The Experts below are selected from a list of 258 Experts worldwide ranked by ideXlab platform

F X Timmes - One of the best experts on this subject based on the ideXlab platform.

  • on Carbon Burning in super asymptotic giant branch stars
    The Astrophysical Journal, 2015
    Co-Authors: R Farmer, C E Fields, F X Timmes
    Abstract:

    We explore the detailed and broad properties of Carbon Burning in Super Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models. The location of first Carbon ignition, quenching location of the Carbon-Burning flames and flashes, angular frequency of the Carbon core, and Carbon core mass are studied as a function of the zero-age main sequence (ZAMS) mass, initial rotation rate, and mixing parameters such as convective overshoot, semiconvection, thermohaline, and angular momentum transport. In general terms, we find that these properties of Carbon Burning in SAGB models are not a strong function of the initial rotation profile, but are a sensitive function of the overshoot parameter. We quasi-analytically derive an approximate ignition density, ρign ≈ 2.1 × 106 g cm−3, to predict the location of first Carbon ignition in models that ignite Carbon off-center. We also find that overshoot moves the ZAMS mass boundaries where off-center Carbon ignition occurs at a nearly uniform rate of ΔMZAMS/Δfov ≈ 1.6 . For zero overshoot, fov = 0.0, our models in the ZAMS mass range ≈8.9–11 show off-center Carbon ignition. For canonical amounts of overshooting, fov = 0.016, the off-center Carbon ignition range shifts to ≈7.2–8.8 . Only systems with fov ≥ 0.01 and ZAMS mass ≈7.2–8.0 show Carbon Burning is quenched a significant distance from the center. These results suggest a careful assessment of overshoot modeling approximations on claims that Carbon Burning quenches an appreciable distance from the center of the Carbon core.

Claudio Ritossa - One of the best experts on this subject based on the ideXlab platform.

  • on the evolution of stars that form electron degenerate cores processed by Carbon Burning v shell convection sustained by helium Burning transient neon Burning dredge out urca cooling and other properties of an 11 m population i model star
    The Astrophysical Journal, 1999
    Co-Authors: E Garciaberro, Claudio Ritossa, Icko Iben
    Abstract:

    A stellar model of mass 11 M☉ and Population I composition is evolved from the hydrogen-Burning main sequence through the core Carbon-Burning phase. In contrast with 9, 10, and 10.5 M☉ models studied in earlier papers of this series, Carbon Burning is ignited at the center of the 11 M☉ model. Like the 10.5 M☉ model, the 11 M☉ model experiences a dredge-out episode at the end of the Carbon-Burning phase. At the beginning of this episode, a semiconvective zone forms at the base of the hydrogen-rich envelope and carries hydrogen inward in mass toward the outer edge of a fully convective zone that is sustained by helium Burning at its base. Hydrogen diffuses into the helium-rich convective zone untila hydrogen shell flash occurs. Helium Burning dies out and the outer edge of the convective layer,sustained by fluxes due to hydrogen Burning, extends outward in mass through hydrogen-rich material, mixing freshly synthesized nuclei outward. Then, hydrogen Burning dies out and the outer edge of the convective shell, now sustained primarily by fluxes due to the release of gravothermal energy, moves outward until it reaches the inner edge of the convective envelope. Freshly synthesized material is then convected to the surface. Mixing during the final phase of homogenization in the convective envelope is maintained by fluxes due to the release of gravothermal energy. At the end of the dredge-out phase, the surface nitrogen abundance has decreased and the C/N ratio has changed from less than unity to larger than unity, showing that mixing has extended into regions where helium Burning has manufactured substantial quantities of 12C and destroyed 14N. Prior to the dredge-out phase, neon Burning is narrowly averted, and, after the dredge-out phase, neutrino losses due to electron capture and decay reactions between A=25 and A=23 isotopes in and above convective Urca shells cool the inner portions of the electron-degenerate oxygen-neon (ONe) core. Ultimately, the model becomes a thermally pulsing super-asymptotic giant branch (TPSAGB) star with an ONe core of mass ~1.368 M☉. Hydrogen and helium Burning over a period of ~1.4×104 yr of TPSAGB evolution add a Carbon-oxygen layer of mass ~0.014 M☉ to the electron-degenerate core. Then, electron captures on products of Carbon Burning lead to the collapse of the core into a neutron star and expulsion of the envelope in a weak Type II supernova explosion. The ratio of helium to hydrogen in the ejecta is approximately twice solar.

