A Tale of Two Stars: The Extreme O7 Iaf+ Supergiant AV 83 and the OC7.5 III((f)) star AV 69
A Tale of Two Stars: The Extreme O7 Iaf+ Supergiant AV 83 and the OC7.5 III((f)) star AV 69
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两颗恒星的故事：极端 O7 Iaf 超巨星 AV 83 和 OC7.5 III((f)) 恒星 AV 69
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发表时间：
2003
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通讯作者：
J. Bouret
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作者：
D. Hillier;T. Lanz;S. Heap;I. Hubeny;Linda J. Smith;C. Evans;D. Lennon;J. Bouret
我们详细研究了SMC中的AV 83，O 7 Iaf+超巨星和AV 69 [OC 7.5 III（（f））]。这两颗恒星有相似的有效温度和光度，但显示出非常不同的风信号。在我们的研究中，我们使用了Hillier和米勒开发的非LTE线覆盖大气代码，该代码明确允许C、N、O、S、Ar、Ne、Fe和其他元素的线覆盖。我们的研究发现，AV 83的有效温度约为33，000 K，log g = 3.25。它有一个扩展的光球层，这是由于它的有效表面重力“低”和比主序星O更密集的风。我们只能通过使用慢速度定律来匹配光谱，β = 2，这个值比标准辐射风理论预测的大约1的值要大得多。此外，我们还证明了AV 83的Hα发射轮廓对所采用的表面重力是敏感的。为了适应AV 83的频谱，我们考虑了风是平滑的传统模型和风是高度聚集的替代模型。这两种类型的风产生一个令人满意的适合大多数线在所观察到的光谱，但是，强紫外光球线和P-V共振转换有利于一个丛生风。如果结块是重要的，则它必须以相对低的速度开始（即，30 km s-1，而不是300 km s-1）。在平稳的风中，线力太小，无法驱动风。在聚集风中，线力通常足以驱动风，尽管在音速点周围仍有一些差异。在AV 83中，相对于正常的SMC丰度，N丰度大幅提高，而C和O都是SMC样的，与恒星表面存在内部处理的CNO材料一致。模型很好地再现了N Ⅲ λ4640多重峰，它是由双电子复合产生的。这些谱线和相邻的C III λ4649多重谱线显示出对表面重力的显著敏感性，以及对丰度和有效温度的通常敏感性。光球内发生的非相干电子散射可以解释这些线上看到的宽大翅膀。我们已经模拟了AV 83和AV 69中UV中的Fe光谱（Fe IV-Fe VI）。对于有效温度约为33，000 K的恒星，Fe IV与Fe V线的比值形成了一个有用的有效温度诊断，并给出了与光学和紫外线诊断结果一致的结果。导出的铁丰度，这是敏感的微湍流速度，是0.2-0.4倍的太阳能铁丰度在AV 83，而0.2太阳能给出了一个很好的适合AV 69。AV 69的风力密度远低于AV 83。由于缺乏合适的诊断，它是不可能的约束质量损失率和速度定律独立。它的光谱表明，它具有与AV 83相似的有效温度（Teff = 34，000 K），比AV 83高得多的重力（log g = 3.5），以及未受内部CNO处理影响的CNO丰度模式。我们发现，N/C丰度比大大低于太阳，与SMC星云和恒星丰度的研究。AV 83和AV 69的光谱之间的差异，以及导出的质量和表面丰度之间的差异是惊人的。我们已经检查了可能的原因，只有一个似乎与观测和我们目前对大质量星星演化的理解一致。AV 83很可能是一个快速旋转体，经历了旋转增强的质量损失。增强的N，但几乎正常的C和O丰度的存在是旋转诱导混合的直接指示。另一方面，AV 69是一个缓慢的旋转体。作为我们分析的一部分，我们系统地研究了H/He丰度比、质量损失率、速度定律、Fe丰度、微湍流和聚集对理论谱的影响。我们说明哪些线提供有用的诊断和突出的一些困难与光谱分析的O星。AV 83的光谱显示出光球层吸收线的存在，在风的底部形成的线的存在，以及许多风线。由于这些线采样的光球和整个风，极端O If超巨星，如AV 83，是理想的候选人，以探测恒星风的条件，因此进一步我们的知识O星星风。
We present a detailed study of AV 83, an O7 Iaf+ supergiant, and AV 69 [OC7.5 III((f))] in the SMC. The stars have similar effective temperatures and luminosities but show very different wind signatures. For our study we have used the non-LTE line-blanketed atmosphere code developed by Hillier and Miller, which explicitly allows for line blanketing by C, N, O, S, Ar, Ne, Fe, and other elements. Our study finds that AV 83 has an effective temperature of approximately 33,000 K and log g ≈ 3.25. It has an extended photosphere as a result of a "low" effective surface gravity and a much denser wind than main-sequence O stars. We can match the spectrum only by using a slow velocity law with β ≈ 2, a value that is much larger than the values of around 1 predicted by standard radiation wind theory. Further, we show that the Hα emission profile in AV 83 is sensitive to the adopted surface gravity. To fit the spectrum of AV 83, we have considered conventional models in which the wind is smooth and alternate models in which the winds are highly clumped. Both types of winds yield a satisfactory fit to the majority of lines in the observed spectrum; however, strong UV photospheric lines and the P V resonance transitions favor a clumped wind. If