Nuclear Time Scale Research Articles

Evolution of Young Neutron Star Envelopes

We extend our initial study of diffusive nuclear burning (DNB) for neutron stars (NSs) with hydrogen (H) atmospheres and an underlying layer of proton-capturing nuclei. Our initial study showed that DNB can alter the photospheric abundance of hydrogen on surprisingly short timescales (102-104 yr). Significant composition evolution impacts the radiated thermal spectrum from the NS as well as its overall cooling rate. In this paper, we consider the case when the rate-limiting step for the H consumption is diffusion to the burning layer rather than the local nuclear timescale. This is relevant for NSs with surface temperatures in excess of 106 K, such as young (<105 yr) radio pulsars and accreting NSs in quiescence. When downward diffusion is the limiting rate in DNB, the rate of H consumption is suppressed by 1-2 orders of magnitude compared to a DNB estimate that assumes diffusive equilibrium. In order to apply our ongoing study to young neutron stars, we also include the important effects of strong magnetic fields (B ~ 1012 G). In this initial study of magnetic modifications to DNB, we find that the H-burning time is lengthened by 2-3 orders of magnitude for a 1012 G field. However, even for NSs with dipole field strengths of 1012 G, we find that all of the H can be burned before the pulsar reaches an age of ~105 yr, thus potentially revealing the underlying proton-capturing elements. Finally, we conclude by providing an overview of what can be learned about fallback and pulsar winds from measuring the surface composition of a young NS.

Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV 16O + 176Yb and 208Pb and 104-MeV 4He + 188Os and 209Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10−21 s.

Diffusive Nuclear Burning in Neutron Star Envelopes

We calculate the rate of hydrogen burning for neutron stars (NSs) with hydrogen atmospheres and an underlying reservoir of nuclei capable of proton capture. This burning occurs in the exponentially suppressed diffusive tail of H that extends to the hotter depths of the envelope where protons are rapidly captured. This process, which we call diffusive nuclear burning (DNB), can change the H abundance at the NS photosphere on timescales as short as $10^{2-4}$ years. In the absence of diffusion, the hydrogen at the photosphere (where $T\approx 10^6 {\rm K}$ and $\rho\sim 0.1 {\rm g cm^{-2}}$) would last for far longer than a Hubble time. Our work impacts the understanding of the evolution of surface abundances of isolated NSs, which is important to their thermal spectrum and their effective temperature-core temperature relation. In this paper, we calculate the rate of H burning when the overall consumption rate is controlled by the nuclear timescales, rather than diffusion timescales. The immediate application is for H burning on millisecond radio pulsars and in quiescence for the accreting NS Cen X-4. We will apply this work to young radio pulsars and magnetars once we have incorporated the effects of strong $B>10^{12} {\rm G}$ magnetic fields.

A New Abundance Analysis of the ONeMg Nova QU Vulpeculae

This paper presents a new determination of the ejecta elemental abundances of the moderately fast ONeMg nova QU Vul 1984 using published ultraviolet, optical, infrared, and radio observations. The technique uses an optimization code in combination with CLOUDY photoionization models to fit the emission-line spectra obtained at multiple epochs during the nebular phase of the outburst. The best-fit model parameters from four epochs of multiwavelength spectra are compatible with a hot white dwarf source with a roughly constant luminosity of 3 × 1037 ergs s-1. We find the following abundances (by number) with respect to solar: He/H = 1.16 ± 0.08, C/H = 0.2, N/H = 12 ± 0.6, O/H = 2.5, Ne/H = 21.7 ± 1.7, Mg/H = 10, Al/H = 53.3 ± 14.7, Si/H = 2.0, Ar/H = 0.28 ± 0.02, and Fe/H = 0.53 ± 0.03. Previous studies of QU Vul yielded considerable differences in the derived elemental abundances. This analysis, which uses a better physical model, efficient optimization technique, and more multiwavelength data than the previous studies, generally finds the lowest abundance enhancements. The ejected mass could not be determined, because of the large range in distance from the different distance estimate methods. However, even the closest distance of 1.9 kpc gives a lower limit to the ejected mass of ~3.5 × 10-4 M☉, which presents a challenge for hydrodynamical models to reproduce. These results, coupled with the long light curve decay time and nuclear burning timescale, relative to other ONeMg novae, imply that the outburst occurred on a low-mass ONeMg white dwarf. The extreme mass and enhanced abundances imply that a QU Vul-like event might play an important role in the nucleosynthesis of the local interstellar medium.

Lasetron: a proposed source of powerful nuclear-time-scale electromagnetic bursts.

