Molecular abundance variations in the Magellanic Clouds
Molecular abundance variations in the Magellanic Clouds
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麦哲伦云中的分子丰度变化
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
H. Olofsson
中科院分区：
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作者：
A. Heikkilä;L. Johansson;H. Olofsson
我们观测了金属丰度和远紫外线(FUV)辐射对小麦哲伦云(SMC)和大麦哲伦云(LMC)的物理条件和星际云中分子丰度的影响。在LMC中位于剑鱼座30度(30Dor)和N159区南部及其之间的一系列云中，以及位于SMC条中的一颗云(n27，也称为LIRS49)中，观察到了一些分子的谱线发射。物理条件和分子丰度由观测数据通过激发和辐射转移计算得到。给出了SMC、LMC和银河系云中分子丰度的比较。我们还报道了首次在河外源中检测到硫化氢(邻位-H2 S)，在热发射中检测到甲醇(CH3OH)和甲基乙炔(CH3CCH)，并在N159W中初步检测到硫代甲醛(H2CS)。LMC源和n27源中分子物种(相对于H2)的丰度(相对于H2)估计分别为5×10−10和1×10−10。这些值适用于由CO管路排放定义的气体体积。相对于银河系云，N159W(我们的参考云)的丰度要低五到二十倍。在两个云中：N27和30Dor(30Dor-10)中最中心的云中，导出的丰度与我们样本中其他云中的丰度显著偏离，分别平均低6倍和8倍。在n27，最可能的解释是SMC的金属丰度较低，而30Dor-10的低丰度可能主要是由于该地区较强的FUV辐射导致的较快的光解离所致。对于n27和30Dor-10中的低丰度，另一种解释是这些分子云中存在较高的H/H2比。乙炔自由基(C2H)是向A.Heikkilä发送退印请求后观测到的最丰富的示踪分子，估计其平均丰度分别为5×10−9和3×10−9。基于使用瑞典-欧洲南方天文台亚毫米望远镜(SEST)在智利拉西拉的欧洲南方天文台进行的观测。?？图3-14仅在本白皮书的电子版中可用。?？?现地址：芬兰赫尔辛基大学FIN-000 14，邮政信箱14号天文台，CO同位素。定性地说，高的C2H丰度可以解释为反映了富C+和富FUV光子的环境，即光子占优势的区域的化学特征。对于n27，我们使用HCO+和H13CO+数据，估计气相12C/13C比率为40-90，这个范围包括在N159W和银河系盘云中发现的值。在我们样本中的所有云中，从CS、SO、HCO+、HCN和H_2CO的激发分析估计的数密度在(1-100)×10~4厘米−3的范围内。CO数据给出了几个×10~3厘米−3的下限。然而，平均密度(从维里质量估计)要低得多，通常是几个×10~2厘米−3，这表明云(由痕量分子探测)非常密集，体积填充因子为1。在N159W，我们的数据库是迄今为止最广泛的，计算得到该气体致密部分分子氢的数密度为(1-10)×10~5 cm−-3，动力学温度为25±10K。与N159W相比，n27中相应的数值为(5-50)×10~4 cm−~3和15±5K。因此，LMC和SMC之间的金属丰度差异似乎对气体的密度和温度影响不大。在SMC中，CO(J=1-0)/HCO+(J=1-0)谱线强度比与LMC具有相同的趋势：在成星活动较强的云中，这一比值较低。LMC的CO(J=1-0)/C2H(N=1-0)谱线强度比也表现出类似的趋势。Ky单词：ISM：分子-星系：丰度-星系：ISM-星系：麦哲伦云-射电线：星系-射电线：ISM
We have observationally studied the effect of metallicity and far-ultraviolet (FUV) radiation on the physical conditions and the molecular abundances in interstellar clouds in the Small and the Large Magellanic Clouds (SMC and LMC, respectively). Spectral line emission from a number of molecules was observed in a sequence of clouds with positions in and between the 30 Doradus (30Dor) and the southern part of the N159 region in the LMC, and in one cloud (N27, also denoted LIRS 49) located in the SMC bar. Physical conditions and molecular abundances were estimated from the observational data by excitation and radiative transfer calculations. A comparison of the molecular abundances in clouds in the SMC, the LMC, and the Galaxy is presented. We also report the first detection of hydrogen sulphide (ortho-H 2S) in an extragalactic source, detections of methanol (CH3OH) in thermal emission and methyl acetylene (CH3CCH), and a tentative detection of thio-formaldehyde (H2CS) in N159W. The abundances (relative to H 2) of molecular species (except CO) in the LMC sources and in N27 are estimated to be typically 5×10−10, and 1×10−10, respectively. These values apply to the gas volume defined by the CO line emission. Relative to Galactic clouds, the abundances in N159W (our reference cloud) are five to twenty times lower. In two of the clouds: N27 and the centremost cloud in 30Dor (30Dor-10), the derived abundances deviate significantly from those in the other clouds in our sample, by being on the average six and eight times lower, respectively. In N27, the most likely explanation is the lower metallicity in the SMC, whereas the underabundance in 30Dor-10 is probably mainly caused by a more rapid photodissociation due to the more intense FUV radiation in this area. An alternative explanation for the underabundances in both N27 and 30Dor-10 would be a higher H/H2 ratio inside these molecular clouds. The ethynyl radical (C2H), with an estimated average abundance of 5 ×10−9 in seven clouds in the LMC and 3 ×10−9 in two clouds in the SMC, is the most abundant observed trace molecule after the Send offprint requests to : A. Heikkilä ? Based on observations using the Swedish-ESO Submillimetre Telescope (SEST) at the European Southern Observatory (ESO), La Silla, Chile. ?? Figs. 3–14 are only available in the electronic version of this paper. ??? Present address: Observatory, P.O. Box 14, FIN-000 14 University of Helsinki, Finland CO isotopomers. Qualitatively, the high C 2H abundance can be explained as reflecting the C +-rich and FUV photon-rich environment, i.e., a chemistry characteristic for photon-dominated regions. For N27 we have, using HCO + and H13CO+ data, estimated the gas-phase 12C/13C ratio to be 40–90, a range that encompasses the values found in N159W and in Galactic disc clouds. In all clouds in our sample, the number density estimates from an excitation analysis of CS, SO, HCO +, HCN and H2CO are in the range (1–100) ×104 cm−3. CO data gives a lower limit of a few×103 cm−3. However, the average densities (estimated from the virial mass) are significantly lower, typically a few×102 cm−3, suggesting that the clouds (as probed by trace molecules) are very clumpy with volume-filling factors 1. In N159W, where our data-base is by far most extensive, the number density and the kinetic temperature of molecular hydrogen in the dense part of the gas are estimated to be (1–10)×105 cm−3 and 25±10 K, respectively. The corresponding numbers in N27 are, although based on less data than in N159W, (5–50) ×104 cm−3 and 15±5 K. Thus, the metallicity difference between the LMC and the SMC does not seem to affect the density and the temperature of the gas dramatically. In the SMC, the CO( J=1–0)/HCO+(J=1–0) line intensity ratio follows the same trend with respect to the star-formation activity as in the LMC: a lower ratio is found in clouds with a more vigorous star-formation activity. A similar trend is also exhibited by the CO( J=1–0)/C2H(N=1–0) line intensity ratio in the LMC. K y words: ISM: molecules – galaxies: abundances – galaxies: ISM – galaxies: Magellanic Clouds – radio lines: galaxies – radio lines: ISM