A spectropolarimetric survey of northern hemisphere Wolf–Rayet stars
A spectropolarimetric survey of northern hemisphere Wolf–Rayet stars
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北半球沃尔夫-拉叶星的光谱偏振测量
DOI：
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发表时间：
1998
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通讯作者：
I. Howarth
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文献类型：
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作者：
T. Harries;D. Hillier;I. Howarth
我们对北半球 16 颗沃尔夫-拉叶 (WR) 恒星进行了均匀、高信噪比的光谱偏振测量。在四颗恒星中发现了发射线波长的偏振减少，即“线效应”：WR 134、137、139 和 141。WR 139 (V444 Cyg) 中的效应强度是可变的，而之前报道显示线效应的 WR 136 并未在我们的数据中显示出来。假设线效应通常是由恒星风的轴对称扭曲引起的，我们表明，所有 WR 具有相同的固有（赤道上）极化的模型，观测到的变化仅是倾斜效应的结果，与观测结果不一致。内在偏振均匀分布的模型更为合理，但如果偏振分布偏向较小的值，则可以获得最佳拟合结果，只有约 20% 的恒星的内在偏振大于约 0.3%。辐射传输计算表明，观测到的连续偏振可以与赤道：极点密度比为 2-3 的模型相匹配。模型光谱的电子散射翼明显强于观察到的强度（强度和偏振通量），证实显示内在偏振的恒星风必须在小尺度上聚集，并且在大尺度上扭曲。 我们将调查结果与文献中的观察结果相结合，给出了 29 颗恒星的样本，这些恒星既有准确的分光偏振观测结果，也有从标准模型分析中得出的物理参数。我们发现，线效应恒星在光度-质量损失率平面中聚集在高M、L处（尽管它们在终端速度-亚型和表面质量通量-温度平面中并不例外）。从这些恒星的射电连续谱观测中得出的质量损失率与光学发射线分析的结果非常一致，这表明（i）线效应恒星的风结构具有与半径有效恒定的密度对比，以及（ii）高M值可能是大规模风结构的产物。假设观测到的线效应恒星的光谱和光度变化与 WR 自转周期有关，我们计算赤道自转速度。根据 Ignace、Cassinelli 和 Bjorkman 的模型，这些速度相当于核心破碎率的 10%，并且可能足够大以产生显着的风压缩效应。
We present a homogeneous, high signal-to-noise spectropolarimetric survey of 16 northern hemisphere Wolf–Rayet (WR) stars. A reduction in polarization at emission-line wavelengths — the ‘line effect’— is identified in four stars: WRs 134, 137, 139, and 141. The magnitude of the effect in WR 139 (V444 Cyg) is variable, while WR 136, previously reported to show the line effect, does not show it in our data. Assuming the line effect generally to arise from axisymmetric distortions of stellar winds, we show that a model in which all WRs have the same intrinsic (equator-on) polarization, with the observed variations solely a result of inclination effects, is inconsistent with the observations. A model in which the intrinsic polarizations are uniformly distributed is more plausible, but best-fitting results are obtained if the distribution of polarizations is biased towards small values, with only ∼ 20 per cent of stars having intrinsic polarizations greater than ∼ 0.3 per cent. Radiative transfer calculations indicate that the observed continuum polarizations can be matched by models with equator:pole density ratios of 2–3. The model spectra have electron-scattering wings that are significantly stronger than observed (in both intensity and polarized flux), confirming that the winds of stars showing intrinsic polarization must be clumped on small scales as well as being distorted on large scales. We combine the results of our survey with observations from the literature to give a sample of 29 stars which have both accurate spectropolarimetric observations and physical parameters derived from standard-model analyses. We find that the line-effect stars are clustered at high M, L in the luminosity–mass-loss rate plane (although they are unexceptional in the terminal velocity–subtype and the surface-mass-flux–temperature planes). The mass-loss rates derived from radio-continuum observations for these stars are in good accord with the results of optical emission-line analyses, suggesting that (i) the wind structure of line-effect stars has a density contrast which is effectively constant with radius, and (ii) the high M values may be artefacts of large-scale wind structure. Assuming that observed spectroscopic and photometric variability of the line-effect stars is related to the WR rotation period, we compute equatorial rotation velocities. These velocities correspond to ∼ 10 per cent of the core breakup rates, and may be large enough to produce significant wind-compression effects according to the models of Ignace, Cassinelli & Bjorkman.