Convective Urca Process Research Articles

Sensitivity of 3D Convective Urca Simulations to Changes in Urca Reactions

A proposed setting for thermonuclear (Type Ia) supernovae is a white dwarf that has gained mass from a companion to the point of carbon ignition in the core. There is a simmering phase in the early stages of burning that involves the formation and growth of a core convection zone. One aspect of this phase is the convective Urca process, a linking of weak nuclear reactions to convection that may alter the composition and structure of the white dwarf. Convective Urca is not well understood and requires 3D fluid simulations to realistically model. Additionally, the convection is relatively slow (Mach number less than 0.005) so a low-Mach method is needed to make simulating computationally feasible. Using the MAESTROeX low-Mach hydrodynamics code, we investigate recent changes to how the weak reactions are modeled in the convective Urca simulations. We present results that quantify the changes to the reaction rates and their impact on the evolution of the simulation.

Type Ia supernovae in NS+He star systems and the isolated mildly recycled pulsars

ABSTRACT Type Ia supernovae (SNe Ia) are successful cosmological distance indicators and important element factories in the chemical evolution of galaxies. They are generally thought to originate from thermonuclear explosions of carbon–oxygen white dwarfs in close binaries. However, the observed diversity among SNe Ia implies that they have different progenitor models. In this article, we performed the long-term evolution of NS+He star binaries with different initial He star masses ($M_{\rm He}^{\rm i}$) and orbital periods ($P_{\rm orb}^{\rm i}$) for the first time, in which the He star companions can explode as SNe Ia eventually. Our simulations indicate that after the He stars develop highly degenerate oxygen–neon (ONe) cores with masses near the Chandrasekhar limit, explosive oxygen burning can be triggered due to the convective Urca process. According to these calculations, we obtained an initial parameter space for the production of SNe Ia in the $\rm log\,$$P^{\rm i}_{\rm orb}{\text {--}}M^{\rm i}_{\rm He}$ plane. Meanwhile, we found that isolated mildly recycled pulsars can be formed after He stars explode as SNe Ia in NS+He star binaries, in which the isolated pulsars have minimum spin periods ($P_{\rm spin}^{\rm min}$) of ∼30–110 ms and final orbital velocities of ${\sim} \rm 60{\!-\!}360\, km\, s^{-1}$, corresponding to initial orbital periods of 0.07–10 d. Our work suggests that the NS+He star channel may contribute to the formation of isolated mildly recycled pulsars with velocity $\rm {\lesssim} 360\, km\, s^{-1}$ in observations, and such isolated pulsars should locate in the region of pulsars with massive white dwarf companions in the $P_{\rm spin} {\!-\!}\dot{P}_{\rm spin}$ diagram.

Pre-explosive Accretion and Simmering Phases of SNe Ia

Abstract In accreting white dwarfs (WDs) approaching the Chandrasekhar limit, hydrostatic carbon burning precedes the dynamical breakout. During this simmering phase, e-captures are energetically favored in the central region of the star, while β-decay are favored more outside, and the two zones are connected by a growing convective instability. We analyze the interplay between weak interactions and convection, the so-called convective URCA process, during the simmering phase of Type Ia supernovae (SNe Ia) progenitors and its effects on the physical and chemical properties at the explosion epoch. At variance with previous studies, we find that the convective core powered by the carbon burning remains confined within the 21(Ne,F) URCA shell. As a result, a much larger amount of carbon has to be consumed before the explosion that eventually occurs at larger density than previously estimated. In addition, we find that the extension of the convective core and its average neutronization depend on the the WD progenitor’s initial metallicity. For the average neutronization in the convective core at the explosion epoch, we obtain η ¯ exp = ( 1.094 ± 0.143 ) × 10−3 + (9.168 ± 0.677) × 10−2 × Z. Outside the convective core, the neutronization is instead determined by the initial amount of C + N + O in the progenitor star. Since S, Ca, Cr, and Mn, the elements usually exploited to evaluate the pre-explosive neutronization, are mainly produced outside the heavily neutronized core, the problem of too high metallicity estimated for the progenitors of the historical Tycho and Kepler SNe Ia remains unsolved.

