[论文解读] Physical conditions in the gas phases of the giant HII region LMC-N11: II. Origin of [CII] and fraction of CO-dark gas

V. Lebouteiller, D. Cormier+12 more|arXiv (Cornell University)|Nov 8, 2019

Astrophysics and Star Formation Studies参考文献 80被引用 1

一句话总结

本研究利用SOFA/GREAT对大麦哲伦云-N11 HII区进行的速度分辨观测，通过将[ C II] 158 µm线轮廓与H I和CO进行比较，厘清了[ C II]发射的起源。结果发现，超过90%的[ C II]发射源自CO暗态H₂气体，该气体构成了主要的分子储层——尤其在致密且受恒星反馈影响的区域中——凸显了[ C II]作为星暴环境中难以探测的分子气体关键示踪剂的重要性。

ABSTRACT

(abridged) The ambiguous origin of [CII] 158um in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions (PDRs). We observed the giant HII region N11 in the Large Magellanic Cloud with SOFIA/GREAT in order to investigate the origin of [CII] to obtain the total H2 gas content, the fraction of CO-dark H2 gas, and the influence of environmental effects such as stellar feedback. We present an innovative spectral decomposition method that allows statistical trends to be derived. The [CII] line is resolved in velocity and compared to HI and CO, using a Bayesian approach to decompose the profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H2 column density traced by C+. The profile of [CII] most closely resembles that of CO, but the integrated [CII] line width lies between that of CO and that of HI. Using various methods, we find that [CII] mostly originates from the neutral gas. We show that [CII] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components. Most of the molecular gas is CO-dark. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric-effect heating efficiency, which we find is constant across regions probed with Herschel, with [CII] and [OI] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with PAH emission tracing the CO-dark H2 gas heating where [CII] and [OI] emit. Our study highlights the importance of velocity-resolved PDR diagnostics and higher spatial resolution for HI observations.

研究动机与目标

• 确定大麦哲伦云中巨大HII区N11的[ C II] 158 µm发射的物理起源。
• 量化HII区不同区域中未被CO追踪的CO暗态H₂气体所占比例。
• 评估恒星反馈和辐射场等环境因素对CO暗态H₂气体分布和可探测性的影响。
• 评估[ C II]作为总分子气体含量示踪剂的可靠性，特别是在低金属度环境（如LMC）中。
• 通过纳入CO暗态H₂气体的贡献，改进CO-H₂转换因子（XCO）的校准。

提出的方法

• 对N11 HII区的[ C II] 158 µm线进行了高光谱分辨率的SOFIA/GREAT观测。
• 采用贝叶斯线轮廓分解技术，将[ C II]发射分解为速度分量，并与H I 21 cm和CO线轮廓进行比较。
• 应用C+在中性原子气体和分子气体中的简单碰撞激发模型，从[ C II]发射推导出H₂柱密度。
• 将[ C II]/CO线比和CO柱密度与气体密度、热压力及辐射场强度相关联，以推断CO暗态气体的物理条件。
• 通过引入[ C II]和[ O I]作为冷却剂，约束气体冷却效率，扩展了先前的XCO测量，纳入了CO暗态H₂气体的贡献。
• 与ALMA和Herschel数据进行比较，以验证分解结果并评估不同示踪剂之间的一致性。

实验结果

研究问题

• RQ1 在N11 HII区中，主导[ C II]发射的ISM相是什么？
• RQ2 N11中总分子气体中CO暗态气体所占比例是多少？该比例如何随局部环境条件变化？
• RQ3 [ C II]/CO线比与CO柱密度和气体密度的相关性如何？这反映了气体的何种物理状态？
• RQ4 大质量恒星的恒星反馈在CO暗态H₂气体的形成和分布中起到多大作用？
• RQ5 当CO光学厚度较薄或完全缺失时，[ C II]发射是否仍可作为总分子气体的可靠示踪剂？”

主要发现

• 在N11 HII区中，超过90%的[ C II]发射源自CO暗态H₂气体，表明[ C II]是探测CO无法检测到的分子气体的主要示踪剂。
• 在CO柱密度较低（<10²⁰.⁵ cm⁻²）的区域，CO暗态H₂气体的比例最高，且在较高柱密度时出现向CO明亮气体的急剧转变。
• CO暗态H₂气体的典型密度约为~200 cm⁻³，热压力范围为10³.⁵–10⁵ K cm⁻³，且不总是与周围中性原子气体处于压力平衡状态。
• 随着CO柱密度增加，CO暗态H₂气体的比例下降，但在CO柱密度超过10²⁰.⁵ cm⁻²后存在显著离散性，提示恒星反馈可能在破坏分子云中起作用。
• 包含CO暗态H₂气体的等效XCO因子范围为10²¹至10²² (K km s⁻¹)⁻¹，适用于最亮的[ C II]成分，表明[ C II]比单独使用CO更能准确追踪总分子气体。
• 有间接证据表明，在相同CO柱密度下，SMC中CO暗态H₂气体的比例高于LMC，尽管该结论受限于气体温度的不确定性。