Carbon-oxygen Core Research Articles

MULTI-DIMENSIONAL SIMULATIONS OF ROTATING PAIR-INSTABILITY SUPERNOVAE

We study the effects of rotation on the dynamics, energetics and Ni-56 production of Pair Instability Supernova explosions by performing rotating two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN progenitors with initial surface rotational velocities 50% that of the critical Keplerian value using the stellar evolution code MESA. We allow for both the inclusion and the omission of the effects of magnetic fields in the angular momentum transport and in chemical mixing, resulting in slowly-rotating and rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation for carbon-oxygen cores of the same mass and chemical stratification leads to less energetic PISN explosions that produce smaller amounts of Ni-56 due to the effect of the angular momentum barrier that develops and slows the dynamical collapse. We find a non-monotonic dependence of Ni-56 production on rotational velocity in situations when smoother composition gradients form at the outer edge of the rotating cores. In these cases, the PISN energetics are determined by the competition of two factors: the extent of chemical mixing in the outer layers of the core due to the effects of rotation in the progenitor evolution and the development of angular momentum support against collapse. Our 2.5-D PISN simulations with rotation are the first presented in the literature. They reveal hydrodynamic instabilities in several regions of the exploding star and increased explosion asymmetries with higher core rotational velocity.

Detonation shock dynamics of Type Ia supernovae

The wavefront propagation of curved detonation waves in carbon-oxygen cores and helium shells of type Ia supernova progenitors may be predicted via a Detonation Shock Dynamics (DSD) approach. DSD is used typically in engineering to model explosives. A level set implementation is used to evolve the front, using intrinsic quasi-steady, quasi-one-dimensional detonation speed-curvature relationships. The effects of curvature are analysed for a number of models from the literature which originally use the local planar detonation speed, and compared. The differences can be very profound in the low density regions where detonation models are exploited to produce intermediate mass elements. In detonable low density regions, the detonation wave speed tends to be much lower than the planar DSD analysis predicts, while the subsonic driving zone controlling the dynamics is many order of magnitude shorter than its planar version. However, the lower shock temperatures ensure that the complete reaction lengths are orders of magnitude longer when curvature effects are properly accounted. Furthermore, the material cannot be detonated in sufficiently low density regions due to a curvature induced extinction limit. The implications for and need to reassess the nucleosynthesis, intermediate mass element production and even the progenitors of SN Ia detonation models, is discussed. In the second part of this thesis, an adapted method for evolving the front when the speed function has a curvature component is introduced. A stationary boundary value problem is adapted by considering the non-stationary form. This method is similar in its implementation to the level set form but is more efficient. Its efficiency and error is investigated and compared to the level set method.

A precision study of two eclipsing white dwarf plus M dwarf binaries

We use a combination of X-shooter spectroscopy, ULTRACAM high-speed photometry and SOFI near-infrared photometry to measure the masses and radii of both components of the eclipsing post common envelope binaries SDSS J1212-0123 and GK Vir. For both systems we measure the gravitational redshift of the white dwarf and combine it with light curve model fits to determine the inclinations, masses and radii. For SDSS J1212-0123 we find a white dwarf mass and radius of 0.439 +/- 0.002 Msun and 0.0168 +/- 0.0003 Rsun, and a secondary star mass and radius of 0.273 +/- 0.002 Msun and 0.306 +/- 0.007 Rsun. For GK Vir we find a white dwarf mass and radius of 0.564 +/- 0.014 Msun and 0.0170 +/- 0.0004 Rsun, and a secondary star mass and radius of 0.116 +/- 0.003 Msun and 0.155 +/- 0.003 Rsun. The mass and radius of the white dwarf in GK Vir are consistent with evolutionary models for a 50,000K carbon-oxygen core white dwarf. Although the mass and radius of the white dwarf in SDSS J1212-0123 are consistent with carbon-oxygen core models, evolutionary models imply that a white dwarf with such a low mass and in a short period binary must have a helium core. The mass and radius measurements are consistent with helium core models but only if the white dwarf has a very thin hydrogen envelope, which has not been predicted by evolutionary models. The mass and radius of the secondary star in GK Vir are consistent with evolutionary models after correcting for the effects of irradiation by the white dwarf. The secondary star in SDSS J1212-0123 has a radius ~9 per cent larger than predicted.

