Distances and ages of NGC 6397, NGC 6752 and 47 Tuc
Distances and ages of NGC 6397, NGC 6752 and 47 Tuc
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DOI：
10.1051/0004-6361:20031003
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发表时间：
2003-07
影响因子：
6.5
通讯作者：
R. Gratton;A. Bragaglia;E. Carretta;G. Clementini;S. Desidera;F. Grundahl;S. L. I. A. D. Padova;
中科院分区：
文献类型：
--
作者：
R. Gratton;A. Bragaglia;E. Carretta;G. Clementini;S. Desidera;F. Grundahl;S. L. I. A. D. Padova;
使用主序拟合方法获得了银河球状星团 NGC 6397、NGC 6752 和 47 Tuc 的新改进距离和绝对年龄。我们使用严格的微分程序对这三个星团的红化和金属丰度进行了准确的估计，其中将属于这些星团的转向星和早期亚巨星的约翰逊 B V 和斯特罗姆格伦颜色和 UVES 高分辨率光谱与由依帕谷测量的精确视差的场亚矮星的类似数据进行比较。在由当地亚矮星定义的尺度中，使用红化自由温度指示剂（H 的轮廓）使我们能够将红化测定的误差线减少到约 0.005 mag，金属丰度的误差线减少到 0.04 dex。然后，每个星团的距离误差条减少到约 0.07 星等，产生典型随机误差约为 1 Gyr 的年龄。我们发现，当使用没有微观弥散的标准等时线时，NGC 6397 和 NGC 6752 的年龄分别为 13:9 1:1 和 13:8 1: 1Gyr，而 47 Tuc 可能年轻约 2.6 Gyr，这与其他对相对年龄敏感的技术获得的结果一致。如果我们使用的模型包含与我们对 NGC 6397 的观察一致的微观扩散造成的沉积效应，并通过统计方法考虑可能的系统误差的各种来源，我们得出的结论是，银河系中最古老的球状星团的年龄为 13:4 0:8 0:6 Gyr，其中第一个误差条说明了随机效应，第二个误差条说明了系统误差。这一年龄估计与 WMAP 的最新结果完全一致，并表明最古老的银河球状星团是在大爆炸后的第一个 1.7 Gyr 内形成的，对应于标准 CDM 模型中 z 2:5 的红移。 （内晕）球状星团的形成时期持续了约 2.6 Gyr，结束于对应于 z 1:3 红移的时间。另一方面，我们的新年龄估计与 WMAP 和 HST 重点项目给出的 H0 值相结合，提供了 M < 0:57 的 95% 置信水平的稳健上限，与 Ia SNe 类型无关，并强烈支持对暗能量的需求。新的簇距离导致了对水平分支光度的新估计，这可用于导出水平分支绝对星等与金属丰度之间关系的零点：我们得到MV (HB)= (0:22 0:05)((Fe=H)+ 1:5)+ (0:56 0:07)。该零点比 Carretta 等人获得的零点短 0.03 星等。 (2000) 并在其同意的误差范围内，但它比大多数先前对天琴座 RR 绝对星等的单独测定更精确。当与 Clementini 等人在 LMC 中的 RR Lyrae 恒星的视平均光度相结合时。 (2003)，这个零点提供了到 LMC 的距离模数的新估计：(m M)0= 18:50 0:09。
New improved distances and absolute ages for the Galactic globular clusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the Main Sequence Fitting Method. We derived accurate estimates of reddening and metal abundance for these three clusters using a strictly dierential procedure, where the Johnson B V and Stromgren b y colours and UVES high resolution spectra of turn-o stars and early subgiants belonging to the clusters were compared to similar data for field subdwarfs with accurate parallaxes measured by Hipparcos. The use of a reddening free temperature indicator (the profile of H) allowed us to reduce the error bars in reddening determinations to about 0.005 mag, and in metal abundances to 0.04 dex, in the scales defined by the local subdwarfs. Error bars in distances are then reduced to about 0.07 mag for each cluster, yielding ages with typical random errors of about 1 Gyr. We find that NGC 6397 and NGC 6752 have ages of 13:9 1:1 and 13:8 1: 1G yr respectively, when standard isochrones without microscopic diusion are used, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by other techniques sensitive to relative ages. If we use models that include the eects of sedimentation due to microscopic diusion in agreement with our observations of NGC 6397, and take into account various sources of possible systematic errors with a statistical approach, we conclude that the age of the oldest globular clusters in the Galaxy is 13:4 0:8 0:6 Gyr, where the first error bar accounts for random eects, and the second one for systematic errors. This age estimate is fully compatible with the very recent results from WMAP, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z 2:5, in a standardCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z 1:3. On the other hand, our new age estimate once combined with values of H0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of M < 0:57, independently of type Ia SNe, and strongly supports the need for a dark energy. The new cluster distances lead to new estimates of the horizontal branch luminosity, that may be used to derive the zero point of the relation between the horizontal branch absolute magnitude and metallicity: we obtain MV (HB)= (0:22 0:05)((Fe=H)+ 1:5)+ (0:56 0:07). This zero point is 0.03 mag shorter than obtained by Carretta et al. (2000) and within the error bar it agrees with, but it is more precise than most of the previous individual determinations of the RR Lyrae absolute magnitude. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m M)0= 18:50 0:09.