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arXiv 1305.3471astro-ph.GA
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高质量恒星形成区外流的演化和激发条件

Evolution and excitation conditions of outflows in high-mass star-forming regions

A. Sanchez-Monge, A. Lopez-Sepulcre, R. Cesaroni, C. M. Walmsley, C. Codella, M. T. Beltran, M. Pestalozzi, S. Molinari

摘要:

理论模型表明,大质量恒星是通过盘介导的吸积形成的,双极外流起着重要作用。最近对大量分子外流的研究表明,随着物体的发展,SiO线强度会降低。本研究的目的是表征随时间的变化的分子外流的性质,并在研究与大规模YSO外流的SiO激发条件。我们使用IRAM 30米望远镜在SiO(2-1),SiO(5-4)和HCO+(1-0)流出线,以及几个致密气体和热核示踪剂中绘制了14个大质量恒星形成区。Hi-GAL数据被用来改善光谱能量分布和L/M比,这被认为是一个很好的指示器的演化阶段的YSO。我们检测SiO和HCO+流出排放的所有来源,和双极结构,其中六个。外流参数与其他大质量YSO的外流参数相似。我们发现HCO+的流出能量随着天体的演化而增加,SiO的丰度随着时间的推移而减少,从10^(-8)减少到10^(-9)。发现SiO(5-4)与(2-1)的线比在环境气体速度下较低,并且随着我们移动到高速度而增加,这表明SiO的激发条件随着气体的速度而变化(对于高速气体组分具有较大的密度和/或温度)。SiO和HCO+外流排放的属性表明,SiO在第一个演化阶段大大增强的情况下,可能是由于原恒星喷流产生的强烈冲击。随着天体的演化,喷流的能量会降低,SiO的丰度也会降低。然而,在这个过程中,原恒星周围的物质会被喷流卷起,流出的活动,被夹带的分子物质(HCO+)追踪,会随着时间的推移而增加。

英文摘要:

Theoretical models suggest that massive stars form via disk-mediated accretion, with bipolar outflows playing a fundamental role. A recent study toward massive molecular outflows has revealed a decrease of the SiO line intensity as the object evolves. The present study aims at characterizing the variation of the molecular outflow properties with time, and at studying the SiO excitation conditions in outflows associated with massive YSOs. We used the IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1), SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core tracers. Hi-GAL data was used to improve the spectral energy distributions and the L/M ratio, which is believed to be a good indicator of the evolutionary stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources, and bipolar structures in six of them. The outflow parameters are similar to those found toward other massive YSOs. We find an increase of the HCO+ outflow energetics as the object evolve, and a decrease of the SiO abundance with time, from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at the ambient gas velocity, and increases as we move to high velocities, indicating that the excitation conditions of the SiO change with the velocity of the gas (with larger densities and/or temperatures for the high-velocity gas component). The properties of the SiO and HCO+ outflow emission suggest a scenario in which SiO is largely enhanced in the first evolutionary stages, probably due to strong shocks produced by the protostellar jet. As the object evolves, the power of the jet would decrease and so does the SiO abundance. During this process, however, the material surrounding the protostar would have been been swept up by the jet, and the outflow activity, traced by entrained molecular material (HCO+), would increase with time.

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