Carbon-oxygen Core Research Articles

Synthetic Spectra of Pair-instability Supernovae in 3D

Pair-Instability Supernovae (PISNe) may signal the deaths of extremely massive stars in the local Universe or massive primordial stars after the end of the Cosmic Dark Ages. Hydrodynamic simulations of these explosions, performed in 1D, 2D, and 3D geometry, have revealed the strong dependence of mixing in the PISN ejecta on dimensionality. This chemical rearrangement is mainly driven by Rayleigh-Taylor instabilities that start to grow shortly after the collapse of the carbon-oxygen core. We investigate the effects of such mixing on the spectroscopic evolution of PISNe by post-processing explosion profiles with the radiation diffusion-equilibrium code SNEC and the implicit Monte Carlo-discrete diffusion Monte Carlo (IMC-DDMC) radiation transport code SuperNu. The first 3D radiation transport calculation of a PISN explosion is presented yielding viewing angle-dependent synthetic spectra and lightcurves. We find that while 2D and 3D mixing does not significantly affect the lightcurves of PISNe, their spectroscopic and color evolution is impacted. Strong features of intermediate mass elements dominated by silicon, magnesium and oxygen appear at different phases and reach different intensities depending on the extent of mixing in the silicon/oxygen interface of the PISN ejecta. On the other hand, we do not find a significant dependence of PISN lightcurves and spectra on viewing angle. Our results showcase the capabilities of SuperNu to handle 3D radiation transport and highlight the importance of modeling time-series of spectra in identifying PISNe with future missions.

Two Predictions of Supernova: GRB 130427A/SN 2013cq and GRB 180728A/SN 2018fip

Abstract On 2018 July 28, GRB 180728A triggered Swift satellites and, soon after the determination of the redshift, we identified this source as a type II binary-driven hypernova (BdHN II) in our model. Consequently, we predicted the appearance time of its associated supernova (SN), which was later confirmed as SN 2018fip. A BdHN II originates in a binary composed of a carbon–oxygen core (COcore) undergoing SN, and the SN ejecta hypercritically accrete onto a companion neutron star (NS). From the time of the SN shock breakout to the time when the hypercritical accretion starts, we infer the binary separation ≃3 × 1010 cm. The accretion explains the prompt emission of isotropic energy ≃3 × 1051 erg, lasting ∼10 s, and the accompanying observed blackbody emission from a thermal convective instability bubble. The new neutron star (νNS) originating from the SN powers the late afterglow from which a νNS initial spin of 2.5 ms is inferred. We compare GRB 180728A with GRB 130427A, a type I binary-driven hypernova (BdHN I) with isotropic energy >1054 erg. For GRB 130427A we have inferred an initially closer binary separation of ≃1010 cm, implying a higher accretion rate leading to the collapse of the NS companion with consequent black hole formation, and a faster, 1 ms spinning νNS. In both cases, the optical spectra of the SNe are similar, and not correlated to the energy of the gamma-ray burst. We present three-dimensional smoothed-particle-hydrodynamic simulations and visualizations of the BdHNe I and II.

Type Ib/Ic Supernovae: Effect of Nickel Mixing on the Early-time Color Evolution and Implications for the Progenitors

Abstract We investigate the effect of mixing of radioactive nickel (56Ni) on the early-time color evolution of Type Ib and Ic supernovae (SNe Ib/Ic) using multigroup radiation hydrodynamics simulations. We consider both helium-rich and helium-poor progenitors. Mixing of 56Ni is parameterized using a Gaussian distribution function. We find that the early-time color evolution with a weak 56Ni mixing is characterized by three different phases: initial rapid reddening, blueward evolution due to the delayed effect of 56Ni heating, and redward evolution thereafter until the transition to the nebular phase. With a strong 56Ni mixing, the color evolution is characterized by a monotonic reddening. We compare our models with the early-time color evolution of several SNe Ib/Ic (SN 1999ex, SN 2008D, SN 2009jf, iPTF13bvn, SN 1994I, SN 2007gr, SN 2013ge, and 2017ein) and find signatures of relatively weak and strong 56Ni mixing for SNe Ib and SNe Ic, respectively. This suggests that SNe Ib progenitors are distinct from SN Ic progenitors in terms of helium content and that 56Ni mixing is generally stronger in the carbon–oxygen core and weaker in the helium-rich envelope. We conclude that the early-time color evolution is a powerful probe of 56Ni mixing in SNe Ib/Ic.

