标题： 球状星团在银河系外围晕中的年龄：第二参数星团帕洛马3、帕洛马4和埃里达诺斯 显示英文标题

标题： Ages for Globular Clusters in the Outer Galactic Halo: The Second Parameter Clusters Palomar~3, Palomar~4 and Eridanus

摘要： 我们使用哈勃空间望远镜上的WFPC2相机对外围晕球状星团Palomar 3、Palomar 4和Eridanus进行了测光。 这三个星团是“第二参数”异常的经典例子，因为它们的红色水平分支形态与其低至中等金属丰度相结合。 我们的(V, V-I)色-光图达到$V_{lim} \simeq 27.0$，清晰地描绘了次巨星和主序转折区域以及约三个星等的未演化主序。 红巨星分支的斜率和去红化颜色与已发表的[Fe/H]估计一致，这些估计将这些星团（Pal 3、Eridanus、Pal 4）按金属丰度递增顺序排列，所有三个星团都接近或介于经典邻近晕星团M3和M5的丰度值之间。 对它们的色-光图进行相对年龄测定，相互之间以及与M3和M5进行微分拟合。 我们发现，这三个外围晕星团的CMD与邻近星团有所不同，这与它们年轻约$\sim 1.5 - 2 $Gyr是一致的，假设我们正确估计了星团的化学丰度比。 相反，如果外围晕星团的[Fe/H]或[$\alpha$/Fe]显著低于我们所假设的，则推断的年龄差异可能更小（$\ltsim 1 $Gyr）。 邻近星团和外围晕星团之间可能存在约1$ $Gyr的年龄弥散。 显示英文摘要

摘要： We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$\alpha$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present.