Exploring the Carbon Simmering Phase: Reaction Rates, Mixing, and the Convective Urca Process

Abstract The neutron excess at the time of explosion provides a powerful discriminant among models of Type Ia supernovae. Recent calculations of the carbon simmering phase in single degenerate progenitors have disagreed about the final neutron excess. We find that the treatment of mixing in convection zones likely contributes to the difference. We demonstrate that in Modules for Experiments in Stellar Astrophysics models, heating from exothermic weak reactions plays a significant role in raising the temperature of the white dwarf. This emphasizes the important role that the convective Urca process plays during simmering. We briefly summarize the shortcomings of current models during this phase. Ultimately, we do not pinpoint the difference between the results reported in the literature, but show that the results are consistent with different net energetics of the convective Urca process. This problem serves as an important motivation for the development of models of the convective Urca process suitable for incorporation into stellar evolution codes.

Hybrid C–O–Ne white dwarfs as progenitors of Type Ia supernovae: dependence on Urca process and mixing assumptions

When carbon is ignited off-centre in a CO core of a super-asymptotic giant branch star, its burning in a convective shell tends to propagate to the centre. Whether the C flame will actually be able to reach the centre depends on the efficiency of extra mixing beneath the C convective shell. Whereas thermohaline mixing is too inefficient to interfere with the C-flame propagation, convective boundary mixing can prevent the C burning from reaching the centre. As a result, a C–O–Ne white dwarf (WD) is formed, after the star has lost its envelope. Such a ‘hybrid’ WD has a small CO core surrounded by a thick ONe zone. In our 1D stellar evolution computations, the hybrid WD is allowed to accrete C-rich material, as if it were in a close binary system and accreted H-rich material from its companion with a sufficiently high rate at which the accreted H would be processed into He under stationary conditions, assuming that He could then be transformed into C. When the mass of the accreting WD approaches the Chandrasekhar limit, we find a series of convective Urca shell flashes associated with high abundances of 23Na and 25Mg. They are followed by off-centre C ignition leading to convection that occupies almost the entire star. To model the Urca processes, we use the most recent well-resolved data for their reaction and neutrino-energy loss rates. Because of the emphasized uncertainty of the convective Urca process in our hybrid WD models of Type Ia supernova (SN Ia) progenitors, we consider a number of their potentially possible alternative instances for different mixing assumptions, all of which reach a phase of explosive C ignition, either off or in the centre. Our hybrid SN Ia progenitor models have much lower C-to-O abundance ratios at the moment of the explosive C ignition than their pure CO counterparts, which may explain the observed diversity of the SNe Ia.

EVOLUTION OF PROGENITORS FOR ELECTRON CAPTURE SUPERNOVAE

We provide progenitor models for electron capture supernovae (ECSNe) with detailed evolutionary calculation. We include minor electron capture nuclei using a large nuclear reaction network with updated reaction rates. For electron captures, the Coulomb correction on the rates is treated and contribution of neutron-rich nuclei is taken into account in the nuclear statistical equilibrium (NSE) composition. We calculate the evolution of the most massive super asymptotic giant branch stars and show that these stars undergo off-center carbon burnings and form ONe cores at the center. These cores get heavier up to the critical mass of 1.367 Msun and keep contracting even after the initiation of O+Ne deflagration. Though inclusion of minor electron capture nuclei causes convective URCA process at the contraction phase, such process will have minor effect on the evolution. On the other hand, electron captures by neutron-rich nuclei in the NSE region have more significant effect. Also, we discuss the uniqueness of the critical core mass for ECSNe and the effect of mass loss on the plausibility of our models for ECSN progenitors.

SIMPLIFIED HYDROSTATIC CARBON BURNING IN WHITE DWARF INTERIORS

We introduce two simplified nuclear networks that can be used in hydrostatic carbon burning reactions occurring in white dwarf interiors. They model the relevant nuclear reactions in carbon-oxygen white dwarfs (COWDs) approaching ignition in Type Ia supernova (SN Ia) progenitors, including the effects of the main e-captures and \beta-decays that drive the convective Urca process. They are based on studies of a detailed nuclear network compiled by the authors and are defined by approximate sets of differential equations whose derivations are included in the text. The first network, N1, provides a good first order estimation of the distribution of ashes and it also provides a simple picture of the main reactions occurring during this phase of evolution. The second network, N2, is a more refined version of N1 and can reproduce the evolution of the main physical properties of the full network to the 5% level. We compare the evolution of the mole fraction of the relevant nuclei, the neutron excess, the photon energy generation and the neutrino losses between both simplified networks and the detailed reaction network in a fixed temperature and density parcel of gas.