Gravitational waves from very massive stars collapsing to a black hole

We compute gravitational waves emitted by the collapse of a rotating very massive star (VMS) core leading directly to a black hole in axisymmetric numerical-relativity simulations. The evolved rotating VMS is derived by a stellar evolution calculation and its initial mass and the final carbon-oxygen core mass are $320M_\odot$ and $\approx 150M_\odot$, respectively. We find that for the moderately rapidly rotating cases, the peak strain amplitude and the corresponding frequency of gravitational waves are $\sim 10^{-22}$ and $f \approx 300$--600\,Hz for an event at the distance of $D=50$~Mpc. Such gravitational waves will be detectable only for $D \lesssim 10$~Mpc by second generation detectors, advanced LIGO, advanced VIRGO, and KAGRA, even if the designed sensitivity for these detectors is achieved. However, third-generation detectors will be able to detect such gravitational waves for an event up to $D \sim 100$~Mpc. The detection of the gravitational-wave signal will provide a potential opportunity for verifying the presence of VMSs with mass $\gtrsim 300M_\odot$ and their pair-unstable collapse in the universe.

SPH Simulations of the Induced Gravitational Collapse Scenario of Long Gamma-Ray Bursts Associated with Supernovae

Abstract We present the first three-dimensional smoothed particle hydrodynamics simulations of the induced gravitational collapse scenario of long-duration gamma-ray bursts (GRBs) associated with supernovae (SNe). We simulate the SN explosion of a carbon–oxygen core (COcore) forming a binary system with a neutron star (NS) companion. We follow the evolution of the SN ejecta, including their morphological structure, subject to the gravitational field of both the new NS (νNS) formed at the center of the SN and the one of the NS companion. We compute the accretion rate of the SN ejecta onto the NS companion, as well as onto the νNS from SN matter fallback. We determine the fate of the binary system for a wide parameter space including different COcore and NS companion masses, orbital periods, and SN explosion geometry and energies. We identify, for selected NS nuclear equations of state, the binary parameters leading the NS companion, by hypercritical accretion, either to the mass-shedding limit or to the secular axisymmetric instability for gravitational collapse to a black hole (BH), or to a more massive, fast-rotating, stable NS. We also assess whether the binary remains gravitationally bound after the SN explosion, hence exploring the space of binary and SN explosion parameters leading to νNS–NS and νNS–BH binaries. The consequences of our results for the modeling of long GRBs, i.e., X-ray flashes and binary-driven hypernovae, are discussed.

The white dwarf mass–radius relation and its dependence on the hydrogen envelope.

We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbon-oxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05 Msun. Computations of the pre-white dwarf evolution uncovers an intrinsic dependence of the maximum mass of the hydrogen envelope with stellar mass, i.e., it decreases when the total mass increases. We find that a reduction of the hydrogen envelope mass can lead to a reduction in the radius of the model of up to ~12%. This translates directly into an increase in log g for a fixed stellar mass, that can reach up to 0.11 dex, mainly overestimating the determinations of stellar mass from atmospheric parameters. Finally, we find a good agreement between the results from the theoretical mass-radius relation and observations from white dwarfs in binary systems. In particular, we find a thin hydrogen mass of MH ~ 2 10^-8 Msun, for 40 Eridani B, in agreement with previous determinations. For Sirius B, the spectroscopic mass is 4.3% lower than the dynamical mass. However, the values of mass and radius from gravitational redshift observations are compatible with the theoretical mass-radius relation for a thick hydrogen envelope of MH = 2 10^-6 Msun.

