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| Fig 1: OGLE14-073 observed on September 24, 2014. The image was taken with the ESO Faint Object Spectrograph (EFOSC) on the NTT telescope operated by the European Sourthern Observatory (ESO). Credit: Terreran et al. (2017) Nature Astronomy |
An international research team discovered a supernova that cannot be explained by the current standard theory of supernova explosions. The supernova, named OGLE-2014-SN-074 (OGLE14-073 hereafter), was more than 10 times more energetic than supernovae of a similar kind. OGLE14-073 could be a new kind of supernovae whose existence has been predicted theoretically but has never been observed.
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| Fig 1: OGLE14-073 observed on September 24, 2014. The image was taken with the ESO Faint Object Spectrograph (EFOSC) on the NTT telescope operated by the European Sourthern Observatory (ESO). Credit: Terreran et al. (2017) Nature Astronomy |
Massive stars end their lives with spectacular explosions called supernovae. A large fraction of massive stars contain hydrogen at the end of their lives and supernovae containing hydrogen are often observed. The brightness of supernovae containing hydrogen usually do not change for about 100 days. However, the brightness of OGLE14-073 kept increasing for 100 days. Such a slow increase in the brightness was observed in SN 1987A which appeared in the Large Magellanic Cloud. It is now known that a slow brightness increase is rarely observed.
What made OGLE14-073 more astonishing was its brightness, rather than its slow brightness increase. It was about 10 times brighter than SN 1987A. The research team led by Dr. Giacomo Terreran from Queen’s University at Belfast and Padua Observatory revealed that OGLE14-073 is more than 10 times more energetic than SN 1987A. This fact led the team to a deeper confusion. According to the standard theory of supernova explosions, such an energetic explosion is impossible.
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| Fig 2: Light curves of OGLE14-073 (green circle) and SN 1987A (black triangles). The shapes of the light curves of OGLE14-073 and SN 1987A are similar but the luminosity of OGLE14-073 is about 10 times brighter. Light curves of some bright supernovae are also presented for comparison. Their light curve shapes are not similar to OGLE14-073. Credit: Terreran et al. (2017) Nature Astronomy |
How could a massive star containing hydrogen explode with such a huge explosion energy? When Dr. Takashi Moriya, a member of the research team from the Division of Theoretical Astronomy at NAOJ, first saw the observational data he pointed out that OGLE14-073 may be a pair-instability supernova(*1). "Pair-instability supernovae are non-standard explosions of very massive stars. The explosions are triggered by the instability initiated by the creation of electron-positron pairs in the very massive stars. Pair-instability supernovae can lead to explosions that are more than 10 times more energetic than SN 1987A," Moriya said. No clear pair-instability supernovae have been observed so far and the research team thought that OGLE14-073 may be the first observed example of pair-instability supernovae. However, the research team also found that some observational features of OGLE14-073 did not match the theoretically predicted features of pair-instability supernovae.
Another possible explanation for the huge energy of OGLE14-073 is that a "magnetar," a strongly magnetized rapidly rotating neutron star that rotates about 1000 times in a second, was formed during the explosion and energized the exploding massive star. However, the current stellar evolution theory has difficulties in making such a rapidly-rotating neutron star in massive stars containing large amounts of hydrogen.
Neither of the hypotheses, pair-instability supernova or magnetar, can perfectly explain the unexpected large energy of OGLE14-073 and the research team was not able to reach a clear conclusion on its origin. Finding many similar supernovae with large transient surveys such as the one currently being conducted by the Subaru Telescope and observing them frequently even after they become faint will help clarifying whether OGLE14-073-like supernovae are pair-instability supernovae or not. The discovery of OGLE14-073 showed that the mechanisms triggering the explosions of massive stars are more diverse than previously thought and provided new insights into the study of the stellar explosions. This research was published online in Nature Astronomy on September 18, 2017.
Notes:
1) Supernovae from massive stars are usually triggered by the collapse of the iron core that could not support its weight. However, in the case of extremely massive stars, it is theoretically predicted that the massive cores cannot support their weight when they are composed of oxygen before they form iron. The collapse of the massive oxygen cores can lead to energetic supernova explosions. These explosions are called pair-instability supernovae.
Title: "Hydrogen-rich supernovae beyond the neutrino-driven core-collapse paradigm"
Journal: Nature Astronomy
DOI: 10.1038/s41550-017-0228-8
Authors: G. Terreran, M. L. Pumo, T.-W. Chen, T. J. Moriya, F. Taddia, L. Dessart, L. Zampieri, S. J. Smartt, S. Benetti, C. Inserra, E. Cappellaro, M. Nicholl, M. Fraser, Ł. Wyrzykowski, A. Udalski, D. A. Howell, C. McCully, S. Valenti, G. Dimitriadis, K. Maguire, M. Sullivan, K. W. Smith, O. Yaron, D. R. Young, J. P. Anderson, M. Della Valle, N. Elias-Rosa, A. Gal-Yam, A. Jerkstrand, E. Kankare, A. Pastorello, J. Sollerman, M. Turatto, Z. Kostrzewa-Rutkowska, S. Kozłowski, P. Mróz, M. Pawlak, P. Pietrukowicz, R. Poleski, D. Skowron, J. Skowron, I. Soszyński, M. K. Szymański & K. Ulaczyk
Related Links
Takashi Moriya Personal Webpage