Recent multi-dimensional simulations suggest that high-entropy buoyant plumes help massive stars to explode1,2. Outwardly protruding iron (Fe)-rich fingers of gas in the galactic supernova remnant3,4 Cassiopeia A seem to match this picture. Detecting the signatures of specific elements synthesized in the high-entropy nuclear burning regime (that is, α-rich freeze out) would constitute strong substantiating evidence. Here we report observations of such elements—stable titanium (Ti) and chromium (Cr)—at a confidence level greater than 5 standard deviations in the shocked high-velocity Fe-rich ejecta of Cassiopeia A. We found that the observed Ti/Fe and Cr/Fe mass ratios require α-rich freeze out, providing evidence of the existence of the high-entropy ejecta plumes that boosted the shock wave at explosion. The metal composition of the plumes agrees well with predictions for strongly neutrino-processed proton-rich ejecta2,5,6. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A.
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All the Chandra and NuSTAR data used in this research are available from the Chandra Data Archive (https://cxc.harvard.edu/cda/) and the NuSTAR Archive (https://heasarc.gsfc.nasa.gov/docs/nustar/nustar_archive.html) in raw and reduced formats.
To analyse X-ray data with Chandra, we used public software, Chandra Interactive Analysis of Observations: CIAO (https://cxc.cfa.harvard.edu/ciao/). We used public atomic data in atomDB (http://www.atomdb.org/) and SPEX (https://www.sron.nl/astrophysics-spex). We fitted the X-ray spectra with a public package, Xspec (https://heasarc.gsfc.nasa.gov/xanadu/xspec/). We have not made publicly available codes for the hydrodynamics and nucleosynthesis of supernova explosions because they are not prepared for open use. Instead, the simulated thermodynamic profiles of the supernova explosions and the composition distributions shown in this paper are available on request.
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T.S. was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant number JP19K14739, the Special Postdoctoral Researchers Program, and FY 2019 Incentive Research Projects in RIKEN. K.M. was supported in part by Grants-in-Aid for the Scientific Research of JSPS (grant numbers JP18H05223 and JP20H00174). S.N. is partially supported by the Grants-in-Aid for the Scientific Research of JSPS (grant KAKENHI (A) 19H00693), the RIKEN programme for Evolution of Matter in the Universe (r-EMU), and the Theoretical and Mathematical Sciences Program of RIKEN (iTHEMS). J.PH. acknowledges support for X-ray studies of supernova remnants from NASA grant NNX15AK71G to Rutgers University. T.Y. is supported in part by a Grant-in-Aid for Scientific Research of Innovative Areas (JP20H05249). H.U. is supported in part by a Grant-in-Aid for Scientific Research (JP17H01130).
The authors declare no competing interests.
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Extended data figures and tables
a, The Fe-K/Si-K ratio map in 2004. The solid white contour shows the region used for the result discussed in the main text. b, The Fe−K image in 2000 (left), 2009 (middle) and 2018 (right). In order to track the proper motions of each structure, we shift the region from epoch to epoch. c, The X-ray spectrum and its best-fit model for the southeastern Fe-rich region. The spectrum (black data) taken is the same as in Fig. 2, but the best-fit thermal model has the Ti and Cr emissions. The residuals around 4.7–4.8 keV and 5.5–5.9 keV in Fig. 2 are well explained by the Ti and Cr emissions. d, The best-fit parameters for the Fe-rich ejecta. The errors show 1σ confidence level (Δχ2 = 1.0). The solar abundance in ref. 29 is used. e, The summary of the Ti measurements in the Fe-rich regions of Cassiopeia A. The errors show 1σ confidence level (Δχ2 = 1.0). d.o.f., degrees of freedom; nH, column density. n is density of plasma, t is ionization time and V is the volume of hot plasma.