The shortest period detached white dwarf + main-sequence binary

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94 minute orbital period eclipsing white dwarf / main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of 85.5 +/- 0.2 deg. The white dwarf has a mass of 0.51 +/- 0.05 Msun and a radius of 0.0247 +/- 0.0008 Rsun. With a temperature of 35,300 +/- 400K the white dwarf is highly over-inflated if it has a carbon-oxygen core, however if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 +/- 0.01 Msun and a radius of 0.110 +/- 0.004 Rsun placing it close to the hydrogen burning limit. The system emerged from a common envelope ~20 million years ago and will reach a semi-detached configuration in ~400 million years, becoming a cataclysmic variable with a period of 66 minutes, below the period minimum.

HELIUM SHELL DETONATIONS ON LOW-MASS WHITE DWARFS AS A POSSIBLE EXPLANATION FOR SN 2005E

Recently several type Ib supernovae (SNe; with the prototypical SN 2005E) have been shown to have atypical properties. These SNe are faint (absolute peak magnitude of ~ -15) and fast SNe that show unique composition. They are inferred to have low ejecta mass (a few tenths of a solar mass) and to be highly enriched in calcium, but poor in silicon elements and nickel. These SNe were therefore suggested to belong to a new class of calcium-rich faint SNe explosions. Their properties were proposed to be the result of helium detonations that may occur on helium accreting white dwarfs. In this paper we theoretically study the scenario of helium detonations, and focus on the results of detonations in accreted helium layers on low mass carbon-oxygen (CO) cores. We present new results from one dimensional simulations of such explosions, including their light curves and spectra. We find that when the density of the helium layer is low enough the helium detonation produces large amounts of intermediate elements, such as calcium and titanium, together with a large amount of unburnt helium. Our results suggest that the properties of calcium-rich faint SNe could indeed be consistent with the helium-detonation scenario on small CO cores. Above a certain density (larger CO cores) the detonation leaves mainly 56Ni and unburnt helium, and the predicted spectrum will unlikely fit the unique features of this class of SNe. Finally, none of our studied models reproduces the bright, fast evolving light curves of another type of peculiar SNe suggested to originate in helium detonations (SNe 1885A, 1939B and 2002bj).

White Dwarf Remnants of Binary Star Evolution

AbstractWhite dwarfs grow as the cores of red giants and, in particular, carbon-oxygen white dwarfs grow in asymptotic giant branch (AGB) stars. The evolution of an AGB star is a competition between growth of the core and loss of the stellar envelope, typically in a wind. It is complicated by thermal pulses driven periodically by unstable helium shell burning. Dredge up following such pulses delays core growth. The compression at the center of a cold carbon-oxygen core means that carbon ignites when it reaches a mass of 1.38 M⊙. This begins the familiar thermonuclear runaway of the Type Ia supernova (SN Ia). At higher temperatures carbon can ignite more gently and burn mostly to neon to leave a core rich in oxygen, neon and magnesium. Such cores can go on to collapse to neutron stars with a release of only neutrinos. Accepted mass-loss prescriptions for giants mean that the range of masses of single stars that leave carbon-oxygen white dwarfs is somewhere from around 1 to 8 M⊙. We investigate how unusual mass loss, perhaps brought about by interaction with a binary companion, can radically alter the single star picture. Though population syntheses treat some possibilities with various prescriptions, there is sufficient doubt over the physics, the observations, and the implementation of mass loss and binary interaction that there is scope for several more unusual progenitors of carbon-oxygen white dwarfs and hence SNe Ia.

