[论文解读] Planetary nebulae and HII regions in the spiral galaxy NGC 300. Clues on the evolution of abundance gradients and on AGB nucleosynthesis

G. Stasińska, M. Peña+2 more|Open Research Online (The Open University)|Jan 22, 2013

Galaxies: Formation, Evolution, Phenomena参考文献 66被引用 32

一句话总结

本研究分析了NGC 300中26个行星状星云（PNe）和9个致密H II区的深谱线数据，发现PNe的N/O比值显著高于H II区，且元素丰度弥散更大，表明其存在超出二次对流包覆的额外核合成过程。PNe中O/H、Ne/H和Ar/He梯度较浅，暗示过去十亿年中金属丰度梯度发生陡化，可能由于径向混合或演化恒星中旋转驱动的混合过程所致。

ABSTRACT

We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact HII regions in the nearby spiral galaxy NGC 300, and analyzed them together with those of the giant HII regions previously observed. We have determined the physical properties of all these objects and their He, N, O, Ne, S and Ar abundances in a consistent way. We find that, globally, compact HII regions have abundance ratios similar to those of giant HII regions, while PNe have systematically larger N/O ratios and similar Ne/O and Ar/O ratios. We demonstrate that the nitrogen enhancement in PNe cannot be only due to second dredge-up in the progenitor stars, since their initial masses are around 2--2.5\,\msun. An extra mixing process is required, perhaps driven by stellar rotation. Concerning the radial abundance distribution, PNe behave differently from HII regions: in the central part of the galaxy their average O/H abundance ratio is 0.15 dex smaller. Their abundance dispersion at any galactocentric radius is significantly larger than that shown by HII regions and many of them have O/H values higher than HII regions at the same galactocentric distance. This suggests that not only nitrogen, but also oxygen is affected by nucleosynthesis in the PN progenitors, by an amount which depends at least on the stellar rotation velocity and possibly other parameters. The formal O/H, Ne/H and Ar/He abundance gradients from PNe are significantly shallower that from HII regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gyr, rather than an effect of radial stellar motions, although the large observed dispersion makes this conclusion only tentative.

研究动机与目标

• 研究邻近旋涡星系NGC 300中行星状星云（PNe）与H II区的化学丰度，以探究金属丰度梯度的演化。
• 确定观测到的PNe中氮元素增强是否源于其前身星中超出二次对流包覆的核合成过程。
• 评估从PNe获得的丰度梯度是否系统性地不同于H II区的梯度，及其对旋涡星系化学演化的影响。
• 检验恒星自转与额外混合过程在塑造PNe前身星化学成分中的作用。
• 评估PNe中丰度测定的可靠性，特别是电离校正因子与尘埃耗竭效应的影响。

提出的方法

• 利用甚大望远镜（VLT）上的FORSS2仪器，对NGC 300中的26个PNe和9个致密H II区获取了信噪比高的光谱数据。
• 通过碰撞激发线（CELs），特别是[O III] λ4363和[S II] λλ6717,6731，确定了电子温度与密度等物理条件。
• 采用基于温度的计算方法求得离子丰度，并应用电离校正因子（ICFs）推导出He、N、O、Ne、S和Ar的总元素丰度。
• 将光学候选PNe与H II区与光谱分类结果进行比对，纠正了先前成像巡天中的误识别问题。
• 通过比较PNe与H II区在不同星系中心半径处的O/H、Ne/H和Ar/He比值，分析径向丰度梯度。
• 评估恒星自转与额外混合过程对观测到的丰度模式的影响，特别是PNe中N/O与O/H的增强现象。

实验结果

研究问题

• RQ1 NGC 300中PNe的N/O比值相较于H II区高出多少？这一现象能否仅由二次对流包覆解释？
• RQ2 NGC 300中从PNe获得的径向丰度梯度与从H II区获得的梯度相比如何？这对金属丰度梯度演化有何启示？
• RQ3 为何PNe在任一星系中心半径处的O/H丰度弥散均大于H II区？
• RQ4 恒星自转在驱动额外混合过程、将CNO循环处理的物质输运至PNe前身星表面的过程中起何种作用？
• RQ5 PNe中观测到的浅梯度是由于恒星的径向运动所致，还是过去十亿年中金属丰度梯度真实发生陡化？

主要发现

• NGC 300中PNe的N/O比值系统性高于H II区，表明氮元素增强无法仅由二次对流包覆解释，这与估算的前身星质量（约2–2.5 M⊙）不一致。
• 在NGC 300中心区域，PNe的平均O/H丰度比H II区低0.15 dex，表明氧元素也受到前身星核合成过程的影响。
• 在任一星系中心半径处，PNe中O/H、Ne/O和Ar/O的丰度弥散显著大于H II区，表明其化学富集历史具有异质性。
• 从PNe推导出的正式O/H、Ne/H和Ar/He丰度梯度比从H II区获得的更平缓（近乎平坦），暗示过去十亿年中金属丰度梯度可能发生了陡化。
• PNe中观测到的丰度模式最合理的解释是存在一种额外混合过程——极有可能由恒星自转驱动——在行星状星云阶段之前将CNO循环处理的物质输运至表面。
• PNe中硫元素的丰度表现出不合理的激发依赖趋势，表明当前S的电离校正因子不足，可能也影响其他元素的丰度测定。