a, The red and black show a plasma model (vvpshock) with Ti and without Ti, respectively. The Heα emissions from Ti (red) are between Ca Heβ and Ca Heγ. The grey area shows the 90% error range of the centroid energy of the Ti line observed by Chandra. The plasma parameters are the same as in Extended Data Fig. 1e (without the line broadening). b, Comparison of 4–5 keV model spectra (vvnei) that have different ionization states. The grey area shows the 90% error range of the centroid energy of the Ti line by Chandra. The most prominent lines are the Ca Heβ and Ca Heγ lines at 4.584 keV and 4.822 keV, respectively. These two Ca lines become stronger at high ionization states (>5 × 1010 cm−3 s). Ly, Lyman. DR, dielectronic recombination. c, The black data and red curves show the observed spectra and the best-fit models, respectively. The fitting range (grey area) is 3.7–7.1 keV. This result is used in the main text. d, The fitting range is 3.7–9.5 keV where the emissions up to Ni are included. To express the lack of emissions around 8.3 keV, one Gaussian line is added. e, The fitting range is 2.2–9.5 keV. Here, we added a thermal bremsstrahlung model to express a low temperature component. The best-fit parameters are summarized in the table in f. f, The best-fit parameters for the Fe-rich ejecta in the spectrum shown in e. The errors show 1σ confidence level (Δχ2 = 1.0). The solar abundance in ref. 29 is used. Some other lighter elements that are not shown here are also included in the model. D is the distance to the source and k is the Boltzmann constant.
Extended Data Fig. 3 The Fe distribution (image) and the 44Ti upper limit map (coloured boxes) around the southeastern region.
We use the 44Ti upper limits estimated in ref. 16. The box size is 45″ × 45″. The box IDs are the same as those in the paper. The white contour regions are the same as in Fig. 1. Almost all of the areas from which we extracted spectra are included in three boxed regions: 31, 39 and 47. M44 is the mass of 44Ti.
Extended Data Fig. 4 The one-dimensional core-collapse supernova nucleosynthesis model used in this study.
The model assumed a high-energy explosion of 3 × 1051 erg for a 15M☉ progenitor. The α-rich freeze out produces some α elements (for example, Fe, Ni, Cr, Ti, Zn) at the deepest layer with high peak temperatures (>5.5 GK). At the QSE (that is, incomplete Si burning) layer, the intermediate-mass elements (such as Si, S, Ar, Ca, Cr, Mn) are abundant. Mr is the Lagrangian mass coordinate.
a, Ti/Fe (top row) and Cr/Fe (bottom row) mass ratios in the peak temperature−density plane. From left to right, the lepton fraction corresponds to Ye = 0.499, 0.5 and 0.55. Here, we used the thermodynamic trajectories taken from our one-dimensional supernova model. All the stable isotopes are included. The production of Ti and Cr is sensitive to the high-entropy environment (toward the bottom right), which is the same as radioactive 44Ti (ref. 71). The dashed lines show the boundary between incomplete and complete Si burning. The black boxes show typical parameter spaces for the complete Si burning (α-rich freeze out) in our one-dimensional supernova model and the three-dimensional supernova model in ref. 19. In the proton-rich environment, α- and p-rich (αp-rich) freezeout occurs (ref. 71). b, Mass fractions of Ti, Cr and Ni isotopes in the nucleosynthesis calculations. n-rich, neutron-rich; p-rich, proton-rich.
The pink-shaded areas show the observed mass ratios. a, The coloured points show parameter studies for hot (Tpeak = 10 GK) and proton-rich environment while changing the peak density from 105.5 g cm−3 to 107.5 g cm−3. The circle, star and square symbols show Ye = 0.53, 0.55 and 0.58, respectively. Here, we used the thermodynamic trajectories taken from our one-dimensional supernova model. b, Parameter studies with power-law thermodynamic evolusion. The star and square symbol data show the Ti/Fe and Cr/Fe mass ratios produced by Tpeak = 8 GK and Tpeak = 10 GK, respectively. To reproduce the observed mass ratios, a higher radiation entropy is needed than that in the model with the thermodynamic trajectories taken from the one-dimensional supernova model.
