Strong neutrino cooling by cycles of electron capture and ß- decay in neutron star crusts.

The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities, neutrino cooling and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, ß(-) decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism has been studied in the context of white dwarfs and type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.

Connecting neutron star observations to three-body forces in neutron matter and to the nuclear symmetry energy.

Using a phenomenological form of the equation of state of neutron matter near the saturation density which has been previously demonstrated to be a good characterization of quantum Monte Carlo simulations, we show that currently available neutron star mass and radius measurements provide a significant constraint on the equation of state of neutron matter. At higher densities we model the equation of state by using polytropes and a quark matter model. We show that observations offer an important constraint on the strength of the three-body force in neutron matter, and thus some theoretical models of the three-body force may be ruled out by currently available astrophysical data. In addition, we obtain an estimate of the symmetry energy of nuclear matter and its slope that can be directly compared to the experiment and other theoretical calculations.

Constraints on the density dependence of the symmetry energy.

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.

Evolution of proto-neutron stars with quarks.

Neutrino fluxes from proto-neutron stars with and without quarks are studied. Observable differences become apparent after 10-20 s of evolution. Sufficiently massive stars containing negatively charged, strongly interacting, particles collapse to black holes during the first minute of evolution. Since the neutrino flux vanishes when a black hole forms, this is the most obvious signal that quarks (or other types of strange matter) have appeared. The metastability time scales for stars with quarks are intermediate between those containing hyperons and kaon condensates.

Endogenous protein phosphorylation in rat brain mitochondria: occurrence of a novel ATP-dependent form of the autophosphorylated enzyme succinyl-CoA synthetase.

When rat brain mitochondria are incubated with [gamma-32P]ATP, there is a rapid (10 s) phosphorylation of proteins designated E1 and F of M.W. 42,000 and 32,000, respectively. Although [gamma-32P]ATP was the preferred substrate for protein F, a small amount of labeling did occur with [gamma-32P]GTP. Phosphorylation of E1 was absolutely ATP-dependent. On the other hand, a 32,000 M.W. protein from rat liver mitoplasts (mitochondria devoid of an outer membrane) was highly phosphorylated when [gamma-32P]GTP was used but not at all phosphorylated within short time periods with [gamma-32P]ATP. Both the ATP-labeled brain phosphoprotein F and GTP-labeled liver protein migrated to identical positions on high-resolution two-dimensional polyacrylamide gels, and both contained acid-labile phosphoryl groups. Furthermore, both phosphoproteins were identified as the autophosphorylated subunit of succinyl-CoA synthetase (SCS, EC 6.2.1.4) by using antibody directed against purified GTP-dependent porcine SCS. However, immunotitration experiments with anti-porcine SCS revealed that ATP- and GTP-labeled protein F in brain differed in their interactions with antibody, suggesting that in rat brain mitochondria two different forms of the enzyme exist that are immunologically distinct and differ in substrate specificity. When mitochondrial preparations enriched in particular brain cell or subcellular types were examined, an unequal distribution of E1 and the two forms of protein F were observed. A brain subfraction containing neuronal cell body and glial mitochondria (CM) was found to contain E1 and approximately equal amounts of the ATP- and GTP-dependent forms of protein F. Light synaptic mitochondria (SM1) contained ATP-dependent protein F almost exclusively and were depleted in E1. Dense synaptic mitochondria (SM2) are rich in the ATP form of SCS but also contain low amounts of the GTP enzyme.

High-performance liquid chromatography of acid-stable and acid-labile phosphoamino acids.

A high-performance liquid chromatographic system has been developed which permits the separation of both acid-stable and acid-labile phosphoamino acids. An anion-exchange resin and two buffers of different ionic strength and near neutral pH are used. A low-ionic-strength buffer is used for the separation of N-omega-phosphoarginine and N-epsilon-phospholysine, while the higher-ionic-strength buffer permits the clear separation of tau-phosphohistidine, omicron-phosphoserine and omicron-phosphothreonine. An in-stream fluorometric detection system using omicron-phthalaldehyde permits the rapid analysis of samples containing as little as 25 pmoles of phosphoamino acid. This method has been applied to the detection of tau-phosphohistidine from alkaline digests of chemically phosphorylated calf thymus histone 4 and bovine myelin basic protein.

A simple preparation of N-phosphorylated lysine and arginine.

An improved specific synthesis of N-epsilon-phospholysine is described. Phosphorylation under basic conditions of the copper chelate of L-lysine with phosphorus oxychloride and subsequent removal of the copper ion affords N-epsilon-phospholysine as the sole phosphorylated product. Similar treatment of L-arginine yields N-omega-phosphoarginine as the sole product which is identical to that produced enzymatically.