A recent study indicated that the inner cores of rapidly accreting ($\dot M > 10^{-7}~M_\odot~{\rm yr}^{-1}$) CO white dwarfs may rotate differentially, with a shear rate near the threshold value for ...the onset of the dynamical shear instability. Such differentially rotating white dwarfs have critical masses for thermonuclear explosion or electron-capture induced collapse that significantly exceed the canonical Chandrasekhar limit. Here, we construct two-dimensional differentially rotating white dwarf models with rotation laws resembling those of the one-dimensional models of the previous work. We derive analytic relations between the white dwarf mass, its angular momentum, and its rotational-, gravitational- and binding energy. We show that these relations are applicable for a wide range of angular velocity profiles, including solid body rotation. Taken at a central density of 2 $\times$ $10^9~{\rm g~cm}^{-3}$ they specify initial models for the thermonuclear explosion of rotating CO white dwarfs. At $\rho_{\rm c} = 10^{10}~{\rm g~cm}^{-3}$ and 4 $\times$ $10^{9}~{\rm g~cm}^{-3}$, they give criteria for the electron-capture induced collapse of rotating CO and ONeMg white dwarfs, respectively. We demonstrate that pre-explosion and pre-collapse conditions of both rigidly and differentially rotating white dwarfs are well established by the present work, which may facilitate future multi-dimensional simulations of type Ia supernova explosions and studies of the formation of millisecond pulsars and gamma-ray bursts from collapsing white dwarfs. Our results lead us to suggest various possible evolutionary scenarios for progenitors of type Ia supernovae, leading to a new paradigm of a variable mass of exploding white dwarfs, at values well above the classical Chandrasekhar mass. Based on our 2D-models, we argue that the supernova peak brightness is proportional to the white dwarf mass, which could explain various aspects of the diversity of type Ia supernovae, such as their variation in brightness, the dependence of their mean luminosity on the host galaxy type, and the weak correlation between ejecta velocity and peak brightness.
Context. So-called superluminous supernovae have been recently discovered in the local Universe. It appears possible that some of them originate in stellar explosions induced by the pair instability ...mechanism. Recent stellar evolution models also predict pair instability supernovae from very massive stars at fairly high metallicities (i.e., Z ~ 0.004). Aims. We provide supernova models and synthetic light curves for two progenitor models, a 150 M⊙ red supergiant and a 250 M⊙ yellow supergiant at a metallicity of Z = 0.001, for which the evolution from the main sequence to collapse and the initiation of the pair instability supernova itself has been previously computed in a realistic and self-consistent way. Methods. We use the radiation hydrodynamics code STELLA to describe the supernova evolution of both models in a time frame of about 500 days. Results. We describe the shock-breakout phases of both supernovae, which are characterized by higher luminosity, longer duration, and a lower effective temperature than those of ordinary Type IIP supernovae. We derive the bolometric, as well as the U, B, V, R, and I, light curves of our pair instability supernova models, which show a long-lasting plateau phase with maxima at Mbol ≃ −19.3 mag and −21.3 mag for our lower and higher mass models, respectively. While we do not produce synthetic spectra, we also describe the photospheric composition and velocity as a function of time. Conclusions. We conclude that the light curve of the explosion of our initially 150 M⊙ star resembles those of relatively bright type IIP supernovae, whereas its photospheric velocity at early times is somewhat lower. Its 56Ni mass of 0.04 M⊙ also falls well into the range found in ordinary core collapse supernovae. The light curve and photospheric velocity of our 250 M⊙ models has a striking resemblance to that of the superluminous SN 2007bi, strengthening its interpretation as pair instability supernova. We conclude that pair instability supernovae may occur more frequently in the local universe than previously assumed.
Exosomes are enclosed compartments that are released from cells and that can transport biological contents for the purpose of intercellular communications. Research into exosomes is hindered by their ...rarity. In this article, we introduce a device that uses centrifugal force and a filter with micro-sized pores to generate a large quantity of cell-derived nanovesicles. The device has a simple polycarbonate structure to hold the filter, and operates in a common centrifuge. Nanovesicles are similar in size and membrane structure to exosomes. Nanovesicles contain intracellular RNAs ranging from microRNA to mRNA, intracellular proteins, and plasma membrane proteins. The quantity of nanovesicles produced using the device is 250 times the quantity of naturally secreted exosomes. Also, the quantity of intracellular contents in nanovesicles is twice that in exosomes. Nanovesicles generated from murine embryonic stem cells can transfer RNAs to target cells. Therefore, this novel device and the nanovesicles that it generates are expected to be used in exosome-related research, and can be applied in various applications such as drug delivery and cell-based therapy.
