arXiv subjects

Takashi J. Moriya

Publications and source records attributed to Takashi J. Moriya.

At least 145 records · Page 8Linked to original sources

arXiv2016

Properties of magnetars mimicking 56Ni-powered light curves in Type Ic superluminous supernovae

Many Type Ic superluminous supernovae have light-curve decline rates after their luminosity peak which are close to the nuclear decay rate of 56Co, consistent with the interpretation that they are powered by 56Ni and possibly pair-instability supernovae. However, their rise times are typically shorter than those expected from pair-instability supernovae, and Type Ic superluminous supernovae are often suggested to be powered by magnetar spin-down. If magnetar spin-down is actually a major mechanism to power Type Ic superluminous supernovae, it should be able to produce decline rates similar to the 56Co decay rate rather easily. In this study, we investigate the conditions for magnetars under which their spin-down energy input can behave like the 56Ni nuclear decay energy input. We find that an initial magnetic field strength within a certain range is sufficient to keep the magnetar energy deposition within a factor of a few of the 56Co decay energy for several hundreds of days. Magnetar spin-down needs to be by almost pure dipole radiation with the braking index close to 3 to mimic 56Ni in a wide parameter range. Not only late-phase 56Co-decay-like light curves, but also rise time and peak luminosity of most 56Ni-powered light curves can be reproduced by magnetars. Bolometric light curves for more than 700 days are required to distinguish the two energy sources solely by them. We expect that more slowly-declining superluminous supernovae with short rise times should be found if they are mainly powered by magnetar spin-down.

astro-ph.HE↗

arXiv2016

Light-curve and spectral properties of ultra-stripped core-collapse supernovae leading to binary neutron stars

We investigate light-curve and spectral properties of ultra-stripped core-collapse supernovae. Ultra-stripped supernovae are the explosions of heavily stripped massive stars which lost their envelopes via binary interactions with a compact companion star. They eject only ~ 0.1 Msun and may be the main way to form double neutron-star systems which eventually merge emitting strong gravitational waves. We follow the evolution of an ultra-stripped supernova progenitor until iron core collapse and perform explosive nucleosynthesis calculations. We then synthesize light curves and spectra of ultra-stripped supernovae using the nucleosynthesis results and present their expected properties. Ultra-stripped supernovae synthesize ~ 0.01 Msun of radioactive 56Ni, and their typical peak luminosity is around 1e42 erg/s or -16 mag. Their typical rise time is 5 - 10 days. Comparing synthesized and observed spectra, we find that SN 2005ek, some of the so-called calcium-rich gap transients, and SN 2010X may be related to ultra-stripped supernovae. If these supernovae are actually ultra-stripped supernovae, their event rate is expected to be about 1 per cent of core-collapse supernovae. Comparing the double neutron-star merger rate obtained by future gravitational-wave observations and the ultra-stripped supernova rate obtained by optical transient surveys identified with our synthesized light-curve and spectral models, we will be able to judge whether ultra-stripped supernovae are actually a major contributor to the binary neutron star population and provide constraints on binary stellar evolution.

astro-ph.HE↗

arXiv2016

Radio transients from accretion-induced collapse of white dwarfs

We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs in radio frequencies. If AIC is triggered by accretion from a companion star, a dense circumstellar medium can be formed around the progenitor system. Then, the ejecta from AIC collide to the dense circumstellar medium, making a strong shock. The strong shock can produce synchrotron emission which can be observed in radio frequencies. Even if AIC occurs as a result of white dwarf mergers, we argue that AIC may cause fast radio bursts if a certain condition is satisfied. If AIC forms neutron stars which are so massive that rotation is required to support themselves (i.e., supramassive neutron stars), the supramassive neutron stars may immediately lose their rotational energy by the r-mode instability and collapse to black holes. If the collapsing supramassive neutron stars are strongly magnetized, they may emit fast radio bursts as previously suggested. The AIC radio transients from the single-degenerate systems may be detected in the future radio transient surveys like Very Large Array Sky Survey or the Square Kilometer Array transient survey. Because AIC is suggested to be a gravitational wave source, gravitational waves from AIC may be accompanied by radio-bright transients which can be used to confirm the AIC origin of observed gravitational waves.

