Theoretical, observational, and isotopic estimates of the lifetime of the solar nebula
Theoretical, observational, and isotopic estimates of the lifetime of the solar nebula
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太阳星云寿命的理论、观测和同位素估计
DOI：
10.1111/j.1945-5100.1994.tb00649.x
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发表时间：
1994
期刊：
影响因子：
--
通讯作者：
P. Cassen
中科院分区：
文献类型：
--
作者：
F. Podosek;P. Cassen
陨石的各种同位素数据表明，原恒星星云存在，并参与了大约10年（exp 7）或更长时间的行星物质的形成。然而，许多宇宙化学家主张对这些数据进行另一种解释，以符合一个已知的约束，即从理论上考虑，星云只存在了很短的时间，通常被认为小于或等于10年（exp 6）年。在本文中，我们回顾了与太阳星云持续时间有关的证据，这些证据可通过三个不同的学科获得：恒星形成的理论模型，来自陨石的同位素数据，以及金牛座T星的天文观测。基于目前恒星形成区域观测的理论模型表明，像太阳这样的恒星是由星际介质中星际云中冷分子云密集核心的动态引力坍缩形成的。坍缩成恒星和盘的过程发生得很快，在10（exp 5）年的时间尺度上。盘的演化是在重新分配角动量的同时耗散能量，但是很难预测演化的速率，特别是对于低质量（与恒星相比）的盘，尽管如此，它仍然包含足够的物质来解释观测到的行星系统。从现有的盘状结构和演化理论来看，没有令人信服的证据表明太阳星云一定是快速演化的，不可能持续超过1亿年。在考虑与陨石年代学相关的同位素数据时，我们侧重于三种方法：U-Pb/Pb-Pb的绝对年龄，短寿命放射性核素（特别是Al-26）和Sr-87/Sr-86的演化的相对年龄。两种陨石材料——难熔包裹体，如cai和差异陨石（长辉石和长辉石）——似乎经历了可能可测定年代的星云事件。在这两种情况下，对现有数据最直接的解释表明，星云事件跨越数Ma。我们还考虑了其他解释，特别是Al-26的完全异质分布的假设，这将避免这些按时间顺序的解释。这种替代解释的主要推动力似乎恰恰是对时间顺序解释的排除（即，假设而不是推断星云的寿命很短（小于或等于1 Ma））。对金牛座T星的天文观测表明，尘埃盘的存在即使不是普遍的特征，也是一个普遍的特征，这些尘埃盘的质量足以容纳像我们这样的行星系统，而且至少有一些可以持续110年（exp 7）以上。结果与从陨石同位素资料推断的时间尺度一致。然而，它们不能被认为是关于太阳星云时间尺度的结论性的，部分原因是很难将圆盘观测与影响陨石的过程联系起来，部分原因是为这些恒星分配的年龄可能在任何方向上都有几个因素是错误的。我们的结论是，现有证据的平衡支持这样的观点，即星云存在并活跃了至少几个Ma。然而，由于证据不是确定的，重要的是要将这个问题视为一个悬而未决的问题，其答案应该寻求而不是假设。
There are a variety of isotopic data for meteorites which suggest that the protostellar nebula existed and was involved in making planetary materials for some 10(exp 7) yr or more. Many cosmochemists, however, advocate alternative interpretations of such data in order to comply with a perceived constraint, from theoretical considerations, that the nebula existed only for a much shorter time, usually stated as less than or equal to 10(exp 6) yr. In this paper, we review evidence relevant to solar nebula duration which is available through three different disciplines: theoretical modeling of star formation, isotopic data from meteorites, and astronomical observations of T Tauri stars. Theoretical models based on observations of present star-forming regions indicate that stars like the Sun form by dynamical gravitational collapse of dense cores of cold molcular clouds in the interstellar clouds in the interstellar medium. The collapse to a star and disk occurs rapidly, on a time scale of the order 10(exp 5) yr. Disks evolve by dissipating energy while redistributing angular momentum, but it is difficult to predict the rate of evolution, particularly for low mass (compared to the star) disks which nonetheless still contain enough material to account for the observed planetary system. There is no compelling evidence, from available theories of disk structure and evolution, that the solar nebula must have evolved rapidly and could not have persisted for more than 1 Ma. In considering chronoloically relevant isotopic data for meteorites, we focus on three methodologies: absolute ages by U-Pb/Pb-Pb, and relative ages by short-lived radionuclides (especially Al-26) and by evolution of Sr-87/Sr-86. Two kinds of meteoritic materials-refractory inclusions such as CAIs and differential meteorites (eucrites and augrites) -- appear to have experienced potentially dateable nebular events. In both cases, the most straightforward interpretations of the available data indicate nebular events spanning several Ma. We also consider alternative interpretations, particularly the hypothesis of radically heterogeneous distribution of Al-26, which would avoid these chronological interpretations. The principal impetus for such alternative interpretations seems to be precisely the obviation of the chronological interpretation (i.e., the presumption rather than the inference of a short (less than or equal to 1 Ma) lifetime of the nebula). Astronomical observations of T Tauri stars indicate that the presence of dusty disks is a common if not universal feature, that the disks are massive enough to accomodate a planetary system such as ours, and that at least some persist for 110(exp 7) yr or more. The results are consistent with the time scales inferred from the meteoritic isotopic data. They cannot be considered conclusive with regard to solar nebula time scales, however, in part because it is difficult to relate disk observations to processes that affect meteorites, and in part because the ages assigned for these stars could be wrong by a factor of several in either direction. We conclude that the balance of available evidence favors the view that the nebula existed and was active for at least several Ma. However, because the evidence is not definitive, it is important that the issue be perceived to be an open question, whose answer should be sought rather than presumed.