超亮超新星SN2019szu中的前兆平台与峰值前[O II]发射：一个脉动对不稳定性候选体
A Precursor Plateau and Pre-Maximum [O II] Emission in the Superluminous SN2019szu: A Pulsational Pair-Instability Candidate
AI总结：
本文研究超亮超新星SN2019szu，发现其爆炸前约40天平台和上升期[O II]等禁戒发射线，提出早期脉动对不稳定性抛射低密度富氧物质并碰撞产生前兆平台，是该机制迄今最佳候选体之一。
摘要：
我们对SN2019szu进行了详细研究，这是一颗位于红移z=0.213处的I型超亮超新星，展现出独特的光度和光谱性质。Pan-STARRS和ZTF的强制测光显示出一个持续约40天的爆炸前平台。与其他随时间显示光球温度下降的SLSNe不同，其光学颜色显示在最初70天内表观温度从约15000 K升高到约20000 K，这很可能是由光谱中额外的伪连续谱造成的。值得注意的是，即使在光变曲线的上升阶段，光谱也显示出一条禁戒发射线，这与表观上致密的光球层不一致。我们证明这一早期特征为[O II] λλ7320,7330。我们还看到了[O III] λλ4959, 5007和[O III] λ4363的证据，进一步强化了这一谱线证认。通过与星云发射模型比较，我们发现氧线流量和比值可以用约0.25 M⊙的富氧物质在约10^-15 g cm^-3的密度下再现。如此低的密度暗示其来源于星周环境，但发射线如此早地出现要求这些物质是在最终爆炸前最后几个月内被抛射出来的，这与前兆平台的时间一致。与更靠近爆炸处的较致密物质相互作用很可能产生了约5500 Å蓝端的伪连续谱。我们认为该事件是迄今为止脉动对不稳定性抛射的最佳候选体之一，早期脉冲提供了禁戒发射线所需的低密度物质，而最后抛射壳层之间的碰撞则产生了爆炸前平台。
英文摘要：
We present a detailed study on SN2019szu, a Type I superluminous supernova at $z=0.213$, that displayed unique photometric and spectroscopic properties. Pan-STARRS and ZTF forced photometry shows a pre-explosion plateau lasting $\sim$ 40 days. Unlike other SLSNe that show decreasing photospheric temperatures with time, the optical colours show an apparent temperature increase from $\sim$15000 K to $\sim$20000 K over the first 70 days, likely caused by an additional pseudo-continuum in the spectrum. Remarkably, the spectrum displays a forbidden emission line even during the rising phase of the light curve, inconsistent with an apparently compact photosphere. We show that this early feature is [O II] $λλ$7320,7330. We also see evidence for [O III] $λλ$4959, 5007, and [O III] $λ$4363 further strengthening this line identification. Comparing with models for nebular emission, we find that the oxygen line fluxes and ratios can be reproduced with $\sim$0.25 M$_{\odot}$ of oxygen rich material with a density of $\sim10^{-15} \rm{g cm}^{-3}$. The low density suggests a circumstellar origin, but the early onset of the emission lines requires that this material was ejected within the final months before the terminal explosion, consistent with the timing of the precursor plateau. Interaction with denser material closer to the explosion likely produced the pseudo-continuum bluewards of $\sim$5500 Å. We suggest that this event is one of the best candidates to date for a pulsational pair-instability ejection, with early pulses providing the low density material needed for the forbidden emission line, and collisions between the final shells of ejected material producing the pre-explosion plateau.
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