Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
复制标题
麦哲伦星云中的星际丰度。
DOI：
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发表时间：
1999
期刊：
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通讯作者：
D. York
中科院分区：
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作者：
D. Welty;P. Frisch;G. Sonneborn;D. York
我们分析了LMC SN 1987A的高分辨率光学吸收线谱和用IUE获得的UV光谱，以确定沿超新星(SN)视线方向的各种中性星际云的丰度和物理条件。我们使用平场方法来提高紫外光谱的信噪比(S/NS)和弱特征的可靠性，并利用由高分辨率、高S/N光谱得到的组分结构来模拟紫外线轮廓。与钙离子、钙离子和钠离子的吸收线谱拟合显示(至少)46个组分，在-24公里S-1≲v≲296公里S-1的速度下，这些组分可以与低分辨率紫外光谱中可分辨的10个组分相关联。根据紫外光谱，我们测定了CI、Mg I、Mg II、Al II、Si II、P II、Cl I、Ti II、Cr II、Mn II、Fe II、Ni II和Zn II-的组成基柱密度，在许多情况下，1σ的不确定度小于0.1Dex。这些是迄今为止在LMC中测量到的中性ISM的最广泛和最准确的丰度。成分速度、相对元素丰度[X/锌]和[X/Fe]模式以及各种诊断比率被用来估计这些成分群的位置和物理特征[N(H)，T，n]。(然而，诊断比率之间的系统差异使导出的物理属性有些不确定。)低速(5公里S-1≲v≲23公里S-1)的分量的相对丰度和诊断比值与温暖、弥漫的银盘云非常相似，很可能是由于银盘中的暖气体和冷气体的混合所致。速度为56公里的S-1≲v≲90公里的S-1的分量是由于银晕中的冷热气体的混合，显然没有明显的损耗。两个中速分量群(109kmS-1≲v≲140kmS-1和155kmS-1≲v≲176kmS-1)的相对丰度与银晕云相似。这些温暖的(T≳4500K)、部分电离的云团可能分别位于银晕和大湄公河。速度为191公里的S-1≲v≲225公里S-1的分量也具有与晕云相似的相对丰度，但可能是由于大湄公河中的气体所致，可能离SN很近。在238公里S-1≲v≲255公里S-1和265公里S-1≲v≲270公里S-1处的成分群可能位于主≲成分群的两侧(速度为275公里S-1≲v LMC 296公里S-1)(利用其他几个相邻视线的吸收线数据和从SN光回波观测推断的结构)。尽管这三个LMC群的相对丰度和诊断比与温暖、低密度的银盘云相似，但在NaI和KI的高分辨率光谱中看到的单个组分的宽度表明，T通常小于约1500K。较高的N(NaI)/N(CaII)比、CH的存在和CI精细结构能级群表明，主要的LMC群既包含冷气体，也包含暖气体。对于LMC组分，从观测到的相对丰度和推断的亏损估计的总N(H)与从Lyα对邻近恒星Sk-69°203的吸收得到的值是一致的，考虑了LMC ISM的红化差异和总的太阳下金属丰度的0.2Dex-0.3Dex。由于在SMC和LMC中确定的恒星和气态星云的相对丰度模式似乎与太阳模式相似(对于我们考虑的星际丰度元素)，我们银河系和麦哲伦星云中相对气相星际丰度的相似之处表明，尘埃的消耗也遵循类似的模式--尽管这三个星系的金属丰度和尘气比不同。因此，这些局部相对丰度/耗竭模式可以用来推断不同红移的类星体吸收线系统的总(气体+尘埃)丰度。
We have analyzed both high-resolution optical absorption-line spectra and UV spectra obtained with IUE of the LMC SN 1987A, in order to determine abundances and physical conditions in the various neutral interstellar clouds along the line of sight to the supernova (SN). We have used a flat-fielding procedure to enhance the signal-to-noise ratios (S/Ns) and the reliability of weak features in the UV spectra and have modeled the UV line profiles using the component structure derived from the higher resolution, high-S/N optical spectra of Ca II and Na I. Fits to the Ca II, Ca I, and Na I absorption-line profiles reveal (at least) 46 components, at velocities -24 km s-1≲v≲296 km s-1, which can be associated with the 10 component groups discernible in the lower resolution UV spectra. From the UV spectra, we determined component-group column densities for C I, Mg I, Mg II, Al II, Si II, P II, Cl I, Ti II, Cr II, Mn II, Fe II, Ni II, and Zn II—with 1 σ uncertainties less than 0.1 dex in many cases. These are the most extensive and accurate abundances yet measured for the neutral ISM in the LMC. The component velocities, the patterns of relative elemental abundances [X/Zn] and [X/Fe], and various diagnostic ratios have been used to estimate the locations and physical characteristics [N(H), T, n] of these component groups. (Systematic differences among the diagnostic ratios make the derived physical properties somewhat uncertain, however.) The components at low velocities (5 km s-1≲v≲23 km s-1) have relative abundances and values for the diagnostic ratios very similar to those found for warm, diffuse Galactic disk clouds and likely are due to a mixture of warm and cool gas in the Galactic disk. The components at velocities 56 km s-1≲v≲90 km s-1 are due to a mixture of warm and cool gas, apparently with negligible depletions, in the Galactic halo. The two intermediate-velocity component groups (109 km s-1≲v≲140 km s-1 and 155 km s-1≲v≲176 km s-1) both have relative abundances similar to those found for Galactic halo clouds. These warm (T≳4500 K), partially ionized clouds are probably located in the Galactic halo and in the LMC, respectively. The components at velocities 191 km s-1≲v≲225 km s-1 also have relative abundances similar to those in the halo clouds but are likely due to gas in the LMC, perhaps very close to the SN. The component groups at 238 km s-1≲v≲255 km s-1 and 265 km s-1≲v≲270 km s-1 are probably located on opposite sides of the main LMC component group (at velocities 275 km s-1≲v≲296 km s-1) (using absorption-line data for several other adjacent lines of sight and the structure inferred from SN light-echo observations). Although the relative abundances and diagnostic ratios for those three LMC groups are similar to those found for warm, low-density Galactic disk clouds, the widths of individual components seen in very high resolution spectra of Na I and K I imply that T is generally less than about 1500 K. Higher N(Na I)/N(Ca II) ratios, the presence of CH, and the C I fine structure level populations suggest that the main LMC group contains both cool and warm gas. For the LMC components, the total N(H) estimated from the observed relative abundances and inferred depletions is consistent with the value obtained from Lyα absorption toward the neighboring star Sk -69°203, after accounting for differences in reddening and for an overall subsolar metallicity of 0.2-0.3 dex for the LMC ISM. Since the relative abundance patterns determined for stars and gaseous nebulae in both the SMC and the LMC appear to be similar to the solar pattern (for the elements whose interstellar abundances we have considered), the similarities in relative gas-phase interstellar abundances in our Galaxy and in the Magellanic Clouds suggest that the dust depletions follow similar patterns as well—despite differences in metallicity and dust-to-gas ratio among the three galaxies. These local relative abundance/depletion patterns may thus be used to infer total (gas+dust) abundances for QSO absorption-line systems at various redshifts.