astro-ph.HE↗

arXiv2016

Supernovae powered by magnetars that transform into black holes

Rapidly rotating, strongly magnetized neutron stars (magnetars) can release their enormous rotational energy via magnetic spin-down, providing a power source for bright transients such as superluminous supernovae. On the other hand, particularly massive (so-called supramassive) neutron stars require a minimum rotation rate to support their mass against gravitational collapse, below which the neutron star collapses to a black hole. We model the light curves of supernovae powered by magnetars which transform into black holes. Although the peak luminosities can reach high values in the range of superluminous supernovae, their post maximum light curves can decline very rapidly because of the sudden loss of the central energy input. Early black hole transformation also enhances the shock breakout signal from the magnetar-driven bubble relative to the main supernova peak. Our synthetic light curves of supernovae powered by magnetars transforming to black holes are consistent with those of some rapidly evolving bright transients recently reported by Arcavi et al. (2016).

astro-ph.HE↗

arXiv2016

Rapidly evolving faint transients from stripped-envelope electron-capture supernovae

We investigate the expected rates and bolometric light-curve properties of stripped-envelope electron-capture supernovae (ECSNe) using stellar models from the Binary Population and Spectral Synthesis (BPASS) code. We find that 0.8 per cent (Z = 0.020) and 1.2 per cent (Z = 0.004) of core-collapse supernovae are stripped-envelope ECSNe. Their typical ejecta masses are estimated to be about 0.3 Msun (Z = 0.020) and 0.6 Msun (Z = 0.004). Assuming ECSN explosion properties from numerical explosion simulations, an explosion energy of 1.5e50 erg and a 56Ni mass of 2.5e-3 Msun, we find that stripped-envelope ECSNe have a typical rise time of around 7 days (Z = 0.020) or 13 days (Z = 0.004) and peak luminosity of around 1e41 erg/s (-13.8 mag, Z = 0.020) or 7e40 erg/s (-13.4 mag, Z = 0.004). Their typical ejecta velocities are around 7000 km/s (Z = 0.020) or 5000 km/s (Z = 0.004). Thus, stripped-envelope ECSNe are observed as rapidly-evolving faint transients with relatively small velocities. SN 2008ha-like supernovae, which are the faintest kind of SN 2002cx-like (a.k.a. Type Iax) supernovae, may be related to stripped-envelope ECSNe.

astro-ph.HE↗

arXiv2016

Constraining the ellipticity of strongly magnetized neutron stars powering superluminous supernovae

Superluminous supernovae (SLSNe) have been suggested to be powered by strongly magnetized, rapidly rotating neutron stars which are often called magnetars. In this process, rotational energy of the magnetar is radiated via magnetic dipole radiation and heats the supernova ejecta. However, if magnetars are highly distorted in their geometric shape, rotational energy is mainly lost as gravitational wave radiation and thus such magnetars cannot power SLSNe. By simply comparing electromagnetic and gravitational wave emission timescales, we constrain upper limits to the ellipticity of magnetars by assuming that they power the observed SLSNe. We find that their ellipticity typically needs to be less than about a few 1e-3. This indicates that the toroidal magnetic field strengths in these magnetars are typically less than a few 1e16 G so that their distortions remain small. Because light-curve modelling of SLSNe shows that their dipole magnetic field strengths are of the order of 1e14 G, the ratio of poloidal to toroidal magnetic field strengths is found to be larger than ~ 0.01 in magnetars powering SLSNe.

astro-ph.HE↗

arXiv2016

Circumstellar and explosion properties of Type Ibn supernovae

We investigate circumstellar and explosion properties of Type Ibn supernovae (SNe) by analyzing their bolometric light curves. Bolometric light curves of Type Ibn SNe generally have a large contrast between peak luminosity and late-phase luminosity, which is much larger than those of 56Ni-powered SNe. Thus, most of them are likely powered by the interaction between SN ejecta and dense circumstellar media. In addition, Type Ibn SNe decline much faster than Type IIn SNe, and this indicates that the interaction in Type Ibn SNe ceases earlier than in Type IIn SNe. Thus, we argue that Type Ibn SN progenitors experience high mass-loss rates in a short period just before explosion, while Type IIn SN progenitors have high mass-loss rates sustained for a long time. Furthermore, we show that rise time and peak luminosity of Type Ibn and Type IIn SNe are similar and thus, they have similar explosion properties and circumstellar density. The similar circumstellar density in the two kinds of SNe may indicate that mass-loss rates of Type Ibn SN progenitors are generally higher than those of Type IIn as the wind velocities inferred from narrow spectral components are generally higher in Type Ibn SNe. We also show that 56Ni mass and explosion energy of Type Ibn SNe may be smaller than those of other stripped-envelope SNe, probably because they tend to suffer large fallback or some of them may not even be terminal stellar explosions.