Electromagnetic bursts of substantial energy on a nuclear time scale of 10(-21)-10(-22) s [zeptosecond (zs) to sub-zs] can theoretically be generated by a perawatt or multiterawatt laser beam focused on a subwavelength-size solid particle or thin wire. Terawatt laser in a similar setup could be instrumental in reaching the subattosecond domain. The system may also generate a half-cycle pulse magnetic field on astrophysical scale up to approximately 10(6) T.

Massive zero-metal stars: Energy production and mixing

Time-dependent nuclear network calculations at constant temperature show that for zero-metal stars >= 20 Msun (i) beta-decay reactions and (ii) the 13N(p,gamma)14O reaction must be included. It is also shown that the nuclear timescale in these zero-metal stars is shorter than the mixing timescale and therefore the assumption of instantaneous mixing across convective regions is not fulfilled. We conclude that proper modeling of these processes may alter the evolution of massive zero-metal stars.

Analytical solution for the evolution of a binary with stable mass transfer from a giant

We derive a simple analytical solution for the evolution of a close binary with nuclear time-scale driven mass transfer from a giant. This solution is based on the well-known fact that the luminosity and the radius of a giant scale to a good approximation as simple power laws of the mass M_c of the degenerate helium core. Comparison with results of numerical calculations by Webbink, Rappaport & Savonije (1983) show the analytical solution and the power law approximation to be quite accurate. The analytical solution presented does also allow (in parametrized form) for non-conservative mass transfer. Furthermore it is shown that the near constancy of the mass transfer rate over most of the mass transfer phase seen in the results by Webbink, Rappaport & Savonije is not a generic feature of this type of evolution but rather a consequence of a particular choice of parameters. The analytical solution also demonstrates that the level of mass transfer is largely set by the core mass of the giant at the onset of mass transfer. Finally we show that the model is selfconsistent and discuss its applicability to low-mass X-ray binaries.

Quantal trajectories for adiabatic and nonadiabatic regimes of vibronic systems

Exact and averaged nuclear pseudorotational quantal trajectories are compared for various adiabatic and vibronic states of the Longuet-Higgins E⊗ϵ Jahn–Teller model. It is argued that the usual averaging over the electronic motion could be understood as being a consequence of ergodicity. The failure of the Born–Oppenheimer factorization to obey the ergodic hypothesis was examined. A quantitative separation of the electronic and nuclear time-scales is, nevertheless, achieved for all regimes. It is shown that the short-time deviations from the global “drift” of the electronic and nuclear motions are perfectly correlated. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 409–416, 1999

Quantal trajectories for adiabatic and nonadiabatic regimes of vibronic systems

Exact and averaged nuclear pseudorotational quantal trajectories are compared for various adiabatic and vibronic states of the Longuet-Higgins E⊗ϵ Jahn–Teller model. It is argued that the usual averaging over the electronic motion could be understood as being a consequence of ergodicity. The failure of the Born–Oppenheimer factorization to obey the ergodic hypothesis was examined. A quantitative separation of the electronic and nuclear time-scales is, nevertheless, achieved for all regimes. It is shown that the short-time deviations from the global “drift” of the electronic and nuclear motions are perfectly correlated. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 409–416, 1999

Tidal friction in triple stars

Tidal friction in close binaries, with periods of a few days, is expected to circularize the orbit on a time-scale long compared with human observation but shorter than, or comparable to, the lifetimes of main-sequence stars. In a hierarchical triple star, however, the perturbing effect of the distant third star may decircularize the inner orbit significantly on a time-scale of the order of days (as in λ Tau) or centuries (as in β Per). If the inner pair is observed to be semidetached, however, it is plausible to assume that the eccentricity is small. This may be because tidal friction is operating on a comparably short time-scale, and so it is in principle amenable to observation. We attempt to determine a lower limit to the strength of tidal friction in λ Tau and β Per, on the basis of this consideration. Tidal friction will also lead to a secular transfer of angular momentum from the inner orbit to the outer orbit. Too rapid a transfer may lead to orbital shrinkage that is fast compared with the nuclear time-scales of the inner systems, and this can also be ruled out on observational grounds. Thus we may be able to set an upper as well as a lower limit to the strength of tidal friction, on the basis of observations. In a young hierarchical triple, provided that the orbits are fairly nearly orthogonal, tidal friction can serve to reduce the inner orbital period from months to days within a fairly short period of time, of order P2out/Pin. This may be a significant mechanism for producing young short-period binaries.

