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Detection of pristine circumstellar material from the Cassiopeia A supernova progenitor


Cassiopeia A is a nearby young supernova remnant that provides a unique laboratory for the study of core-collapse supernova explosions1. Cassiopeia A is known to be a type IIb supernova from the optical spectrum of its light echo2, but the immediate progenitor of the supernova remains uncertain3. Here, we report results of near-infrared, high-resolution spectroscopic observations of Cassiopeia A, where we detected the pristine circumstellar material of the supernova progenitor. Our observations revealed a strong emission line of iron (Fe) from a circumstellar clump that has not yet been processed by the supernova shock wave. A comprehensive analysis of the observed spectra, together with a Hubble Space Telescope image, indicates that the majority of Fe in this unprocessed circumstellar material is in the gas phase, not depleted onto dust grains as in the general interstellar medium4. This result is consistent with a theoretical model5,6 of dust condensation in material that is heavily enriched with carbon–nitrogen–oxygen cycle nuclear reaction products, supporting the idea that the clump originated near the helium core of the progenitor7,8. It has recently been found that type IIb supernovae can result from the explosion of a blue supergiant with a thin hydrogen envelope9,10,11, and our results support such a scenario for Cassiopeia A.

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Fig. 1: A deep [Fe ii] 1.644 μm image of Cas A.
Fig. 2: Average spectra of knot 24.
Fig. 3: Integrated intensity [Fe ii] 1.644 μm maps of the NLC and BLC of knot 24 compared with a HST image.
Fig. 4: Internal chemical structure of a non-rotating 20 \({M}_{\odot }\) single star in the RSG stage.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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We thank R. Fesen for comments on the manuscript. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2017R1A2A2A05001337). This work used the IGRINS, which was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI), with the financial support of the US National Science Foundation under grant AST-1229522, of the University of Texas at Austin and of the Korean GMT Project of KASI. These results made use of the DCT at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. This work is based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

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Authors and Affiliations



B.-C.K. led the project, analysis and discussion and wrote the manuscript. H.-J.K. performed the observation and data reduction and contributed to the data analysis and manuscript writing. H.O. performed the observation and contributed to the IGRINS data analysis. J.C.R. contributed to the shock emission analysis and scientific interpretation. S.-C.Y. contributed to the scientific interpretation and the manuscript writing. Y.-H.L. contributed to the HST data analysis. D.T.J. contributed to the project set-up. All authors commented on the manuscript.

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Correspondence to Bon-Chul Koo.

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Supplementary Information, Tables 1 and 2 and Figs. 1–3.

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Koo, BC., Kim, HJ., Oh, H. et al. Detection of pristine circumstellar material from the Cassiopeia A supernova progenitor. Nat Astron 4, 584–589 (2020).

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