astro-ph.HE↗

arXiv2016

Rapidly Rising Transients from Subaru Hyper Suprime-Cam Transient Survey

We present rapidly rising transients discovered by a high-cadence transient survey with Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z=0.384-0.821 showing the rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly rising transients is ~9 % of core-collapse supernova rates, assuming a duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from dense wind with the mass loss rate of ~10^{-3} Msun yr^{-1}, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is >~1 % of core-collapse supernova rate, and thus, our study implies that more than ~1 % of massive stars can experience an intensive mass loss at a few years before the explosion.

astro-ph.HE↗

arXiv2015

Revealing the binary origin of Type Ic superluminous supernovae through nebular hydrogen emission

We propose that nebular H-alpha emission as detected in the Type Ic superluminous supernova iPTF13ehe stems from matter which is stripped from a companion star when the supernova ejecta collide with it. The temporal evolution, the line broadening, and the overall blueshift of the emission are consistent with this interpretation. We scale the nebular H-alpha luminosity predicted for Type Ia supernovae in single-degenerate systems to derive the stripped mass required to explain the H-alpha luminosity of iPTF13ehe. We find a stripped mass of 0.1 - 0.9 solar masses, assuming that the supernova luminosity is powered by radioactivity or magnetar spin down. Because a central heating source is required to excite the H-alpha emission, an interaction-powered model is not favored for iPTF13ehe if the H-alpha emission is from stripped matter. We derive a companion mass of more than 20 solar masses and a binary separation of less than about 20 companion radii based on the stripping efficiency during the collision, indicating that the supernova progenitor and the companion formed a massive close binary system. If Type Ic superluminous supernovae generally occur in massive close binary systems, the early brightening observed previously in several Type Ic superluminous supernovae may also be due to the collision with a close companion. Observations of nebular hydrogen emission in future Type Ic superluminous supernovae will enable us to test this interpretation.

astro-ph.HE↗

arXiv2015

On the nature of rapidly fading Type II supernovae

It has been suggested that Type II supernovae with rapidly fading light curves (a.k.a. Type IIL supernovae) are explosions of progenitors with low-mass hydrogen-rich envelopes which are of the order of 1 Msun. We investigate light-curve properties of supernovae from such progenitors. We confirm that such progenitors lead to rapidly fading Type II supernovae. We find that the luminosity of supernovae from such progenitors with the canonical explosion energy of 1e51 erg and 56Ni mass of 0.05 Msun can increase temporarily shortly before all the hydrogen in the envelope recombines. As a result, a bump appears in their light curves. The bump appears because the heating from the nuclear decay of 56Ni can keep the bottom of hydrogen-rich layers in the ejecta ionized, and thus the photosphere can stay there for a while. We find that the light-curve bump becomes less significant when we make explosion energy larger (>~ 2e51 erg), 56Ni mass smaller (<~ 0.01 Msun), 56Ni mixed in the ejecta, or the progenitor radius larger. Helium mixing in hydrogen-rich layers makes the light-curve decline rates large but does not help reducing the light-curve bump. Because the light-curve bump we found in our light-curve models has not been observed in rapidly fading Type II supernovae, they may be characterized by not only low-mass hydrogen-rich envelopes but also higher explosion energy, larger degrees of 56Ni mixing, and/or larger progenitor radii than slowly fading Type II supernovae, so that the light-curve bump does not become significant.

astro-ph.HE↗

arXiv2015

The interaction of core-collapse supernova ejecta with a companion star

The progenitors of many CCSNe are expected to be in binary systems. After the SN explosion, the companion may suffer from mass stripping and be shock heated as a result of the impact of the SN ejecta. If the binary system is disrupted, the companion is ejected as a runaway and hypervelocity star. By performing a series of 3D hydrodynamical simulations of the collision of SN ejecta with the companion star, we investigate how CCSN explosions affect their companions. We use the BEC code to construct the detailed companion structure at the time of SN explosion. The impact of the SN blast wave on the companion is followed by means of 3D SPH simulations using the Stellar GADGET code. For main-sequence (MS) companions, we find that the amount of removed mass, impact velocity, and chemical contamination of the companion that results from the impact of the SN ejecta, strongly increases with decreasing binary separation and increasing explosion energy. Their relationship can be approximately fitted by power laws, which is consistent with the results obtained from impact simulations of SNe~Ia. However, we find that the impact velocity is sensitive to the momentum profile of the outer SN ejecta and, in fact, may decrease with increasing ejecta mass, depending on the modeling of the ejecta. Because most companions to Ib/c CCSNe are in their MS phase at the moment of the explosion, combined with the strongly decaying impact effects with increasing binary separation, we argue that the majority of these SNe lead to inefficient mass stripping and shock heating of the companion star following the impact of the ejecta. Our simulations show that the impact effects of Ib/c SN ejecta on the structure of MS companions, and thus their long-term post-explosion evolution, is in general not dramatic. We find that at most 10% of their mass is lost, and their resulting impact velocities are less than 100 km/s.

