在z~0.45处由弱MgII吸收追踪的冷贫金属云。在QSO吸收器中首次检测到SiI、CaI和FeI
A cold metal-poor cloud traced by a weak MgII absorption at z~0.45. First detection of SiI, CaI and FeI in a QSO absorber
摘要：
我们在类星体HE 0001 -2340的UVES高分辨光谱中观测到了z~0.452处的弱MgII吸收系统。较弱的两个MgII组成部分形成的系统显示相关的反射由于SiI，CaI和FeI观察到的第一次在类星体光谱。我们调查这种吸收剂的性质，通过比较其性质与不同类别的吸收剂（弱镁II，阻尼Ly-alpha系统和本地星际云）和再现其电离条件与光电离模型。所观察到的吸收体属于类的弱MgII系统的基础上，其等效宽度，然而，通常观察到的过渡的相对强度偏离上述吸收体显着。通过类星体视线穿过这样一个系统的概率的粗略估计是P~0.03。罕见的中性跃迁的存在表明云被大量的中性氢屏蔽。详细比较所观察到的柱密度与阻尼Ly-alpha系统和本地星际冷云的平均性能，特别是在我们的云缺乏MgII/MgI和CaII/CaI的比例。光电离模型的计算结果表明，云可以被紫外背景电离。然而，一个简单的模型，一个单一的云与均匀的密度不能重现所观察到的离子丰度比，这表明一个更复杂的密度结构的吸收剂。假设电离修正可以忽略不计，最令人困惑的结果是镁相对于铁的丰度不足，这很难用核合成和差分尘埃损耗来解释。[删节]
英文摘要：
We present the observations of a weak MgII absorption system detected at z~0.452 in the UVES high resolution spectrum of the QSO HE0001-2340. The weaker of the two MgII components forming the system shows associated absorptions due to SiI, CaI and FeI observed for the first time in a QSO spectrum. We investigate the nature of this absorber by comparing its properties with those of different classes of absorbers (weak MgII, Damped Ly-alpha systems and local interstellar clouds) and reproducing its ionization conditions with photoionization models. The observed absorber belongs to the class of weak MgII systems on the basis of its equivalent width, however the relative strength of commonly observed transitions deviates significantly from those of the above mentioned absorbers. A rough estimate of the probability to cross such a system with a QSO line of sight is P~0.03. The presence of rare neutral transitions suggests that the cloud is shielded by a large amount of neutral hydrogen. A detailed comparison of the observed column densities with the average properties of damped Ly-alpha systems and local interstellar cold clouds shows, in particular, deficient MgII/MgI and CaII/CaI ratios in our cloud. The results of photoionization models indicate that the cloud could be ionized by the UV background. However, a simple model of a single cloud with uniform density cannot reproduce the observed ionic abundance ratios, suggesting a more complex density structure for the absorber. Supposing that ionization corrections are negligible, the most puzzling result is the underabundance of magnesium with respect to iron which is hard to explain both with nucleosynthesis and with differential dust depletion. [Abridged]
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