Thermonuclear Instability Research Articles

Remnants of Subdwarf Helium Donor Stars Ejected from Close Binaries with Thermonuclear Supernovae

Abstract Some binary systems composed of a white dwarf (WD) and a hot subdwarf (sdB) helium star will make contact within the helium burning lifetime of the sdB star. The accreted helium on the WD inevitably undergoes a thermonuclear instability, causing a detonation that is expected to transition into the WD core and lead to a thermonuclear supernova (SN) while the donor orbits nearby with high velocity. Motivated by the recent discovery of fast moving objects that occupy unusual locations on the HR diagram, we explore the impact of the thermonuclear SNe on the donors in this specific double detonation scenario. We use MESA to model the binary up to the moment of detonation, then 3D Athena++ to model the hydrodynamic interaction of the SN ejecta with the donor star, calculating the amount of mass that is stripped and the entropy deposited in the deep stellar interior by the strong shock that traverses it. We show that these donor remnants are ejected with velocities primarily set by their orbital speeds: 700–900 km s−1. We model the long-term thermal evolution of remnants by introducing the shock entropy into MESA models. In response to this entropy change, donor remnants expand and brighten for timescales ranging from 106 to 108 yr, giving ample time for these runaway stars to be observed in their inflated state before they leave the galaxy. Even after surface layers are stripped, some donors retain enough mass to resume core helium burning and further delay fading for more than 108 yr.

Analysis of the thermonuclear instability including low-power ICRH minority heating in IGNITOR

The nonlinear thermal balance equation for classical plasma in a toroidal geometry is analytically and numerically investigated including ICRH power. The determination of the equilibrium temperature and the analysis of the stability of the solution are performed by solving the energy balance equation that includes the transport relations obtained by the classical kinetic theory. An estimation of the confinement time is also provided. We show that the ICRH heating in the IGNITOR experiment, among other applications, is expected to be used to trigger the thermonuclear instability. Here a scenario is considered where IGNITOR is led to operate in a slightly sub-critical regime by adding a small fraction of 3He to the nominal 50%-50% deuterium-tritium mixture. The difference between power lost and alpha heating is compensated by additional ICRH heating, which should be able to increase the global plasma temperature via collisions between 3He minority and the background D-T ions.

PHOTOIONIZATION HEATING OF NOVA EJECTA BY THE POST-OUTBURST SUPERSOFT SOURCE

The expanding ejecta from a classical nova remains hot enough ($\sim10^{4}\, {\rm K}$) to be detected in thermal radio emission for up to years after the cessation of mass loss triggered by a thermonuclear instability on the underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the hot temperatures observed in radio observations. During this same period, the unstable thermonuclear burning transitions to a prolonged period of stable burning of the remnant hydrogen-rich envelope, causing the WD to become, temporarily, a super-soft X-ray source. We show that photoionization heating of the expanding ejecta by the hot WD maintains the observed nearly constant temperature of $(1-4)\times10^4\mathrm{~K}$ for up to a year before an eventual decline in temperature due to either the cessation of the supersoft phase or the onset of a predominantly adiabatic expansion. We simulate the expanding ejecta using a one-zone model as well as the Cloudy spectral synthesis code, both incorporating the time-dependent WD effective temperatures for a range of masses from $0.60\ M_{\odot}$ to $1.10\ M_{\odot}$. We show that the duration of the nearly isothermal phase depends most strongly on the velocity and mass of the ejecta and that the ejecta temperature depends on the WD's effective temperature, and hence its mass.

Perspectives for the high field approach in fusion research and advances within the Ignitor Program

The Ignitor Program maintains the objective of approaching D–T ignition conditions by incorporating systematical advances made with relevant high field magnet technology and with experiments on high density well confined plasmas in the present machine design. An additional objective is that of charting the development of the high field line of experiments that goes from the Alcator machine to the ignitor device. The rationale for this class of experiments, aimed at producing poloidal fields with the highest possible values (compatible with proven safety factors of known plasma instabilities) is given. On the basis of the favourable properties of high density plasmas produced systematically by this line of machines, the envisioned future for the line, based on novel high field superconducting magnets, includes the possibility of investigating more advanced fusion burn conditions than those of the D–T plasmas for which Ignitor is designed. Considering that a detailed machine design has been carried out (Coppi et al 2013 Nucl. Fusion 53 104013), the advances made in different areas of the physics and technology that are relevant to the Ignitor project are reported. These are included within the following sections of the present paper: main components issues, assembly and welding procedures; robotics criteria; non-linear feedback control; simulations with three-dimensional structures and disruption studies; ICRH and dedicated diagnostics systems; anomalous transport processes including self-organization for fusion burning regimes and the zero-dimensional model; tridimensional structures of the thermonuclear instability and control provisions; superconducting components of the present machine; envisioned experiments with high field superconducting magnets.

