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

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.Comment: 5 pages, 2 figures, accepted by Astronomy & Astrophysic

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.Comment: 11 pages, 10 figures, 1 table, accepted by Astronomy & Astrophysics, proofed in v

Systematic investigation of the fallback accretion powered model for hydrogen-poor superluminous supernovae

The energy liberated by fallback accretion has been suggested as a possible engine to power hydrogen-poor superluminous supernovae. We systematically investigate this model using the Bayesian light-curve fitting code MOSFiT (Modular Open Source Fitter for Transients), fitting the light curves of 37 hydrogen-poor superluminous supernovae assuming a fallback accretion central engine. We find that this model can yield good fits to their light curves, with a fit quality that rivals the popular magnetar engine models. Examining our derived parameters for the fallback model, we find the total energy requirements from the accretion disk are estimated to be 0.002 - 0.7 Msun c^2. If we adopt a typical conversion efficiency ~ 1e-3, the required mass to accrete is thus 2 - 700 Msun. Many superluminous supernovae, therefore, require an unrealistic accretion mass, and so only a fraction of these events could be powered by fallback accretion unless the true efficiency is much greater than our fiducial value. The superluminous supernovae that require the smallest amounts of fallback mass still remain to be the fallback accretion powered supernova candidates, but they are difficult to be distinguished solely by their light curve properties.Comment: 12 pages, 8 figures, 3 tables, accepted by The Astrophysical Journa

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.Comment: 5 pages, 3 figures, 1 table, accepted by Astronomy & Astrophysics Letters, proofed in v