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arXiv 1004.2822astro-ph.SR
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中等质量（IM）0类原恒星的化学研究。CO耗竭和N2H +氘化

Chemical study of intermediate-mass (IM) Class 0 protostars. CO depletion and N2H+ deuteration

T. Alonso-Albi, A. Fuente, N. Crimier, P. Caselli, C. Ceccarelli, D. Johnstone, P. Planesas, J. R. Rizzo, F. Wyrowski, M. Tafalla, B. Lefloch, S. Maret, C. Dominik

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摘要：

我们正在对IM 0类原恒星的代表性样本中的原恒星封套进行物理和化学研究。在我们的第一篇论文（Criminal et al. 2010）中，我们确定了原恒星包层的物理结构（密度-温度径向分布）。在这里，我们研究了CO耗尽和N2 H+氘代。我们用IRAM 30米望远镜观测到了C_（18）O、C_（17）O、N_2 H ~+和N_2D ~+的毫米谱线。在此基础上，我们利用辐射转移程序计算了C18 O、N2 H+和N2 D+在其包层上的径向丰度分布。此外，我们模拟了原恒星包层的化学性质。所有的C18 O 1-0图都很好地拟合，假设C18 O丰度在原恒星包层内向内减少，直到气体和尘埃达到CO蒸发温度20- 25 K，CO被释放回气相。0类IM中的N2 H+氘分馏为[N2 D +]/[N2 H +]=0.005-0.014，比星际介质中的元素[D/H]值高两个数量级，但比星前团块低10倍。化学模型占C18 O和N2 H+的意见，如果我们假设的CO丰度是2倍低于内信封的典型值。这可能是由于CO在蒸发和/或CO被恒星UV辐射光解之前在颗粒表面转化为CH 3OH的结果。氘分馏不能用化学模型来拟合。这种差异很可能是由于我们的模型假设为球形几何形状，忽略了双极外流和恒星紫外辐射等重要现象的影响。更重要的是，氘分馏是依赖于邻-对氢的比例，这是不可能达到稳态值在这些原恒星的动态时间尺度。

英文摘要：

We are carrying out a physical and chemical study of the protostellar envelopes in a representative sample of IM Class 0 protostars. In our first paper (Crimier et al. 2010), we determined the physical structure (density-temperature radial profiles) of the protostellar envelopes. Here, we study the CO depletion and N2H+ deuteration. We observed the millimeter lines of C18O, C17O, N2H+ and N2D+ toward the protostars using the IRAM 30m telescope. Based on these observations, we derived the C18O, N2H+ and N2D+ radial abundance profiles across their envelopes using a radiative transfer code. In addition, we modeled the chemistry of the protostellar envelopes. All the C18O 1-0 maps are well fit assuming that the C18O abundance decreases inwards within the protostellar envelope until the gas and dust reach the CO evaporation temperature, 20-25K, where the CO is released back to the gas phase. The N2H+ deuterium fractionation in Class 0 IMs is [N2D+]/[N2H+]=0.005-0.014, two orders of magnitude higher than the elemental [D/H] value in the interstellar medium, but a factor of 10 lower than in pre-stellar clumps. Chemical models account for the C18O and N2H+ observations if we assume the CO abundance is 2 times lower than the canonical value in the inner envelope. This could be the consequence of the CO being converted into CH3OH on the grain surfaces prior to the evaporation and/or the photodissociation of CO by the stellar UV radiation. The deuterium fractionation is not fitted by chemical models. This discrepancy is very likely caused by the simplicity of our model that assumes spherical geometry and neglects important phenomena like the effect of bipolar outflows and UV radiation from the star. More important, the deuterium fractionation is dependent on the ortho-to-para H2 ratio, which is not likely to reach the steady-state value in the dynamical time scales of these protostars.

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