On the Ultra-relativistic Prompt Emission, the Hard and Soft X-Ray Flares, and the Extended Thermal Emission in GRB 151027A

Abstract We analyze GRB 151027A within the binary-driven hypernova approach, with a progenitor of a carbon–oxygen core on the verge of a supernova (SN) explosion and a binary companion neutron star (NS). The hypercritical accretion of the SN ejecta onto the NS leads to its gravitational collapse into a black hole (BH), to the emission of the gamma-ray burst (GRB), and to a copious e + e − plasma. The impact of this e + e − plasma on the SN ejecta explains the early soft X-ray flare observed in long GRBs. Here, we apply this approach to the ultra-relativistic prompt emission (UPE) and to the hard X-ray flares. We use GRB 151027A as a prototype. From the time-integrated and the time-resolved analysis, we identify a double component in the UPE and confirm its ultra-relativistic nature. We confirm the mildly relativistic nature of the soft X-ray flare, of the hard X-ray flare, and of the extended thermal emission (ETE). We show that the ETE identifies the transition from an SN to a hypernova (HN). We then address the theoretical justification of these observations by integrating the hydrodynamical propagation equations of the e + e − into the SN ejecta, with the latter independently obtained from 3D smoothed particle hydrodynamics simulations. We conclude that the UPE, the hard X-ray flare, and the soft X-ray flare do not form a causally connected sequence. Within our model, they are the manifestation of the same physical process of the BH formation as seen through different viewing angles, implied by the morphology and the ∼300 s rotation period of the HN ejecta.

On the Induced Gravitational Collapse: SPH Simulations

The Induced Gravitational Collapse (IGC) paradigm points to a binary origin for the longduration gamma-ray burst (GRBs) associated with supernovae (SN). In this one, a carbon-oxygen core (COcore) explodes in a Type Ib/c SN in presence of a close neutron star (NS) companion. The SN triggers a hypercritical accretion into the NS and depending on the initial binary parameters, two outcomes are possible givimg place to two family of long GRBs: binary-driven hypernova (BdHNe), where the NS reaches its critical mass, and collapses to a black hole (BH), emitting a GRB; and x-ray flashes (XRFs) where the hypercritical accretion onto the NS is not sufficient to induce its gravitational collapse. We perform 3-dimensional (3D) numerical simulations of the IGC paradigm with the smoothed particle hydrodynamics (SPH) technique. We determine whether the star gravitational collapse is possible and assess if the binary holds gravitationally bound or it becomes unbound by the SN explosion.

Three-dimensional Simulation of Double Detonations in the Double-degenerate Model for Type Ia Supernovae and Interaction of Ejecta with a Surviving White Dwarf Companion

Abstract We study the hydrodynamics and nucleosynthesis in the double-detonation model of Type Ia supernovae (SNe Ia) and the interaction between the ejecta and a surviving white dwarf (WD) companion in the double-degenerate scenario. We set up a binary star system with 1.0 and 0.6 M ⊙ carbon–oxygen (CO) WDs, where the primary WD consists of a CO core and helium (He) shell with 0.95 and 0.05 M ⊙, respectively. We follow the evolution of the binary star system from the initiation of an He detonation, ignition and propagation of a CO detonation, and the interaction of SN ejecta with the companion WD. The companion (or surviving) WD gets a flung-away velocity of ∼1700 km s−1 and captures 56Ni of ∼0.03 M ⊙ and He of . Such He can be detected on the surface of surviving WDs. The SN ejecta contains a “companion-origin stream” and unburned materials stripped from the companion WD ( ), although the stream compositions would depend on the He shell mass of the companion WD. The ejecta has also a velocity shift of ∼1000 km s−1 due to the binary motion of the exploding primary WD. These features would be prominent in nebular-phase spectra of oxygen emission lines from the unburned materials like SN 2010lp and iPTF14atg and of blue- or redshifted Fe-group emission lines from the velocity shift like a part of subluminous SNe Ia. We expect that SN Ia counterparts to the D6 model would leave these fingerprints for SN Ia observations.

