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Register a new userType I Superluminous Supernovae as Explosions inside Non-Hydrogen Circumstellar Envelopes
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A number of Type I (hydrogenless) superluminous supernova (SLSN) events have been discovered recently. However, their nature remains debatable. One of the most promising ideas is the shock-interaction mechanism, but only simplified semi-analytical models have been applied so far. We simulate light curves for several Type I SLSN (SLSN-I) models enshrouded by dense, non-hydrogen circumstellar envelopes, using a multi-group radiation hydrodynamics code that predicts not only bolometric, but also multicolor light curves. We demonstrate that the bulk of SLSNe-I including those with relatively narrow light curves like SN 2010gx or broad ones like PTF09cnd can be explained by the interaction of the SN ejecta with he CS envelope, though the range of parameters for these models is rather wide. Moderate explosion energy ($sim (2 - 4)cdot 10^{51}$ ergs) is sufficient to explain both narrow and broad SLSN-I light curves, but ejected mass and envelope mass differ for those two cases. Only 5 to 10 $M_odot$ of non-hydrogen material is needed to reproduce the light curve of SN 2010gx, while the best model for PTF09cnd is very massive: it contains almost $ 50 M_odot $ in the CS envelope and only $ 5 M_odot $ in the ejecta. The CS envelope for each case extends from 10 $R_odot$ to $sim 10^5R_odot$ ($7cdot 10^{15} $ cm), which is about an order of magnitude larger than typical photospheric radii of standard SNe near the maximum light. We briefly discuss possible ways to form such unusual envelopes.
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Previous studies have shown that the radiation emitted by a rapidly rotating magnetar embedded in a young supernova can greatly amplify its luminosity. These one-dimensional studies have also revealed the existence of an instability arising from the piling up of radiatively accelerated matter in a thin dense shell deep inside the supernova. Here we examine the problem in two dimensions and find that, while instabilities cause mixing and fracture this shell into filamentary structures that reduce the density contrast, the concentration of matter in a hollow shell persists. The extent of the mixing depends upon the relative energy input by the magnetar and the kinetic energy of the inner ejecta. The light curve and spectrum of the resulting supernova will be appreciably altered, as will the appearance of the supernova remnant, which will be shellular and filamentary. A similar pile up and mixing might characterize other events where energy is input over an extended period by a centrally concentrated source, e.g. a pulsar, radioactive decay, a neutrino-powered wind, or colliding shells. The relevance of our models to the recent luminous transient ASASSN-15lh is briefly discussed.
The near-maximum spectra of most superluminous supernovae that are not dominated by interaction with a H-rich CSM (SLSN-I) are characterised by a blue spectral peak and a series of absorption lines which have been identified as OII. SN2011kl, associated with the ultra-long gamma-ray burst GRB111209A, also had a blue peak but a featureless optical/UV spectrum. Radiation transport methods are used to show that the spectra (not including SN2007bi, which has a redder spectrum at peak, like ordinary SNe Ic) can be explained by a rather steep density distribution of the ejecta, whose composition appears to be typical of carbon-oxygen cores of massive stars which can have low metal content. If the photospheric velocity is ~10000-15000 km/s, several lines form in the UV. OII lines, however, arise from very highly excited lower levels, which require significant departures from Local Thermodynamic Equilibrium to be populated. These SLSNe are not thought to be powered primarily by 56Ni decay. An appealing scenario is that they are energised by X-rays from the shock driven by a magnetar wind into the SN ejecta. The apparent lack of evolution of line velocity with time that characterises SLSNe up to about maximum is another argument in favour of the magnetar scenario. The smooth UV continuum of SN2011kl requires higher ejecta velocities (~20000 km/s): line blanketing leads to an almost featureless spectrum. Helium is observed in some SLSNe after maximum. The high ionization near maximum implies that both He and H may be present but not observed at early times. The spectroscopic classification of SLSNe should probably reflect that of SNe Ib/c. Extensive time coverage is required for an accurate classification.
Supernovae (SNe) are stellar explosions driven by gravitational or thermonuclear energy, observed as electromagnetic radiation emitted over weeks or more. In all known SNe, this radiation comes from internal energy deposited in the outflowing ejecta by either radioactive decay of freshly-synthesized elements (typically 56Ni), stored heat deposited by the explosion shock in the envelope of a supergiant star, or interaction between the SN debris and slowly-moving, hydrogen-rich circumstellar material. Here we report on a new class of luminous SNe whose observed properties cannot be explained by any of these known processes. These include four new SNe we have discovered, and two previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as members. These SNe are all ~10 times brighter than SNe Ia, do not show any trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods of time, and have late-time decay rates which are inconsistent with radioactivity. Our data require that the observed radiation is emitted by hydrogen-free material distributed over a large radius (~10^15 cm) and expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous events can be observed out to redshifts z>4 and offer an excellent opportunity to study star formation in, and the interstellar medium of, primitive distant galaxies.
In certain mass ranges, massive stars can undergo a violent pulsation triggered by the electron/positron pair instability that ejects matter, but does not totally disrupt the star. After one or more of these pulsations, such stars are expected to undergo core-collapse to trigger a supernova explosion. The mass range susceptible to this pulsational phenomena may be as low as 50-70 Msun if the progenitor is of very low metallicity and rotating sufficiently rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and composition of the matter ejected in the pulsation are important aspects to determine the subsequent observational characteristics of the explosion. We examine the dynamics of a sample of stellar models and rotation rates and discuss the implications for the first stars, for LBV-like phenomena, and for superluminous supernovae. We find that the shells ejected by pulsational pair-instability events with rapidly rotating progenitors (>30% the critical value) are hydrogen-poor and helium and oxygen-rich.
We study explosion characteristics of ultra-stripped supernovae (SNe), which are candidates of SNe generating binary neutron stars (NSs). As a first step, we perform stellar evolutionary simulations of bare carbon-oxygen cores of mass from 1.45 to 2.0 $M_odot$ until the iron cores become unstable and start collapsing. We then perform axisymmetric hydrodynamics simulations with spectral neutrino transport using these stellar evolution outcomes as initial conditions. All models exhibit successful explosions driven by neutrino heating. The diagnostic explosion energy, ejecta mass, Ni mass, and NS mass are typically $sim 10^{50}$ erg, $sim 0.1 M_odot$, $sim 0.01M_odot$, and $approx 1.3 M_odot$, which are compatible with observations of rapidly-evolving and luminous transient such as SN 2005ek. We also find that the ultra-stripped SN is a candidate for producing the secondary low-mass NS in the observed compact binary NSs like PSR J0737-3039.
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