The Star Formation History of the Local Group Dwarf Galaxy Leo I
The Star Formation History of the Local Group Dwarf Galaxy Leo I
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本星系群狮子座 I 的恒星形成历史
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发表时间：
1998
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通讯作者：
C. Chiosi
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作者：
C. Gallart;W. Freedman;A. Aparicio;G. Bertelli;C. Chiosi
利用哈勃空间望远镜[（V-I），I]色星等图（CMD）的信息，对本星系群dSph星系Leo I的星星形成史（SFH）进行了定量分析.它达到了最古老的主序星关闭的水平，这使我们能够相当详细地检索SFH。我们使用的方法是基于比较，通过合成CMD，CMD中的恒星的预期分布与观察到的分布不同的演化场景。我们认为SFH由SFR（t）、化学富集定律Z（t）、初始质量函数（IMF）和控制双星质量比分布和分数f的函数β（f，q）组成。我们分析了四个Z（t）、三个IMF和四个以上β（f，q）的一组约50种组合。对于每一个模型，在106 × 107个模型中搜索最佳SFR（t）。观测到的CMD和模型CMD之间的比较是通过在CMD的一组区域中的恒星数量差异的χ最小化来完成的，这些区域被选择来采样不同年龄或特定恒星演化阶段的恒星。我们经验性地确定了χ值的范围，该范围指示使用具有已知SFH的模型进行测试的数据集的可接受模型。我们对Leo I的SFH的解决方案在参数空间的明确定义的位置中定义了χ的最小值，并且导出的SFR（t）是鲁棒的，在这个意义上，其主要特征对于其余参数的不同组合是不变的。然而，只有对Z（t）、IMF和β（f，q）的一个很窄的假设范围才能使数据和模型之间有很好的一致性，即Z = 0.0004，IMF Kroupa等人或稍陡，双星的比例相对较大，f = 0.3-0.6，q > 0.6，秒的IMF近似平坦，或者这些参数的特定组合，其将产生相似质量双星的相似部分。大多数星星形成活动（70%到80%）发生在7到1 Gyr之前。在1 Gyr前，它突然下降到一个可以忽略不计的值，但似乎一直活跃到至少3.3亿年前。我们的结果并不能明确回答狮子座I是否在大约15光年前开始形成恒星的问题，但似乎这种星星形成的数量（如果存在的话）将会很小。
We present a quantitative analysis of the star formation history (SFH) of the Local Group dSph galaxy Leo I, from the information in its Hubble Space Telescope [(V-I), I] color-magnitude diagram (CMD). It reaches the level of the oldest main-sequence turnoffs, and this allows us to retrieve the SFH in considerable detail. The method we use is based on comparing, via synthetic CMDs, the expected distribution of stars in the CMD for different evolutionary scenarios with the observed distribution. We consider the SFH to be composed by the SFR(t), the chemical enrichment law Z(t), the initial mass function (IMF), and a function β(f, q) controlling the fraction f and mass ratio distribution q of binary stars. We analyze a set of ≃50 combinations of four Z(t), three IMFs, and more than four β(f, q). For each of them, the best SFR(t) is searched for among ≃6 × 107 models. The comparison between the observed CMD and the model CMDs is done through χ minimization of the differences in the number of stars in a set of regions of the CMD, chosen to sample stars of different ages or in specific stellar evolutionary phases. We empirically determine the range of χ values that indicate acceptable models for our set of data using tests with models with known SFHs. Our solution for the SFH of Leo I defines a minimum of χ in a well-defined position of the parameter space, and the derived SFR(t) is robust, in the sense that its main characteristics are unchanged for different combinations of the remaining parameters. However, only a narrow range of assumptions for Z(t), IMF, and β(f, q) result in a good agreement between the data and the models, namely, Z = 0.0004, a IMF Kroupa et al. or slightly steeper, and a relatively large fraction of binary stars, with f = 0.3–0.6, q > 0.6, and an approximately flat IMF for the secondaries, or particular combinations of these parameters that would produce a like fraction of similar mass binaries. Most star formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago, it abruptly dropped to a negligible value, but seems to have been active until at least ≃300 million years ago. Our results do not unambiguously answer the question of whether Leo I began forming stars around 15 Gyr ago, but it appears that the amount of this star formation, if it existed at all, would be small.