Showing papers by "Takashi J. Moriya published in 2014"

Mass-loss histories of Type IIn supernova progenitors within decades before their explosion

Abstract: We present results of a systematic study of the mass-loss properties of Type IIn supernova progenitors within decades before their explosion. We apply an analytic light-curve model to 11 Type IIn supernova bolometric light curves to derive the circumstellar medium properties. We reconstruct the mass-loss histories based on the estimated circumstellar medium properties. The estimated mass-loss rates are mostly higher than 10(-3) M-circle dot yr(-1) and they are consistent with those obtained by other methods. The mass-loss rates are often found to be constantly high within decades before their explosion. This indicates that there exists some mechanism to sustain the high mass-loss rates of Type IIn supernova progenitors for at least decades before their explosion. Thus, the shorter eruptive mass-loss events observed in some Type IIn supernova progenitors are not always responsible for creating their dense circumstellar media. In addition, we find that Type IIn supernova progenitors may tend to increase their mass-loss rates as they approach to the time of their explosion. We also show a detailed comparison between our analytic prediction and numerical results.

Detection of the Gravitational Lens Magnifying a Type Ia Supernova

Abstract: Objects of known brightness, like type Ia supernovae (SNIa), can be used to measure distances. If a massive object warps spacetime to form multiple images of a background SNIa, a direct test of cosmic expansion is also possible. However, these lensing events must first be distinguished from other rare phenomena. Recently, a supernova was found to shine much brighter than normal for its distance, which resulted in a debate: Was it a new type of superluminous supernova or a normal SNIa magnified by a hidden gravitational lens? Here, we report that a spectrum obtained after the supernova faded away shows the presence of a foreground galaxy-the first found to strongly magnify a SNIa. We discuss how more lensed SNIa can be found than previously predicted.

Interacting supernovae from photoionization-confined shells around red supergiant stars

Abstract: A model in which the stellar wind of the fast-moving red supergiant Betelgeuse is photoionized by radiation from external sources can explain the dense, almost static shell recently discovered around the star, and predicts both that debris from Betelgeuse’s eventual supernova explosion will violently collide with the shell and that other red supergiants should have similar, but much more massive, shells. The discovery in 2012 of a static, dense gaseous shell around the nearby red supergiant Betelgeuse raised doubts about the assumption that it was a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings. These two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Hilding Neilson and colleagues describe a model in which Betelgeuse's wind is photoionized by radiation from external sources, where pressure induced by photoionization generates a standing shock in the neutral part of the wind. This forms an almost-static photoionization-confined shell, confining gas close to the star, where it can interact with ejecta from a future supernova explosion. This provides a natural explanation for the many supernovae with the signatures of circumstellar interaction. Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings1,2,3,4. This picture has been challenged by the discovery of a dense and almost static shell5 that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse’s wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind6 and forms an almost static, photoionization-confined shell. Other red supergiants should have much more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova light curve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.

Electron-capture supernovae exploding within their progenitor wind

Abstract: The most massive stars on the asymptotic giant branch (AGB), or the so-called super-AGB stars, are thought to produce supernovae triggered by electron captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to have slow winds with high mass-loss rates, so their circumstellar density is high. The explosions of super-AGB stars are therefore presumed to occur in this dense circumstellar environment. We provide the first synthetic light curves for such events by exploding realistic electron-capture supernova progenitors within their super-AGB winds. We find that the early light curve – that is, before the recombination wave reaches the bottom of the hydrogen-rich envelope of supernova ejecta (the plateau phase) – is not affected by the dense wind. However, after the luminosity drop following the plateau phase, the luminosity remains much higher when the super-AGB wind is taken into account. We compare our results to the historical light curve of SN 1054, the progenitor of the Crab Nebula, and show that the explosion of an electron-capture supernova within an ordinary super-AGB wind can explain the observed light curve features. We conclude that SN 1054 could have been a Type IIn supernova without any extra extreme mass loss, which was previously suggested to be necessary to account for its early high luminosity. We also show that our light curves match Type IIn supernovae with an early plateau phase or the so-called Type IIn-P supernovae, and suggest that they are electron-capture supernovae within super-AGB winds. Although some electron-capture supernovae can be bright in the optical spectral range due to the large progenitor radius, their X-ray luminosity from the interaction does not necessarily get as bright as other Type IIn supernovae whose optical luminosities are also powered by the interaction. Thus, we suggest that optically bright X-ray-faint Type IIn supernovae can emerge from electron-capture supernovae. Optically faint Type IIn supernovae, such as SN 2008S, can also originate from electron-capture supernovae if their hydrogen-rich envelope masses are small. We argue that some of them can be observed as Type IIn-b supernovae due to the small hydrogen-rich envelope mass.

