I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany.
1] Department of Physics, PO Box 64, 00014 University of Helsinki, Finland [2] Max-Planck Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85741 Garching bei München, Germany.
Nature. 2014 Dec 11;516(7530):219-21. doi: 10.1038/nature13924. Epub 2014 Nov 17.
The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse, whereas others predict timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, H2, which decreases monotonically with age. This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium (N2H(+)) was used as a chemical clock to infer that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-H2D(+) and para-H2D(+), have the most direct chemical connections to H2 (refs 8, 9, 10, 11, 12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-H2D(+) is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory, and recent progress in instrumentation technology. Here we report observations of ortho- and para-H2D(+) emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier N2H(+) work arises because that chemical clock turns off sooner than the H2D(+) clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years.
密集星际云核的年龄是恒星形成的关键参数，很难测量。一些模型预测会迅速坍塌，而另一些则预测需要超过一百万年的时间（参考文献 3）。确定年龄的一种可能方法是通过云收缩时发生的化学变化，特别是通过间接测量分子氢（H2）两种旋转异构体（顺/反）的比例来实现，随着年龄的增长，该比例会单调下降。对于密集云核 L183，已经进行了这种测量，其中叠氮阳离子（N2H(+）的氘同位素分馏被用作化学时钟，以推断该核心已经迅速收缩（在不到 700,000 年的时间尺度上）。在天文学家可观测的分子中，氘代三氢阳离子顺式-H2D(+)和反式-H2D(+)的旋转异构体与 H2 具有最直接的化学联系，它们的丰度比提供了对云核更老年龄敏感的化学时钟。到目前为止，尚未确定该比值，因为反式-H2D(+)非常难以观测。由于在实验室中准确测量了其跃迁频率，并得益于仪器技术的最新进展，现在才有可能探测到其转动基态线。在这里，我们报告了分别来自密集云核 IRAS 16293-2422 A/B 的顺式和反式-H2D(+)发射和吸收的观测结果，IRAS 16293-2422 A/B 是一组新生的太阳型恒星（年龄小于 100,000 年）。使用顺/反比值与化学模型相结合，我们发现密集核心至少已经经历了一百万年的化学处理。与早期的 N2H(+)工作出现明显差异的原因是，该化学时钟比 H2D(+)时钟更早关闭，但这两个结果都表明，恒星形成的密集核心的年龄约为一百万年，而不是 100,000 年。
