Nuclear Burning Processes Research Articles

A Study of Magnetized White Dwarf + Helium Star Binary Evolution to Type Ia Supernovae

The white dwarf (WD) + helium (He) star binary channel plays an important role in the single degenerate scenario for the progenitors of type Ia supernovae (SNe Ia). Previous studies on the WD + main sequence star evolution have shown that the magnetic fields of WDs may significantly influence their accretion and nuclear burning processes. In this work we focus on the evolution of magnetized WD + He star binaries with detailed stellar evolution and binary population synthesis (BPS) calculations. In the case of magnetized WDs, the magnetic fields may disrupt the inner regions of the accretion disk, funnel the accretion flow onto the polar caps and even confine helium burning within the caps. We find that, for WDs with sufficiently strong magnetic fields, the parameter space of the potential SN Ia progenitor systems shrinks toward shorter orbital periods and lower donor masses compared with that in the non-magnetized WD case. The reason is that the magnetic confinement usually works with relatively high mass transfer rates, which can trigger strong wind mass loss from the WD, thus limiting the He-rich mass accumulation efficiency. The surviving companion stars are likely of low-mass at the moment of the SN explosions, which can be regarded as a possible explanation for the non-detection of surviving companions after the SNe or inside the SN remnants. However, the corresponding birthrate of Galactic SNe Ia in our high-magnetic models is estimated to be ∼(0.08–0.13) × 10−3 yr−1 ( ∼0.17–0.28 × 10−3 yr−1 for the non-magnetic models), significantly lower than the observed Galactic SN Ia birthrate.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

Context. Next to H and He, carbon is, together with oxygen, the most abundant element in the Universe and widely used when modelling the formation and evolution of galaxies and their stellar populations. For the Milky Way bulge, there are currently essentially no measurements of carbon in un-evolved stars, hampering our abilities to properly compare Galactic chemical evolution models to observational data for this still enigmatic stellar population. Aims. We aim to determine carbon abundances for our sample of 91 microlensed dwarf and subgiant stars in the Galactic bulge. Together with new determinations for oxygen this forms the first statistically significant sample of bulge stars that have C and O abundances measured, and for which the C abundances have not been altered by the nuclear burning processes internal to the stars. Methods. Our analysis is based on high-resolution spectra for a sample of 91 dwarf and subgiant stars that were obtained during microlensing events when the brightnesses of the stars were highly magnified. Carbon abundances were determined through spectral line synthesis of six C I lines around 9100 Å, and oxygen abundances using the three O I lines at about 7770 Å. One-dimensional (1D) MARCS model stellar atmospheres calculated under the assumption of local thermodynamic equilibrium (LTE) were used, and non-LTE corrections were applied when calculating the synthetic spectra for both C and O. Results. Carbon abundances was possible to determine for 70 of the 91 stars in the sample and oxygen abundances for 88 of the 91 stars in the sample. The [C/Fe] ratio evolves essentially in lockstep with [Fe/H], centred around solar values at all [Fe/H]. The [O/Fe]–[Fe/H] trend has an appearance very similar to that observed for other α-elements in the bulge, with the exception of a continued decrease in [O/Fe] at super-solar [Fe/H], where other α-elements tend to level out. When dividing the bulge sample into two sub-groups, one younger than 8 Gyr and one older than 8 Gyr, the stars in the two groups follow exactly the elemental abundance trends defined by the solar neighbourhood thin and thick disks, respectively. Comparisons with recent models of Galactic chemical evolution in the [C/O]–[O/H] plane show that the models that best match the data are the ones that have been calculated with the Galactic thin and thick disks in mind. Conclusions. We conclude that carbon, oxygen, and the combination of the two support the idea that the majority of the stars in the Galactic bulge have a secular origin; that is, they are formed from disk material. We cannot exclude that a fraction of stars in the bulge could be classified as a classical bulge population, but it would have to be small. More dedicated and advanced models of the inner region of the Milky Way are needed to make more detailed comparisons to the observations.

