Narrow Results

The history of science can be recounted in many ways: by addressing the work of one person or school; by starting with the ancients and working chronologically up to the present; by focusing on a particular century; or by tracing a particular important idea as far back and forward as it can be found. The present discussion does none of these. Rather, it adopts the ordering of a standard introductory astronomy textbook, from the solar system via stars and galaxies, to the universe as a whole, and in each regime picks out a few issues that were controversial or wrongly decided for a long time. For each, I attempt to identify a duration of the period of uncertainty or error and some of the causes of the confusion. This is surely not an original idea, though I am not aware of having encountered it elsewhere, and it is not one that is likely to appeal to most 21st century historians of science, for whom the question "Who first got it right?" is not necessarily an important, or even appropriate, one. Some of the stories have been told as historical introductions to conferences and are here summarized and brought up to date. Others I had not previously addressed.

Stars that evolve near the Galactic massive black hole show strange behaviors. The spectroscopic features of these stars show that they must be old. But their luminosities are much higher than the amounts that are predicted by the current stellar evolutionary models, which means that they must be active and young stars. In fact, this group of stars shows signatures of old and young stars, simultaneously. This is a paradox known as the “paradox of youth problem” (PYP). Some people tried to solve the PYP without supposing dark matter (DM) effects on stars. But, in this work, we implemented Weakly Interacting Massive Particles (WIMPs) annihilation as a new source of energy inside such stars. This implementation is logical for stars that evolve at high DM density environments. The new source of energy causes stars to follow different evolutionary paths on the H-R diagram in comparison with classical stellar evolutionary models. Increasing DM density in stellar evolutionary simulations causes the deviations from the standard H-R diagrams becomes more pronounced. By investigating the effects of WIMPs density on stellar structures and evolutions, we concluded that by considering DM effects on stars at the Galactic center, it is possible to solve the PYP. In addition to DM effect, complete solutions to PYP must consider all extreme and unique physical conditions that are present near the Galactic massive black hole.

Understanding how massive stars die as supernovae (SNe) is a crucial question in modern astrophysics. SNe are powerful stellar explosions and key drivers in the cosmic baryonic cycles by injecting their explosion energy and heavy elements to the interstellar medium that forms new stars. After decades of effort, astrophysicists have built up a stand model for the explosion mechanism of massive stars. However, this model is challenged by new kinds of stellar explosions discovered in the recent transit surveys. In particular, the new population called superluminous SNe, which are a hundred times brighter than typical SNe, is revolutionizing our understanding of SNe. New studies suggest the superluminous SNe are associated with the unusual demise of very massive stars and their extreme SNe powered by the radioactive isotopes or compact objects formed after the explosion. Studying these SNe fills a gap of knowledge between the death of massive stars and their explosions; furthermore, we may apply their intense luminosity to light up the distant universe. This paper aims to provide a timely review of superluminous SNe physics, focusing on the latest development of their theoretical models.

The diffusion coefficients that are related to the neutrino opacities are calculated in such a way that the formation of nuclear pasta and homogeneous matter at low densities are taken into account. Two methods are developed to build the pasta phase and their differences are outlined. One of them is chosen as part of a complete equation of state used in the calculation of the diffusion coefficients. Our results show that the mean free paths are significantly altered by the presence of nuclear pasta in stellar matter when compared with the results obtained with pure homogeneous matter. These differences in neutrino opacities will have consequences in the calculation of the Kelvin-Helmholtz phase of protoneutron stars.

Charge-changing transitions play a significant role in stellar weak-decay processes. The fate of the massive stars is decided by these weak-decay rates including lepton (positron and electron) captures rates, which play a consequential role in the dynamics of core collapse. As per previous simulation results, weak interaction rates on nickel (Ni) isotopes have significant influence on the stellar core vis-à-vis controlling the lepton content of stellar matter throughout the silicon shell burning phases of high mass stars up to the presupernova stages. In this paper, we perform a microscopic calculation of Gamow–Teller (GT) charge-changing transitions, in the $\beta$-decay and electron capture (EC) directions, for neutron-rich Ni isotopes (${}^{66-71}$Ni). We further compute the associated weak-decay rates for these selected Ni isotopes in stellar environment. The computations are accomplished by employing the deformed proton–neutron quasiparticle random phase approximation (pn-QRPA) model. A recent study showed that the deformed pn-QRPA theory is well suited for the estimation of GT transitions. The astral weak-decay rates are determined over densities in the range of 10–10${}^{11}$g/cm³ and temperatures in the range of 0.01$\times 10^9$–30$\times 10^9$K. The calculated lepton capture rates are compared with the previous calculation of Pruet and Fuller (PF). The overall comparison demonstrates that, at low stellar densities and high temperatures, our EC rates are bigger by as much as two orders of magnitude. Our results show that, at higher temperatures, the lepton capture rates are the dominant mode for the stellar weak rates and the corresponding lepton emission rates may be neglected.