astro-ph.SR↗

arXiv2015

Early ultraviolet signatures from the interaction of Type Ia supernova ejecta with a stellar companion

The progenitors of SNe Ia are not yet fully understood. The two leading progenitor scenarios are the single-degenerate (SD) scenario and the double-degenerate scenario. In the SD scenario, the collision of the SN Ia ejecta with its companion star is expected to produce detectable ultraviolet (UV) emission in the first few days after the SN explosion within certain viewing angles. A strong UV flash has recently been detected in an SN 2002es-like peculiar SN Ia iPTF14atg by Cao et al., which is interpreted as evidence of an early-time UV signature due to SN ejecta interacting with its companion star, supporting the SD scenario. In this paper, we present the expected luminosity distributions of early-time UV emission arising from SN Ia ejecta-companion interaction by performing binary population synthesis calculations for different progenitor systems in the SD scenario. Our theoretical predictions will be helpful for future early-time observations of SNe Ia to constrain their possible progenitors. Assuming the observed strong UV pulse of iPTF14atg was indeed produced by the SN ejecta-companion interaction, our population synthesis model suggests that the progenitor system of iPTF14atg is most likely a red-giant donor binary system, and it is unlikely to have been a main-sequence or helium-star donor system.

astro-ph.SR↗

arXiv2015

Envelope inflation in Wolf-Rayet stars and extended supernova shock breakout signals

Massive, luminous stars reaching the Eddington limit in their interiors develop very dilute, extended envelopes. This effect is called envelope inflation. If the progenitors of Type Ib/c supernovae, which are believed to be Wolf-Rayet (WR) stars, have inflated envelopes then the shock breakout signals diffuse in them and can extend their rise times significantly. We show that our inflated, hydrogen-free, WR stellar models with a radius of ~Rsun can have shock breakout signals longer than ~60 s. The puzzlingly long shock breakout signal observed in the Type Ib SN 2008D can be explained by an inflated progenitor envelope, and more such events might argue in favour of existence of inflated envelopes in general.

astro-ph.SR↗

arXiv2015

Observable fractions of core-collapse supernova light curves brightened by binary companions

Many core-collapse supernova progenitors are presumed to be in binary systems. If a star explodes in a binary system, the early supernova light curve can be brightened by the collision of the supernova ejecta with the companion star. The early brightening can be observed when the observer is in the direction of the hole created by the collision. Based on a population synthesis model, we estimate the fractions of core-collapse supernovae in which the light-curve brightening by the collision can be observed. We find that 0.19% of core-collapse supernova light curves can be observed with the collisional brightening. Type Ibc supernova light curves are more likely to be brightened by the collision (0.53%) because of the high fraction of the progenitors being in binary systems and their proximity to the companion stars. Type II and IIb supernova light curves are less affected (~1e-3% and ~1e-2%, respectively). Although the early, slow light-curve declines of some Type IIb and Ibc supernovae are argued to be caused by the collision with the companion star (e.g. SN 2008D), the small expected fraction, as well as the unrealistically small separation required, disfavour the argument. The future transient survey by the Large Synoptic Survey Telescope is expected to detect ~10 Type Ibc supernovae with the early collisional brightening per year, and they will be able to provide information on supernova progenitors in binary systems.

astro-ph.HE↗

arXiv2015

SN 2009ip: Constraining the latest explosion properties by its late-phase light curve

We constrain the explosion and circumstellar properties at the 2012b event of SN 2009ip based on its late-phase bolometric light curve recently reported. The explosion energy and ejected mass at the 2012b event are estimated as 0.01 Msun and 2e49 erg, respectively. The circumstellar medium is assumed to have two components: an inner shell and an outer wind. The inner shell which is likely created at the 2012a event has 0.2 Msun. The outer wind is created by the wind mass loss before the 2012a mass ejection, and the progenitor is estimated to have had the mass-loss rate about 0.1 Msun/yr with the wind velocity 550 km/s before the 2012a event. The estimated explosion energy and ejected mass indicate that the 2012b event is not caused by a regular supernova.

