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arXiv 1402.2329astro-ph.GA
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原恒星激波L1157-B1中的分子离子

Molecular ions in the protostellar shock L1157-B1

L. Podio, B. Lefloch, C. Ceccarelli, C. Codella, R. Bachiller

摘要:

我们进行了一个完整的人口普查的分子离子丰度大于1e-10的原恒星激波L1157-B1通过一个公正的高灵敏度的调查与IRAM-30米和Herschel/HIFI。利用LVG辐射转移程序导出了气体物理条件和分子离子丰度。后者进行了比较与估计的稳态丰度在云和它们的演变计算的冲击与化学模型Astrochem。我们检测到HCO+,H13CO+,N2H+,HCS+的排放,并首次在冲击,HOCO+,SO+。大部分发射峰的蓝移速度约为0.5 - 3 km/s,宽度约为4 - 8 km/s,与外流空腔有关(T_kin~20 - 70 K,n(H2)~1e~5 cm~(-3))。观测到的HCO+和N2H+丰度与云中的稳态丰度以及它们在宇宙射线电离率Z = 3e-16 s-1的冲击波中压缩和加热气体中的演化一致。相反,观测到的HOCO+、SO+和HCS+丰度比云中预测的大1 - 2个数量级;另一方面,如果CO2、S或H2S和OCS从激波中的尘埃颗粒上溅射下来,它们在短于激波年龄(~2000年)的时间尺度上会强烈增强。分析表明,HCO+和N2H+是流出腔中震前气体的化石记录,而HOCO+、SO+和HCS+是有效的震前示踪剂,可用于推断震前尘幔释放的CO2和含硫物质的量。HCS+(和CS)丰度的观测表明,OCS应该是一个主要的硫载体上的粮食地幔。然而,所需的外大陆架丰度,以适应观察是1 - 2个数量级大于观察。需要进一步研究,以充分了解含硫物质的化学性质。

英文摘要:

We perform a complete census of molecular ions with an abundance larger than 1e-10 in the protostellar shock L1157-B1 by means of an unbiased high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means of an LVG radiative transfer code the gas physical conditions and fractional abundances of molecular ions are derived. The latter are compared with estimates of steady-state abundances in the cloud and their evolution in the shock calculated with the chemical model Astrochem. We detect emission from HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with respect to systemic, has a width of ~ 4-8 km/s, and is associated with the outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+ abundances are in agreement with steady-state abundances in the cloud and with their evolution in the compressed and heated gas in the shock for cosmic rays ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances, instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the other hand they are strongly enhanced on timescales shorter than the shock age (~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are effective shock tracers and can be used to infer the amount of CO2 and sulphur-bearing species released from dust mantles in the shock. The observed HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur carrier on grain mantles. However, the OCS abundance required to fit the observations is 1-2 orders of magnitude larger than observed. Further studies are required to fully understand the chemistry of sulphur-bearing species.

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