Thermal stability studies of an experimental nuclear fusion reactor

The conditions for the operation of a tokamak fusion reactor are studied considering a fiducial operating state and analyze variations about this state. We use a volume averaged 0-D two-temperature model and consider the case of a fusion thermonuclear reactor of the ITER type which will operate with a burning plasma at an energy gain factor of Q=10. The possible operation states are represented in POPCON plots of density n versus electron temperature Te by varying parameters such as the fusion power, the energy gain and the auxiliary heating power, but keeping the ratio (Te – Ti)/Te fixed, in order to determine the optimal operation point. Then, the thermonuclear instability of the burning plasma around this point is studied with a linear analysis. It is found that this instability cannot be excited, due to the fact that the plasma is far from ignition.

Optimization and thermal stability studies of Ignitor and ITER

A comparison between the operational states of Ignitor and ITER needed to obtain a maximal energy gain is presented for a range of temperatures and densities, under the assumption that the ratio of electron to ion temperatures is held fixed for all the steady states considered. A criterion for the optimal operation of ITER is obtained in terms of the auxiliary heating to ions. The optimal states for Ignitor are achieved when auxiliary heating to both ions and electrons is minimized. The thermal stability is also studied for the same range of parameters, finding that the development of this instability is not a concern for ITER, while a certain range of plasma densities and temperatures has to be maintained in order to avoid the thermonuclear instability in Ignitor.

The thermonuclear instability of the solar core

I present here a list of the most important astrophysical problems related to the energy and neutrino producing solar core. Attempting to find a physical connection between the solar activity and the nuclear processes of the solar core, I present the relation between the LOCAL thermal instability and the conditions in the solar core. The result show that if a mechanism would be able to produce a local heating above 10 8 K, this perturbation would push the local region of the core into the instability regime. Tidal-electromagnetic inductive coupling may elicit such a large local heating. Some consequences of the dynamic solar model are discussed.

Thermonuclear Burning on the Accreting X‐Ray Pulsar GRO J1744−28

We investigate the thermal stability of nuclear burning on the accreting X-ray pulsar GRO J1744-28. The neutron star's dipolar magnetic field is 3 × 1011 G if persistent spin-up implies that the magnetospheric radius is less than the corotation radius. Bildsten earlier noted that magnetic fields this weak might not quench the vigorous convection sometimes associated with thermonuclear instabilities, so that a convective burning front can propagate around the star in a few seconds and rapidly release the accumulated nuclear energy. After inferring the properties of the neutron star, we study the thermal stability of hydrogen/helium burning and show that thermonuclear instabilities are unlikely causes of the hourly bursts seen at very high accretion rates. Then we discuss how the stability of the thermonuclear burning depends on both the global accretion rate and the neutron star's magnetic field strength. If the accreted matter spreads over the star prior to ignition, then the burning will become unstable when the accretion rate is below 6 × 10-9 M☉ yr-1. We emphasize that the appearance of the instability (i.e., whether it looks like a type I X-ray burst or a flare lasting a few minutes) will yield crucial information on the neutron star's surface magnetic field and the role of magnetic fields in convection. We suggest that a thermal instability in the accretion disk is the origin of the long (~300 days) outburst and that the recurrence time of these outbursts is more than 50 yr. We also discuss the nature of the binary and point out that a velocity measurement of the stellar companion (most likely a Roche lobe filling giant with mK 17) will constrain the neutron star mass. This is of interest in this binary, where the inferred low mass of the companion implies that 0.5 M☉ of transfer occurred.

Propagation of nuclear burning fronts on accreting neutron stars: X-ray bursts and sub-hertz noise