The nuclear diversity of Type Ia supernova explosions

While Type Ia supernovae (SNe Ia) have been successfully used as cosmological distance candles, there is a large diversity of explosion properties that is presently not understood, nor how this diversity is linked to the properties of their progenitors. Here we review the present status of our understanding of SN Ia progenitors, the main classes of progenitor models and recent observational constraints, the origin of the lightcurve peak – lightcurve width relation (the ‘Phillips relation’) and its metallicity dependence, and illustrate how different evolutionary histories produce a diversity of pre-supernova properties. We particularly emphasize the importance of the final carbon simmering phase preceding the final runaway and the role of the convective Urca process which will alter the immediate pre-supernova conditions in the exploding white dwarf.

Neutronization during Type Ia Supernova Simmering

Prior to the incineration of a white dwarf (WD) that makes a Type Ia supernova (SN Ia), the star simmers for ~1000 yr in a convecting, carbon-burning region. We have found that weak interactions during this time increase the neutron excess by an amount that depends on the total quantity of carbon burned prior to the explosion. This contribution is in addition to the metallicity (Z) dependent neutronization through the 22Ne abundance (as studied by Timmes, Brown, & Truran). The main consequence is that we expect a floor to the level of neutronization that dominates over the metallicity contribution when Z/Z☉ 2/3, and it can be important for even larger metallicities if substantial energy is lost to neutrinos via the convective Urca process. This would mask any correlations between SN Ia properties and galactic environments at low metallicities. In addition, we show that recent observations of the dependences of SNe Ia on galactic environments make it clear that metallicity alone cannot provide for the full observed diversity of events.

The Convective Urca Process with Implicit Two‐dimensional Hydrodynamics

Consideration of the role of the convective flux in the thermodymics of the convective Urca neutrino loss process in degenerate, convective, quasi-static, carbon-burning cores shows that the convective Urca process slows down the convective current around the Urca-shell but, unlike the "thermal" Urca process, does not reduce the entropy or temperature for a given convective volume. Here we demonstrate these effects with two-dimensional numerical hydrodynamical calculations. These two-dimensional implicit hydrodynamics calculations invoke an artificial speeding up of the nuclear and weak rates. They should thus be regarded as indicative but still qualitative. We find that, compared to a case with no Urca-active nuclei, the case with Urca effects leads to a higher entropy in the convective core because the energy released by nuclear burning is confined to a smaller volume by the effective boundary at the Urca shell. All else being equal, this will tend to accelerate the progression to dynamical runaway. We discuss the open issues regarding the impact of the convective Urca process on the evolution to the "smoldering phase" and then to dynamical runaway.

The convective Urca process

One possible fate of an accreting white dwarf is explosion in a type Ia supernova. However, the route to the thermonuclear runaway has always been uncertain owing to the lack of a convective model consistent with the Urca process. We derive a formalism for convective motions involving two radial flows. This formalism provides a framework for convective models that guarantees self-consistency for chemistry and energy budget, allows time-dependence and describes the interaction of convective motions with the global contraction or expansion of the star. In the one-stream limit, we reproduce several already existing convective models and allow them to treat chemistry. We also suggest as a model easy to implement in a stellar evolution code. We apply this formalism to convective Urca cores in Chandrasekhar mass white dwarfs. We stress that in degenerate matter, nuclear reactions that change the number of electrons strongly influence the convective velocities. We point out the sensitivity of the energy budget on the mixing. We illustrate our model by computing {\it stationary} convective cores with Urca nuclei. We show that even a very small mass fraction of Urca nuclei ($10^{-8}$) strongly influences the convective velocities. Finally, we present preliminary computations of the late evolution of a close to Chandrasekhar mass C+O white dwarf including the convective Urca process.