Discovery of a strong 6.6 keV emission feature from EXO 1745−248 after the superburst in 2011 October

Abstract We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745−248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 × 10−3 ph cm−2 s−1. The 3–20 keV spectrum was reproduced successfully by a power-law continuum with narrow and broad (2.7 keV in full width at half maximum) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV, and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick material, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 hr before the RXTE observation with a line flux upper limit of 1.5 × 10−3 ph cm−2 s−1. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall-back ionized burst wind and an accretion disk, at a distance of ∼60 km from the neutron star. If this interpretation is correct, the results provide new information on nuclear burning processes during thermonuclear X-ray bursts.

XMM-Newton and NICER Measurement of the Rms Spectrum of the Millihertz Quasiperiodic Oscillations in the Neutron-star Low-mass X-Ray Binary 4U 1636–53

Abstract We used two XMM-Newton and six Neutron Star Interior Composition Explorer observations to investigate the fractional rms amplitude of the millihertz quasiperiodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636–53. We studied, for the first time, the fractional rms amplitude of the mHz QPOs versus energy in 4U 1636–53 down to 0.2 keV. We find that, as the energy increases from ∼0.2 to ∼3 keV, the rms amplitude of the mHz QPOs increases, different from the decreasing trend that has been previously observed above 3 keV. This finding has not yet been predicted by any current theoretical model; however, it provides an important observational feature to speculate whether a newly discovered mHz oscillation originates from the marginally stable nuclear burning process on the neutron-star surface.

Time evolution of rotating and magnetized white dwarf stars

We investigate the evolution of isolated, zero and finite temperature, massive, uniformly rotating and highly magnetized white dwarf stars under angular momentum loss driven by magnetic dipole braking. We consider the structure and thermal evolution of white dwarf isothermal cores taking also into account the nuclear burning and neutrino emission processes. We estimate the white dwarf lifetime before it reaches the condition either for a type Ia supernova explosion or for the gravitational collapse to a neutron star. We study white dwarfs with surface magnetic fields from $10^6$ to $10^{9}$~G and masses from $1.39$ to $1.46~M_\odot$ and analyze the behavior of the white dwarf parameters such as moment of inertia, angular momentum, central temperature and magnetic field intensity as a function of lifetime. The magnetic field is involved only to slow down white dwarfs, without affecting their equation of state and structure. In addition, we compute the characteristic time of nuclear reactions and dynamical time scale. The astrophysical consequences of the results are discussed.

SkyNet: A Modular Nuclear Reaction Network Library

Almost all of the elements heavier than hydrogen that are present in our solar system were produced by nuclear burning processes either in the early universe or at some point in the life cycle of stars. In all of these environments, there are dozens to thousands of nuclear species that interact with each other to produce successively heavier elements. In this paper, we present SkyNet, a new general-purpose nuclear reaction network that evolves the abundances of nuclear species under the influence of nuclear reactions. SkyNet can be used to compute the nucleosynthesis evolution in all astrophysical scenarios where nucleosynthesis occurs. SkyNet is free and open-source and aims to be easy to use and flexible. Any list of isotopes can be evolved and SkyNet supports various different types of nuclear reactions. SkyNet is modular so that new or existing physics, like nuclear reactions or equations of state, can easily be added or modified. Here, we present in detail the physics implemented in SkyNet with a focus on a self-consistent transition to and from nuclear statistical equilibrium (NSE) to non-equilibrium nuclear burning, our implementation of electron screening, and coupling of the network to an equation of state. We also present comprehensive code tests and comparisons with existing nuclear reaction networks. We find that SkyNet agrees with published results and other codes to an accuracy of a few percent. Discrepancies, where they exist, can be traced to differences in the physics implementations.

