THOR: The H I, OH, Recombination line survey of the Milky Way. The pilot study: H I observations of the giant molecular cloud W43-猫眼课题宝

DOI：

10.1051/0004-6361/201425370

发表时间：

2015-05

期刊：

Astronomy and Astrophysics

影响因子：

6.5

通讯作者：

中科院分区：

物理与天体物理2区

文献类型：

作者：

S. Bihr;H. Beuther;J. Ott;K. Johnston;A. Brunthaler;L. Anderson;F. Bigiel;P. Carlhoff;E. Churchwell;S. Glover;P. Goldsmith;F. Heitsch;T. Henning;M. Heyer;T. Hill;A. Hughes;R. Klessen;R. Klessen;R. Klessen;H. Linz;S. Longmore;N. McClure-Griffiths;K. Menten;F. Motte;Q. Nguyen-Luong;R. Plume;S. Ragan;N. Roy;N. Roy;P. Schilke;Nathan Schneider;Rowan J. Smith;J. Stil;J. Urquhart;A. Walsh;F. Walter

为了研究银河系中巨分子云（GMCs）的原子、分子和电离辐射，我们利用卡尔·G·央斯基甚大阵（VLA）启动了一个大型项目：“THOR：银河系的氢Ⅰ、羟基、复合线观测”。我们绘制了银河系相当一部分区域（银经 = 15° - 67°，|银纬| ≤ 1°）在1 - 2千兆赫兹的21厘米氢Ⅰ线、4条羟基线、多达19条氢α复合线以及连续谱，角分辨率约为20″。从2012年开始，作为一项试点研究，我们绘制了与W43恒星形成复合体相关的4平方度的巨分子云。THOR观测区域的其余部分在2013年和2014年进行了观测。在本文中，我们重点关注W43巨分子云复合体的氢Ⅰ辐射。传统上，氢Ⅰ的21厘米线被视为光学薄的，并在此假设下计算诸如柱密度等属性。这种方法对于低质量恒星形成区域可能会产生合理的结果，但不足以描述巨分子云。我们分析了强连续谱源以测量沿视线方向的光学深度，从而校正氢Ⅰ的21厘米辐射的光学深度效应和微弱的漫射连续谱辐射。因此，我们能够更准确地测量该区域的氢Ⅰ质量，我们的分析揭示了氢Ⅰ质量的下限为M = 6.6 - 1.8×10⁶ M⊙（视向速度 = 60 - 120千米/秒⁻¹），这比在光学薄辐射假设下估计的质量大2.4倍。氢Ⅰ柱密度高达Nₕᵢ ~ 150 M⊙/pc⁻² ≈ 1.9×10²² cm⁻²，比低质量恒星形成区域高一个数量级。这一结果对理论模型提出了挑战，这些理论模型预测氢Ⅰ柱密度的阈值约为10 M⊙/pc⁻²，在此阈值下分子氢应该开始形成。通过假设W43为椭圆层状结构，我们估算了粒子密度分布。对于原子气体粒子密度，我们发现朝着W43中心呈线性下降，其值从云边缘附近的nₕᵢ = 20 cm⁻³下降到中心几乎为0 cm⁻³。另一方面，通过赫歇尔空间天文台的尘埃观测追踪的分子氢，朝着中心呈指数增长，在约10 pc的区域内平均密度增加到nₕ₂ > 200 cm⁻³。虽然原子氢和分子氢在云边缘混合良好，但云的中心强烈地由H₂辐射主导。我们没有发现原子氢和分子氢之间的急剧转变。我们的结果对当前的理论模型提出了挑战，是对极端环境中原子氢到分子氢转变的重要表征。

To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): “THOR: The H i, OH, Recombination line survey of the Milky Way”. We map the 21 cm H i line, 4 OH lines, up to 19 Hα recombination lines and thecontinuum from 1 to 2 GHz of a significant fraction of the Milky Way (l = 15°−67°, | b | ≤ 1°) at an angular resolution of ~ 20″. Starting in 2012, as a pilot study we mapped 4 square degrees of the GMC associated with the W43 star formation complex. The rest of the THOR survey area was observed during 2013 and 2014. In this paper, we focus on the H i emission from the W43 GMC complex. Classically, the H i 21 cm line is treated as optically thin with properties such as the column density calculated under this assumption. This approach might yield reasonable results for regions of low-mass star formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth along the line of sight, and thus correct the H i 21 cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the H i mass of this region more accurately and our analysis reveals a lower limit for the H i mass of M = 6.6-1.8 × 106 M⊙ (vLSR = 60−120 km s-1), which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The H i column densities are as high as NH i ~ 150 M⊙ pc-2 ≈ 1.9 × 1022 cm-2, which is an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a threshold for the H i column density of ~10 M⊙ pc-2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profile. For the atomic gas particle density, we find a linear decrease toward the center of W43 with values decreasing from nH i = 20 cm-3 near the cloud edge to almost 0 cm-3 at its center. On the other hand, the molecular hydrogen, traced via dust observations with the Herschel Space Observatory, shows an exponential increase toward the center with densities increasing to nH2> 200 cm-3, averaged over a region of ~10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H2 emission. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.

THOR: The H I, OH, Recombination line survey of the Milky Way. The pilot study: H I observations of the giant molecular cloud W43

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