A two-stream formalism for the convective Urca process

We derive a new formalism for convective motions involving two radial flows. This formalism provides a framework for convective models that guarantees consistency for the chemistry and the energy budget in the flows, allows time dependence and accounts for the interaction of the convective motions with the global contraction or expansion of the star. In the one-stream limit, the formalism reproduces several existing convective models and allows them to treat the chemistry in the flows. We suggest a version of the formalism that can be implemented easily in a stellar evolution code. We then apply the formalism to convective Urca cores in Chandrasekhar-mass white dwarfs and compare it to previous studies. We demonstrate that, in degenerate matter, nuclear reactions which change the number of electrons strongly influence the convective velocities, and we show that the net energy budget is sensitive to the mixing. We illustrate our model by computing stationary convective cores with Urca nuclei. Even a very small mass fraction of Urca nuclei (as little as 10 -8 ) strongly influences the convective velocities. We conclude that the proper modelling of the Urca process is essential for determining the ignition conditions for the thermonuclear runaway in Chandrasekhar-mass white dwarfs.

The Role of Kinetic Energy Flux in the Convective Urca Process

The previous analysis of the convective Urca neutrino loss process in degenerate, convective, quasi-static, carbon-burning cores by Barkat and Wheeler omitted specific consideration of the role of the kinetic energy flux. The arguments of Barkat and Wheeler that steady-state composition gradients exist are correct, but chemical equilibrium does not result in net cooling. Barkat and Wheeler included a work term that effectively removed energy from the total energy budget that could only have come from the kinetic energy, which must remain positive. Consideration of the kinetic energy in the thermodynamics of the convective Urca process shows that the convective Urca neutrinos reduce the rate of increase of entropy that would otherwise be associated with the input of nuclear energy and slow down the convective current, but, unlike the thermal Urca process do not reduce the entropy or temperature.

Thermonuclear Ignition and Runaway in Type Ia Supernovaea

Recent theoretical models of type Ia SN explosions are examined analytically. Qualitatively, the process described involves mass transfer to a C/O white dwarf in a binary system, bringing it near the Chandrasekhar limit and initiating runaway carbon burning. Particular attention is given to (1) carbon ignition and the convective Urca process, which acts to delay runaway, (2) detonation and deflagration models of SN dynamics, and (3) the application of observational data (light curves and spectra) to place limits on dynamical models. It is shown that most deflagration-type models do not give good agreement with spectroscopic observations, while those that do fail to explain density profiles deduced from other observations. Further research on damped-detonation models is recommended. 32 refs.

The convective URCA mechanism

The convective Urca process of Paczynski (1972, 1973) as applied to carbon burning in degenerate carbon/oxygen cores is reexamined. On time scales long when compared with the convective circulation time but short compared with the evolutionary time scale, a quasi-steady exists in which the convective currents impose a mu dN term that precisely cancels the local heating described by Bruenn. The results is that the convective Urca process must lead to net cooling and to a control of degenerate carbon burning, at least temporarily. The subsequent evolution is complex. Stability arguments show that the evolution depends on the precise treatment of the recession of the boundary of the convective core and on the advance of the Urca shell due to the contraction of the whole core. The fundamental conclusion is that if carbon ignites quasi-statically at a density above the Na-23 threshold at rho = 1.714 x 10 to the 9th g/cu cm, the convective Urca process will be a major effect which cannot be ignored in discussions of degenerate carbon burning, specifically in models for Type Ia supernovae. The limitations of previous treatments are outlined, and possible evolutionary outcomes are discussed. 30 refs.

More on carbon burning in electron-degenerate matter - Within single stars of intermediate mass and within accreting white dwarfs

Carbon burning in highly electron-degenerate matter is followed in two astrophysically interesting cases; an intermediate-mass star that, on the asymptotic giant branch, has developed a large carbon-oxygen core; and an accreting white dwarf composed primarily of carbon and oxygen. Calculations are continued until heating at the edge of the region within which a dominant Urca pair is assumed to be mixed uniformly makes it impossible to achieve coincidence between this edge and the edge of an associated convectively unstable region within which convection is expected to promote mixing. Thereafter, global heating occurs at a rate that exceeds the rate of cooling by neutrino losses, and it does not appear that the convective Urca process can prevent a runaway toward a dynamic event. The major effect of the convective Urca process is thus only to delay the dynamic event until core densities are somewhat larger than they would have been had the process been negelected. One consequence of this delay is that the average energy of potentially detectable neutrinos resulting from electron capture is larger than would otherwise have been the case.

