Andrew W. Steiner

Our new paper has been posted on the arXiv:

Observationally inferred superburst ignition depths are shallower than models predict. We address this discrepancy by reexamining the superburst trigger mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic electron captures trigger superbursts. We find that all electron capture reactions are thermally stable in accreting neutron star oceans and thus are not a viable trigger mechanism. Fusion reactions other than ¹²C + ¹²C are infeasible as well since the possible reactants either deplete at much shallower depths or have prohibitively large Coulomb barriers. Thus we confirm the proposal of Cumming & Bildsten and Strohmayer & Brown that ¹²C + ¹²C triggers superbursts. We then examine the ¹²C + ¹²C fusion rate. The reaction cross-section is experimentally unknown at astrophysically relevant energies, but resonances exist in the ¹²C + ¹²C system throughout the entire measured energy range. Thus it is likely, and in fact has been predicted, that a resonance exists near the Gamow peak energy ~ 1.5 MeV. For such a hypothetical 1.5 MeV resonance, we derive both a fiducial value and upper limit to the resonance strength and find that such a resonance could decrease the theoretically predicted superburst ignition depth by up to a factor of 4; in this case, observationally inferred superburst ignition depths would accord with model predictions for a range of plausible neutron star parameters. Said differently, such a resonance would decrease the temperature required for unstable ¹²C ignition at a column depth 10¹² g/cm² from 6 x 10⁸ K to 5 x 10⁸ K. Determining the existence of a strong resonance in the Gamow window requires measurements of the ¹²C + ¹²C cross-section down to a center-of-mass energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.

Our paper has been published in Phys. Rev. Lett.:

Collisions involving ¹¹²Sn and ¹²⁴Sn nuclei have been simulated with the improved quantum molecular dynamics transport model. The results of the calculations reproduce isospin diffusion data from two different observables and the ratios of neutron and proton spectra. By comparing these data to calculations performed over a range of symmetry energies at saturation density and different representations of the density dependence of the symmetry energy, constraints on the density dependence of the symmetry energy at subnormal density are obtained. The results from the present work are compared to constraints put forward in other recent analyses.

Our new paper on crustal shear oscillations is now on the arXiv:

We show that the fundamental seismic shear mode, observed as a quasi-periodic oscillation in giant flares emitted by highly-magnetized neutron stars, is particularly sensitive to the nuclear physics of the crust. The identification of an oscillation at ~ 30 Hz as the fundamental crustal shear mode requires a nuclear symmetry energy that depends very weakly on density near saturation. If the nuclear symmetry energy varies more strongly with density, then lower frequency oscillations, previously identified as torsional Alfven modes of the fluid core, could instead be associated with the crust. If this is the case, then future observations of giant flares should detect oscillations at around 18 Hz. An accurate measurement of the neutron skin thickness of lead will also constrain the frequencies predicted by the model.

is now published in Phys. Rev. C 79, 015802 (2009).

Abstract: We calculate the neutrino emissivity of superfluid neutron matter in the inner crust of neutron stars. We find that neutrino emission due to fluctuations resulting from the formation of Cooper pairs at finite temperature is highly suppressed in non-relativistic systems. This suppression of the pair breaking emissivity in a simplified model of neutron matter with interactions that conserve spin is of the order of v_F⁴ for density fluctuations and v_F² for spin fluctuations, where v_F is the Fermi velocity of neutrons. The larger suppression of density fluctuations arises because the dipole moment of the density distribution of a single component system does not vary in time. For this reason, we find that the axial current response (spin fluctuations) dominates. In more realistic models of neutron matter which include tensor interactions where the neutron spin is not conserved, neutrino radiation from bremsstrahlung reactions occurs at order v_F⁰. Consequently, even with the suppression factors due to superfluidity, this rate dominates near T_C. Present calculations of the pair-breaking emissivity are incomplete because they neglect the tensor component of the nucleon-nucleon interaction.

Our new paper on r-mode oscillations is now published in Phys. Rev. D 78, 123007 (2008).

Abstract: We determine characteristic time scales for the viscous damping of r-mode oscillations in rapidly rotating compact stars that contain quark matter. We present results for the color-flavor-locked (CFL) phase of dense quark matter, in which the up, down, and strange quarks are gapped, as well as the normal (ungapped) quark phase. While the ungapped quark phase supports a temperature window 10⁸ K<=T<=5×10⁹ K where the r mode is damped even for rapid rotation, the r mode in a rapidly rotating pure CFL star is not damped in the temperature range 10¹⁰ K<=T<=10¹¹ K. Rotating hybrid stars with quark matter cores display an instability window whose width is determined by the amount of quark matter present, and they can have large spin frequencies outside this window. Except at high temperatures T>=10¹⁰ K, the presence of a quark phase allows for larger critical frequencies and smaller spin periods compared to rotating neutron stars. If low-mass x-ray binaries contain a large amount of ungapped or CFL quark matter, then our estimates of the r-mode instability suggest that there should be a population of rapidly rotating binaries at nu>~1000 Hz which have not yet been observed.

This week I’ll be in Athens, Ohio working on the minimal neutron star cooling model.

This week I’m at the 5th ANL/MSU/JINA/INT FRIB Workshop on Bulk Nuclear Properties.

I’m away and will be back next week!

Our paper on the equation of state for core-collapse supernovae is on the arXiv:

Abstract: Neutrinos emitted during the collapse, bounce and subsequent explosion provide information about supernova dynamics. The neutrino spectra are determined by weak interactions with nuclei and nucleons in the inner regions of the star, and thus the neutrino spectra are determined by the composition of matter. The composition of stellar matter at temperature ranging from T=1-3 MeV and densities ranging from 10^-5 to 0.1 times the saturation density is explored. We examine the single-nucleus approximation commonly used in describing dense matter in supernova simulations and show that, while the approximation is accurate for predicting the energy and pressure at most densities, it fails to predict the composition accurately. We find that as the temperature and density increase, the single nucleus approximation systematically overpredicts the mass number of nuclei that are actually present and underestimates the contribution from lighter nuclei which are present in significant amounts.

This week I’m at the workshop entitled, “The Equation of State at Nonzero Density and Temperature, and its Application in Astrophysics” at Argonne Nat’l Lab.