The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983-2000
The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983-2000
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DOI：
10.1051/0004-6361:20031109
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发表时间：
2003-10
影响因子：
6.5
通讯作者：
W. Hamann;M. Peña;G. Gräfener;M. Ruiz
中科院分区：
文献类型：
--
作者：
W. Hamann;M. Peña;G. Gräfener;M. Ruiz
大麦哲伦星云中行星状星云 N 66（别名 WS 35、SMP 83 和 HV 5967）的中心恒星在 1993 年和 1994 年其亮度急剧增强。在随后的四年内，它又回到了之前的水平。它的光谱类似于氮序列的沃尔夫-拉叶星（WN4.5）的光谱。我们从地面和哈勃太空望远镜对这个物体进行了集中监测。现在我们展示N 66亮度爆发之前、期间和之后不同历元的完整光谱观测数据。利用最先进的非LTE膨胀恒星大气模型对不同历元的恒星光谱进行详细分析。主要结果是：对于行星状星云的中心恒星来说，爆发前后的光度 $\log L/L_\odot = 4.6$ 非常高。在1994年的爆发期间，它甚至攀升至$\log L/L_\odot = 5.4$，持续了大约一年。约112 kK的有效温度大致保持不变，即光度主要是由于有效恒星半径较大而增加。质量损失率从安静状态下的 $10^{-5.7} ~M_{\odot}\,{\rm yr^{-1}}$ 增加到爆发期间的 $10^{-5.0}~ M_{\odot}\,{\rm yr^{-1}}$。恒星大气的化学成分是未完全经过碳氮氧化物处理的物质：主要是氦气，其余是氢，氮含量略有增强，碳含量严重贫化。我们广泛讨论了 N 66 的性质和进化起源的可能情景，这应该解释异常的恒星参数、大气成分、爆发机制，以及仅在几千年前喷射并含有约 0.6 个太阳质量的富氢物质的双极星云的存在。如果是一颗单星，N 66 可能是 (i) 渐近巨星分支之后的一颗低质量恒星，通常用于行星状星云的中心恒星，(ii) 一颗大质量恒星，即非简并恒星，或 (iii) 由两颗白矮星合并而成。尽管没有直接的双星迹象，我们还是讨论 N 66 是否可能是 (iv) 一颗大质量恒星，在最近的共包络相中与质量较小的伴星失去了氢包络，或者 (v) 一颗白矮星以高速率从伴星吸积质量。所有场景都与至少一个观察到的事实存在任何矛盾。然而，二进制场景带来的问题不太严重。如果N 66是近双星系统中的一颗白矮星吸积物质，那么它目前的吸积速度将使其在几十万年内达到钱德拉塞卡极限。因此，N 66 可能是我们宇宙附近未来 Ia 型超新星爆炸的候选者。
The central star of the planetary nebula N 66 (alias WS 35, SMP 83 and HV 5967) in the Large Magellanic Cloud enhanced its brightness dramatically in 1993 and 1994. Within the subsequent four years it returned to the previous level. Its spectrum resembles that of a Wolf-Rayet star of the nitrogen sequence (WN4.5). We monitored the object intensively from ground and with the Hubble Space Telescope. Now we present the complete set of spectroscopic observations from the different epochs before, during and after the brightness outburst of N 66. The stellar spectra from the different epochs are analyzed in detail by means of most advanced non-LTE models for expanding stellar atmospheres. The main results are: the luminosity, $\log L/L_\odot = 4.6$, before and after the outburst is exceptionally high for a central star of a planetary nebula. During the outburst in 1994, it even climbed up to $\log L/L_\odot = 5.4$ for about one year. The effective temperature of about 112 kK remained roughly constant, i.e. the luminosity mainly increased because of a larger effective stellar radius. The mass loss rate increased from $10^{-5.7} ~M_{\odot}\,{\rm yr^{-1}}$ in the quiet state to $10^{-5.0}~ M_{\odot}\,{\rm yr^{-1}}$ during the outburst. The chemical composition of the stellar atmosphere is that of incompletely CNO-processed matter: it is dominated by helium with a rest of hydrogen, nitrogen being slightly enhanced and carbon strongly depleted. We extensively discuss possible scenarios for the nature and evolutionary origin of N 66, which should explain the exceptional stellar parameters, the atmospheric composition, the outburst mechanism, and the existence of the bipolar nebula which was ejected only a few thousand years ago and contains about 0.6 solar masses of hydrogen-rich matter. If being a single star, N 66 might be (i) a low-mass star after the Asymptotic Giant Branch, as usually adopted for central stars of planetary nebulae, (ii) a massive, i.e. non-degenerate star, or (iii) a merger produced from two white dwarfs. Although there are no direct indications for binarity, we alternatively discuss whether N 66 might be (iv) a massive star which lost its hydrogen envelope in a recent common-envelope phase with a less massive companion, or (v) a white dwarf accreting mass from a companion with a high rate. None of the scenarios is free of any contradiction to at least one of the observational facts. However, the binary scenarios pose less severe problems. If N 66 is a white dwarf accreting matter in a close-binary system, its present accretion rate would bring it to the Chandrasekhar limit within a few hundred thousand years. Thus N 66 might be a candidate for a future type Ia supernova explosion in our cosmic neighborhood.