The abundances of nitrogen and oxygen in damped lyman alpha systems
The abundances of nitrogen and oxygen in damped lyman alpha systems
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DOI：
10.1051/0004-6361:20020809
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发表时间：
2002-05
影响因子：
6.5
通讯作者：
M. Petitjean
中科院分区：
文献类型：
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作者：
M. Petitjean
我们对氧丰度在太阳的1/10到1/100之间的阻尼莱曼系统（DLAs）中的氮丰度进行了新的研究。这是一个在当地宇宙中取样不足的金属丰度体系，QSO吸收剂可能为N的核合成起源和高红移星系的化学演化提供线索。我们将新的VLT UVES观测结果与其他文献中的观测结果结合起来，形成了10个dla样本，其中N和双捕获元素O或S中的一种的丰度已经测量。样本完全由高分辨率、高信噪比的数据组成，这些数据是用8-10米级望远镜获得的。我们证实了先前的报道，即（N/O）比dla中其他重元素的比值范围更大；然而，所有10个dla都落在以初级和次级氮生产水平划分的（N/O） vs （O/H）图区域。我们的研究结果提供了经验证据，支持中等质量恒星（4 < M=M < 7）是初级氮的主要来源，因此它在星际介质中的释放滞后于II型超新星产生的氧气。在我们的样品中，很大比例（40%）的dla显然尚未达到完全的初级氮富集水平；这一发现可能表明，我们正在研究的金属丰度体系优先挑选出最近才从星系间介质中凝聚出来并开始形成恒星的星系。或者，如果质量逐渐低于4M的恒星能够在其较热的内部合成N，那么在恒星形成后释放N的延迟可能会随着金属丰度的降低而增加。在这张总图中，在缺乏金属的恒星形成星系中局部测量到的统一的（N/O）值意味着它们并不年轻，并且与哈勃太空望远镜成像研究显示的较老恒星群的存在相一致。
We take a fresh look at the abundance of nitrogen in damped Lyman systems (DLAs) with oxygen abundances between1/10 and1/100 of solar. This is a metallicity regime poorly sampled in the local universe and where QSO absorbers may hold clues to both the nucleosynthetic origin of N and the chemical evolution of high redshift galaxies. We combine new VLT UVES observations with others from the literature to form a sample of 10 DLAs in which the abundances of N and of one of the two-capture elements O or S have been measured. The sample consists exclusively of high resolution, high signal-to-noise ratio data obtained with 8-10 m class telescopes. We confirm earlier reports that the (N/O) ratio exhibits a larger range of values than other ratios of heavy elements in DLAs; however, all 10 DLAs fall in the region of the (N/O) vs. (O/H) plot delimited by the primary and secondary levels of nitrogen production. Our results provide empirical evidence in support of the proposal that intermediate mass stars (4 < M=M < 7) are the main source of primary nitrogen, so that its release into the interstellar medium lags behind that of oxygen, which is produced by Type II supernovae. A high proportion (40%) of the DLAs in our sample have apparently not yet attained the full primary level of N enrichment; this finding may be an indication that the metallicity regime we are studying preferentially picks out galaxies which have only recently condensed out of the intergalactic medium and begun forming stars. Alternatively, the delay in the release of N following an episode of star formation may increase with decreasing metallicity, if stars of progressively lower masses than 4M can synthesize N in their hotter interiors. In this general picture, the uniform value of (N/O) measured locally in metal-poor star-forming galaxies implies that they are not young, and is consistent with the presence of older stellar populations revealed by imaging studies with the Hubble Space Telescope.