The Timescale Correlation Method: Distances to Planetary Nebulae with Halos

We present deep narrow-band CCD images of six PNe obtained with the Palomar 5 m telescope in the light of [N II] ?6584 + H? and [O III] ?5007. Several previously undetected structures are found, including faint multiple envelope structures surrounding three of the targets. The inclusion of these sources with data for other multiple envelope PNe published in the literature permit a statistical evaluation of the relationship between PN shells and the thermal pulses of the PN nucleus. In this paper, we investigate the dynamical timescale between successive envelopes, ?dyn, and the nuclear interpulse timescale, ?ip. Our results show that the deviations from the relation ?dyn = ?ip can be explained by the uncertainty in the Shklovsky distance to individual PNe, which is ?dsh/dsh${r sh}$ --> ~2. By imposing the constraint that ?dyn = ?ip, we find that it is possible to derive a PN distance indicator, which we name the timescale correlation distance, dTC. The derivation for dTC is independent of ad hoc and often unsupported assumptions inherent to the Shklovsky method.

Semiconvection mixing and its influence on case B mass exchange in massive binaries

In case of moderate matter mixing in the semi-convective zone the primary loses part of its mass on a nuclear time-scale. It looks like a CNO-supergiant during the first part of the helium burning stage. Another part of the mass is lost on the thermal time-scale. The primary looks like a WR star during the second part of the helium burning stage.

Impulsive effects of phase-locked pulse pairs on nuclear motion in the electronic ground state

The nonlinear effects of ultrashort phase-locked electronically resonant pulse pairs on the ground state nuclear motion are investigated theoretically. The pulse-pair propagator, momentum impulse, and displacement are determined in the weak field limit for pulse pairs separated by a time delay short on a nuclear time scale. Possible application to large amplitude vibrational excitation of the 104 cm−1 mode of α-perylene is considered and comparisons are made to other Raman excitation methods.

Parity violation in neutron statistical scattering

For nearly 100 nuclei, the rms expectation values of three neutron transmission observables sensitive to parity violation are predicted. Total enhancement factors {similar to}10{sup 2--}10{sup 4} in comparison to related observables in nucleon-nucleon scattering are found. To this end, a model of parity violation in the framework of statistical scattering theory is formulated and worked out in detail. General expressions are deduced from it, which connect the expectation values of parity forbidden {ital S}-matrix elements to average input parameters, namely, level densities, transmission coefficients, and spreading widths. The general results are discussed in terms of the relevant nuclear time scales, and the numerical predictions in terms of a parity forbidden strength function'' and several enhancement factors. Additionally, the role of finite energy resolution, crucial for energy averaging experiments, is studied.

Large Magellanic Cloud helium-rich peculiar blue supergiants and SN 1987A

The theoretical distribution of massive stars in the H-R diagram is compared to the revised data of Fitzpatrick and Garmany for the LMC. Preferred models of about 20 M solar masses undergo a thermal contraction at T(eff) about 35,000 K at the end of core hydrogen burning but reestablish thermal equilibrium to the red of the main sequence at T(eff) about 20,000 K after ignition of a hydrogen-burning shell. They then evolve on a nuclear time scale to T(eff) about 6000 K where they lose thermal equilibrium and jump to the Hayashi track. The theoretical and observed distributions agree with two significant exceptions: the blue thermal contraction gap is overpopulated compared to the theory, and there is a ledge crossing the center of the H-R diagram. The hypothesis that some of the observed stars in the blue gap are secondaries that have accreted helium-rich matter from deep within the hydrogen envelope of a red supergiant primary is explored. Some preliminary observational justification is given. 27 refs.

RAPID MASS TRANSFER IN BINARY SYSTEMS

This thesis investigates the conditions for rapid mass transfer in binary stars. Previous theoretical calculations and observations of binaries imply the existence of several different time scales for mass transfer: nuclear, thermal, or dynamical. Since the mass transfer rates differ by several orders of magnitude, it is important to know which time scales are relevant to different systems. Dynamical time-scale mass transfer is thought to cause substantial decreases in the orbital period through mass and angular momentum losses. Thermal time-scale mass transfer is thought to transform the appearance ofthe binary as mass exchange occurs in a short time. Binaries currently transferring mass are doing so on the longest, nuclear time scale. The characteristics of binaries which divide the three time scales are estimated by calculating the response of potential mass donors in two idealized limits: an adiabatic response, where the entropy profile does not change with mass loss, and a thermal response, where thermal relaxation is allowed but nuclear burning is not. A comparison ofthe changing surface radius, £ = d'nR/d'n3)l to the Roche-lobe radius implies the critical mass ratios for dynamically and thermally unstable mass transfer. These calculations are performed here for donors between 0.25 and 20 TtQ which would fill their Roche lobes before helium burning. The adiabatic mass-loss calculations have provided a clear relation between £ad and the donor's convective envelope mass fraction (fce), with a smaller dependence on the state of the interior. Low-mass ZAMS donors and models near the base ofthe giant branch have £ad ^> 1 change to £ad ~

