Many physically motivated extensions to general relativity (GR) predict significant deviations at energies present in massive neutron stars. We report the measurement of a 2.01 \(\pm \) 0.04 solar mass (M\(_\odot \)) pulsar in a 2.46-h orbit around a 0.172 \(\pm \) 0.003 M\(_\odot \) white dwarf. The high pulsar mass and the compact orbit make this system a sensitive laboratory of a previously untested strong-field gravity regime. Thus far, the observed orbital decay agrees with GR, supporting its validity even for the extreme conditions present in the system. The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detection experiments. Additionally, the system strengthens recent constraints on the properties of dense matter and provides novel insight to binary stellar astrophysics and pulsar recycling.

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A massive pulsar in a compact relativistic binary

The temporal evolution of the optical spectra of various types of supernovae (SNe) is illustrated, in part to aid observers classifying supernova candidates. Type II SNe are defined by the presence of hydrogen, and they exhibit a very wide variety of photometric and spectroscopic properties. Among hydrogen-deficient SNe (Type I), three subclasses are now known: those whose early-time spectra show strong Si II (Ia), prominent He I (Ib), or neither Si II nor He I (Ic). The late-time spectra of SNe Ia consist of a multitude of blended emission lines of iron-group elements; in sharp contrast, those of SNe Ib and SNe Ic (which are similar to each other) are dominated by several relatively unblended lines of intermediatemass elements. Although SNe Ia, which result from the thermonuclear runaway of white dwarfs, constitute a rather homogeneous subclass, important variations in their photometric and spectroscopic properties are undeniably present. SNe Ib/Ic probably result from core collapse in massive stars largely stripped of their hydrogen (Ib) and helium (Ic) envelopes, and hence they are physically related to SNe II. Indeed, the progenitors of some SNe II seem to have only a low-mass skin of hydrogen; their spectra gradually evolve to resemble those of SNe Ib. In addition to the two well-known photometric subclasses (linear and plateau) of SNe II, which may exhibit minor spectroscopic differences, there is a new subclass (SNe IIn) distinguished by relatively narrow emission lines with little or no P Cygni absorption component and slowly declining light curves. These objects probably have unusually dense circumstellar gas with which the ejecta interact.

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Optical spectra of supernovae

The standard model of particle physics predicts two phase transitions that are relevant for the evolution of the early Universe. One, the quantum chromodynamics transition, involves the strong force that binds quarks into protons and neutrons. Despite much theoretical effort, the nature of this transition remains ambiguous. Now Aoki et al. report computationally demanding calculations that suggest that there was no true phase transition. Instead, an analytic crossover took place, involving a rapid, continuous change with temperature as opposed to a jump. This means that it will be difficult to find experimental evidence of a transition from astronomical observations. The standard model of particle physics predicts two transitions that are relevant for the evolution of the early Universe. Computationally demanding calculations now reveal that a real phase transition did not occur, but rather an analytic crossover, involving a rapid change (as opposed to a jump) as the temperature varies. Quantum chromodynamics (QCD) is the theory of the strong interaction, explaining (for example) the binding of three almost massless quarks into a much heavier proton or neutron—and thus most of the mass of the visible Universe. The standard model of particle physics predicts a QCD-related transition that is relevant for the evolution of the early Universe. At low temperatures, the dominant degrees of freedom are colourless bound states of hadrons (such as protons and pions). However, QCD is asymptotically free, meaning that at high energies or temperatures the interaction gets weaker and weaker1,2, causing hadrons to break up. This behaviour underlies the predicted cosmological transition between the low-temperature hadronic phase and a high-temperature quark–gluon plasma phase (for simplicity, we use the word ‘phase’ to characterize regions with different dominant degrees of freedom). Despite enormous theoretical effort, the nature of this finite-temperature QCD transition (that is, first-order, second-order or analytic crossover) remains ambiguous. Here we determine the nature of the QCD transition using computationally demanding lattice calculations for physical quark masses. Susceptibilities are extrapolated to vanishing lattice spacing for three physical volumes, the smallest and largest of which differ by a factor of five. This ensures that a true transition should result in a dramatic increase of the susceptibilities. No such behaviour is observed: our finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied). As such, it will be difficult to find experimental evidence of this transition from astronomical observations.

https://arxiv.org/pdf/hep-lat/0611014.pdf

The order of the quantum chromodynamics transition predicted by the standard model of particle physics

The major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae (SNe Ia) are related to the companion star of their accreting white dwarf progenitor (which determines the accretion rate and consequently the carbon ignition density) and the flame speed after the carbon ignition. We calculate explosive nucleosynthesis in relatively slow deflagrations with a variety of deflagration speeds and ignition densities to put new constraints on the above key quantities. The abundance of the Fe group, in particular of neutron-rich species like 48Ca,50Ti,54Cr,54,58Fe, and 58Ni, is highly sensitive to the electron captures taking place in the central layers. The yields obtained from such a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. To avoid excessively large ratios of 54Cr/56Fe and 50Ti/56Fe, the central density of the average white dwarf progenitor at ignition should be as low as 2 ? 109 g cm-3. To avoid the overproduction of 58Ni and 54Fe, either the flame speed should not exceed a few percent of the sound speed in the central low Ye layers or the metallicity of the average progenitors has to be lower than solar. Such low central densities can be realized by a rapid accretion as fast as -->img1.gif 1 ? 10-7 M? yr-1. In order to reproduce the solar abundance of 48Ca, one also needs progenitor systems that undergo ignition at higher densities. Even the smallest laminar flame speeds after the low-density ignitions would not produce sufficient amount of this isotope. We also found that the total amount of 56Ni, the Si-Ca/Fe ratio, and the abundance of some elements like Mn and Cr (originating from incomplete Si burning), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2 ? 107 g cm-3.