  • On the Evolution of Stars that Form Electron-degenerate Cores Processed by Carbon Burning. IV. Outward Mixing During the Second Dredge-up Phase and Other Properties of a 10.5 M☉ Model Star*
    The Astrophysical Journal, 1997
    Co-Authors: Icko Iben, Claudio Ritossa, Enrique García-berro
    Abstract:

    A 10.5 M☉ model of Population I composition is evolved from the main sequence through the core Carbon-Burning phase. As in 9 and 10 M☉ models studied in earlier papers of this series, Carbon is ignited off center, but more Carbon flashes occur before hydrogen is reignited and Carbon Burning dies out. Beginning with the second Carbon flash, a Carbon-Burning flame propagates to the stellar center. The flame is divided into two parts by the flame "front" which is defined to coincide with the base of an associated convective shell. Ahead of the front is a "precursor" flame in which nuclear energy is converted into heat and the work of expansion at a rate comparable to the rate of release of nuclear energy in the convective shell. The width in mass of the precursor flame relative to the distance of the front from the center varies from ~0.01 when the front is at ~0.04 M☉ to ~1 as the front reaches ~10-7 M☉. Toward the end of the Carbon-Burning phase, the 10.5 M☉ model mixes helium- and Carbon-rich matter with hydrogen-rich matter, but, in contrast to the other models, mixing does not occur across the base of a hydrogen-rich convective envelope which moves steadily inward through the helium-rich layer below it. Rather, a convective shell extending outward from the Burning layers meets with the inward moving base of the convective envelope; then, hydrogen diffuses inward convectively and ignites as Carbon and helium are diffusing outward through a region of variable composition. The luminous flux that forces convective motions at the base of the convective region is contributed to by Carbon-Burning, gravothermal, and helium-Burning energy. The mixing process is modeled with a diffusion equation in which the diffusion coefficient is assumed to be a fraction of the local pressure scale height times a convective velocity that is estimated in the mixing-length approximation. When Carbon Burning dies out, the electron-degenerate core consists of an inner oxygen-neon (ONe) part of mass MONe ~ 1.263 M☉ in which 20Ne is more abundant than 12C, and an outer Carbon-oxygen (CO) layer of mass ΔMCO ~ 0.0065 M☉ in which the reverse is true. Over most of the outer ~0.005 M☉ of the CO layer, the abundances of all neon isotopes is much less than 10-4 by number, and the number abundance of 25Mg and of other neutron-rich isotopes is equal to the total abundance of CNO elements in the initial main-sequence model. Final interior abundance characteristics in the deep interior of the ONe part of the core are very similar to those in the other models, the principal difference being that the maximum abundance by mass of 12C in the 10.5 M☉ model (X12 ~ 0.006) is significantly smaller than in the 10 M☉ model (X12 ~ 0.012) and in the 9 M☉ model (X12 ~ 0.048). This has ramifications for the question of the accretion-induced collapse of massive white dwarfs in cataclysmic variables and ultrasoft X-ray sources.

  • on the evolution of stars that form electron degenerate cores processed by Carbon Burning iii the inward propagation of a Carbon Burning flame and other properties of a 9 m model star
    The Astrophysical Journal, 1997
    Co-Authors: E Garciaberro, Claudio Ritossa, Icko Iben
    Abstract:

    A 9 M☉ stellar model of Population I composition is evolved from the hydrogen-Burning main sequence to the thermally pulsing "super" asymptotic giant branch stage, where it has an electron-degenerate core composed of an inner oxygen-neon (ONe) part of mass ~1.066 M☉ and an outer Carbon-oxygen (CO) layer of mass ~0.05 M☉ and is experiencing thermal pulses driven by helium-Burning thermonuclear flashes. The Carbon-Burning phase of the 9 M☉ model is in many respects similar to, but differs importantly from that of a 10 M☉ model studied earlier. In both cases, Carbon is ignited off center, and a series of Carbon flashes accompanied by a convective shell occur. In contrast to the 10 M☉ model, the 9 M☉ model experiences the second dredge-up phenomenon (the penetration of the base of the hydrogen-rich convective envelope inward into helium- and Carbon-rich material) near the beginning rather than near the end of the Carbon-Burning phase. The first Carbon-Burning flash causes helium Burning to shut down and the release of gravothermal energy (compressional and thermal energy) between the helium-Carbon discontinuity and the base of the convective envelope plays a dominant role in the dredge-up event. Beginning with the third Carbon-Burning shell flash, the "flame front," defined as being coincident with the base of the convective shell, propagates inward with a speed close to theoretical predictions that relate flame speed to local thermodynamic, opacity, and energy-generation rate characteristics. Ahead of the inward moving front, most of the nuclear energy released in a "precursor flame" goes into heating and expanding matter. As the precursor flame moves toward the center, its radial thickness decreases and, to follow the progress of the front with standard techniques, both the spatial grid size and the time step must be continually decreased. Following the front gives one the opportunity to ponder Zeno's paradox, which is averted because the thickness of the precursor flame remains finite. On reaching the center, the Carbon-Burning flame reverses direction and continues moving outward until it is within ~0.03 M☉ of the helium-Burning shell. After Carbon Burning is completed,12C remains at a finite abundance throughout the electron-degenerate core of mass ~1.116 M☉ and is more abundant than 20Ne in the outer ~0.05 M☉ of this core. Over most of the ONe interior of both the 9 and 10 M☉ models,23Na is more abundant than 24Mg, but the maximum 12C abundance in the 9 M☉ model ONe interior (X[12C] ~ 0.048) is significantly larger than in the 10 M☉ model (X[12C] ~ 0.012). For an ONe white dwarf that accretes enough matter to reach the Chandrasekhar limiting mass, this may make the difference between total explosive disruption (large 12C abundance) and collapse to neutron-star dimensions (small 12C abundance). The abundances in the CO part of the core have relevance for understanding the abundances in the ejecta of classical novae produced by massive ONe white dwarfs in close binaries. In the outer ~0.014 M☉ of the CO part of the core, the abundances of all neon isotopes are much less than solar, and 25Mg and the neutron-rich isotopes made during the formation of 25Mg are at a total abundance equal to the initial abundance of CNO elements in the model. As in the 10 M☉ case, thermal pulses occasioned by helium shell flashes begin after hydrogen is reignited and the Carbon-Burning luminosity drops below ~100 L☉. The time between pulses is ~400 yr, roughly twice as large as in the 10 M☉ model. After the ejection of the hydrogen-rich envelope as a planetary nebula, the remnant of the 9 M☉ model is expected to evolve into a white dwarf of mass ~1.15 M☉, the outer ~0.08 M☉ of which is composed of Carbon and oxygen.

Bo Wang - One of the best experts on this subject based on the ideXlab platform.

  • off centre Carbon Burning in he accreting Carbon oxygen white dwarfs
    Monthly Notices of the Royal Astronomical Society, 2019
    Co-Authors: Chengyuan Wu, Bo Wang
    Abstract:

    The Carbon-oxygen white dwarf (CO WD) + He star channel is one of the promising ways for producing type Ia supernovae (SNe Ia) with short delay times. Recent studies found that Carbon under the He shell can be ignited if the mass-accretion rate of CO WD is higher than a critical rate (about 2 x 10(-6)M(circle dot) yr(-1)), triggering an inwardly propagating Carbon flame. Previous studies usually supposed that the off-centre Carbon flame would reach the centre, resulting in the formation of an oxygen-neon (ONe) WD that will collapse into a neutron star. However, the process of off-centre Carbon Burning is not well studied. This may result in some uncertainties on the final fates of CO WDs. By employing MESA, we simulated the long-term evolution of off-centre Carbon Burning in He-accreting CO WDs. We found that the inwardly propagating Carbon flame transforms the CO WDs into OSi cores directly but not ONe cores owing to the high temperature of the Burning front. We suggest that the final fates of the CO WDs may be OSi WDs under the conditions of off-centre Carbon Burning, or explode as iron-core-collapse SNe if the mass accretion continues. We also found that the mass fractions of silicon in the OSi cores are sensitive to the mass-accretion rates.