A common envelope binary star origin of long gamma-ray bursts

The stellar origin of gamma-ray bursts can be explained by the rapid release of energy in a highly collimated, extremely relativistic jet. This in turn appears to require a rapidly spinning highly magnetised stellar core that collapses into a magnetic neutron star or a black hole within a relatively massive envelope. They appear to be associated with type Ib/c supernovae but, with a birthrate of around 10^{-6}-10^{-5} per year per galaxy, they are considerably rarer than such supernovae in general. To satisfy all these requirements we hypothesize a binary star model that ends with the merging of an oxygen neon white dwarf with the carbon-oxygen core of a naked helium star during a common envelope phase of evolution. The rapid spin and high magnetic field are natural consequences of such a merging. The evolution that leads to these progenitors is convoluted and so naturally occurs only very rarely. To test the hypothesis we evolve a population of progenitors and find that the rate is as required. At low metallicity we calculate that a similar fraction of stars evolve to this point and so would expect the gamma-ray burst rate to correlate with the star formation rate in any galaxy. This too is consistent with observations. These progenitors, being of intermediate mass, differ radically from the usually postulated high-mass stars. Thus we can reconcile observations that the bursts occur close to but not within massive star associations.

Mode trapping in sdO models

Mode trapping of high-radial order gravity modes was found for a particular sdO model. The trapping is caused by the change in composition from the helium radiative shell to the hydrogen burning shell. A non-adiabatic effect of this trapping is the higher tendency to instability of the trapped modes. Low- to intermediate-radial order pressure modes (in sdO models they correspond to mixed modes with most nodes in the P-mode region) are found to be trapped by the chemical transition from the carbon-oxygen core to the He burning shell. As the trapping is produced in the deep interior of the star, where energy interchange is negligible at the p-mode frequencies, it has no particular effect on the driving.

What helium and lithium can tell us about CEMP stars?

AbstractWe show that the peculiar surface abundance patterns of Carbon Enhanced Metal Poor (CEMP) stars has been inherited from material having been processed by H- and He-burning phases in a previous generation of stars (hereafter called the “Source Stars”). In this previous generation, some mixing must have occurred between the He- and the H-burning regions in order to explain the high observed abundances of nitrogen. In addition, it is necessary to postulate that a very small fraction of the carbon-oxygen core has been expelled (either by winds or by the supernova explosion). Therefore only the outermost layers should have been released by the Source Stars. Some of the CEMP stars may be He-rich if the matter from the Source Star is not too much diluted with the InterStellar Medium (ISM). Those stars formed from nearly pure ejecta would also be Li-poor.

THE IMPACT OF NEUTRINO MAGNETIC MOMENTS ON THE EVOLUTION OF MASSIVE STARS

We explore the sensitivity of massive stars to neutrino magnetic moments. We find that the additional cooling due to the neutrino magnetic moments bring about qualitative changes to the structure and evolution of stars in the mass window 7 Msun < M < 18 Msun, rather than simply changing the time scales for the burning. We describe some of the consequences of this modified evolution: the shifts in the threshold masses for creating core-collapse supernovae and oxygen-neon-magnesium white dwarfs and the appearance of a new type of supernova in which a partial carbon-oxygen core explodes within a massive star. The resulting sensitivity to the magnetic moment is at the level of (2-4) * 10^{-11} \mu_B.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low‐Mass White Dwarfs

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, <M> = 0.43 +/- 0.06 Msun, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash, and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

Stellar fusion doesn’t stop at helium

Nelemans replies: Thanks to Mason Osborne for the clarification. Indeed, stars that are initially lighter than 8 solar masses also fuse helium into carbon and oxygen and end up with a degenerate carbon–oxygen core. Stars with initial masses of less than about 2 solar masses first develop a degenerate helium core, which in the helium flash is turned into a helium-burning core. Apart from the RR Lyrae stars, several other types, such as subdwarf B stars and horizontal-branch stars in globular clusters, are in the helium-burning-core stage (references in the original article and in Osborne’s letter). The point of my simplification in the box text was to distinguish between stars that develop a degenerate core to withstand gravity and more massive stars that do not form such a core. The less massive stars form white dwarfs after losing their hydrogen mantle, and the heavier ones ultimately become neutron stars or black holes. © 2007 American Institute of Physics.

