Formation of methyl formate after cosmic ion irradiation of icy grain mantles
Formation of methyl formate after cosmic ion irradiation of icy grain mantles
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DOI：
10.1051/0004-6361/201014101
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发表时间：
2010-09-01
影响因子：
6.5
通讯作者：
Palumbo, M. E.
中科院分区：
文献类型：
--
作者：
Modica, P.;Palumbo, M. E.
上下文。甲酸甲酯（Methyl formate, HCOOCH3）是一种复杂的有机分子，存在于热岩心和热玉米粒中。气相化学不能再现其观测到的丰度，相对于h -2 - aims，其丰度通常在10（-7）和10（-9）之间变化。为了研究甲酸甲酯的固态形成途径，进行了实验室实验，以获得可以形成的量的估计，并验证它是否可以解释观察到的丰度。在4400-400 cm（-1）范围内用红外光谱分析了几个天体物理学感兴趣的固体样品（16k）。通过推导其波段强度值，研究了冷冻甲酸甲酯的红外光谱特性。比较了样品在不同温度下的光谱，分析了加热对样品的影响。为了研究星际冰中甲酸甲酯的形成和破坏机理，用200 keV质子在16 K下照射甲醇（CH3OH）和一氧化碳（CO）冰的二元混合物和纯甲醇样品。在1160 cm（-1）处通过基本模式（CH3摇摆）鉴定出甲酸甲酯。我们展示了甲酸甲酯冰的中红外光谱，显示了无定形（16 K）和晶体（110 K）结构。我们报告了六个主要甲酸甲酯带强度值的新测量。我们证明了在CH3OH和CO:CH3OH混合物的照射下甲酸甲酯的形成和破坏。将我们的结果外推到星际介质条件下，我们发现甲酸甲酯的产生时间尺度与分子云的演化时间吻合得很好。与观测资料的比较表明，辐照后形成的甲酸甲酯量可以解释观测到的丰度。本研究结果表明，在致密分子云中观测到的气相甲酸甲酯是在含CO和CH3OH的冰粒地幔的宇宙离子辐照后以固态形成的，并在冰粒地幔解吸后释放到气相。
Context. Methyl formate (HCOOCH3) is a complex organic molecule detected in hot cores and hot corinos. Gas-phase chemistry fails to reproduce its observed abundance, which usually varies between 10(-7) and 10(-9) with respect to H-2.Aims. Laboratory experiments were performed in order to investigate a solid-state route of methyl formate formation, to obtain an estimate of the amount that can be formed, and to verify whether it can account for the observed abundances.Methods. Several solid samples (16 K) of astrophysical interest were analyzed by infrared spectroscopy in the 4400-400 cm(-1) range. The infrared spectral characteristics of frozen methyl formate were studied by deriving their band strength values. The effects produced upon warm-up of the samples were analyzed comparing the spectra taken at different temperatures. In order to study the formation and destruction mechanism of methyl formate in the interstellar ices, a binary mixture of methanol (CH3OH) and carbon monoxide (CO) ice and a sample of pure methanol were irradiated at 16 K with 200 keV protons. Methyl formate was identified through its fundamental mode (CH3 rocking) at about 1160 cm(-1).Results. We present the mid-infrared methyl formate ice spectrum showing both the amorphous (16 K) and the crystalline (110 K) structure. We report novel measurements of the band strength values of the six main methyl formate bands. We prove the formation and the destruction of methyl formate after irradiation of CH3OH and a CO:CH3OH mixture. Extrapolating our results to the interstellar medium conditions we found that the production timescale of methyl formate agrees well with the evolutionary time of molecular clouds. The comparison with the observational data indicates that the amount of methyl formate formed after irradiation can account for the observed abundances.Conclusions. The present results allow us to suggest that gas phase methyl formate observed in dense molecular clouds is formed in the solid state after cosmic ion irradiation of icy grain mantles containing CO and CH3OH and released to the gas phase after desorption of icy mantles.