DESIRED HII区域数据库的密度偏差与温度关系
Density biases and temperature relations for DESIRED HII regions
AI总结：
基于DESIRED数据库，本文发现常用光学密度指示会低估HII区电子密度并高估部分温度，提出更灵敏的密度诊断方法，并给出河外HII区的温度关系。
摘要：
我们首次基于电离区域深光谱数据库（DESIRED）的分析开展研究。该数据库汇集了190条高信噪比的HII区及其他光致电离星云光学光谱，主要使用8至10米级望远镜观测获得，包含约29380条发射线。我们发现，当仅以[SII] λ6731/λ6716和/或[OII] λ3726/λ3729作为密度指示时，天体的电子密度ne会被低估。这是因为在存在密度不均匀性的情况下，这些指示对密度具有非线性依赖关系。在河外HII区中，平均低估约为300 cm⁻³，从而相对于Te([NII])，系统性地高估了Te([OII])和Te([SII])。[OII] λλ7319+20+30+31/λλ3726+29和[SII] λλ4069+76/λλ6716+31对密度的高灵敏度使其更适合诊断高密度团块的存在。若采用Te([NII])，密度低估对光学光谱导出的离子丰度影响较小，当使用极光[SII]和/或[OII]线时，偏差最多约为0.1 dex。然而，这些密度效应对红外精细结构线的分析至关重要，例如JWST在本地恒星形成区观测到的那些谱线，会导致离子丰度被严重低估。我们给出了河外HII区中Te([OIII])、Te([ArIII])、Te([SIII])与Te([NII])之间的温度关系。我们确认Te([OIII])与Te([NII])之间存在非线性依赖关系，原因是Te([OIII])在较低金属丰度下增长更快。
英文摘要：
We present a first study based on the analysis of the DEep Spectra of Ionized REgions Database (DESIRED). This is a compilation of 190 high signal-to-noise ratio optical spectra of HII regions and other photoionized nebulae, mostly observed with 8-10m telescopes and containing $\sim$29380 emission lines. We find that the electron density --$n_{\rm e}$-- of the objects is underestimated when [SII] $\lambda6731/\lambda6716$ and/or [OII] $\lambda3726/\lambda3729$ are the only density indicators available. This is produced by the non-linear density dependence of the indicators in the presence of density inhomogeneities. The average underestimate is $\sim 300$ cm$^{-3}$ in extragalactic HII regions, introducing systematic overestimates of $T_{\rm e}$([OII]) and $T_{\rm e}$([SII]) compared to $T_{\rm e}$([NII]). The high-sensitivity of [OII] $λ\lambda7319+20+30+31/λ\lambda3726+29$ and [SII] $λ\lambda4069+76/λ\lambda6716+31$ to density makes them more suitable for the diagnosis of the presence of high-density clumps. If $T_{\rm e}$([NII]) is adopted, the density underestimate has a small impact in the ionic abundances derived from optical spectra, being limited to up to $\sim$0.1 dex when auroral [SII] and/or [OII] lines are used. However, these density effects are critical for the analysis of infrared fine structure lines, such as those observed by the JWST in local star forming regions, implying strong underestimates of the ionic abundances. We present temperature relations between $T_{\rm e}$([OIII]), $T_{\rm e}$([ArIII]), $T_{\rm e}$([SIII]) and $T_{\rm e}$([NII]) for the extragalactic HII regions. We confirm a non-linear dependence between $T_{\rm e}$([OIII])-$T_{\rm e}$([NII]) due to a more rapid increase of $T_{\rm e}$([OIII]) at lower metallicities.
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