Simulating the induced gravitational collapse scenario of long gamma-ray bursts

We present the state-of-the-art of the numerical simulations of the supernova (SN) explosion of a carbon-oxygen core [Formula: see text] that forms a compact binary with a neutron star (NS) companion, following the induced gravitational collapse (IGC) scenario of long gamma-ray bursts (GRBs) associated with type Ic supernovae (SNe). We focus on the consequences of the hypercritical accretion of the SN ejecta onto the NS companion which either becomes a more massive NS or gravitationally collapses forming a black hole (BH). We summarize the series of results on this topic starting from the first analytic estimates in 2012 all the way up to the most recent three-dimensional (3D) smoothed-particle-hydrodynamics (SPH) numerical simulations in 2018. We present a new SN ejecta morphology, highly asymmetric, acquired by binary interaction and leading to well-defined, observable signatures in the gamma- and X-rays emission of long GRBs.

On the minimum mass of neutron stars

We investigate remnant neutron star masses (in particular, the minimum allowed mass) by performing advanced stellar evolution calculations and neutrino-radiation hydrodynamics simulations for core-collapse supernova explosions. We find that, based on standard astrophysical scenarios, low-mass carbon-oxygen cores can have sufficiently massive iron cores that eventually collapse, explode as supernovae, and give rise to remnant neutron stars that have a minimum mass of 1.17 M⊙ – compatible with the lowest mass of the neutron star precisely measured in a binary system of PSR J0453+1559.

Possible white dwarf progenitors of Type Ia supernovae

We examine catalogs of white dwarfs (WDs) and find that there are sufficient number of massive WDs, M_WD > 1.35Mo, that might potentially explode as type Ia supernovae (SNe Ia) in the frame of the core degenerate scenario. In the core degenerate scenario a WD merges with the carbon-oxygen core of a giant star, and they form a massive WD that might explode with a time delay of years to billions of years. If the core degenerate scenario accounts for all SNe Ia, then we calculate that about 0.2 per cent of the present WDs in the Galaxy are massive. Furthermore, we find from the catalogs that the fraction of massive WDs relative to all WDs is about 1-3 per cent, with large uncertainties. Namely, five to ten times the required number. If there are many SNe Ia that result from lower mass WDs, M_WD < 1.3Mo, for which another scenario is responsible for, and the core degenerate scenario accounts only for the SNe Ia that explode as massive WDs, then the ratio of observed massive WDs to required is larger even. Our finding leaves the core degenerate scenario as a viable and promising SN Ia scenario.

Production of Silicon on Mass-increasing White Dwarfs: Possible Origin of High-velocity Features in Type Ia Supernovae

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon-oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 $M_\odot$, 1.2 $M_\odot$, and 1.35 $M_\odot$ CO WDs accreting helium-rich matter with several mass-accretion rates ranging from $1 \times 10^{-7}~M_\odot$ yr$^{-1}$ to $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$. For the $1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer developed as helium burning ash and consisted of 40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by weight. For a higher mass accretion rate of $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and 0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). No substantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si II HVFs, its characteristics are consistent with that of mass increasing WDs. We also discuss possible Ca production on very massive WDs ($ \gtrsim 1.38~M_\odot$).

WD+AGB star systems as the progenitors of type Ia supernovae

AbstractWD+AGB star systems have been suggested as an alternative way for producing type Ia supernovae (SNe Ia), known as the core-degenerate (CD) scenario. In the CD scenario, SNe Ia are produced at the final phase during the evolution of common-envelope through a merger between a carbon-oxygen (CO) WD and the CO core of an AGB secondary. However, the rates of SNe Ia from this scenario are still uncertain. In this work, I carried out a detailed investigation on the CD scenario based on a binary population synthesis approach. I found that the Galactic rates of SNe Ia from this scenario are not more than 20% of total SNe Ia due to more careful treatment of mass transfer, and that their delay times are in the range of ∼90 − 2500 Myr, mainly contributing to the observed SNe Ia with short and intermediate delay times.