Context. Type Ia supernovae (SNe Ia) have been an important tool for astronomy for quite some time; however, the nature of their progenitors remains somewhat mysterious. Recent theoretical studies ...indicated the possibility of producing thermonuclear detonations of carbon-oxygen white dwarfs (CO WDs) at masses less than the Chandrasekhar mass through accretion of helium-rich matter, which would, depending on mass accretion rate, mass, and initial temperature of the WD, spectrally resemble either a normal SN Ia or a peculiar one. Aims. This study aims to further resolve the state of binary systems comprised of a sub-Chandrasekhar-mass CO WD and a helium star at the point where an accretion-induced detonation occurs and constrains the part of the initial parameter space where this kind of phenomenon is possible. Methods. Preexisting data obtained through simulations of single, constantly accreting CO WDs is used as an indicator for the behavior of new binary models in which the WD is treated as a point mass and which include the non-degenerate partner as a fully resolved stellar model. We parameterize the ignition of the accumulated helium layer, changes in the WD temperature, and changes in the CO core mass depending on the mass transfer rate. Results. The initial conditions allowing for detonation do not form a single contiguous area in the parameter space, whose shape is profoundly influenced by the behavior of the donor star. Mass loss due to Nova outbursts acts in favor of detonation. According to our criteria, about 10% of the detonations in this study can be expected to show spectra consistent with ordinary SNe Ia; the rest exhibit peculiar features.
Context. Recently it has been found that models of massive stars reach the Eddington limit in their interior, which leads to dilute extended envelopes. Aims. We perform a comparative study of the ...envelope properties of massive stars at different metallicities, with the aim to establish the impact of the stellar metallicity on the effect of envelope inflation. Methods. We analysed published grids of core-hydrogen burning massive star models computed with metallicities appropriate for massive stars in the Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud, the very metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition. Results. Stellar models of all the investigated metallicities reach and exceed the Eddington limit in their interior, aided by the opacity peaks of iron, helium, and hydrogen, and consequently develop inflated envelopes. Envelope inflation leads to a redward bending of the zero-age main sequence and a broadening of the main-sequence band in the upper part of the Hertzsprung-Russell diagram. We derive the limiting L/M-values as a function of the stellar surface temperature above which inflation occurs, and find them to be higher for lower metallicity. While Galactic models show inflation above ~29 M⊙, the corresponding mass limit for Population III stars is ~150 M⊙. While the masses of the inflated envelopes are generally low, we find that they can reach 1−100 M⊙ in models with effective temperatures below ~8000 K, with higher masses reached by models of lower metallicity. Conclusions. Envelope inflation is expected to occur in sufficiently massive stars at all metallicities, and is expected to lead to rapidly growing pulsations, high macroturbulent velocities, and might well be related to the unexplained variability observed in luminous blue variables such as S Doradus and η Carina.
High-power, long-life carbon-coated TiO2 microsphere electrodes were synthesized by a hydrothermal method for sodium ion batteries, and the electrochemical properties were evaluated as a function of ...carbon content. The carbon coating, introduced by sucrose addition, had an effect of suppressing the growth of the TiO2 primary crystallites during calcination. The carbon coated TiO2 (sucrose 20 wt % coated) electrode exhibited excellent cycle retention during 50 cycles (100%) and superior rate capability up to a 30 C rate at room temperature. This cell delivered a high discharge capacity of 155 mAh gcomposite –1 at 0.1 C, 149 mAh gcomposite –1 at 1 C, and 82.7 mAh gcomposite –1 at a 10 C rate, respectively.