view Abstract Citations (124) References (84) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Propagation of Nuclear Burning Fronts on Accreting Neutron Stars: X-Ray Bursts and Sub-Hertz Noise Bildsten, Lars Abstract We identify a new regime of time dependent helium burning for high accretion rate neutron stars and suggest that this burning is the origin of the low-level luminosity variations (on timescales of 10-104 s, designated the 'very low-frequency noise'(VLFN) by van der Klis and collaborators) always detected in the brightest accreting X-ray sources. Only two nuclear burning regimes were previously recognized. At accretion rates in excess of the Eddington limit (dot-M approximately greater than (1-3) x 10-8 solar mass/yr), the accreted matter fuses steadily. At very low dot-M, the star's entire surface is rapidly (approximately less than 10 s) burned by a fast propagating convective burning front at regular intervals, giving quasi-periodic Type I X-ray bursts. We show that for the observationally interesting range of 5 x 10-10 solar mass/yr approximately less than dot-M approximately less than 10-8 solar mass/yr, parts of the stellar surface burn slowly. At these accretion rates, a local thermonuclear instability starts a fire which propagates horizontally at v approximately 300 cm/s. The fire propagates around the flammable surface in roughly the same time it takes to accrete enough fuel for the next instability (approximately 103-104, so that only a few fires are burning at once, giving rise to large luminosity flares. Nuclear burning is always time dependent for sub-Eddington local accretion rates: a local patch undergoes a recurrent cycle, accumulation fuel for hours until it becomes thermally unstable or is 'ignited' by a nearby burning region. The global pattern of burning and the resulting luminosity are thus very dependent on how fast nuclear fires spread around the star. The nuclear burning luminosity is not uniform over the stellar surface and so may provide a handle on measuring, or constraining, the spin periods of these neutron stars. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175128 Bibcode: 1995ApJ...438..852B Keywords: Accretion Disks; Combustion; Neutron Stars; Nuclear Reactions; Stellar Mass Accretion; X Rays; Helium; Stellar Atmospheres; Stellar Luminosity; Stellar Physics; Stellar Temperature; Astrophysics; ACCRETION; ACCRETION DISKS; INSTABILITIES; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES; STARS: NEUTRON; X-RAYS: BURSTS full text sources ADS | data products SIMBAD (10)

Rings of Fire: Nuclear Burning as the Origin of Sub-Hertz Noise and Weak X-Ray Bursts in Accreting Neutron Stars

We identify a new regime of time-dependent nuclear burning on accreting neutron stars. Only two regimes were previously recognized. At high accretion rates (M˙), the accreted matter fuses steadily. At very low M˙, the entire surface of the star is rapidly (≤10 s) burned by a fast propagating thermonuclear instability at regular intervals, giving type I X-ray bursts. We show that for the observationally interesting range 5×10 −10 M ○. yr −1 ≤M≤10 −8 M ○. yr −1 , small patches of the star burn intermittently. At these rates, a local thermonuclear instability starts a fire which propagates horizontally at v∼300 cm s −1

Stellar black holes in globular clusters

Following the discovery of X-ray sources in globular clusters, the accretion of matter onto a central massive black hole was suggested as a possible explanation. Subsequently, it was found that these sources could be readily explained by thermonuclear instabilities on neutron-star surfaces and the black-hole models were abandoned. We show here, however, that the recent discovery of large populations of millisecond pulsars—and hence neutron stars—in globular clusters implies that several hundred stellar black holes (of about ten solar masses) should form within a typical cluster. In clusters of high central density, we find that the rapid dynamical evolution of the black-hole population will cause ejection of nearly all of the holes on a relatively short time-scale. But in systems of intermediate density, some of the surviving holes may capture a normal star to form a low-mass X-ray binary. We suggest that there may be one or more such binaries in the globular clusters surrounding our Galaxy. These systems will be quiescent most of the time—with only occasional X-ray outbursts—but future observations of the hard X-ray spectrum may indirectly establish their existence.

Spectroscopy of the recurrent Nova U Scorpii

U Sco, the recurrent nova with the shortest known recurrence time, has been argued to be a strong case for a very massive accreting white dwarf undergoing thermonuclear explosions. Such massive white dwarfs have also been proposed as the progenitors of millisecond pulsars in the disk and in globular clusters. We present optical spectroscopic observations of the system and measure both emission- and absorption-line velocities. We derive an upper limit to the mass for the white dwarf of ≤0.9 M ⊙ (3 σ) in direct contradiction to the thermonuclear instability models

Excitation of thermonuclear cone instabilities by toroidal equilibrium distributions of high-energy alpha particles in tokamaks

The possibility of a new set of thermonuclear cone instabilities in tokamaks is demonstrated. These instabilities are caused by an inherent anisotropy existing in the alpha-particle velocity distribution function owing to the finite width of the particle orbits in combination with a spatially varying particle source. The conditions for the excitation of shear Alfvén waves by well-trapped alpha particles are studied.

Authors

Authors

Thermonuclear Cone Alfvén Instability in Tokamak Reactors

In this paper possible excitation of thermonuclear cone instabilities cause by anisotropy in alpha-particle velocity distribution is considered in tokamak reactor plasmas for frequencies much lower than electron cyclotron frequency. In the ignite tokamak reactor experiment, for example the International Thermonuclear Experimental Reactor (ITER), Alfven waves are excited at frequencies comparable to alpha-particle cyclotron frequency. The growth rate, the region of instability localization in the plasma cross section, and the marginal stability boundaries in the n-T plane are determined for cone Alfven were instability.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. IV. Inhomogeneous plasma.

