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arXiv 1404.1202astro-ph.GA
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M51螺旋臂3 mm和2 mm波段谱线测量

Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands

Yoshimasa Watanabe, Nami Sakai, Kazuo Sorai, Satoshi Yamamoto

摘要:

我们用IRAM 30 m望远镜对M51(NGC 5194)旋臂的两个位置进行了3 mm和2 mm波段的谱线测量。在本次调查中,我们已经确定了13种分子物种,包括CN,CCH,N2 H+,HNCO,和CH 3OH。此外,还检测到主要物种的6种同位素。另一方面,未检测到SiO、HC 3 N、CH 3CN和氘代物质如DCN和DCO+。DCN/HCN和DCO+/HCO+的氘分馏比分别小于0.8%和1.2%。通过比较两个不同恒星形成活动位置的结果,我们发现观测到的化学成分并不强烈地依赖于恒星形成活动。它们似乎反映了在1千秒差距尺度范围内平均的化学成分,包括许多巨大的分子云。在检测到的分子CN,CCH和CH 3OH被发现是丰富的。CN和CCH的高丰度与上述分子广泛分布的图像一致,因为它们可以通过光解产生。另一方面,CH 3OH似乎可能通过与大尺度现象(如云-云碰撞)相关的冲击和/或通过非热解吸过程(如宇宙射线诱导的UV光子引起的光蒸发)释放到气相中。目前的结果表明,一个巨大的分子云复杂的螺旋臂的特征化学成分,它可以作为一个标准的参考研究活动星系核和星爆的化学。

英文摘要:

We have conducted a spectral line survey in the 3 mm and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the IRAM 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N2H+, HNCO, and CH3OH. Furthermore, 6 isotopologues of the major species have been detected. On the other hand, SiO, HC3N, CH3CN, and the deuterated species such as DCN and DCO+ are not detected. The deuterium fractionation ratios are evaluated to be less than 0.8 % and 1.2 % for DCN/HCN and DCO+/HCO+, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1-kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH3OH are found to be abundant. High abundances of CN, and CCH are consistent with the above picture of a wide spread distribution of molecules, because they can be produced by photodissociation. On the other hand, it seems likely that CH3OH is liberated into the gas phase by shocks associated with large scale phenomena such as cloud-cloud collisions and/or by non-thermal desorption processes such as photoevaporation due to cosmic-ray induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in AGNs and starbursts.

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