Multidimensional simulations of ultrastripped supernovae to shock breakout

The recent discoveries of many double neutron star systems and their detection as LIGO-Virgo merger events call for a detailed understanding of their origin. Explosions of ultra-stripped stars in binary systems have been shown to play a key role in this context and have also generated interest as a potential explanation for rapidly evolving hydrogen-free transients. Here we present the first attempt to model such explosions based on binary evolution calculations that follow the mass transfer to the companion to obtain a consistent core-envelope structure as needed for reliable predictions of the supernova transient. We simulate the explosion in 2D and 3D, and confirm the modest explosion energies ~10^50erg and small kick velocities reported earlier in 2D models based on bare carbon-oxygen cores. The spin-up of the neutron star by asymmetric accretion is small in 3D with no indication of spin-kick alignment. Simulations up to shock breakout show the mixing of sizeable amounts of iron group material into the helium envelope. In view of recent ideas for a mixing-length treatment (MLT) of Rayleigh-Taylor instabilities in supernovae, we perform a detailed analysis of the mixing, which reveals evidence for buoyancy-drag balance, but otherwise does not support the MLT approximation. The mixing may have implications for the spectroscopic signatures of ultra-stripped supernovae that need to be investigated in the future. Our stellar evolution calculation also predicts presupernova mass loss due to an off-centre silicon deflagration flash, which suggests that supernovae from extremely stripped cores may show signs of interactions with circumstellar material.

On the Rate and on the Gravitational Wave Emission of Short and Long GRBs

Abstract On the ground of the large number of gamma-ray bursts (GRBs) detected with cosmological redshift, we classified GRBs in seven subclasses, all with binary progenitors which emit gravitational waves (GWs). Each binary is composed of combinations of carbon–oxygen cores (COcore), neutron stars (NSs), black holes (BHs), and white dwarfs (WDs). The long bursts, traditionally assumed to originate from a BH with an ultrarelativistic jetted emission, not emitting GWs, have been subclassified as (I) X-ray flashes (XRFs), (II) binary-driven hypernovae (BdHNe), and (III) BH–supernovae (BH–SNe). They are framed within the induced gravitational collapse paradigm with a progenitor COcore–NS/BH binary. The SN explosion of the COcore triggers an accretion process onto the NS/BH. If the accretion does not lead the NS to its critical mass, an XRF occurs, while when the BH is present or formed by accretion, a BdHN occurs. When the binaries are not disrupted, XRFs lead to NS–NS and BdHNe lead to NS–BH. The short bursts, originating in NS–NS, are subclassified as (IV) short gamma-ray flashes (S-GRFs) and (V) short GRBs (S-GRBs), the latter when a BH is formed. There are (VI) ultrashort GRBs (U-GRBs) and (VII) gamma-ray flashes (GRFs) formed in NS–BH and NS–WD, respectively. We use the occurrence rate and GW emission of these subclasses to assess their detectability by Advanced LIGO-Virgo, eLISA, and resonant bars. We discuss the consequences of our results in view of the announcement of the LIGO/Virgo Collaboration of the source GW 170817 as being originated by an NS–NS.

Asteroseismology of ZZ Ceti stars with full evolutionary white dwarf models

Context. The thermally pulsing phase on the asymptotic giant branch (TP-AGB) is the last nuclear burning phase experienced by most low- and intermediate-mass stars. During this phase, the outer chemical stratification above the C/O core of the emerging white dwarf (WD) is built up. The chemical structure resulting from progenitor evolution strongly impacts the whole pulsation spectrum exhibited by ZZ Ceti stars, which are pulsating C/O core white dwarfs located on a narrow instability strip at Teff ~ 12 000 K. Several physical processes occurring during progenitor evolution strongly affect the chemical structure of these stars; those found during the TP-AGB phase are the most relevant for the pulsational properties of ZZ Ceti stars. Aims. We present a study of the impact of the chemical structure built up during the TP-AGB evolution on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Methods. Our analysis is based on a set of carbon–oxygen core white dwarf models with masses from 0.534 to 0.6463 M⊙ derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We computed evolutionary sequences that experience different number of thermal pulses (TP). Results. We find that the occurrence or not of thermal pulses during AGB evolution implies an average deviation in the asteroseimological effective temperature of ZZ Ceti stars of at most 8% and on the order of ≲5% in the stellar mass. For the mass of the hydrogen envelope, however, we find deviations up to 2 orders of magnitude in the case of cool ZZ Ceti stars. Hot and intermediate temperature ZZ Ceti stars show no differences in the hydrogen envelope mass in most cases. Conclusions. Our results show that, in general, the impact of the occurrence or not of thermal pulses in the progenitor stars is not negligible and must be taken into account in asteroseismological studies of ZZ Ceti stars.

