大质量年轻恒星天体中的扭转激发甲醇：中红外辐射场的作用

中文摘要 AI 辅助

在中红外（mid-IR）波段观测大质量恒星形成的最早阶段具有挑战性，因为消光较高且分辨率有限。若干分子的毫米波振动和扭转激发跃迁已被提出作为年轻恒星天体（YSOs）周围局部中红外辐射场的间接探针。我们研究了12个处于不同演化阶段的大质量恒星形成团块中扭转激发甲醇（$\mathrm{CH_3OH}$）的激发和空间分布，评估其作为嵌入原恒星中红外光度示踪物的潜力。我们分析了阿塔卡马大型毫米/亚毫米波阵列（ALMA）890 $\mathrm{\mu m}$ 观测中同一扭转激发态 $v_\mathrm{t}$$\geq$1 内的甲醇转动跃迁，并通过绘制以甲醇丰度归一化的谱线光度与中红外光度的关系，将其与斯皮策（Spitzer）24 $\mathrm{\mu m}$ 数据进行比较。扭转激发甲醇发射通常致密，并系统性地与ALMA尘埃连续谱峰值和24 $\mathrm{\mu m}$ 发射重合，即使在微弱源中也是如此。唯一没有红外对应体的团块也显示出无 $v_\mathrm{t}$$\geq$1 发射，支持辐射泵浦作为主导激发机制。我们发现 $v_\mathrm{t}$$\geq$1 甲醇谱线与24 $\mathrm{\mu m}$ 光度之间存在有前景的幂律趋势，且该趋势在很大程度上与甲醇丰度无关。$\mathrm{CH_3OH}$ $v_\mathrm{t}$$\geq$1 跃迁似乎是嵌入原恒星局部中红外辐射场的有前景的示踪物，即使在高度消光区域也能提供核尺度信息。作为一项初步研究，我们的结果表明，将研究扩展到更大的统计样本对于测试和改进该方法至关重要。

英文摘要

Observing the earliest stages of high-mass star formation in the mid-infrared (mid-IR) is challenging because the extinction is high and the resolution limited. Millimeter vibrationally and torsionally excited transitions of several molecules have been proposed as indirect probes of local mid-IR radiation fields around young stellar objects (YSOs). We investigate the excitation and spatial distribution of torsionally excited methanol ($\mathrm{CH_3OH}$) in 12 high-mass star-forming clumps in different evolutionary stages, assessing their potential as tracers of the mid-IR luminosity of embedded protostars. We analyzed rotational transitions of methanol within the same torsionally excited states $v_\mathrm{t}$$\geq$1 in the Atacama Large Millimeter/submillimeter Array (ALMA) 890 $\mathrm{μm}$ observations and compared them with Spitzer 24 $\mathrm{μm}$ data by plotting line luminosities normalized by methanol abundance against mid-IR luminosities. The torsionally excited methanol emission is generally compact and systematically coincident with ALMA dust continuum peaks and 24 $\mathrm{μm}$ emission, even in faint sources. The only clump without an IR counterpart also shows no $v_\mathrm{t}$$\geq$1 emission, supporting radiative pumping as the dominant excitation mechanism. We found a promising power-law trend between $v_\mathrm{t}$$\geq$1 methanol lines and 24 $\mathrm{μm}$ luminosities, largely independent of the methanol abundance. $\mathrm{CH_3OH}$ $v_\mathrm{t}$$\geq$1 transitions appear to be promising tracers of local mid-IR radiation fields of embedded protostars, providing core-scale information even in highly extincted regions. As a pilot study, our results suggest that extending our study to a larger statistical sample is crucial for testing and improving this method.

发表机构

• Università degli Studi di Cagliari（卡利亚里大学）
• INAF - Osservatorio Astronomico di Cagliari（意大利国家天体物理研究所卡利亚里天文台）
• INAF - Istituto di Radioastronomia di Bologna（意大利国家天体物理研究所博洛无线电天文研究所）
• INAF - Istituto di Astrofisica e Planetologia Spaziali（意大利国家天体物理研究所天体物理学与行星科学研究所）
• Alma Mater Studiorum – Università di Bologna（博洛尼亚大学）
• INAF-Osservatorio Astrofisico di Arcetri（意大利国家天体物理研究所阿切特里天文台）
• Centre for Astrophysics and Planetary Science, University of Kent（肯特大学天体物理学与行星科学中心）
• Haystack Observatory, Massachusetts Institute of Technology（麻省理工学院海斯塔克天文台）
• European Southern Observatory（欧洲南方天文台）
• Osservatorio Astronomico di Brera（布雷拉天文台）

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