Context. The progenitors of many Type II supernovae have been observationally identified but the search for Type Ibc supernova (SN Ibc) progenitors has thus far been unsuccessful, despite the ...expectation that they are luminous Wolf-Rayet (WR) stars. Aims. We investigate how the evolution of massive helium stars affects their visual appearances, and discuss the implications for the detectability of SN Ibc progenitors. Methods. Evolutionary models of massive helium stars are analysed and their properties compared to Galactic WR stars. Results. Massive WR stars that rapidly lose their helium envelopes through stellar-wind mass-loss end their lives when their effective temperatures – related to their hydrostatic surfaces – exceed about 150 kK. At their pre-supernova stage, their surface properties resemble those of hot Galactic WR stars of WO sub-type. These are visually faint with narrow-band visual magnitudes Mv = −1.5 ··· −2.5, despite their high bolometric luminosities (log L/L⊙ = 5.6···5.7), compared to the bulk of Galactic WR stars (Mv < −4). In contrast, relatively low-mass helium stars that retain a thick helium envelope appear fairly bright in optical bands, depending on the final masses and the history of the envelope expansion during the late evolutionary stages. Conclusions. We conclude that SNe Ibc observations have so far not provided strong constraints on progenitor bolometric luminosities and masses, even with the deepest searches. We also argue that Ic progenitors are more challenging to identify than Ib progenitors in any optical images.
Aims.Pair creation supernovae (PCSN) are thought to be produced from very massive low metallicity stars. The spectacularly bright SN 2006gy does show several signatures expected from PCSNe. Here, we ...investigate the metallicity threshold below which PCSN can form and estimate their occurrence rate. Methods.We perform stellar evolution calculations for stars of 150 ${M}_\odot$ and 250 ${M}_\odot$ of low metallicity ($Z_{\odot}$/5 and $Z_{\odot}$/20), and analyze their mass loss rates. Results.We find that the bifurcation between quasi-chemically homogeneous evolution for fast rotation and conventional evolution for slower rotation, which has been found earlier for massive low metallicity stars, persists in the mass range considered here. Consequently, there are two separate PCSN progenitor types: (I) fast rotators produce PCSNe from very massive Wolf-Rayet stars, and (II) slower rotators that generate PCSNe in hydrogen-rich massive yellow hypergiants. Conclusions.We find that hydrogen-rich PCSNe could occur at metallicities as high as $Z_{\odot}$/3, which – assuming standard IMFs are still valid to estimate their birth rates – results in a rate of about one PCSN per 1000 supernovae in the local universe, and one PCSN per 100 supernovae at a redshift of $z = 5$. PCSNe from WC-type Wolf-Rayet stars are restricted to much lower metallicity.
Context. Helium accretion induced explosions in CO white dwarfs (WDs) are considered promising candidates for a number of observed types of stellar transients, including supernovae (SNe) of Type Ia ...and Type Iax. However, a clear favorite outcome has not yet emerged. Aims. We explore the conditions of helium ignition in the WD and the final fates of helium star-WD binaries as functions of their initial orbital periods and component masses. Methods. We computed 274 model binary systems with the Binary Evolution Code, in which both components are fully resolved. Both stellar and orbital evolution were computed including mass and angular momentum transfer, tides, gravitational wave emission, differential rotation, and internal hydrodynamic and magnetic angular momentum transport. We worked out the parts of the parameter space leading to detonations of the accreted helium layer on the WD, likely resulting in the complete disruption of the WD to deflagrations, where the CO core of the WD may remain intact and where helium ignition in the WD is avoided. Results. We find that helium detonations are expected only in systems with the shortest initial orbital periods, and for initially massive WDs (MWD ≥ 1.0 M⊙) and lower mass donors (Mdonor ≤ 0.8 M⊙), which have accumulated helium layers mostly exceeding 0.1 M⊙. Upon detonation, these systems would release the donor as a hypervelocity pre-WD runaway star, for which we predict the expected range of kinematic and stellar properties. Systems with more massive donors or initial periods exceeding 1.5 h likely undergo helium deflagrations after accumulating 0.1 − 0.001 M⊙ of helium. Helium ignition in the WD is avoided in systems with helium donor stars below ∼0.6 M⊙, and leads to three distinctly different groups of double WD systems. Conclusions. The size of the parameter space open to helium detonation corresponds to only about 3% of the galactic SN Ia rate and to 10% of the SN Iax rate, while the predicted large amounts of helium (0.1 M⊙) in progenitors cannot easily be reconciled with observations of archetypical SN Ia. However, the transients emerging from these systems may contribute significantly to massive helium novae, calcium-rich SNe Ib, and, potentially, very close double degenerate systems that may eventually produce either ordinary or peculiar SNe Ia, or, for the smallest considered masses, R Coronae Borealis stars.
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