SkyNet: A Modular Nuclear Reaction Network Library

Almost all of the elements heavier than hydrogen that are present in our solar system were produced by nuclear burning processes either in the early universe or at some point in the life cycle of stars. In all of these environments, there are dozens to thousands of nuclear species that interact with each other to produce successively heavier elements. In this paper, we present SkyNet, a new general-purpose nuclear reaction network that evolves the abundances of nuclear species under the influence of nuclear reactions. SkyNet can be used to compute the nucleosynthesis evolution in all astrophysical scenarios where nucleosynthesis occurs. SkyNet is free and open source, and aims to be easy to use and flexible. Any list of isotopes can be evolved, and SkyNet supports different types of nuclear reactions. SkyNet is modular so that new or existing physics, like nuclear reactions or equations of state, can easily be added or modified. Here, we present in detail the physics implemented in SkyNet with a focus on a self-consistent transition to and from nuclear statistical equilibrium to non-equilibrium nuclear burning, our implementation of electron screening, and coupling of the network to an equation of state. We also present comprehensive code tests and comparisons with existing nuclear reaction networks. We find that SkyNet agrees with published results and other codes to an accuracy of a few percent. Discrepancies, where they exist, can be traced to differences in the physics implementations.

Superburst Oscillations: ocean and crustal modes excited by X-Ray bursts

AbstractSuperburst oscillations are high frequency X-ray variations observed during hours’ long superbursts on accreting neutron stars. We investigate a potential mechanism to explain these observations; a buoyant r-mode, excited in the ocean layers of the star. These modes are affected by ash composition in the ocean so are a good probe of nuclear burning processes. The phenomenon could be used in pulse profile modelling as a way of measuring neutron star mass and radius, and so the dense matter equation of state.

Millihertz quasi-periodic oscillations in 4U 1636−53 associated with bursts with positive convexity only

We investigated the convexity of all type I X-ray bursts with millihertz quasi-periodic oscillations (mHz QPOs) in 4U 1636-53 using archival observations with the Rossi X-ray Timing Explorer. We found that, at a 3.5 $\sigma$ confidence level, in all 39 cases in which the mHz QPOs disappeared at the time of an X-ray burst, the convexity of the burst is positive. The convexity measures the shape of the rising part of the burst light curve and, according to recent models, it is related to the ignition site of bursts on the neutron star surface. This finding suggests that in 4U 1636-53 these 39 bursts and the marginally-stable nuclear burning process responsible for the mHz QPOs take place at the neutron-star equator. This scenario could explain the inconsistency between the high accretion rate required for triggering mHz QPOs in theoretical models and the relatively low accretion rate derived from observations.

Asteroseismology of the DAV star HS 0507+0434B, including core-composition profiles

The DAV star, HS 0507+0434B, was observed by Fu et al. in 2007 and from 2009 December to 2010 January. There were a total of six triplets with nearly equal split, which were identified as l = 1 modes caused by rotation. In order to fit the six l = 1 modes, grids of white dwarf models are generated using the white dwarf evolution code (wdec). For the core-composition profiles, we choose the linear fittings to carbon profile of white dwarf models from modules for experiments in stellar astrophysics (mesa), which can be considered as the results of the real nuclear-burning process. Coupled with diffusion equilibrium H/He and He/C mixtures, we make grids of models in wdec and perform asteroseismology of HS 0507+0434B. There is a total error of 9.50 s for our best-fitting model, which is smaller than the result of Fu et al. (i.e. a total error of 22.1 s), when fitting the six l = 1 modes. The two other previously identified pulsation mode frequencies (286.1 and 743.40 s) can also be well fitted by our best-fitting model. The model parameters are Teff = 11 450 K, log g = 8.088, M* = 0.640 M⊙, log (MH/M*) = −6 and log (MHe/M*) = −3.

Phenomenological and semi-microscopic analysis for 16O and 12C elastically scattering on the nucleus of 16O and 12C at Energies near the Coulomb barrier

The nuclear burning process proceeds from the conservation of the most abundant element hydrogen to helium, then from helium to carbon and oxygen, and then from these to heavier elements. Some of the key reactions for the carbon and oxygen burning stages of the nucleosynthesis are 12C+12C and 16O+16O leading to all possible final states. This paper contains the experimental measurements of 12C+12C and 16O+16O angular distributions performed at the cyclotron DC-60 in Astana, Kazakhstan. The extracted beam of 16O and 12C was accelerated up to two energies 1.75 and 1.5 MeV/n and then directed to an Al2O3 target of thickness 20 μg/cm2 and a carbon self-supporting target of thickness 17.4 μg/cm2. The angular distribution calculations were performed using both the phenomenological optical potential (SPI-GENOA) code and the double folding potential (FRESCO) code.

Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473−2721

We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an X-ray transient that in 2008 underwent a six month long outburst, starting (unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range of inferred accretion rates (between <1% and about 20% of the Eddington accretion rate m-dot_Edd) spanned during the outburst allows us to study changes in the nuclear burning processes and to identify up to seven different phases. The burst rate increased gradually with the accretion rate until it dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few days before the outburst peak, after which bursts were not detected for a month. As the persistent emission subsequently decreased, the bursting activity resumed with a much lower rate than during the outburst rise. This hysteresis may arise from the thermal effect of the accretion on the surface nuclear burning processes, and the timescale is roughly consistent with that expected for the neutron star crust thermal response. On the other hand, an undetected superburst, occurring within a data gap near the outburst peak, could have produced a similar quenching of burst activity.

AstrophysicalSfactor for the radiative captureN12(p,γ)O13determined from theN14(N12,O13)C13proton transfer reaction

The cross section of the radiative proton capture reaction on the drip line nucleus $^{12}\mathrm{N}$ was investigated using the asymptotic normalization coefficient (ANC) method. We have used the $^{14}\mathrm{N}(^{12}\mathrm{N},^{13}\mathrm{O})^{13}\mathrm{C}$ proton transfer reaction at 12 MeV/nucleon to extract the ANC for $^{13}\mathrm{O}\ensuremath{\rightarrow}^{12}\mathrm{N}+p$ and calculate from it the direct component of the astrophysical $S$ factor of the $^{12}\mathrm{N}(p,\ensuremath{\gamma})^{13}\mathrm{O}$ reaction. The optical potentials used and the distorted-wave Born approximation analysis of the proton transfer reaction are discussed. For the entrance channel, the optical potential was inferred from an elastic scattering measurement carried out at the same time as the transfer measurement. From the transfer, we determined the square of the ANC, ${C}_{{p}_{1/2}}^{2}(^{13}\mathrm{O}{}_{\mathrm{g}.\mathrm{s}.})=2.53\ifmmode\pm\else\textpm\fi{}0.30 {\mathrm{fm}}^{\ensuremath{-}1}$, and hence a value of $0.33(4) \mathrm{keV}$ b was obtained for the direct astrophysical $S$ factor at zero energy. Constructive interference at low energies between the direct and resonant captures leads to an enhancement of ${S}_{\mathrm{total}}(0)=0.42(6) \mathrm{keV}$ b. The $^{12}\mathrm{N}(p,\ensuremath{\gamma})^{13}\mathrm{O}$ reaction was investigated in relation to the evolution of hydrogen-rich massive Population III stars, for the role that it may play in the hot $\mathit{pp}$-chain nuclear burning processes, possibly occurring in such objects.

The new intermediate long-bursting source XTE J1701-407

XTE J1701-407 is a newly discovered X-ray transient source. In this work we investigate its flux variability and study the intermediate long and short bursts discovered by Swift on July 17, and 27, 2008, respectively. So far, only one intermediate long burst, with a duration of ~18 minutes and ten days later a short burst, have been recorded from XTE J1701-407. We analyzed the public available data from Swift and RXTE, and compared the observed properties of the intermediate long burst with theoretical ignition condition and light curves to investigate the possible nuclear burning processes. The intermediate long burst may have exhibited a photospheric radius expansion, allowing us to derive the source distance at 6.2 kpc assuming the empirically derived Eddington luminosity for pure helium. The intermediate long burst decay was best fit by using two exponential functions with e-folding times of \tau_1=40(3) s and \tau_2=221(9) s. The bursts occurred at a persistent luminosity of L_{per}=8.3x10E36 erg/s. For the intermediate long burst the mass accretion rate per unit area onto the NS was \dot{m}=4x10E3 g/cm2/s, and the total energy released was E_{burst}=3.5x10E40 erg. This corresponds to an ignition column depth of y_{ign}=1.8x10E9 g/cm2, for a pure helium burning. We find that the energetics of this burst can be modeled in different ways, as (i) pure helium ignition, as the result of either pure helium accretion or depletion of hydrogen by steady burning during accumulation, or (ii) as ignition of a thick layer of hydrogen-rich material in a source with low metallicity. However, comparison of the burst duration with model light curves suggests that hydrogen burning plays a role during the burst, and therefore this source is a low accretion rate burster with a low metallicity in the accreted material.