The thermal thermonuclear instability of a deuterium plasma with inhomogeneous temperature and number density in a strong magnetic field is studied theoretically. The distribution of the number density is supposed to be proportional to that of the temperature. By solving the basic equation which governs the reactions, the growth rates in a pure DD fusion reactor and a catalyzed DD fusion reactor are obtained. It is shown that the plasmas with inhomogeneous temperature and number density in the above reactors are unstable to the thermal instability.

Searches for low-energy counterparts of gamma-ray bursters

I report on the results of six studies related to identifying low-energy counterparts of gamma-ray bursters (GRBs). These studies have used the Very Large Array in New Mexico, the 3.0m IRTF telescope in Hawaii, the 4.0m, 1.5m, and 0.6m telescopes at CTIO in Chile, the 1.3m telescope at KPNO in Arizona, the Point Source Catalog of the Infrared Astronomy Satellite, the collection of archival photographs at Harvard, and a collaboration with the Gamma Ray Burst Source Monitoring System (GMS) of Pedersen and coworkers at ESO in Chile. The main results of these studies are: (1) The near infrared observations pose serious difficulties for several GRB models. (2) I can find no precedent or plausible explanation for the 8 February 1984 optical flash of Pedersen et al . /1/ other than that the flash originated on a GRB. (3) The star in the middle of the 1944 optical transient (OT) error box has an ultraviolet excess which is significant at the 95% confidence level-a result which strongly suggests that the object is indeed the quiescent GRB optical counterpart. (4) If the previous result is true, then most likely, GRBs are low-mass contact binaries caused by thermonuclear or accretion instabilities at distances of roughly 1 kpc.

Thermal instability of a thermonuclear plasma in a DD fusion reactor. III Inhomogeneous temperature.

A thermal thermonuclear instability of a deuterium plasma with an inhomogeneous temperature in a strong magnetic field is studied theoretically. The growth rate of the instability is obtained. The plasmas with an inhomogeneous temperature in a DD fusion reactor and in a catalyzed DD fusion reactor are unstable for the thermal instability.

The influence of nuclear burning on the stability of degenerate and nondegenerate accretion disks

view Abstract Citations (11) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The influence of nuclear burning on the stability of degenerate and nondegenerate accretion disks. Taam, R. E. ; Fryxell, B. A. Abstract The structure and stability of accretion disks composed of hydrogen-rich matter rotating about a central neutron star have been investigated for known sources of viscosity. Two general classes of solutions have been found. For one class the energy generated in the disk is provided by hydrogen burning, whereas for the other class the gravitational binding energy released by viscous dissipation dominates. The former solutions are thermally unstable (stable) whenever hydrogen burns via the normal CNO cycle (pp chain) in a partially or fully degenerate region of the disk. Solutions characterized by nuclear-burning via the Beta-limited CNO cycle or by viscous dissipation only are always stable. On the basis of a local analysis it is shown that modulations of the mass flow in the disk are possible for a range of mass inflow rates into the disk. In such circumstances the disk can undergo a phase transition from a cold, low-viscosity state to a hot, high-viscosity state as a result of the thermonuclear flash instability. Phase transitions from the hot state to the cold state also occur whenever the mass input rate into the disk is less than the equilibrium mass flow rate corresponding to the hot state. It is also shown that for sufficiently high mass flow rates all the hydrogen-rich matter can be processed to helium in the inner regions of the disk before it can be accreted by a neutron star. Publication: The Astrophysical Journal Pub Date: July 1985 DOI: 10.1086/163298 Bibcode: 1985ApJ...294..303T Keywords: Accretion Disks; Mass Flow; Neutron Stars; Nuclear Astrophysics; Nuclear Fusion; Stellar Models; Thermal Stability; Mass Flow Rate; Viscosity; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (1)

A search for nuclear-burning instabilities in the sun

A search for coupled thermal-nuclear oscillations in the solar core is made using a stellar evolution code with a full nuclear reaction network. The Kelvin-Helmholtz time sclae for the whole Sun is 3 x 10/sup 7/ yr, but only 3 x 10/sup 6/ yr for the nuclear-burning core. The lifetime of /sup 3/He in the solar core is > or approx. =10/sup 5/ yr and increases rapidly outward. Processing of /sup 3/He accounts for about two-thirds of the energy released in the dominant p-p reaction chains in the Sun. Oscillations of the nuclear species could generate solar luminosity variations on time scales of 10/sup 5/--10/sup 7/ yr. Evolution sequences following large-amplitude perturbations of /sup 3/He and other minor nuclear species show strong damping with equilibrium being restored on the Kelvin-Helmhotz time scale. Consistent with previous linear analyses, no evidence for thermonuclear instability in the Sun is found.