计时到超生物矮人星系网的R-Process富集II

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计时到超生物矮人星系网的R-Process富集II

Timing the r-Process Enrichment of the Ultra-Faint Dwarf Galaxy Reticulum II

论文作者

Simon, Joshua D., Brown, Thomas M., Mutlu-Pakdil, Burçin, Ji, Alexander P., Drlica-Wagner, Alex, Avila, Roberto J., Martínez-Vázquez, Clara E., Li, Ting S., Balbinot, Eduardo, Bechtol, Keith, Frebel, Anna, Geha, Marla, Hansen, Terese T., James, David J., Pace, Andrew B., Aguena, M., Alves, O., Andrade-Oliveira, F., Annis, J., Bacon, D., Bertin, E., Brooks, D., Burke, D. L., Rosell, A. Carnero, Kind, M. Carrasco, Carretero, J., Costanzi, M., da Costa, L. N., De Vicente, J., Desai, S., Doel, P., Everett, S., Ferrero, I., Frieman, J., García-Bellido, J., Gatti, M., Gerdes, D. W., Gruen, D., Gruendl, R. A., Gschwend, J., Gutierrez, G., Hinton, S. R., Hollowood, D. L., Honscheid, K., Kuehn, K., Kuropatkin, N., Marshall, J. L., Mena-Fernández, J., Miquel, R., Palmese, A., Paz-Chinchón, F., Pereira, M. E. S., Pieres, A., Malagón, A. A. Plazas, Raveri, M., Rodriguez-Monroy, M., Sanchez, E., Santiago, B., Scarpine, V., Sevilla-Noarbe, I., Smith, M., Suchyta, E., Swanson, M. E. C., Tarle, G., To, C., Vincenzi, M., Weaverdyck, N., Wilkinson, R. D.

论文摘要

超生物矮星系网状II（RET II）表现出独特的化学进化史，其中72 +10/-12％的恒星在R过程中强烈增强。我们介绍了RET II的深哈勃太空望远镜光度法并分析其恒星形成历史。与其他超生物矮人一样，颜色磁性图最适合由两种恒星形成的模型组成。如果我们假设爆发是瞬时的，那么较旧的爆发发生在电离时代周围，并在银河系中形成约80％的恒星，而其余的恒星后来形成〜3 Gyr。当允许爆发的持续时间非零时，我们会得到稍好的拟合。在这种情况下，最合适的模型由两次爆发组成，从回报开始之前，大约一半的恒星在短（100 Myr）爆发中形成，而另一半则在更长的时间内持续2.6 Gyr。考虑到完整的可行恒星形成历史模型，我们发现恒星形成发作的500 +/- 200 MYR内形成了28％的恒星。恒星形成史与R-Process增强恒星的流行率的结合表明，RET II中的R过程元素必须在其初始恒星形成阶段早期合成。因此，我们将RET II中第一颗恒星与R-Process核合成的延迟时间限制为小于500 MYR。该测量结果排除了一个R-Process源，其延迟时间为几个或更多的GW170817。

The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with 72 +10/-12% of its stars strongly enhanced in r-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color-magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization and formed ~80% of the stars in the galaxy, while the remainder of the stars formed ~3 Gyr later. When the bursts are allowed to have nonzero durations we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 +/- 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence of r-process-enhanced stars demonstrates that the r-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and the r-process nucleosynthesis to be less than 500 Myr. This measurement rules out an r-process source with a delay time of several Gyr or more such as GW170817.

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