The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
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DOI：
10.1086/375398
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发表时间：
2003-03
期刊：
影响因子：
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通讯作者：
Robert C. Kennicutt Jr.;F. Bresolin;D. Garnett
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文献类型：
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作者：
Robert C. Kennicutt Jr.;F. Bresolin;D. Garnett
我们使用巨型螺旋星系M101中20个HⅡ区的高信噪比光谱来推导H II区的电子温度和半径R=0.19-1.25R0(6-41kpc)上的稳健金属丰度。我们比较了从[O III]λ4363，[N II]λ5755，[S III]λ6312和[O II]λ7325极光线测量的电子温度的一致性。来自[O III]、[S III]和[N II]的温度与相对偏移量相关联，这些偏移量与星云光电离模型的预期一致。然而，从[O II]λ7325线得到的温度表现出很大的散射，并且与其他离子得到的温度几乎不相关。我们暂时将这一结果归因于观测和物理效应，这可能会给仅由[O II]温度得出的丰度引入较大的随机和系统误差。我们得到的氧丰度在六个盘尺度上符合指数分布，从中心的大约1.3(O/H)☉到最外层的1/15(O/H)☉[太阳12+log(O/H)=8.7时]。我们在N/O和He/H丰度比中测量了显著的径向梯度，但相对恒定的S/O和Ar/O。我们的结果与以前发表的基于几个HⅡ区温度测量的M101丰度研究大致一致。然而，我们的丰度比从最广泛使用的强线“经验”丰度指标得出的丰度系统地低0.2-0.5dex，这也与之前基于较小的H II区样本的研究一致。从紫外线吸收线独立测量的银河系星际氧丰度与基于Te的星云丰度很好地一致。我们怀疑，与强线丰度的大部分不一致源于用于校准“经验”标度的星云模型中的不确定性，并且为H II区和发射线星系得出的强线丰度比实际氧丰度高出2倍之多。然而，其他解释，如温度波动对极光线丰度的影响，不能完全排除。这些结果表明，需要对河外H II区的更大样本进行直接丰度测定，特别是对金属含量比太阳更丰富的物体。
We use high signal-to-noise ratio spectra of 20 H II regions in the giant spiral galaxy M101 to derive electron temperatures for the H II regions and robust metal abundances over radii R = 0.19-1.25R0 (6-41 kpc). We compare the consistency of electron temperatures measured from the [O III] λ4363, [N II] λ5755, [S III] λ6312, and [O II] λ7325 auroral lines. Temperatures from [O III], [S III], and [N II] are correlated with relative offsets that are consistent with expectations from nebular photoionization models. However, the temperatures derived from the [O II] λ7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions. We tentatively attribute this result to observational and physical effects, which may introduce large random and systematic errors into abundances derived solely from [O II] temperatures. Our derived oxygen abundances are well fitted by an exponential distribution over six disk scale lengths, from approximately 1.3 (O/H)☉ in the center to 1/15 (O/H)☉ in the outermost region studied [for solar 12 + log(O/H) = 8.7]. We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O. Our results are in approximate agreement with previously published abundances studies of M101 based on temperature measurements of a few H II regions. However, our abundances are systematically lower by 0.2-0.5 dex than those derived from the most widely used strong-line "empirical" abundance indicators, again consistent with previous studies based on smaller H II region samples. Independent measurements of the Galactic interstellar oxygen abundance from ultraviolet absorption lines are in good agreement with the Te-based nebular abundances. We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the "empirical" scale, and that strong-line abundances derived for H II regions and emission-line galaxies are as much as a factor of 2 higher than the actual oxygen abundances. However, other explanations, such as the effects of temperature fluctuations on the auroral line based abundances, cannot be completely ruled out. These results point to the need for direct abundance determinations of a larger sample of extragalactic H II regions, especially for objects more metal-rich than solar.