CONSTRAINING PHYSICAL PROPERTIES OF TYPE IIn SUPERNOVAE THROUGH RISE TIMES AND PEAK LUMINOSITIES

Abstract: We investigate the diversity in the wind density, supernova ejecta energy, and ejecta mass in Type IIn supernovae based on their rise times and peak luminosities. We show that the wind density and supernova ejecta properties can be estimated independently if both the rise time and peak luminosity are observed. The peak luminosity is mostly determined by the supernova properties and the rise time can be used to estimate the wind density. We find that the ejecta energies of Type IIn supernovae need to vary by factors of 0.2-5 from the average if their ejecta masses are similar. The diversity in the observed rise times indicates that their wind densities vary by factors of 0.2-2 from the average. We show that Type IIn superluminous supernovae should have not only large wind density but also large ejecta energy and/or small ejecta mass to explain their large luminosities and the rise times at the same time. We also note that shock breakout does not necessarily occur in the wind even if it is optically thick, except for the case of superluminous supernovae, and we analyze the observational data both with and without assuming that the shock breakout occurs in the dense wind of Type IIn supernovae.

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

Abstract: 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.

Mass loss of massive stars near the Eddington luminosity by core neutrino emission shortly before their explosion

Abstract: We present a novel mechanism for enhancing the mass-loss rates of massive stars shortly before their explosion. The neutrino luminosities of the stellar core of massive stars increase as they get closer to the time of the core collapse. As emitted neutrinos escape freely from the core, the core mass is significantly reduced when the neutrino luminosity is high. If a star is near the Eddington luminosity when the neutrino luminosity is high, the star can exceed the Eddington luminosity because of the core neutrino mass loss. We suggest that the stellar surface mass-loss rates due to the core neutrino emission can be higher than 10-4 M ⊙ yr-1 from ~1 year before the core collapse. The mass-loss rates can exceed 10-2 M ⊙ yr-1 ~ 10 days before the core collapse. This mass-loss mechanism may be able to explain the enhanced mass loss observed in some supernova progenitors shortly before their explosion. Even if the star is not close enough to the Eddington luminosity to enhance the mass loss, the star can still expand because of the reduced gravitational force. This mechanism can be activated in Wolf-Rayet stars, and it can create the hydrogen-poor, as well as hydrogen-rich, dense circumstellar media observed in some supernovae.

Constraining Physical Properties of Type IIn Supernovae through Rise Times and Peak Luminosities

Abstract: We investigate the diversity in the wind density, supernova ejecta energy, and ejecta mass in Type IIn supernovae based on their rise times and peak luminosities. We show that the wind density and supernova ejecta properties can be estimated independently if both the rise time and peak luminosity are observed. The peak luminosity is mostly determined by the supernova properties and the rise time can be used to estimate the wind density. We find that the ejecta energy of Type IIn supernovae needs to vary by factors of 0.2-5 from the average if their ejecta mass is similar. The diversity in the observed rise times indicates that their wind density varies by factors of 0.2-2 from the average. We show that Type IIn superluminous supernovae should have not only large wind density but also large ejecta energy and/or small ejecta mass to explain their large luminosities and the rise times at the same time. We also note that the shock breakout does not necessarily occur in the wind even if it is optically thick, except for the case of superluminous supernovae, and we analyze the observational data both with and without assuming that the shock breakout occurs in the dense wind of Type IIn supernovae.

