Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae
Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae
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II 型超新星星云相光谱中高速散射体的偏振特征
DOI：
10.1051/0004-6361/202140409
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发表时间：
2021
影响因子：
6.5
通讯作者：
Leonard, Douglas C.
中科院分区：
文献类型：
--
作者：
Dessart, Luc;John Hillier, D.;Leonard, Douglas C.
II型超新星（SNe）通常表现出线性极化，这是由自由电子散射引起的，具有复杂的光学特征，无论是在连续体中还是在线中。聚焦于星云早期阶段，在SN年龄为200d时，我们对a56Ni“斑点”的偏振特征进行了系统的研究，该“斑点”打破了球对称。我们的分析得到了非局域热力学平衡辐射传递计算的支持，即这种a56Ni团块的主要作用是提高局域自由电子的密度，否则在II型SN抛射中重组后自由电子密度会降低。利用二维极化辐射传输模型，我们探索了这种电子密度增强的影响，改变了它的大小、面、速度位置和空间范围。对于似是而非的几十个facvalues，高速斑点可以在200天内提供0.5-1.0%的连续偏振控制。我们的模拟再现了pcont的解析缩放，特别是斑点径向光学深度的线性增长。然而，最具限制性的信息是由偏振线光子携带的。对于高vblob，极化光谱呈现为全光谱的复制品，按比例缩小100-1000倍（即1∕Pcont），并红移amountblob(1−cosαlos)，其中α表示视线角度。随着vblobis的减少，红移减少，复制变差。形成区域与斑点重叠的线在极化通量中显得较弱和较窄。由于偏振光与倾角（∝sin2αlos）的关系，偏振光在垂直于视距（αlos= 90°）的平面上优先呈现不对称。这种特性也减弱了极化通量中谱线的展宽。如果选择适当的电子密度增强，这些结果中的一些可能适用于一般的不对称爆炸或外部喷射物中新形成的尘埃的极化特征。
Type II supernovae (SNe) often exhibit a linear polarization, arising from free-electron scattering, with complicated optical signatures, both in the continuum and in lines. Focusing on the early nebular phase, at a SN age of 200 d, we conduct a systematic study of the polarization signatures associated with a56Ni “blob” that breaks spherical symmetry. Our ansatz, supported by nonlocal thermodynamic equilibrium radiative transfer calculations, is that the primary role of such a56Ni blob is to boost the local density of free electrons, which is otherwise reduced following recombination in Type II SN ejecta. Using 2D polarized radiation transfer modeling, we explore the influence of such an electron-density enhancement, varying its magnitudeNe, fac, its velocity locationVblob, and its spatial extent. For plausibleNe, facvalues of a few tens, a high-velocity blob can deliver a continuum polarizationPcontof 0.5–1.0% at 200 d. Our simulations reproduce the analytic scalings forPcont, and in particular the linear growth with the blob radial optical depth. The most constraining information is, however, carried by polarized line photons. For a highVblob, the polarized spectrum appears as a replica of the full spectrum, scaled down by a factor of 100–1000 (i.e., 1∕Pcont) and redshifted by an amountVblob(1 − cosαlos), whereαlosis the line-of-sight angle. AsVblobis reduced, the redshift decreases and the replication deteriorates. Lines whose formation region overlaps with the blob appear weaker and narrower in the polarized flux. Because of its dependence on inclination (∝ sin2αlos), the polarization preferentially reveals asymmetries in the plane perpendicular to the line-of-sight (αlos= 90 deg). This property also weakens the broadening of lines in the polarized flux. With the adequate choice of electron-density enhancement, some of these results may apply to asymmetric explosions in general or to the polarization signatures from newly formed dust in the outer ejecta.