  • The outcomes of Carbon-oxygen white dwarfs accreting CO-rich material
    Monthly Notices of the Royal Astronomical Society, 2018
    Co-Authors: Chengyuan Wu, Bo Wang
    Abstract:

    The double-degenerate model, involving the merger of double Carbon-oxygen white dwarfs (CO WDs), is one of the two classic models for the progenitors of type Ia supernovae (SNe Ia). Previous studies suggested that off-centre Carbon Burning would occur if the mass-accretion rate (Macc) is relatively high during the merging process, leading to the formation of oxygen-neon (ONe) cores that may collapse into neutron stars. However, the off-centre Carbon Burning is still incompletely understood, especially when the inwardly propagating Burning wave reaches the centre. In this paper, we aim to investigate the propagating characteristics of Burning waves and the subsequently evolutionary outcomes of these CO cores. We simulated the long-term evolution of CO WDs that accrete CO-rich material by employing the stellar evolution code MESA on the basis of the thick-disc assumption. We found that the final outcomes of CO WDs strongly depend on Macc (Msun/yr) based on the thick-disc assumption, which can be divided into four regions: (1) explosive Carbon ignition in the centre, then SNe Ia (Macc 1.05*10^-5). Our results indicate that the final fates of double CO WD mergers are strongly dependent on the merging processes (e.g. slow merger, fast merger, composite merger, violent merger, etc.).

  • Mass retention efficiencies of He accretion onto Carbon-oxygen white dwarfs and type Ia supernovae
    Astronomy and Astrophysics, 2017
    Co-Authors: C. Y. Wu, Bo Wang
    Abstract:

    Context. Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and galactic chemical evolution. They are thought to be thermonuclear explosions of Carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar mass limit in binaries. Previous studies have suggested that He novae may be progenitor candidates of SNe Ia. However, the mass retention efficiencies during He nova outbursts are still uncertain. Aims. In this article, we aim to study the mass retention efficiencies of He nova outbursts and to investigate whether SNe Ia can be produced through He nova outbursts. Methods. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics, we simulated a series of multicycle He-layer flashes, in which the initial WD masses range from 0.7 to 1.35 M ⊙ with various accretion rates. Results. We obtained the mass retention efficiencies of He nova outbursts for various initial WD masses, which can be used in the binary population synthesis studies. In our simulations, He nova outbursts can increase the mass of the WD to the Chandrasekar mass limit and the explosive Carbon Burning can be triggered in the center of the WD; this suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass retention efficiencies in the present work are lower than those of previous studies, which leads to a lower birthrates of SNe Ia through the WD + He star channel. Furthermore, we obtained the elemental abundances distribution at the moment of explosive Carbon Burning, which can be used as the initial input parameters in studying explosion models of SNe Ia.

Icko Iben - One of the best experts on this subject based on the ideXlab platform.

  • Carbon-Oxygen White Dwarfs Accreting CO-rich Matter. I. A Comparison between Rotating and Nonrotating Models
    The Astrophysical Journal, 2003
    Co-Authors: Luciano Piersanti, Icko Iben, Simona Gagliardi, Amedeo Tornambe
    Abstract:

    We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for Carbon Burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 M☉. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as ~1 rad s-1, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive Carbon Burning at the center so that a Type Ia supernova event is produced.

  • on the evolution of stars that form electron degenerate cores processed by Carbon Burning v shell convection sustained by helium Burning transient neon Burning dredge out urca cooling and other properties of an 11 m population i model star
    The Astrophysical Journal, 1999
    Co-Authors: E Garciaberro, Claudio Ritossa, Icko Iben
    Abstract:

    A stellar model of mass 11 M☉ and Population I composition is evolved from the hydrogen-Burning main sequence through the core Carbon-Burning phase. In contrast with 9, 10, and 10.5 M☉ models studied in earlier papers of this series, Carbon Burning is ignited at the center of the 11 M☉ model. Like the 10.5 M☉ model, the 11 M☉ model experiences a dredge-out episode at the end of the Carbon-Burning phase. At the beginning of this episode, a semiconvective zone forms at the base of the hydrogen-rich envelope and carries hydrogen inward in mass toward the outer edge of a fully convective zone that is sustained by helium Burning at its base. Hydrogen diffuses into the helium-rich convective zone untila hydrogen shell flash occurs. Helium Burning dies out and the outer edge of the convective layer,sustained by fluxes due to hydrogen Burning, extends outward in mass through hydrogen-rich material, mixing freshly synthesized nuclei outward. Then, hydrogen Burning dies out and the outer edge of the convective shell, now sustained primarily by fluxes due to the release of gravothermal energy, moves outward until it reaches the inner edge of the convective envelope. Freshly synthesized material is then convected to the surface. Mixing during the final phase of homogenization in the convective envelope is maintained by fluxes due to the release of gravothermal energy. At the end of the dredge-out phase, the surface nitrogen abundance has decreased and the C/N ratio has changed from less than unity to larger than unity, showing that mixing has extended into regions where helium Burning has manufactured substantial quantities of 12C and destroyed 14N. Prior to the dredge-out phase, neon Burning is narrowly averted, and, after the dredge-out phase, neutrino losses due to electron capture and decay reactions between A=25 and A=23 isotopes in and above convective Urca shells cool the inner portions of the electron-degenerate oxygen-neon (ONe) core. Ultimately, the model becomes a thermally pulsing super-asymptotic giant branch (TPSAGB) star with an ONe core of mass ~1.368 M☉. Hydrogen and helium Burning over a period of ~1.4×104 yr of TPSAGB evolution add a Carbon-oxygen layer of mass ~0.014 M☉ to the electron-degenerate core. Then, electron captures on products of Carbon Burning lead to the collapse of the core into a neutron star and expulsion of the envelope in a weak Type II supernova explosion. The ratio of helium to hydrogen in the ejecta is approximately twice solar.

