Multiepoch VLT–FORS spectropolarimetric observations of supernova 2012aw reveal an asymmetric explosion
Multiepoch VLT–FORS spectropolarimetric observations of supernova 2012aw reveal an asymmetric explosion
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DOI：
10.1051/0004-6361/202140281
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发表时间：
2021-01
影响因子：
6.5
通讯作者：
L. Dessart;D. Leonard;D. Hillier;G. Pignata
中科院分区：
文献类型：
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作者：
L. Dessart;D. Leonard;D. Hillier;G. Pignata
我们提出了VLT-FORS光谱偏振观测的II型超新星（SN）2012 aw在7个时期的光球阶段，从16至120天后爆炸。我们通过假设SN偏振在最强谱线的静止波长附近为零来校正星际偏振-这后来被我们的建模所证实。SN 2012 aw表现出内在的极化，在谱线之间有很强的变化，并且在7000 μ m的无谱线区域内，其幅度从16 d时的0.1%增长到120 d时的1.2%。这种行为在定性上类似于其他II型SNe的观察结果。斯托克斯矢量的适当旋转将大部分极化置于q中，这表明SN 2012 aw的喷出物主要是轴对称的。使用我们的二维偏振辐射传输代码的升级版本，我们模拟了SN 2012 aw的波长和时间相关的偏振。关键的观测值可以解释为在~4000 km s−1处存在一个增强的56 Ni受限区域，它增加了一个开口角为~50度的圆锥中的电子密度，并且观察者的倾斜度为~70度对称轴。由于时间上的这种固定不对称性，SN 2012 aw偏振的观测演变源于喷出物光学深度、电离以及多次散射与单次散射的相对重要性的演变。然而，偏振特征表现出许多简并性。在早期的抵消效应意味着，低极化甚至可能发生喷出物具有较大的不对称性。一个轴对称的喷出物与纬度依赖的爆炸能量也可以产生类似的极化签名作为不对称的56镍分布。尽管存在这些不确定性，SN 2012 aw为II型SN喷出物的一般不对称性提供了额外的证据，其中VLT-FORS分光偏振观测是一个决定性的和精致的探测器。
We present VLT–FORS spectropolarimetric observations of the type II supernova (SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to 120 d after explosion. We corrected for interstellar polarization by postulating that the SN polarization is naught near the rest wavelength of the strongest lines – this is later confirmed by our modeling. SN 2012aw exhibits intrinsic polarization, with strong variations across lines, and with a magnitude that grows in the 7000 Å line-free region from 0.1% at 16 d up to 1.2% at 120 d. This behavior is qualitatively similar to observations gathered for other type II SNe. A suitable rotation of Stokes vectors places the bulk of the polarization in q, suggesting the ejecta of SN 2012aw is predominantly axisymmetric. Using an upgraded version of our 2D polarized radiative transfer code, we modeled the wavelength- and time-dependent polarization of SN 2012aw. The key observables may be explained by the presence of a confined region of enhanced 56Ni at ~4000 km s−1, which boosts the electron density in a cone having an opening angle of ~50 deg and an observer’s inclination of ~70 deg to the axis of symmetry. With this fixed asymmetry in time, the observed evolution of the SN 2012aw polarization arises from the evolution of the ejecta optical depth, ionization, and the relative importance of multiple versus single scattering. However, the polarization signatures exhibit numerous degeneracies. Cancellation effects at early times imply that low polarization may even occur for ejecta with a large asymmetry. An axisymmetric ejecta with a latitudinal-dependent explosion energy can also yield similar polarization signatures as asymmetry in the 56Ni distribution. In spite of these uncertainties, SN 2012aw provides additional evidence for the generic asymmetry of type II SN ejecta, of which VLT–FORS spectropolarimetric observations are a decisive and exquisite probe.