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arXiv 1902.06181astro-ph.HE
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NGC 6946中IIP型超新星2017 eaw的末阶段祖先演化

Probing the Final-stage Progenitor Evolution for Type IIP Supernova 2017eaw in NGC 6946

Liming Rui, Xiaofeng Wang, Jun Mo, Danfeng Xiang, Jujia Zhang, Justyn R. Maund, Avishy Gal-Yam, Lifan Wang, Tianmeng Zhang

摘要:

我们基于爆炸前的图像和爆炸后立即获得的早期观测结果,详细分析了NGC 6946中IIP型超新星2017 eaw的祖先属性。在2016年哈勃太空望远镜(HST)拍摄的发现前图像中,可以在SN位置识别出一颗不寻常的红色恒星,M $_{F814W}$ = $-$ 6.9等,m $_{F606W}$ $-$ m $_{F814W}=$ 2.9 $\pm$ 0.2等。观测到的这颗恒星的光谱能量分布覆盖了0.6-2.0 $\ \mathrm{μm}$的波长,与温度约为3550 K的M4型红超巨星(RSG)的光谱能量分布相匹配。这些结果表明SN 2017 eaw具有初始质量为12 $\pm$ 2 M $_\odot$的RSG祖先。这颗恒星的F814 W波段绝对星等从2004年的7.2等演化到2016年的6.9等。然而,这种调光效应对于处于氖/氧燃烧阶段的RSG来说是不可预测的,此时其亮度应该适度增加。SN 2017 eaw在发现后几小时拍摄的光谱清楚地显示了一个狭窄的H $_α$发射特征,被$\sim$蓝移了160 km s $^{-1}$。这一狭窄的成分在两天后拍摄的光谱中消失了,表明存在一个拱星物质(CSM)壳(即,距离为$<$ 2.1-4.3 $\times$ 10 $^{14}$ cm)。结合推断的距离和CSM的膨胀速度,我们认为SN 2017 eaw的前身应该在爆炸前1-2年经历了剧烈的质量损失,这可能是由脉动包络抛射引起的。这一机制可能有助于解释它在2016年的光度下降,以及没有检测到初始质量在17 M $_\odot<$ M $<$ 25 M $_\odot$范围内的RSG作为SNe IIP的祖先。

英文摘要:

We presented a detailed analysis of progenitor properties of type IIP supernova 2017eaw in NGC 6946, based on the pre-explosion images and early-time observations obtained immediately after the explosion. An unusually red star, with M$_{F814W}$ = $-$6.9 mag and m$_{F606W}$$-$ m$_{F814W}=$2.9$\pm$0.2 mag, can be identified at the SN position in the pre-discovery Hubble Space Telescope(HST) images taken in 2016. The observed spectral energy distribution of this star, covering the wavelength of 0.6-2.0$\ \mathrm{μm}$, matches that of an M4-type red supergiant (RSG) with a temperature of about 3550 K. These results suggest that SN 2017eaw has a RSG progenitor with an initial mass of 12$\pm$2 M$_\odot$. The absolute F814W-band magnitude of this progenitor star is found to evolve from $-$7.2 mag in 2004 to $-$6.9 mag in 2016. Such a dimming effect is, however, unpredicted for a RSG in its neon/oxygen burning phase when its luminosity should modestly increase. The spectrum of SN 2017eaw taken a few hours after discovery clearly shows a narrow H$_α$ emission feature blueshifted by $\sim$160 km s$^{-1}$. This narrow component disappeared in the spectrum taken two days later, suggesting the presence of a circumstellar material (CSM) shell (i.e., at a distance of $<$2.1-4.3$\times$10$^{14}$ cm). Combining the inferred distance with the expansion velocity of the CSM, we suggest that the progenitor of SN 2017eaw should have experienced violent mass loss at about 1-2 years prior to explosion, perhaps invoked by pulsational envelop ejection. This mechanism may help explain its luminosity decline in 2016 as well as the lack of detections of RSGs with initial mass in the range of 17 M$_\odot<$ M $<$ 25 M$_\odot$ as progenitors of SNe IIP.

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