Constraints on Grain Formation around Carbon Stars from Laboratory Studies of Presolar Graphite
Constraints on Grain Formation around Carbon Stars from Laboratory Studies of Presolar Graphite
复制标题
太阳系前石墨的实验室研究对碳星周围晶粒形成的限制
DOI：
--
复制
发表时间：
2005
期刊：
影响因子：
--
通讯作者：
R. Cowsik
中科院分区：
文献类型：
--
作者：
T. Bernatowicz;O. Akande;T. Croat;R. Cowsik
我们报告的物理条件的调查结果，在质量流出的渐近巨型分支（AGB）碳星，所需的微米级的前太阳石墨颗粒的形成，有和没有以前形成的内部晶体的碳化钛（TiC）。导出了能为太阳星云贡献颗粒的恒星的质量下限为1.1M。这个质量极限，结合碳星的质光关系，确定了与太阳前石墨产生相关的H-R图区域。详细的动力学模型的AGB流出，沿着与动力学和平衡热力学提供的约束，表明颗粒形成发生在半径从2.3到3.7 Au的AGB碳星在1.1-5 M的范围。该分析还产生了几年左右的石墨生长可用的时间间隔。通过考虑碳星的光度变化，我们表明，在最低光度期间形成的颗粒很可能随后被蒸发，而那些形成在光度最大值将生存。我们计算严格的上限颗粒尺寸的石墨和TiC在球对称AGB流出。石墨颗粒只有在理想的生长条件下（完美的粘附效率，没有蒸发，没有耗尽有助于颗粒生长的气体物质）才能达到观察到的微米尺寸范围（1-2 μm）的直径，然后只有在质量为2.5 M的碳星流出时才能达到。对于在太阳前石墨中发现的TiC颗粒也是如此，其平均直径为24 ± 14 nm。一般来说，在球对称流出物中产生观察到的粒度所需的质量损失率至少比观察到的最大AGB碳星星质量损失率大一个数量级。这些结果，以及来自平衡热力学的压力约束，迫使我们得出结论，太阳前石墨和TiC必须形成在区域的密度增强（团块，射流）在AGB流出具有小的角度尺度。如Croat等人的配套论文所示，在许多AGB石墨中12 C的富集，以及在其中发现的碳化物中s-过程元素Mo、Zr和Ru的过量，经常大大超过在AGB流出物中天文观测到的值。这些观测结果不仅进一步支持了外流是凝块的观点，而且还意味着外流物质在拱星包层中没有很好地混合到发生颗粒凝结的半径。
We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 M☉ for stars capable of contributing grains to the solar nebula is derived. This mass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium thermodynamics, indicate that grain formation occurs at radii from 2.3 to 3.7 AU for AGB carbon stars in the 1.1-5 M☉ range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 μm) only under ideal growth conditions (perfect sticking efficiency, no evaporation, no depletion of gas species contributing to grain growth), and then only in outflows from carbon stars with masses ≲2.5 M☉. The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24 ± 14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium thermodynamics, force us to conclude that presolar graphite and TiC must form in regions of enhanced density (clumps, jets) in AGB outflows having small angular scales. As shown in the companion paper by Croat et al., the enrichment of 12C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations not only lend further support to the idea that the outflows are clumpy, but also imply that the outflowing matter is not well mixed in the circumstellar envelope out to the radii where grain condensation takes place.