H-poor Type I superluminous supernovae (SLSNe I) are characterized by O ii lines around 4000–4500 Å in pre-/near-maximum spectra, the so-called W-shaped O ii lines. As these lines are from relatively high excitation levels, they have been considered a sign of nonthermal processes, which may provide a hint of the power sources of SLSNe I. However, the conditions for these lines to appear have not been well understood. In this work, we systematically calculate synthetic spectra to reproduce the observed spectra of eight SLSNe I, parameterizing departure coefficients from the nebular approximation in the supernova ejecta (expressed as b _neb). We find that most of the observed spectra can be well reproduced with b _neb ≲ 10, which means that no strong departure is necessary for the formation of the W-shaped O ii lines. We also show that the appearance of the W-shaped O ii lines is sensitive to temperature; only spectra with temperatures in the range of T ∼ 14,000–16,000 K can produce the W-shaped O ii lines without large departures. Based on this, we constrain the nonthermal ionization rate near the photosphere. Our results suggest that spectral features of SLSNe I can give independent constraints on the power source through the nonthermal ionization rates.

中文翻译：

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I型超光度超新星光谱中W形O ii 谱线的形成


贫氢 I 型超光度超新星 (SLSNe I) 的特征是前/近最大光谱中 4000-4500 Å 左右的 O ii 谱线，即所谓的 W 形 O ii 谱线。由于这些谱线来自相对较高的激发能级，因此它们被认为是非热过程的标志，这可能为SLSNe I的能量来源提供线索。然而，这些谱线出现的条件尚未得到很好的了解。在这项工作中，我们系统地计算合成光谱，以再现八个 SLSNe I 的观测光谱，参数化超新星喷射物中星云近似的偏离系数（表示为 b _neb ）。我们发现大多数观测到的光谱都可以在 b _neb ≲ 10 的情况下很好地再现，这意味着 W 形 O ii 谱线的形成不需要强烈的偏离。我们还表明，W 形 O ii 线的出现对温度很敏感；只有温度在 T ∼ 14,000–16,000 K 范围内的光谱才能产生没有大偏差的 W 形 O ii 谱线。基于此，我们限制了光球层附近的非热电离率。我们的结果表明，SLSNe I 的光谱特征可以通过非热电离率对电源给出独立的约束。