Supernova 1987A - The evolution from blue to red

The evolution of stars with mass comparable to that of SK -69^deg^202, the progenitor of SN 1987A, from the main sequence to the Hayashi track is critically examined to determine why some models evolve to the red on nuclear time scales, some on thermal time scales, and some not at all. A parameterized series of structural models is used with the constraint that there be an active hydrogen burning shell. Thermal equilibrium solutions show there can be two stable solutions with different T_eff_ for the same helium core mass, M_He_, and a minimum M_He_ below which no blue thermal equilibrium solution is possible. The dependence of the equilibrium solutions on stellar mass, envelope composition, and mass loss are investigated. Forbidden and allowed regions of parameter space are defined, which account for the evolutionary behavior of various models in the literature. The resulting solutions quantitatively account for the "gap" in the H-R diagrams of massive stars for the Galaxy and the LMC and suggest that the outer envelopes are not substantially enriched in helium during the first passage from the main sequence to the Hayashi track. This work also shows that published evolutionary calculations for SN 1987A that go to the red and return to the blue by invoking the Ledoux criterion for convection evolve to the red on a thermal time scale and therefore are in quantitative disagreement with the observed distribution of stars across the H-R diagram and, in particular, with the observed location of the blue edge of the gap. In addition, published models using the Ledoux criterion do not have sufficiently red Hayashi tracks or avoid the thermal time scale jump to the true Hayashi track altogether and therefore do not properly account for the well-populated Hayashi track in the LMC. Evolutionary models for SN 1987A must account for the number of stars in the middle of the H-R diagram and for the return to the blue from the Hayashi track if SK -69^deg^202 is to be typical of LMC evolution.

A nuclear fluiddynamical approximation with one-plus-two-body viscosity

A semiclassical fluiddynamical model based on an usual scaling approximation (SCA) is extended to investigate the role of one and two-body dissipation in the widths of nuclear collective modes. The competition between one and two-body viscosity in:i) the collisionless (elastic) limit;ii) the hydrodynamical case andiii) the general viscoelastic regime is examined over the whole range of nuclear collision time scales. Numerical solutions are investigated for the first magnetic 2− twist mode in208Pb.

The decline and fall of classical novae

We address questions concerning the evolution of classical nova systems through outburst and the time scale for the return to their preoutburst state. Observations indicate that evolution on a purely nuclear burning time scale is not sufficient. We call attention to the sequence of events involving common envelope evolution which naturally occurs following thermonuclear runaways in the accreted hydrogen shells on the white dwarf components of these close binary systems. We estimate that a significant fraction of the envelope may be lost as a result of dynamical friction during a relatively short-lived common envelope phase which is experienced by most classical nova systems. The residual hydrogen-rich envelope matter may be consumed by nuclear burning or wind-driven mass loss or both on time scales compatible with observations of nova systems. As mass is consumed, the photospheric radius decreases and consequently the photospheric temperature rises. The associated hardening of the radiation from novae causes them progressively to become UV, EUV, and ultimately soft X-ray sources. Estimates of the fluxes during these late stages are provided and shown to be compatible with their being detected with EXOSAT or equivalent X-ray facilities. Such detection would provide strong confirmation of theoretical models for the classicalmore » novae. We also point out that infrared emission from X-ray heated grains may be detectable during the soft X-ray phase.« less

On turbulent mixing

Several authors (myself included!) have suggested that turbulent mixing takes place in some, if not all, stars, and in particular that such mixing can explain the low solar neutrino flux. This turbulence is thought to be caused by differential rotation produced by braking due to angular momentum loss in a stellar wind, and/or to the effect of meridional circulation currents in redistributing angular momentum. Whilst such instabilities may exist even in the presence of a stabilizing distribution of chemical composition, they do not necessarily cause mixing. To be effective in mixing, the energy available to the instability be it differential rotation or any other mechanism, has to be sufficient to lift the helium rich matter in the interior of the star to the outer regions. This requires where Erot is the kinetic energy in rotation, Eg the gravitational energy, τth the thermal time scale and τnuc the nuclear evolution time scale of the star.