In a recent study by Cole et al. [A. L. Cole et al., Phys. Rev. C86 (2012) 015809], it was concluded that quasi-particle random phase approximation (QRPA) calculations show larger deviations and overestimate the total experimental Gamow–Teller (GT) strength. It was also concluded that QRPA calculated electron capture rates exhibit larger deviation than those derived from the measured GT strength distributions. The main purpose of this study is to probe the findings of the Cole et al. paper. This study gives useful information on the performance of QRPA-based nuclear models. As per simulation results, the capturing of electrons that occur on medium heavy isotopes have a significant role in decreasing the ratio of electron-to-baryon content of the stellar interior during the late stages of core evolution. We report the calculation of allowed charge-changing transitions strength for odd-$A$$fp$-shell nuclei (${}^{45}$Sc and ${}^{55}$Mn) by employing the deformed pn-QRPA approach. The computed GT transition strength is compared with previous theoretical calculations and measured data. For stellar applications, the corresponding electron capture rates are computed and compared with rates using previously calculated and measured GT values. Our finding shows that our calculated results are in decent accordance with measured data. At higher stellar temperature, our calculated electron capture rates are larger than those calculated by independent particle model (IPM) and shell model. It was further concluded that at low temperature and high density regions, the positron emission weak-rates from ${}^{45}$Sc and ${}^{55}$Mn may be neglected in simulation codes.

In several classes of modified gravity theories, extra degrees of freedom are not completely screened in the interiors of stellar and substellar objects. In such theories, the hydrostatic equilibrium condition inside these objects is altered. Moreover, the interior structures of these objects might have a small pressure anisotropy induced by several physical phenomena, including rotation and magnetic fields. All these effects, both individually and collectively, induce changes in predicted stellar observables. Such changes have an impact on different phases of the stellar life cycle, starting from its birth to its death, covering almost all the branches of the Hertzsprung–Russell diagram. The aim of this work is to systematically review the current literature on the topic. We discuss the main results and constraints obtained on a class of modified gravity theories.

The astronomical dark matter is an essential component of the Universe and yet its nature is still unresolved. It could be made of neutral and massive elementary particles which are their own antimatter partners. These dark matter species undergo mutual annihilations whose effects are briefly reviewed in this article. Dark matter annihilation plays a key role at early times as it sets the relic abundance of the particles once they have decoupled from the primordial plasma. A weak annihilation cross section naturally leads to a cosmological abundance in agreement with observations. Dark matter species subsequently annihilate — or decay — during Big Bang nucleosynthesis and could play havoc with the light element abundances unless they offer a possible solution to the ⁷Li problem. They could also reionize the intergalactic medium after recombination and leave visible imprints in the cosmic microwave background. But one of the most exciting aspects of the question lies in the possibility to indirectly detect the dark matter species through the rare antimatter particles — antiprotons, positrons and antideuterons — which they produce as they currently annihilate inside the galactic halo. Finally, the effects of dark matter annihilation on stars is discussed.

New direct experimental methods and techniques, combined with the development of new theoretical tools have opened new avenues to explore nuclear reactions of significance for nucleosynthesis at or near the actual temperatures of stellar burning. The main difficulty of direct measurements is determined by the background, which, together with the low cross sections, set a limit on the energy range that can be investigated with a simple setup on the earth’s surface. Essentially there are three sources of background, cosmic rays, environmental radioactivity and beam-target induced nuclear reactions. Each of these sources produces background of a different nature and energy, so that each reaction studied needs special care to suppress the relevant background component. We will show two different experimental approaches that have been used to study processes of astrophysical interest. In particular, we will focus our attention on underground experiments and the recoil mass separator approach used to measure ³He(³He,2p)⁴He and ³He(⁴He,γ)⁷Be.

The possibility of measuring the internal rotation of the Sun and stars thanks to helio- and asteroseismology offers tremendous constraints on hydro- and magnetohydrodynamical processes acting in stellar interiors. Understanding the processes responsible for the transport of angular momentum in stellar interiors is crucial as they will also influence the transport of chemicals and thus the evolution of stars. Here we present some of the key results obtained in both fields and how detailed seismic analyses can provide stringent constraints on the physics of angular momentum transport in the interior of low mass stars and potentially rule out some candidates.

A scenario for the formation of an isolated X-ray pulsar 1E161348-5055 with an anomalously long period of 6.7 hours is proposed. It is shown that this pulsar can be a descendant of a massive X-ray binary system, which disintegrated about 2000 years ago after a supernova explosion caused by the core collapse of a massive component. X-ray radiation of this object in the present epoch is generated as a result of accretion of matter onto (about 10 million years old) neutron star from the residual non-Keplerian accretion disk. The pulsar’s nebula RCW 103 is a supernova remnant formed by the explosion of its massive companion in the final evolutionary phase of a massive binary system.

