Mixing of 56Ni, whose nuclear decay energy is a major luminosity source in stripped-envelope supernovae, is known to affect the observational properties of stripped-envelope supernovae such as light-curve and color evolution. Here we systematically investigate the effect of 56Ni mixing on the photospheric velocity evolution in stripped-envelope supernovae. We show that 56Ni mixing significantly affects the early photospheric velocity evolution. The photospheric velocity, which is often used to constrain the ejecta mass and explosion energy, significantly varies by just changing the degree of 56Ni mixing. In addition, the models with a small degree of 56Ni mixing show a flattening in the early photospheric velocity evolution, while the fully mixed models show a monotonic decrease. The velocity flattening appears in both helium and carbon+oxygen progenitor explosions with a variety of ejecta mass, explosion energy, and 56Ni mass. Some stripped-envelope supernovae with early photospheric velocity information do show such a flattening. We find that Type Ib SN 2007Y, which has early photospheric velocity information, has a signature of a moderate degree of 56Ni mixing in the photospheric velocity evolution and about a half of the ejecta is mixed in it. The immediate spectroscopic follow-up observations of stripped-envelope supernovae shortly after the explosion providing the early photospheric evolution give an important clue to constrain 56Ni mixing in the ejecta.

中文翻译：

---

56Ni混合对剥离包络超新星早期光球速度演化影响的系统研究

众所周知，56Ni 的核衰变能量是剥离包络超新星的主要光度来源，其混合会影响剥离包络超新星的观测特性，例如光变曲线和颜色演化。在这里，我们系统地研究了 56Ni 混合对剥离包络超新星光球速度演化的影响。我们表明 56Ni 混合显着影响早期光球速度演化。通常用于约束喷射物质量和爆炸能量的光球速度仅通过改变 56Ni 混合的程度而显着变化。此外，56Ni混合程度较小的模型在早期光球速度演化中表现出平坦化，而完全混合的模型则表现出单调递减。速度平坦化出现在氦气和碳+氧前体爆炸中，喷射物质量、爆炸能量和 56Ni 质量各不相同。一些具有早期光球速度信息的剥离包络超新星确实表现出这种扁平化。我们发现具有早期光球速度信息的 Ib 型 SN 2007Y 在光球速度演化中具有中等程度的 56Ni 混合特征，并且大约有一半的喷射物混合在其中。爆炸后不久对剥离包络超新星的光谱跟踪观测提供了早期光球演化，为限制喷射物中 56Ni 的混合提供了重要线索。一些具有早期光球速度信息的剥离包络超新星确实表现出这种扁平化。我们发现具有早期光球速度信息的 Ib 型 SN 2007Y 在光球速度演化过程中具有中等程度的 56Ni 混合特征，并且大约有一半的喷射物混合在其中。爆炸后不久对剥离包络超新星的光谱跟踪观测提供了早期光球演化，为限制喷射物中 56Ni 的混合提供了重要线索。一些具有早期光球速度信息的剥离包络超新星确实表现出这种扁平化。我们发现具有早期光球速度信息的 Ib 型 SN 2007Y 在光球速度演化过程中具有中等程度的 56Ni 混合特征，并且大约有一半的喷射物混合在其中。爆炸后不久对剥离包络超新星的光谱跟踪观测提供了早期光球演化，为限制喷射物中 56Ni 的混合提供了重要线索。