https://iopscience.iop.org/article/10.1086/313278/pdf

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

▪ Abstract We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with...

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Nucleosynthesis in asymptotic giant branch stars: Relevance for galactic enrichment and solar system formation

Some eclipsing variables are observed to undergo orbital period modulations of amplitude ΔP/P∼10 −5 over time scales of decades or longer. These modulations can be explained by the gravitational coupling of the orbit to variations in the shape of a magnetically active star in the system. The variable deformation of the active star is produced by variations in the distribution of angular momentum as the star goes through its activity cycle. This mechanism typically requires that the active star be variable at the ΔL/L⇒0.1 level, and be differentially rotating at the ΔΩ/Ω⇒0.01 level

A Mechanism for Orbital Period Modulation in Close Binaries

This review volume provides an overview of the current status of nuclear astrophysics. Experts in their respective fields provide in ten contributions detailed surveys of our understanding of different astrophysical environments and their connection to nuclear processes. Special emphasis is given to the interdisciplinar nature of the field: astronomy, nuclear physics, astrophysics and elementary particle physics are equally involved. The topics of the book range from the collection of observational facts with astronomical methods to experimental work and theoretical models to describe the nuclear processes involved in different astrophysical scenarios. Astrophysical models and calculations simulate how nuclear processes contribute to driving the evolution of stars, interstellar matter and the whole universe.

Nuclei in the cosmos

The diffusion rate of baryons through the big-bang plasma is calculated Fluctuations in baryon density in the early Universe lead to inhomogeneities in the neutron-proton ratio, due to the differential diffusion of these particles through the radiation plasma For certain types of nonlinear fluctuations, some nucleosynthesis would occur in very neutron-rich regions Nuclear products of homogeneous neutron-enriched regions are evaluated numerically using a standard reaction network and these results are used to estimate final abundances in an inhomogeneous universe Net deuterium and lithium abundances tend to increase and the net helium abundance tends to decrease compared to an unperturbed standard model It is suggested that pronounced nonlinear baryon-density fluctuations produced in QCD- or electroweak-epoch phase transitions could alter abundances sufficiently to make a closed baryonic universe consistent with current observations of these elements In such a model the abundance of heavier elements (C,N,O, etc) increases significantly and approaches observable levels Abundances can be used to place constraints on extreme scenarios for phase transitions at these epochs

Cosmological baryon diffusion and nucleosynthesis

The first results are reported from a program to reanalyze the cooling of neutron stars by including the direct Urca process in calculations. It is found that the surface temperature of a young neutron star drops dramatically after about 100 yr if the direct Urca process is allowed and nucleons do not become superfluid. If nucleon superfluidity occurs throughout the direct Urca region, the surface temperature drops to a value determined by the superfluid transition temperature after about 100 yr and decreases slowly for the next 100,000 yr, at which time surface photon cooling takes over. By comparison with observational data, it is found that superfluid transition temperatures of the order of 10 exp 9 K are required in the whole direct Urca inner core.

The cooling of neutron stars by the direct Urca process

The influence of the QCD phase transition on the standard cosmological model is examined. Physical mechanisms are analyzed which transport energy and baryon number during the cosmological transition from free-quark (unconfined) to hadron (confined) phases of matter, with particular attention to an effect described by Witten---the concentration of baryons in low-entropy bubbles. Two limiting regimes of transport, hydrodynamic flow and neutrino conduction, are discussed and their relative importance under various circumstances is clarified. The spatial distribution of specific entropy at the end of the transition, and its subsequent evolution, is described using a spherical shell model. Inhomogeneities which persist until the epoch of nucleosynthesis can lead to nuclear products with very different chemical composition from the standard hot big bang, both because of contamination from regions with entropy orders of magnitude less than the cosmic average and because of variation in neutron-to-proton ratio caused by differential diffusion of these reactants into voids of higher-than-average entropy. These events may produce observable distortions in cosmic light-element abundances; in particular, the deuterium abundance may be increased by orders of magnitude. Thus, it is perhaps more appropriate to regard nucleosynthesis as a constraint on the parameters of the phase transition than as a precise probe of the cosmic baryon density or the number of neutrino species.

James H. Applegate

Papers

A Mechanism for Orbital Period Modulation in Close Binaries

Nuclei in the cosmos

Cosmological baryon diffusion and nucleosynthesis

The cooling of neutron stars by the direct Urca process

Relics of cosmic quark condensation.