Relics of Metal-free Low-Mass Stars Exploding as Thermonuclear Supernovae

Renewed interest in the first stars that were formed in the universe has led to the discovery of extremely iron-poor stars. Since several competing scenarios exist, our understanding of the mass range that determines the observed elemental abundances remains unclear. In this study, we consider three well-studied metal-poor stars in terms of the theoretical supernova (SN) model. Our results suggest that the observed abundance patterns in the metal-poor star BD +80 245 and the pair of stars HD 134439/40 agree strongly with the theoretical possibility that these stars inherited their heavy-element abundance patterns from SNe initiated by thermonuclear runaways in the degenerate carbon-oxygen cores of primordial asymptotic giant branch stars with masses of ~3.5-5 M☉. Recent theoretical calculations have predicted that such SNe could be originated from metal-free stars in the intermediate-mass range. On the other hand, intermediate-mass stars containing some metals would end their lives as white dwarfs after expelling their envelopes in the wind due to intense momentum transport from outgoing photons to heavy elements. This new pathway for the formation of SNe requires that stars be formed from the primordial gas. Thus, we suggest that stars of a few solar masses were formed from the primordial gas and that some of them caused thermonuclear explosions when the mass of their degenerate carbon-oxygen cores increased to the Chandrasekhar limit without experiencing efficient mass loss.

The gravitational wave radiation of pulsating white dwarfs revisited: the case of BPM 37093 and PG 1159-035

We compute the emission of gravitational radiation from pulsating white dwarfs. This is done by using an up-to-date stellar evolutionary code coupled with a state-of-the-art pulsational code. The emission of gravitational waves is computed for a standard 0.6 M ○. white dwarf with a liquid carbon-oxygen core and a hydrogen-rich envelope, for a massive DA white dwarf with a partially crystallized core for which various l = 2 modes have been observed (BPM 37093) and for PG 1159-035, the prototype of the GW Vir class of variable stars, for which several quadrupole modes have been observed as well. We find that these stars do not radiate sizeable amounts of gravitational waves through their observed pulsation g-modes, in line with previous studies. We also explore the possibility of detecting gravitational waves radiated by the /-mode and the p-modes. We find that in this case the gravitational wave signal is very large and, hence, the modes decay very rapidly. We also discuss the possible implications of our calculations for the detection of gravitational waves from pulsating white dwarfs within the framework of future space-borne interferometers like LISA.

Can pulsating PG 1159 stars place constraints on the occurrence of core overshooting?

The present letter is aimed at exploring the influence of overshooting during the central helium burning in pre-white dwarf progenitors on the pulsational properties of PG 1159 stars. To this end we follow the complete evolution an intermediate-mass white dwarf progenitor from the zero age main sequence through the thermally pulsing and born-again phases to the domain of the PG 1159 stars. Our results suggest that the presence of mode-trapping features in the period spacings of these hot pulsating stars could result from structure in the carbon-oxygen core. We find in particular that in order to get enough core structure consistent with observational demands, the occurrence of overshoot episodes during the central helium burning is needed. This conclusion is valid for thick helium envelopes like those predicted by our detailed evolutionary calculations. If the envelope thickness were substantially smaller, then the occurrence of core overshooting would be more difficult to disentangle from the effects related to the envelope transition zones.

Mass distribution of DA white dwarfs in the First Data Release of the Sloan Digital Sky Survey

We investigate the sample of 1175 new nonmagnetic DA white dwarfs with the effective temperatures T_eff > 12000 K, which were extracted from the Data Release 1 of the Sloan Digital Sky Survey. We determined masses, radii, and bolometric luminosities of stars in the sample. The above parameters were derived from the effective temperatures T_eff and surface gravities log g published in the DR1, and the new theoretical M - R relations for carbon-core and oxygen-core white dwarfs. Mass distribution of white dwarfs in this sample exhibits the peak at M = 0.562 M_sol (carbon core stars), and the tail towards higher masses. Both the shape of the mass distribution function and the empirical mass - radius relation are practically identical for white dwarfs with either pure carbon or pure oxygen cores.