  • On the Evolution of Stars that Form Electron-degenerate Cores Processed by Carbon Burning. IV. Outward Mixing During the Second Dredge-up Phase and Other Properties of a 10.5 M☉ Model Star*
    The Astrophysical Journal, 1997
    Co-Authors: Icko Iben, Claudio Ritossa, Enrique García-berro
    Abstract:

    A 10.5 M☉ model of Population I composition is evolved from the main sequence through the core Carbon-Burning phase. As in 9 and 10 M☉ models studied in earlier papers of this series, Carbon is ignited off center, but more Carbon flashes occur before hydrogen is reignited and Carbon Burning dies out. Beginning with the second Carbon flash, a Carbon-Burning flame propagates to the stellar center. The flame is divided into two parts by the flame "front" which is defined to coincide with the base of an associated convective shell. Ahead of the front is a "precursor" flame in which nuclear energy is converted into heat and the work of expansion at a rate comparable to the rate of release of nuclear energy in the convective shell. The width in mass of the precursor flame relative to the distance of the front from the center varies from ~0.01 when the front is at ~0.04 M☉ to ~1 as the front reaches ~10-7 M☉. Toward the end of the Carbon-Burning phase, the 10.5 M☉ model mixes helium- and Carbon-rich matter with hydrogen-rich matter, but, in contrast to the other models, mixing does not occur across the base of a hydrogen-rich convective envelope which moves steadily inward through the helium-rich layer below it. Rather, a convective shell extending outward from the Burning layers meets with the inward moving base of the convective envelope; then, hydrogen diffuses inward convectively and ignites as Carbon and helium are diffusing outward through a region of variable composition. The luminous flux that forces convective motions at the base of the convective region is contributed to by Carbon-Burning, gravothermal, and helium-Burning energy. The mixing process is modeled with a diffusion equation in which the diffusion coefficient is assumed to be a fraction of the local pressure scale height times a convective velocity that is estimated in the mixing-length approximation. When Carbon Burning dies out, the electron-degenerate core consists of an inner oxygen-neon (ONe) part of mass MONe ~ 1.263 M☉ in which 20Ne is more abundant than 12C, and an outer Carbon-oxygen (CO) layer of mass ΔMCO ~ 0.0065 M☉ in which the reverse is true. Over most of the outer ~0.005 M☉ of the CO layer, the abundances of all neon isotopes is much less than 10-4 by number, and the number abundance of 25Mg and of other neutron-rich isotopes is equal to the total abundance of CNO elements in the initial main-sequence model. Final interior abundance characteristics in the deep interior of the ONe part of the core are very similar to those in the other models, the principal difference being that the maximum abundance by mass of 12C in the 10.5 M☉ model (X12 ~ 0.006) is significantly smaller than in the 10 M☉ model (X12 ~ 0.012) and in the 9 M☉ model (X12 ~ 0.048). This has ramifications for the question of the accretion-induced collapse of massive white dwarfs in cataclysmic variables and ultrasoft X-ray sources.