Mass loss of massive stars near the Eddington luminosity by core neutrino emission shortly before their explosion

Abstract: We present a novel mechanism to enhance the mass-loss rates of massive stars shortly before their explosion. The neutrino luminosities of the stellar core of massive stars become larger as they get closer to the time of the core collapse. As emitted neutrinos escape freely from the core, the core mass is significantly reduced when the neutrino luminosity is large. If a star is near the Eddington luminosity when the neutrino luminosity is large, the star can exceed the Eddington luminosity because of the core neutrino mass loss. We suggest that the stellar surface mass-loss rates due to the core neutrino emission can be higher than 1e-4 Msun/yr from ~ 1 year before the core collapse. The mass-loss rates can exceed 1e-2 Msun/yr in ~ 10 days before the core collapse. This mass-loss mechanism may be able to explain the enhanced mass loss observed in some supernova progenitors shortly before their explosion. Even if the star is not close enough to the Eddington luminosity to enhance mass loss, the star can still expand because of the reduced gravitational force. This mechanism can be activated in Wolf-Rayet stars and it can make hydrogen-poor, as well as hydrogen-rich, dense circumstellar media observed in some supernovae.

Electron-capture supernovae exploding within their progenitor wind

Abstract: The most massive stars on the asymptotic giant branch (AGB), so called super-AGB stars, are thought to produce supernovae (SNe) triggered by electron captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to have slow winds with high mass-loss rates, so their wind density is high. The explosions of super-AGB stars are therefore presumed to occur in this dense wind. We provide the first synthetic light curves (LCs) for such events by exploding realistic electron-capture supernova (ecSN) progenitors within their super-AGB winds. We find that the early LC, i.e. before the recombination wave reaches the bottom of the H-rich envelope of SN ejecta (the plateau phase), is not affected by the dense wind. However, after the plateau phase, the luminosity remains higher when the super-AGB wind is taken into account. We compare our results to the historical LC of SN 1054, the progenitor of the Crab Nebula, and show that the explosion of an ecSN within an ordinary super-AGB wind can explain the LC features. We conclude that SN 1054 could have been a Type IIn SN without any extra extreme mass loss which was previously suggested to be necessary to account for its early high luminosity. We also show that our LCs match Type IIn SNe with an early plateau phase (`Type IIn-P') and suggest that they are ecSNe within super-AGB winds. Although some ecSNe can be bright in the optical spectral range due to the large progenitor radius, their X-ray luminosity from the interaction does not necessarily get as bright as other Type IIn SNe whose optical luminosities are also powered by the interaction. Thus, we suggest that optically-bright X-ray-faint Type IIn SNe can emerge from ecSNe. Optically-faint Type IIn SNe, such as SN 2008S, can also originate from ecSNe if their H-rich envelope masses are small. Some of them can be observed as `Type IIn-b' SNe due to the small H-rich envelope mass.

On the 'snow-plow' phase of supernovae interacting with dense circumstellar media

Abstract: Recently, Moriya et al. (2013) developed an analytic bolometric light curve model for supernovae interacting with dense circumstellar media. Because of the dense circumstellar medium, the shocked region is assumed to be radiative and make a thin dense shell. The model is based on the conservation of momentum in the shocked dense shell. However, the analytic model was mentioned to neglect the 'snow-plow' phase of the shocked dense shell by Ofek et al. (2014). The 'snow-plow' or momentum-conserving phase refers to the period in which the momentum injection from the supernova ejecta is almost terminated and the radiative shocked dense shell keeps moving due only to the momentum previously provided by the supernova ejecta. In this Note, I clarify that the analytic model of Moriya et al. (2013) does take the 'snow-plow' phase into account and the criticism of Ofek et al. (2014) is incorrect. In addition, Ofek et al. (2014) related the sudden luminosity break observed in the light curve of Type IIn SN 2010jl to the transition to the 'snow-plow' phase. However, I argue that the sudden transition to the 'snow-plow' phase is not consistent with the luminosity break observed in SN 2010jl. The luminosity break is likely to be related to other phenomena like the dense shell exiting the dense part of the circumstellar medium.

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

Abstract: 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 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.