On the Origin of HE 0107-5240, the Most Iron-deficient Star Presently Known

We show that the puzzling chemical composition observed in the extremely metal-poor star HE 0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model, a supernova of quite low mass (~15 M☉) underwent a normal explosion and ejected ~0.06 M☉ of 56Ni while a second one was massive enough (~35 M☉) to experience a strong fallback that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 M☉). The successive explosion of a quite massive star experiencing an extended fallback would have largely raised the abundances of the light elements in its close neighborhood.

FG Sagittae

AbstractFG Sagittae has evolved from a hot central star of a planetary nebula to an K-type supergiant within approximately the last 100 years. The generally accepted interpretation for this redward evolution is that of a late thermal pulse during the planetary nebula stage, wherein helium burning at the surface of the electron-degenerate carbon-oxygen core is reignited. As the star expands in response to the energy released by helium burning, envelope convection digs deeper and deeper until nuclearly processed material may get dredged-up to the stellar surface. Analysing the spectra as FG Sge is evolving would then give unique information about the temporal development of mixing processes occurring inside the star that are otherwise impossible to obtain. The existing abundance analyses do not give, however, a consistent picture. Especially the question about FG Sge’s hydrogen abundance is still unsettled. We present a critical assessment of all the existing data, trying to find a self-consistent picture of the evolution of FG Sge, based on the latest evolutionary models.

A New Formation Channel for Double Neutron Stars Without Recycling: Implications for Gravitational Wave Detection

We report on a new evolutionary path leading to the formation of close double neutron stars (NSs), with the unique characteristic that none of the two NSs ever had the chance to be recycled by accretion. The existence of this channel stems from the evolution of helium-rich stars (cores of massive NS progenitors), which has been neglected in most previous studies of double compact object formation. We find that these nonrecycled NS-NS binaries are formed from bare carbon-oxygen cores in tight orbits, with formation rates comparable to or maybe even higher than those of recycled NS-NS binaries. On the other hand, their detection probability as binary pulsars is greatly reduced (by ~103) relative to recycled pulsars because of their short lifetimes. We conclude that, in the context of gravitational wave detection of NS-NS in-spiral events, this new type of binaries calls for an increase of the rate estimates derived from the observed NS-NS systems with recycled pulsars, typically by factors of 1.5-3 or even higher.

Wolf-Rayet stars and relativistic objects: Distinctions between the mass distributions in close binary systems

The observed properties of Wolf-Rayet stars and relativistic objects in close binary systems are analyzed. The final masses MCOf for the carbon-oxygen cores of WR stars in WR + O binaries are calculated taking into account the radial loss of matter via stellar wind, which depends on the mass of the star. The analysis includes new data on the clumpy structure of WR winds, which appreciably decreases the required mass-loss rates \(\dot M_{WR}\) for the WR stars. The masses MCOf lie in the range (1–2)M⊙–(20–44)M⊙ and have a continuous distribution. The masses of the relativistic objects Mx are 1–20M⊙ and have a bimodal distribution: the mean masses for neutron stars and black holes are 1.35 ± 0.15M⊙ and 8–10M⊙, respectively, with a gap from 2–4M⊙ in which no neutron stars or black holes are observed in close binaries. The mean final CO-core mass is \(\overline M _{CO}^f = 7.4 - 10.3M_ \odot\), close to the mean mass for the black holes. This suggests that it is not only the mass of the progenitor that determines the nature of the relativistic object, but other parameters as well-rotation, magnetic field, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binaries that may have low-mass companions (main-sequence or subgiant M-A stars) are identified; these could be the progenitors of low-mass X-ray binaries with neutron stars and black holes.