Thermal oscillations during carbon burning in an electron-degenerate stellar core

view Abstract Citations (40) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal oscillations during carbon burning in an electron-degenerate stellar core. Iben, I., Jr. Abstract The response of the electron-degenerate carbon-oxygen core of an intermediate-mass star (3 to 8 solar masses) to the initiation of carbon-burning reactions and to the resulting Urca neutrino losses is investigated. It is shown that the core oscillates thermally between two unstable states: (1) a heating phase, during which heating by nuclear reactions and by the release of Fermi energy in response to electron capture exceeds cooling by Urca neutrinos, carries the core from a low-temperature to a high-temperature state and (2) a cooling phase, during which the reverse is the case, carries the core back to the low-temperature state. The time dependence of stellar structure variables and energetics during the thermal oscillations is examined, along with stellar structure and nuclear reaction rates near the beginning and the end of a heating phase, the nature of the convective Urca process at its peak, and the behavior of the convective core when the central density approaches the threshold for electron capture on Ne-21. Publication: The Astrophysical Journal Pub Date: December 1978 DOI: 10.1086/156680 Bibcode: 1978ApJ...226..996I Keywords: Carbon; Neutrinos; Nuclear Fusion; Stellar Mass; Stellar Models; Abundance; Carbon Isotopes; Decay Rates; Electron Capture; Oxygen Isotopes; Time Dependence; Astrophysics; Convection:Stellar Interiors; Neutrinos:Stellar Interiors; Nucleosynthesis:Stellar Interiors; Oscillations:Stellar Interiors; Stellar Evolution:Carbon Burning; Stellar Interiors: Urca Processes full text sources ADS |

The URCA process in dense stellar interiors

view Abstract Citations (15) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Urca process in dense stellar interiors. Couch, R. G. ; Loumos, G. L. Abstract An investigation has been made to determine the effect which the Coulomb interaction energy between an ion and the background plasma has on the Urca process in dense stellar interiors. It was found that the change in interaction energy which occurs when an ion changes its charge by one unit can increase significantly the threshold for electron capture. The exact threshold depends on the particular ionic configuration which exists at the time the beta-interaction occurs. There thus exists a distribution of possible thresholds about some mean value. Monte Carlo techniques were used to investigate this distribution. The effect on the energy generation rate of the Urca process was found to be small. The thermal effects of the Urca process are discussed, and calculations are presented which indicate several interesting properties of the convective Urca process. Publication: The Astrophysical Journal Pub Date: December 1974 DOI: 10.1086/153255 Bibcode: 1974ApJ...194..385C Keywords: Energy Dissipation; Neutrinos; Nuclear Reactions; Stellar Evolution; Stellar Interiors; Stellar Structure; Thermal Energy; Astronomical Models; Convective Heat Transfer; Coulomb Collisions; Electron Capture; Energy Distribution; Ionic Reactions; Plasma-Particle Interactions; Astrophysics full text sources ADS |

On the Thermal Properties of the Convective URCA Process

view Abstract Citations (17) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Thermal Properties of the Convective URCA Process Couch, Richard G. ; Arnett, David W. Abstract Bruenn's analysis of heating due to Urca processes during degenerate carbon burning is supplemented by consideration of the additional work necessary to maintain the convective flow. We conclude that at least initially Urca processes (especially the 23Na-23Ne pair) do stabilize degenerate carbon burning; the net effect of the convective Urca processes is cooling. Subject headings: nucleosynthesis - stellar evolution - supernovae Publication: The Astrophysical Journal Pub Date: December 1974 DOI: 10.1086/153272 Bibcode: 1974ApJ...194..537C full text sources ADS |

Neutron-Star Formation in the Carbon-Detonation Supernova

Neutrino losses, such as those driven by the convective Urca process, may affect the evolution of stars in the mass range from 4 to 8 solar masses so as to lead to collapse of their degenerate carbon/oxygen cores. A corresponding hydrodynamic model is computed which leads to the formation of a 1.3 to 1.4 solar mass neutron star with the expulsion of a small fraction of the mass, about 0.l solar mass at about 20,000 km/sec into the overlying hydrogen envelope. This sets the stage for the Ostriker-Gunn mechanism in which Type II supernovae and pulsars are formed.