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arXiv 1305.2292astro-ph.GA
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猎户座B9-SMM 6星前核碎片内致密凝聚的动力学、CO损耗和氘分馏

Dynamics, CO depletion, and deuterium fractionation of the dense condensations within the fragmented prestellar core Orion B9-SMM 6

Oskari Miettinen, Stella S. R. Offner

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Accepted to A&A; 12 pages, 3 figures, 5 tables
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摘要：

我们目前的APEX观测C17 O（2-1），N2 H+（3-2），和N2 D+（3-2）对亚碎片内部的星前核心SMM 6在猎户座B 9。我们将这些谱线数据与我们以前的SABOCA 350 μm尘埃连续谱图相结合。亚碎片的特征是亚音速内部非热运动（σNT~0.5cs），它们中的大多数似乎是引力束缚的。N_2H ~+速度质心在凝聚体之间的分散度很低（0.02 km/s）。我们推导出的CO消耗因子fD=0.8+/-0.4至3.6+/-1.5，并不表明任何显著的CO冻结，但这可能是由于我们采用的典型CO丰度。N2 H+和N2 D+相对于H2的丰度分数分别为~0.9-2.3x10^-9和~4.9-9.9x10^-10。N2 H+的氘分馏或N2 D +/N2 H+柱密度比在0.30+/-0.07-0.43 +/-0.09的范围内。在SMM 6内部检测到的子结构很可能是圆柱形琼斯型引力碎片的结果。我们估计这种碎裂的时间尺度约为1.8x10^5年。在局部的引力坍缩膨胀之前，凝聚体不太可能相互作用并合并。此外，显着的质量增长的凝聚通过竞争性的吸积从母核似乎是不可行的。在冷凝的分子氘的高水平表明，气相CO应强烈耗尽。它还指向化学进化的高级阶段。因此，SMM 6的子碎片可能接近引力坍缩的开始，但它们是否能形成原恒星或亚恒星物体（褐矮星）取决于当地的恒星形成效率，仍有待澄清。

英文摘要：

We present APEX observations of C17O(2-1), N2H+(3-2), and N2D+(3-2) towards the subfragments inside the prestellar core SMM 6 in Orion B9. We combined these spectral line data with our previous SABOCA 350-μm dust continuum map of the source. The subfragments are characterised by subsonic internal non-thermal motions (σNT~0.5cs), and most of them appear to be gravitationally bound. The dispersion of the N2H+ velocity centroids among the condensations is very low (0.02 km/s). The CO depletion factors we derive, fD=0.8+/-0.4 - 3.6+/-1.5, do not suggest any significant CO freeze-out but this may be due to the canonical CO abundance we adopt. The fractional abundances of N2H+ and N2D+ with respect to H2 are found to be ~0.9-2.3x10^-9 and ~4.9-9.9x10^-10, respectively. The deuterium fractionation of N2H+, or the N2D+/N2H+ column density ratio, lies in the range 0.30+/-0.07 - 0.43+/-0.09. The detected substructure inside SMM 6 is likely the result of cylindrical Jeans-type gravitational fragmentation. We estimate the timescale for this fragmentation to be ~1.8x10^5 yr. The condensations are unlikely to be able to interact with one another and coalesce before local gravitational collapse ensues. Moreover, significant mass growth of the condensations via competitive-like accretion from the parent core seems unfeasible. The high level of molecular deuteration in the condensations suggests that gas-phase CO should be strongly depleted. It also points towards an advanced stage of chemical evolution. The subfragments of SMM 6 might therefore be near the onset of gravitational collapse, but whether they can form protostellar or substellar objects (brown dwarfs) depends on the local star formation efficiency and remains to be clarified.

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