Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
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DOI：
10.1051/0004-6361/201425455
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发表时间：
2015-03
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通讯作者：
E. Fern'andez-Alvar;C. Prieto;K. Schlesinger;T. Beers;A. Robin;D. Schneider;Y. Lee;D. Bizyaev;G. Ebelke;E. Malanushenko;V. Malanushenko;D. Oravetz;K. Pan;A. Simmons
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作者：
E. Fern'andez-Alvar;C. Prieto;K. Schlesinger;T. Beers;A. Robin;D. Schneider;Y. Lee;D. Bizyaev;G. Ebelke;E. Malanushenko;V. Malanushenko;D. Oravetz;K. Pan;A. Simmons
我们分析了斯隆数字巡天(SDSS)的3,944个低分辨率(R~2000)光谱样本，重点分析了有效温度为5800<；Tef<；6300K，距离银河系平面超过5kpc的恒星，并确定了它们的Fe、Ca和Mg丰度。在本系列中，我们遵循与上一篇论文相同的方法，通过CHI2最小化得到大气参数，但这一次我们通过拟合单个元素的相关谱线来获得它们的丰度。距离是通过我们的恒星参数与恒星演化模型的统计比较而获得的绝对星等来计算的。观测显示，在距离银河系中心很远的地方，铁、钙和镁的丰度有所下降。所分析的晕星丰度的中位数在银心距离r~20kpc时相当恒定，在r~20kpc和r~40kpc之间迅速下降，在较大距离时趋于平缓，与以前的研究一致。此外，我们还考察了[Ca/Fe]和[Mg/Fe]作为Fe/H和银心距的函数。我们的结果表明，在晕的最远的部分，[Ca/Fe]和[Mg/Fe]随铁的变化较大。我们发现在-1.6<；[Fe/H]<；-0.4范围内，[Ca/Fe]随距离减小，这与早期基于局部恒星的结果是一致的。然而，对于[镁/铁]来说，相反的趋势是明显的。根据对遥远恒星的现场观测，我们得出的结论是，光晕外部区域比内部区域更缺乏金属，这与最近基于更多本地恒星运动学推断得出的结果一致，也与最近对类似银河系的星系形成模拟的预测一致。
We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by chi2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine the [Ca/Fe] and [Mg/Fe] as a function of Fe/H and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of the [Ca/Fe] and [Mg/Fe] with iron. We found that at the range -1.6 < [Fe/H] < -0.4 [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.