Selected scientific highlights are presented from the first 5 years of observations of the gamma-ray sky by ESA's INTEGRAL space telescope. Its unprecedented angular resolution and sensitivity at high energies (≳ 20 keV) has allowed INTEGRAL to detect around 500 objects, many of which are new. Sources that have been classified are predominantly represented by active galactic nuclei (AGN) and X-ray binaries (XRBs) whereby a compact object (a supermassive black hole in AGN, usually a neutron star in XRBs) accretes matter from a large disk (AGN) or from a stellar companion (XRBs, often mediated by a disk). Together with unclassified sources, they account for nearly all of the diffuse Galactic background emission. Furthermore, INTEGRAL has created an all-sky map of the 511 keV distribution helping to identify potential dark matter sites. The distribution of Al-26 follows massive star-forming regions and reflects the rotation of the Galaxy. Gamma-ray bursts (GRBs) are detected in the wide field of view (FOV) at a rate of 1 per month, but INTEGRAL's design also enables it to detect GRBs outside its FOV. Previously rare, XRBs with supergiant companions are an emerging class. This underscores INTEGRAL's ability to peer through the dust that enshrouds these sources and which made them invisible to previous X-ray surveys. Their increasing numbers (as well as those of other classes) offer larger samples on which to perform statistical analyses. A synthetic view of populations of γ-ray sources is instrumental for highlighting signatures of stellar and galactic evolution. In addition, it permits a speculation on the nature of the roughly 100 sources that remain unclassified.

Recent work has indicated that WIMP annihilation in stellar cores has the potential to contribute significantly to a star's total energy production. We report on progress in simulating the effects of WIMP capture and annihilation upon stellar structure and evolution near supermassive black holes, using the new DarkStars code. Preliminary results indicate that low-mass stars are the most influenced by WIMP annihilation, which could have consequences for upcoming observational programs.

The evolution of close-orbit progenitor binaries of double neutron star (DNS) systems leads to supernova (SN) explosions of ultra-stripped stars. The amount of SN ejecta mass is very limited from such, more or less, naked metal cores with envelope masses of only 0.01 - 0.2 M_⊙. The combination of little SN ejecta mass and the associated possibility of small NS kicks is quite important for the characteristics of the resulting DNS systems left behind. Here, we discuss theoretical predictions for DNS systems, based on Case BB Roche-lobe overflow prior to ultra-stripped SNe, and briefly compare with observations.

The explosion of ultra-stripped stars in close binaries may explain new discoveries of weak and fast optical transients. We have demonstrated that helium star companions to neutron stars (NSs) may evolve into naked metal cores as low as ∼ 1.5 M_⊙, barely above the Chandrasekhar mass limit, by the time they explode. Here we present a new systematic investigation of the progenitor evolution leading to such ultra-stripped supernovae (SNe), in some cases yielding pre-SN envelopes of less than 0.01 M_⊙. We discuss the nature of these SNe (electron-capture vs iron core-collapse) and their observational light-curve properties. Ultra-stripped SNe are highly relevant for binary pulsars, as well as gravitational wave detection of merging NSs by LIGO/VIRGO, since these events are expected to produce mainly low-kick NSs in the mass range 1.10 − 1.80 M_⊙.

We announce the public release of the 'dark' stellar evolution code DarkStars. The code simultaneously solves the equations of WIMP capture and annihilation in a star with those of stellar evolution assuming approximate hydrostatic equilibrium. DarkStars includes the most extensive WIMP microphysics of any dark evolution code to date. The code employs detailed treatments of the capture process from a range of WIMP velocity distributions, as well as composite WIMP distribution and conductive energy transport schemes based on the WIMP mean-free path in the star. We give a brief description of the input physics and practical usage of the code, as well as examples of its application to dark stars at the Galactic centre.

Using a fully general relativistic implementation of ideal magnetohydrodynamics with no assumed symmetries in three spatial dimensions, the dynamics of magnetized, rigidly rotating neutron stars are studied. Beginning with fully consistent initial data constructed with Magstar, part of the Lorene project, we study the dynamics and stability of rotating, magnetized polytropic stars as models of neutron stars. Evolutions suggest that some of these rotating, magnetized stars may be minimally unstable occurring at the threshold of black hole formation.

We discuss the identification of SGR/AXPs progenitors and associated birth events. We argue that a possible interval of 18 — 40M_⊙ of rotating progenitors is indicated, and that associated supernovae may be driven by the magnetar under certain conditions. This does not guarantee a super-energetic event, although it is shown that this may be the case of the recent identification CXOU J171405.7-381031/CTB37B. Magnetars are predicted to be massive, M ≥ 1.6M_⊙ right at their birth.