激波）或外部辐射泵浦（例如，邻近星团成员）。我们推测垂直混合抑制了光球层中的CH4并增强了大型烷烃，但CH4在上层大气中仍然存在，在那里它能够经历所观测到的发射。

英文摘要

In recent studies, we have used low-resolution spectroscopy (R~100) with the James Webb Space Telescope (JWST) to uncover low-mass brown dwarfs (2-10 MJup) in the young cluster IC 348 that exhibit the so-called 3.4um absorption band from aliphatic hydrocarbons. Atmospheric models have not predicted the presence of that band in brown dwarfs at any age. Instead, methane absorption was expected in the coolest newborn brown dwarfs, but it was not detected. We have performed JWST spectroscopy at higher resolution (R~1000) on one of the brown dwarfs in IC 348, LRL 11001 (~6 MJup). The new measurement of the 3.4um band is fit well with four Gaussians that represent the CH stretching modes of CH2 and CH3 groups, indicating that the carrier is a large alkane (CnH2n+2). In addition, the spectrum contains emission lines at 3.324 and 3.333um, which we attribute to Q branches of CH4 hot bands between the octad and dyad regions (primarily nu3+nu4->nu4) and the tetradecad and pentad regions. This is the first detection of 3.33um CH4 emission from a young brown dwarf. The emission probably arises from the photosphere rather than a circumstellar disk given that no disks around stars or brown dwarfs are known to exhibit emission in these transitions. Modeling of the emission is needed to constrain the excitation mechanism, which could be either thermal (e.g., an accretion shock) or pumping by external radiation (e.g., neighboring cluster members). We speculate that vertical mixing has suppressed CH4 and enhanced large alkanes in the photosphere, but CH4 survives in the upper atmosphere, where it is available to experience the observed emission.

发表机构

• The Pennsylvania State University（宾夕法尼亚州立大学）
• European Space Agency, European Space Astronomy Centre（欧洲空间局，欧洲空间天文中心）
• Museum of Natural History（自然历史博物馆）
• Department of Physics, The Graduate Center City University of New York（纽约城市大学研究生院物理学系）
• Spectros Associates

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accepted for publication in Astronomy & Astrophysics Letters