Molecular freeze-out as a tracer of the thermal and dynamical evolution of pre- and protostellar cores
Molecular freeze-out as a tracer of the thermal and dynamical evolution of pre- and protostellar cores
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分子冻结作为前恒星核心和原恒星核心热和动力学演化的示踪剂
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
S. Observatory
中科院分区：
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作者：
J. Jørgensen;F. Schoeier;E. F. V. D. L. Observatory;S. Observatory
多跃迁观测的辐射传输模型被用来确定前恒星和原恒星核中分子丰度作为位置的函数。这些数据需要一个“下降”的丰度曲线半径，高丰度的最外层地区探测低激发3毫米线，和低得多的丰度在中间地带探测更高的频率线。详细分析CO和HCO +线的一个子集的对象的结果进行说明。我们提出了一个方案，其中分子被冻结在温度足够低（40 K），以防止立即解吸，但密度足够高（>10 4 - 10 5 cm-3），冻结的时间尺度比核心的寿命短的信封的区域。因此，冻结区的大小是岩心热演化和动力演化的记录。16个天体样本的CO数据拟合表明，冻结区的大小在0类和I类天体之间显著减小，这解释了例如CO丰度随包封质量的变化。然而，相应的时间尺度是10 ^{5 pm 0.5}$年，在0类和I类对象之间没有显著差异。这些时间尺度表明，密集的恒星前阶段与沉重的消耗只持续很短的时间，10 - 5年的顺序，与最近的化学动力学模型。
Radiative transfer models of multi-transition observations are used to determine molecular abundances as functions of position in pre- and protostellar cores. The data require a “drop” abundance profile with radius, with high abundances in the outermost regions probed by low excitation 3 mm lines, and much lower abundances at intermediate zones probed by higher frequency lines. The results are illustrated by detailed analysis of CO and HCO + lines for a subset of objects. We propose a scenario in which the molecules are frozen out in a region of the envelope where the temperature is low enough ($la$40 K) to prevent immediate desorption, but where the density is high enough (>10 4 –10 5 cm -3 ) that the freeze-out timescales are shorter than the lifetime of the core. The size of the freeze-out zone is thereby a record of the thermal and dynamical evolution of the cores. Fits to CO data for a sample of 16 objects indicate that the size of the freeze-out zone decreases significantly between class 0 and I objects, explaining the variations in, for example, CO abundances with envelope masses. However, the corresponding timescales are $10^{5pm 0.5}$ years, with no significant difference between class 0 and I objects. These timescales suggest that the dense pre-stellar phase with heavy depletions lasts only a short time, of the order of 10 5 yr, in agreement with recent chemical-dynamical models.