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Register a new userProduction and Distribution of $^{44}$Ti and $^{56}$Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A
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Annop Wongwathanarat
Publication date
2016
fields
Physics
and research's language is
English
Authors
A. Wongwathanarat
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The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive $^{44}$Ti with high-energy satellite telescopes (Nuclear Spectroscopic Telescope Array [NuSTAR], INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in SN 1987A and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here we discuss a 3D simulation of a neutrino-driven explosion, using a parameterized neutrino engine, whose $^{44}$Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither the progenitor nor the explosion was fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high $^{44}$Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions generically eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tugboat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly one-sided Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.
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The nucleosynthetic yield from a supernova explosion depends upon a variety of effects: progenitor evolution, explosion process, details of the nuclear network, and nuclear rates. Especially in studies of integrated stellar yields, simplifications reduce these uncertainties. But nature is much more complex, and to actually study nuclear rates, we will have to understand the full, complex set of processes involved in nucleosynthesis. Here we discuss a few of these complexities and detail how the NuGrid collaboration will address them.
Due to its proximity, SN 1987A offers a unique opportunity to directly observe the geometry of a stellar explosion as it unfolds. Here we present spectral and imaging observations of SN 1987A obtained ~10,000 days after the explosion with HST/STIS and VLT/SINFONI at optical and near-infrared wavelengths. These observations allow us to produce the most detailed 3D map of H-alpha to date, the first 3D maps for [Ca II] lambda lambda 7292, 7324, [O I] lambda lambda 6300, 6364 and Mg II lambda lambda 9218, 9244, as well as new maps for [Si I]+[Fe II] 1.644 mu m and He I 2.058 mu m. A comparison with previous observations shows that the [Si I]+[Fe II] flux and morphology have not changed significantly during the past ten years, providing evidence that it is powered by 44Ti. The time-evolution of H-alpha shows that it is predominantly powered by X-rays from the ring, in agreement with previous findings. All lines that have sufficient signal show a similar large-scale 3D structure, with a north-south asymmetry that resembles a broken dipole. This structure correlates with early observations of asymmetries, showing that there is a global asymmetry that extends from the inner core to the outer envelope. On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644 mu m, show substructures at the level of ~ 200 - 1000 km/s and clear differences in their 3D geometries. We discuss these results in the context of explosion models and the properties of dust in the ejecta.
Context: Tracing unstable isotopes produced in supernova nucleosynthesis provides a direct diagnostic of supernova explosion physics. Theoretical models predict an extensive variety of scenarios, which can be constrained through observations of the abundant isotopes $^{56}$Ni and $^{44}$Ti. Direct evidence of the latter was previously found only in two core-collapse supernova events, and appears to be absent in thermonuclear supernovae.Aims: We aim to to constrain the supernova progenitor types of Cas A, SN 1987A, Vela Jr., G1.9+0.3, SN1572, and SN1604 through their $^{44}$Ti ejecta masses and explosion kinematics. Methods: We analyzed INTEGRAL/SPI observations of the candidate sources utilizing an empirically motivated high-precision background model. We analyzed the three dominant spectroscopically resolved de-excitation lines at 68, 78, and 1157,keV emitted in the decay chain of $^{44}$Ti. The fluxes allow the determination of the production yields of $^{44}$Ti. Remnant kinematics were obtained from the Doppler characteristics of the lines. Results: We find a significant signal for Cas A in all three lines with a combined significance of 5.4$sigma$. The fluxes are $(3.3 pm 0.9) times 10^{-5}$ ph cm$^{-2}$ s$^{-1}$, and $(4.2 pm 1.0) times 10^{-5}$ ph cm$^{-2}$ s$^{-1}$ for the $^{44}$Ti and $^{44}$Sc decay, respectively. We obtain higher fluxes for $^{44}$Ti with our analysis of Cas A than were obtained in previous analyses. We discuss potential differences. Conclusions: We obtain a high $^{44}$Ti ejecta mass for Cas A that is in disagreement with ejecta yields from symmetric 2D models. Upper limits for the other core-collapse supernovae are in agreement with model predictions and previous studies. The upper limits we find for the three thermonuclear supernovae consistently exclude the double detonation and pure helium deflagration models as progenitors.
$^{56}$Ni is an important indicator of the supernova explosions, which characterizes light curves. Nevertheless, rather than $^{56}$Ni, the explosion energy has often been paid attention from the explosion mechanism community, since it is easier to estimate from numerical data than the amount of $^{56}$Ni. The final explosion energy, however, is difficult to estimate by detailed numerical simulations because current simulations cannot reach typical timescale of saturation of explosion energy. Instead, the amount of $^{56}$Ni converges within a short timescale so that it would be a better probe of the explosion mechanism. We investigated the amount of $^{56}$Ni synthesized by explosive nucleosynthesis in supernova ejecta by means of numerical simulations and an analytic model. For numerical simulations, we employ Lagrangian hydrodynamics code in which neutrino heating and cooling terms are taken into account by light-bulb approximation. Initial conditions are taken from Woosley & Hegel (2007), which have 12, 15, 20, and 25 $M_odot$ in zero age main sequence. We additionally develop an analytic model, which gives a reasonable estimate of the amount of $^{56}$Ni. We found that, in order to produce enough amount of $^{56}$Ni, $mathcal{O}(1)$ Bethe s$^{-1}$ of growth rate of the explosion energy is needed, which is much larger than that found in recent exploding simulations, typically $mathcal{O}(0.1)$ Bethe s$^{-1}$.
We present the discovery and the photometric and spectroscopic study of H-rich Type II supernova (SN) KSP-SN-2016kf (SN2017it) observed in the KMTNet Supernova Program in the outskirts of a small irregular galaxy at $zsimeq0.043$ within a day from the explosion. Our high-cadence, multi-color ($BVI$) light curves of the SN show that it has a very long rise time ($t_text{rise}simeq 20$ days in $V$ band), a moderately luminous peak ($M_Vsimeq -$17.6 mag), a notably luminous and flat plateau ($M_Vsimeq -$17.4 mag and decay slope $ssimeq0.53$ mag per 100 days), and an exceptionally bright radioactive tail. Using the color-dependent bolometric correction to the light curves, we estimate the $^{56}$Ni mass powering the observed radioactive tail to be $0.10pm0.01$ M$_odot$, making it a H-rich Type II SN with one of the largest $^{56}$Ni masses observed to date. The results of our hydrodynamic simulations of the light curves constrain the mass and radius of the progenitor at the explosion to be $sim$15 M$_odot$ (evolved from a star with an initial mass of $sim$ 18.8 M$_odot$) and $sim1040$ R$_odot$, respectively, with the SN explosion energy of $sim 1.3times 10^{51}$ erg s$^{-1}$. The above-average mass of the KSP-SN-2016kf progenitor, together with its low metallicity $ Z/Z_odot simeq0.1-0.4$ obtained from spectroscopic analysis, is indicative of a link between the explosion of high-mass red supergiants and their low-metallicity environment. The early part of the observed light curves shows the presence of excess emission above what is predicted in model calculations, suggesting there is interaction between the ejecta and circumstellar material. We further discuss the implications of the high progenitor initial mass and low-metallicity environment of KSP-SN-2016kf on our understanding of the origin of Type II SNe.
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