astro-ph.HE↗

arXiv2015

Extended supernova shock breakout signals from inflated stellar envelopes

Stars close to the Eddington luminosity can have large low-density inflated envelopes. We show that the rise times of shock breakout signals from supernovae can be extended significantly if supernova progenitors have an inflated stellar envelope. If the shock breakout occurs in such inflated envelopes, the shock breakout signals diffuse in them, and their rise time can be significantly extended. Then, the rise times of the shock breakout signals are dominated by the diffusion time in the inflated envelope rather than the light-crossing time of the progenitors. We show that our inflated Wolf-Rayet star models whose radii are of the order of the solar radius can have shock breakout signals which are longer than ~100 sec. The existence of inflated envelopes in Wolf-Rayet supernova progenitors may be related to the mysterious long shock breakout signal observed in Type Ib SN 2008D. Extended shock breakout signals may provide evidence for the existence of inflated stellar envelopes and can be used to constrain the physical properties of these enigmatic structures.

astro-ph.HE↗

arXiv2014

Constraints on single-degenerate Chandrasekhar mass progenitors of Type Iax supernovae

SNe Iax are proposed as one new sub-class of SNe Ia. SNe Iax have been estimated to account for ~5-30% of the total SN Ia rate, and most SNe Iax have been discovered in late-type galaxies. In addition, observations constrain the progenitor systems of some SN Iax progenitors have ages of <80Myr. Recently, the weak deflagration explosions of Ch-mass CO WDs seem to provide a viable physical scenario for SNe Iax. Here, comparing theoretical predictions from BPS calculations with SN Iax observations, we put constraints on the SD Ch-mass model as a possible Iax progenitor. A long delay times of >3Gyr and low SN rates of ~3e-5/yr are found in our RG donor channel, indicating that this channel is unlikely to produce SNe Iax. We predict that the Galactic rate from the MS (He) donor channel is ~1.5e-3/yr (3e-4/yr), which is consistent with the observed Iax rate. The short delay times in the He channel support the young host environments of SNe Iax. However, the relative long delay times in the MS donor channel are less favourable for the observational constraints on the Iax progenitor ages. Finally, we set an upper limit on the pre-SN mass-loss rate at < 10e-4 M_sun/yr (for V_wind=100 km/s). The delay times in the SD Ch-mass model do not account for that most SNe Iax are located in late-type galaxies. However, at least one Iax event (SN 2008ge) is hosted by a S0 galaxy with no signs of star formation. Moreover, current X-ray observations cannot rule out the SD Ch-mass model. Taking all these into account and considering the uncertainty of the observed rate for SNe Iax, we suggest that some SNe Iax may be produced from weak deflagrations of Ch-mass CO WDs in SD progenitors, especially in the He-star channel. This is consistent with recent analysis of HST observations for the SN Iax SN2012Z. However, this SD deflagration model is still unlikely to be the most common progenitor scenario for SNe Iax.

astro-ph.SR↗

arXiv2014

Pulsations of red supergiant pair-instability supernova progenitors leading to extreme mass loss

Recent stellar evolution models show consistently that very massive metal-free stars evolve into red supergiants shortly before they explode. We argue that the envelopes of these stars, which will form pair-instability supernovae, become pulsationally unstable and that this will lead to extreme mass-loss rates despite the tiny metal content of the envelopes. We investigate the pulsational properties of such models and derive pulsationally induced mass-loss rates, which take the damping effects of the mass loss on the pulsations selfconsistently into account. We find that the pulsations may induce mass-loss rates of ~ 1e-4 - 1e-2 Msun/yr shortly before the explosions, which may create a dense circumstellar medium. Our results show that very massive stars with dense circumstellar media may stem from a wider initial mass range than pulsational-pair instability supernovae. The extreme mass loss will cease when so much of the hydrogen-rich envelope is lost that the star becomes more compact and stops pulsating. The helium core of these stars therefore remains unaffected, and their fate as pair-instability supernovae remains unaltered. The existence of dense circumstellar media around metal-free pair-instability supernovae can make them brighter and bluer, and they may be easier to detect at high redshifts than previously expected. We argue that the mass-loss enhancement in pair-instability supernova progenitors can naturally explain some observational properties of superluminous supernovae: the energetic explosions of stars within hydrogen-rich dense circumstellar media with little 56Ni production and the lack of a hydrogen-rich envelope in pair-instability supernova candidates with large 56Ni production.

astro-ph.SR↗