Time – Frequency Analysis of GALLEX and GNO Solar Neutrino Data

Time – frequency analysis of data from the GALLEX and GNO solar neutrino experiments shows that some features in power-spectrum analyses of those datasets are due to aliasing (a result of the fact that run durations tend to be small multiples of one week). Displays formed from the published GALLEX data show a sharp discontinuity that we attribute to some systematic effect. We therefore normalize data for each of the four experiments in the GALLEX series and concatenate the resulting normalized data. This step effectively removes the presumed systematic effect. To help understand the effect of aliasing, we form time – frequency displays of the two principal modulations found in the data, at 11.87 year−1 and at 13.63 year−1. We also form time – frequency displays of datasets formed by subtracting these modulations from the actual (normalized) data. The results suggest that the true principal modulation is that at 11.87 year−1. Comparison with helioseismology data suggests that modulation may be occurring in the core, perhaps resulting from inhomogeneities and fluctuations in the nuclear-burning process, and that the sidereal rotation rate of the core is 12.87 year−1, or 408 nHz.

Exposing the Nuclear Burning Ashes of Radius Expansion Type I X‐Ray Bursts

We solve for the evolution of the vertical extent of the convective region of a neutron star atmosphere during a type I X-ray burst. The convective region is well mixed with ashes of nuclear burning, and its extent determines the rise time of the burst light curve. Using a full nuclear reaction network, we show that the maximum vertical extent of the convective region during photospheric radius expansion (RE) bursts can be sufficiently great that (1) some ashes of burning are ejected by the radiation-driven wind during the RE phase and (2) some ashes of burning are exposed at the neutron star surface following the RE phase. We find that ashes with mass numbers in the range A ~ 30-60 are mixed in with the ejected material. We calculate the expected column density of ejected and surface ashes in hydrogen-like states and determine the equivalent widths of the resulting photoionization edges from both the wind and the neutron star surface. We find that these can exceed 100 eV and are potentially detectable. A detection would probe the nuclear burning processes and might enable a measurement of the gravitational redshift of the neutron star. In addition, we find that in bursts with pure helium burning layers, protons from (α, p) reactions cause a rapid onset of the 12C(p, γ)13N(α, p)16O reaction sequence. The sequence bypasses the relatively slow 12C(α, γ)16O reaction and leads to a sudden surge in energy production that is directly observable as a rapid (~millisecond) increase in flux during burst rise.

Supernova 1987A from 1 keV to 10 MeV: The status as of 1990

Satellite, rocket, and balloon borne instruments have obtained a wealth of high-energy data from SN 1987A. These have added significantly to our understanding of Type II supernovae. Besides the exciting direct detection of just-synthesized radioactivity via γ-ray line observations, the light curves of the lines and the extremely hard continuum which results from them have provided clues to the structure of the newly disrupted star. Mysteries still remain. We do not yet understand how the radioactive matter reached low depths in the envelope at such early times. We have not yet identified the mechanism which produced the softest observed X-ray emission. We do not know how much of other radioactive species were produced. This could help us begin to understand in detail the nuclear burning and mass ejection processes. We have not yet seen the signature of the compact remnant of the collapse. At this point we have great hope that some of these questions will be answered with the state-of-the-art experiments already flying and scheduled for launch soon.