Tidal double detonation: a new mechanism for the thermonuclear explosion of a white dwarf induced by a tidal disruption event

Abstract We suggest tidal double detonation as a new mechanism for the thermonuclear explosion of a white dwarf (WD) induced by a tidal disruption event (TDE). Tidal detonation is also a WD explosion induced by a TDE. In this case, helium (He) and carbon–oxygen (CO) detonation waves incinerate He WDs and CO WDs, respectively. On the other hand, for tidal double detonation, He detonation is first excited in the He shell of a CO WD, which then drives CO detonation in the CO core. We name this mechanism after the double detonation scenario in the context of type Ia supernovae. In this paper, by performing numerical simulations for CO WDs of mass 0.60 M⊙ with and without a He shell, we show that tidal double detonation occurs in the shallower encounter of a CO WD with an intermediate-mass black hole (IMBH) compared to simple tidal detonation. We expect tidal double detonation will increase the possibility of the occurrence of WD TDEs, which can help us to understand IMBHs.

On the Induced Gravitational Collapse

The induced gravitational collapse (IGC) paradigm has been applied to explain the long gamma ray burst (GRB) associated with type Ic supernova, and recently the Xray flashes (XRFs). The progenitor is a binary systems of a carbon-oxygen core (CO) and a neutron star (NS). The CO core collapses and undergoes a supernova explosion which triggers the hypercritical accretion onto the NS companion (up to 10-2 M⊙s-1). For the binary driven hypernova (BdHNe), the binary system is enough bound, the NS reach its critical mass, and collapse to a black hole (BH) with a GRB emission characterized by an isotropic energy Eiso &gt; 1052 erg. Otherwise, for binary systems with larger binary separations, the hypercritical accretion onto the NS is not sufficient to induced its gravitational collapse, a X-ray flash is produced with Eiso &lt; 1052 erg. We’re going to focus in identify the binary parameters that limits the BdHNe systems with the XRFs systems.

Early X-Ray Flares in GRBs

Abstract We analyze the early X-ray flares in the GRB “flare–plateau–afterglow” (FPA) phase observed by Swift-XRT. The FPA occurs only in one of the seven GRB subclasses: the binary-driven hypernovae (BdHNe). This subclass consists of long GRBs with a carbon–oxygen core and a neutron star (NS) binary companion as progenitors. The hypercritical accretion of the supernova (SN) ejecta onto the NS can lead to the gravitational collapse of the NS into a black hole. Consequently, one can observe a GRB emission with isotropic energy erg, as well as the associated GeV emission and the FPA phase. Previous work had shown that gamma-ray spikes in the prompt emission occur at cm with Lorentz Gamma factors . Using a novel data analysis, we show that the time of occurrence, duration, luminosity, and total energy of the X-ray flares correlate with E iso. A crucial feature is the observation of thermal emission in the X-ray flares that we show occurs at radii ∼1012 cm with . These model-independent observations cannot be explained by the “fireball” model, which postulates synchrotron and inverse-Compton radiation from a single ultrarelativistic jetted emission extending from the prompt to the late afterglow and GeV emission phases. We show that in BdHNe a collision between the GRB and the SN ejecta occurs at ≃1010 cm, reaching transparency at ∼1012 cm with . The agreement between the thermal emission observations and these theoretically derived values validates our model and opens the possibility of testing each BdHN episode with the corresponding Lorentz Gamma factor.