The Mn/Fe mass ratio is derived from the best-fit model in the left column of Extended Data Fig. 1d. The shaded areas show the observed mass ratios with 90% and 99% error range. The coloured circles show the mass ratios in our one-dimensional supernova model (a 15 M☉ progenitor, Eexp = 3 × 1051 erg, Z = 0.5Z☉). For the square data symbols, the same one-dimensional supernova model was used, but the lepton fraction at the α-rich freeze out is modified to Ye = 0.5. The modification from circle to square symbols (increase of Ye) suppresses the synthesized amount of neutron-rich elements like Mn. The coloured stars show a parameter study for hot (Tpeak = 10 GK) and proton-rich (Ye = 0.55) environment while changing the peak density from 105.5 g cm−3 to 107.5 g cm−3. SN, supernova.
a, Two-colour image around the southeastern Fe-rich region. The red and green show the Fe and Si images, respectively. The green box is defined as the Si-rich ejecta region. b, The X-ray spectrum extracted from the Si-rich region. The blue curve (Si-rich component) shows the best-fit thermal model. c, The X-ray spectrum at the Fe-rich region. The Si-rich component has the same plasma parameters as in the model of b. The red model shows the Fe-rich component that has emissions from H, He, Ti, Cr, Mn, Fe and Ni. d, Comparisons of the observed Ca/Si and Fe/Si mass ratios in the Si-rich ejecta region with those by theoretical calculations. The faint orange shading shows the observed mass ratios (99% confidence level, Δχ2 = 6.64). The coloured points show the mass ratios of the nucleosynthesis calculations in Fig. 3. e, The best-fit parameters for the Si-rich ejecta in the spectrum of b. The spectrum is extracted from the 2004 data. The errors show 1σ confidence level (Δχ2 = 1.0). The solar abundance in ref. 29 is used.
Extended Data Fig. 9 Comparisons of the observed Ni/Fe and Cr/Fe mass ratios in the Fe-rich ejecta region with those by theoretical calculations.
a, The blue and red dashed lines show the best-fit Ni/Fe mass ratios with Xspec (AtomDB79, version 3.0.9) and SPEX80 (version 3.0.5), respectively. The best-fit Ni/Fe mass ratios are different from each other because the emissivities of the Fe Kβ,γ,δ, … emissions are different depending on the atomic code. The faint orange shading indicates the observed Cr/Fe mass ratios in Fig. 3. The coloured points show the mass ratios of the nucleosynthesis calculations in Fig. 3. b, The Ni/Fe dependence on the lepton (electron) fraction, Ye. The blue and red dashed lines show the best-fit Ni/Fe mass ratios with Xspec and SPEX, respectively. Here, we assumed an explosion energy of 3 × 1051 erg, and a region with the peak temperature of Tpeak = 6.5 GK is analysed. On the neutron-rich side, the Ni/Fe ratio changes more dramatically because the neutron-rich element, 58Ni, is efficiently synthesized. On the other hand, on the proton-rich side, the Ni is not as sensitive to the lepton fraction. Here, 60Ni is dominantly synthesized (see Extended Data Fig. 5b).
a, three-colour image of Cassiopeia A. The red, green and blue colours show the Fe−K, Si−K and the continuum (approximately non-thermal) emissions, respectively. b, X-ray spectra in the southeast (red), north (black) and southwest (green) regions. c, Comparison of spectra between XRISM and Chandra. We assumed an energy resolution of 7 eV (FWHM) and exposure time of 1 Ms for the XRISM simulation. In the simulated XRISM spectrum, we do not consider the line broadening effects (either thermal and Doppler). d, Zoom of area around the Ti emissions in c. Here we simulated a spectrum with the thermal broadening assuming kTion = 125 keV (data with error bars). The black and red lines show the thermal models with kTion = 125 keV and kTion = 780 keV, respectively.
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Sato, T., Maeda, K., Nagataki, S. et al. High-entropy ejecta plumes in Cassiopeia A from neutrino-driven convection. Nature 592, 537–540 (2021). https://doi.org/10.1038/s41586-021-03391-9