clumping is important, it must begin at relatively low velocities (i.e., 30 km s-1, not 300 km s-1). In the smooth wind, the line force is too small to drive the wind. In the clumped wind, the line force is generally sufficient to drive the wind, although there are still some discrepancies around the sonic point. In AV 83, the N abundance is substantially enhanced relative to normal SMC abundances, while both C and O are SMC-like, consistent with the presence of internally processed CNO material at the stellar surface. The N III λ4640 multiplet, which is known to be produced by dielectronic recombination, is well reproduced by the models. These lines, and the adjacent C III λ4649 multiplet, show a significant sensitivity to surface gravity, as well as the usual sensitivity to abundance and effective temperature. Incoherent electron scattering, occurring within the photosphere, can explain the broad wings seen on these lines. We have modeled the Fe spectrum (Fe IV-Fe VI) in the UV in both AV 83 and AV 69. For stars with an effective temperature around 33,000 K, the Fe IV-to-Fe V line ratios form a useful effective temperature diagnostic and give results consistent with those found from optical and UV line diagnostics. The derived iron abundance, which is sensitive to the adopted microturbulent velocity, is 0.2-0.4 times the solar iron abundance in AV 83, while 0.2 solar gives a good fit for AV 69. The wind of AV 69 is substantially less dense than that of AV 83. Because of the lack of suitable diagnostics, it is impossible to constrain the mass-loss rate and velocity law independently. Its spectrum indicates that it has a similar effective temperature to AV 83 (Teff ≈ 34,000 K), a substantially higher gravity (log g = 3.5) than AV 83, and a CNO abundance pattern that has not been influenced by internal CNO processing. We show that the N/C abundance ratio is substantially below solar, in agreement with SMC nebular and stellar abundance studies. The differences between the spectra of AV 83 and AV 69, and between the derived masses and surface abundances, are striking. We have examined possible causes, and only one seems consistent with the observations and our current understanding of massive star evolution. AV 83 was most likely a fast rotator that experienced rotationally enhanced mass loss. The presence of enhanced N but almost normal C and O abundances is a direct indication of rotationally induced mixing. On the other hand, AV 69 is a slow rotator. As part of our analyses, we have systematically examined the influence of the H/He abundance ratio, the mass-loss rate, the velocity law, the Fe abundance, microturbulence, and clumping on the theoretical spectrum. We illustrate which lines provide useful diagnostics and highlight some of the difficulties associated with spectroscopic analyses of O stars. The spectrum of AV 83 shows the presence of photospheric absorption lines, the presence of lines formed at the base of the wind, and numerous wind lines. Since these lines sample the photosphere and the entire wind, extreme O If supergiants, such as AV 83, are ideal candidates to probe conditions in stellar winds and hence further our knowledge of O star winds.