  • on the evolution of stars that form electron degenerate cores processed by Carbon Burning iii the inward propagation of a Carbon Burning flame and other properties of a 9 m model star
    The Astrophysical Journal, 1997
    Co-Authors: E Garciaberro, Claudio Ritossa, Icko Iben
    Abstract:

    A 9 M☉ stellar model of Population I composition is evolved from the hydrogen-Burning main sequence to the thermally pulsing "super" asymptotic giant branch stage, where it has an electron-degenerate core composed of an inner oxygen-neon (ONe) part of mass ~1.066 M☉ and an outer Carbon-oxygen (CO) layer of mass ~0.05 M☉ and is experiencing thermal pulses driven by helium-Burning thermonuclear flashes. The Carbon-Burning phase of the 9 M☉ model is in many respects similar to, but differs importantly from that of a 10 M☉ model studied earlier. In both cases, Carbon is ignited off center, and a series of Carbon flashes accompanied by a convective shell occur. In contrast to the 10 M☉ model, the 9 M☉ model experiences the second dredge-up phenomenon (the penetration of the base of the hydrogen-rich convective envelope inward into helium- and Carbon-rich material) near the beginning rather than near the end of the Carbon-Burning phase. The first Carbon-Burning flash causes helium Burning to shut down and the release of gravothermal energy (compressional and thermal energy) between the helium-Carbon discontinuity and the base of the convective envelope plays a dominant role in the dredge-up event. Beginning with the third Carbon-Burning shell flash, the "flame front," defined as being coincident with the base of the convective shell, propagates inward with a speed close to theoretical predictions that relate flame speed to local thermodynamic, opacity, and energy-generation rate characteristics. Ahead of the inward moving front, most of the nuclear energy released in a "precursor flame" goes into heating and expanding matter. As the precursor flame moves toward the center, its radial thickness decreases and, to follow the progress of the front with standard techniques, both the spatial grid size and the time step must be continually decreased. Following the front gives one the opportunity to ponder Zeno's paradox, which is averted because the thickness of the precursor flame remains finite. On reaching the center, the Carbon-Burning flame reverses direction and continues moving outward until it is within ~0.03 M☉ of the helium-Burning shell. After Carbon Burning is completed,12C remains at a finite abundance throughout the electron-degenerate core of mass ~1.116 M☉ and is more abundant than 20Ne in the outer ~0.05 M☉ of this core. Over most of the ONe interior of both the 9 and 10 M☉ models,23Na is more abundant than 24Mg, but the maximum 12C abundance in the 9 M☉ model ONe interior (X[12C] ~ 0.048) is significantly larger than in the 10 M☉ model (X[12C] ~ 0.012). For an ONe white dwarf that accretes enough matter to reach the Chandrasekhar limiting mass, this may make the difference between total explosive disruption (large 12C abundance) and collapse to neutron-star dimensions (small 12C abundance). The abundances in the CO part of the core have relevance for understanding the abundances in the ejecta of classical novae produced by massive ONe white dwarfs in close binaries. In the outer ~0.014 M☉ of the CO part of the core, the abundances of all neon isotopes are much less than solar, and 25Mg and the neutron-rich isotopes made during the formation of 25Mg are at a total abundance equal to the initial abundance of CNO elements in the model. As in the 10 M☉ case, thermal pulses occasioned by helium shell flashes begin after hydrogen is reignited and the Carbon-Burning luminosity drops below ~100 L☉. The time between pulses is ~400 yr, roughly twice as large as in the 10 M☉ model. After the ejection of the hydrogen-rich envelope as a planetary nebula, the remnant of the 9 M☉ model is expected to evolve into a white dwarf of mass ~1.15 M☉, the outer ~0.08 M☉ of which is composed of Carbon and oxygen.

Amedeo Tornambe - One of the best experts on this subject based on the ideXlab platform.

  • Carbon-Oxygen White Dwarfs Accreting CO-rich Matter. I. A Comparison between Rotating and Nonrotating Models
    The Astrophysical Journal, 2003
    Co-Authors: Luciano Piersanti, Icko Iben, Simona Gagliardi, Amedeo Tornambe
    Abstract:

    We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for Carbon Burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 M☉. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as ~1 rad s-1, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive Carbon Burning at the center so that a Type Ia supernova event is produced.

  • Carbon‐Oxygen White Dwarfs Accreting CO‐rich Matter. I. A Comparison between Rotating and Nonrotating Models
    The Astrophysical Journal, 2003
    Co-Authors: Luciano Piersanti, Icko Iben, Jr., Stefania Gagliardi, Amedeo Tornambe
    Abstract:

    We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for Carbon Burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 Msolar. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as ~1 rad s-1, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive Carbon Burning at the center so that a Type Ia supernova event is produced.