Astrophysical Sites Research Articles

Particle acceleration mechanisms

High-energy particle acceleration is observed to proceed in a diverse variety of astrophysical sites ranging from the terrestrial aurorae to the most distant quasars. Particle acceleration is a fairly common channel for the release of large-scale kinetic, rotational, and magnetic energy. Physical mechanisms include electrostatic acceleration, stochastic processes, and diffusive shock energization. Cosmic-ray energy spectra have shapes which reflect escape, collisional, and radiative losses. The overall acceleration efficiency is controlled by the low-energy particle injection which may, in turn, feed back into the energization. Recent observational developments, which illustrate these general principles and raise fresh questions, are briefly summarized.

Particle Acceleration Mechanisms

AbstractHigh-energy particle acceleration is observed to proceed in a diverse variety of astrophysical sites ranging from the terrestrial aurorae to the most distant quasars. Particle acceleration is a fairly common channel for the release of large-scale kinetic, rotational, and magnetic energy. Physical mechanisms include electrostatic acceleration, stochastic processes and diffusive shock energization. Cosmic-ray energy spectra have shapes which reflect escape, collisional, and radiative losses. The overall acceleration efficiency is controlled by the low-energy particle injection which may, in turn, feed back into the energization. Recent observational developments, which illustrate these general principles and raise fresh questions, are briefly summarized.Subject heading: acceleration of particles

Parametric study of heavy element isotopic abundances in SiC grains

Abstract— This paper presents a new interpretation of the non‐solar isotopic composition of a series of elements from Si to Sm found in small‐sized (μm) SiC grains of presumably circumstellar origin. It relies on a parametrized model that aims at investigating in as broad a way as possible the neutron exposures or range of such exposures that could replicate the series of measured isotopic compositions without relying on any specific (and sometimes highly uncertain) astrophysical scenario.It is found that the general trends of the isotopic data of Si, Ca, Ti, Sr, Kr, Xe, Ba, Nd and Sm can be reproduced with a much weaker neutron exposure than contemplated in previous studies. Such a neutron irradiation history does not necessitate the classical assumption of an admixture of a s‐process component to normal solar abundances. Instead, the linear correlations observed in three‐isotope plots are interpreted in terms of a mixture of components having experienced slightly different neutron exposures. In this framework, predictions are provided for all the elements from Si to Bi, and implications for possible astrophysical sites are discussed.

On the stellar progenitor of the type Ib supernova 1984L

view Abstract Citations (19) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Stellar Progenitor of the Type Ib Supernova 1984L Baron, E. ; Young, Timothy R. ; Branch, David Abstract Photometric and spectroscopic data on the Type Ib SN 1984L and the probable Type Ib SN 1985F near maximum light and at late times appear to lead to contradictory conclusions. At early times a low mass and/or a high explosion energy is indicated, while at late times a high mass and/or a low explosion is indicated. Several possibilities for resolving the discrepancy are considered. The ejection of ~50 M_sun_ of helium with a kinetic energy of ~2 x 10^52^ ergs, little intermediate mass elements, and ~0.2 M_sun_ of ^56^Ni, might satisfy the observational constraints. The astrophysical site of such an explosion is elusive although a massive star that leaves behind a massive black hole is a possibility. Some problems that make this scenario unappealing are discussed. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172674 Bibcode: 1993ApJ...409..417B Keywords: Astronomical Photometry; Astronomical Spectroscopy; Stellar Evolution; Supernovae; Light Curve; Polytropic Processes; Astrophysics full text sources ADS | data products SIMBAD (7) NED (4)

Operation of the r-process and cosmochronology

The rapid neutron capture process produced about half of the heavy elements in nature beyond the Fe-peak. In the past quite a number of astrophysical sites have been suggested, but none of them has yet been uniquely identified. Without assuming a particular site or model, we deduce the conditions responsible for the production of r-process nuclei by making use of the following information: (1) the solar r-process abundances and (2) nuclear masses and beta decay half lives for nuclei far from stability - in particular experimental information near magic neutron numbers, which determines the shape of the r-process peaks. In addition, we review briefly galactic age determinations based on the long-lived r-process chronometer nuclei 232Th, 235U and 238U.

Galactic Ba Enrichment from TP-AGB Stars

The production of the bulk of barium has long been ascribed to the main component of the s-process, whose astrophysical site has been envisaged in the convective He shell of Thermally Pulsing Asymptotic Giant Branch (TP-AGB) stars of low mass (1–3 M⊙; see Käppeler et al. 1990). The main neutron source is the 13C(α, n)16O reaction, operating at the thermal energy of kT = 12keV. We have calculated neutron captures in such environment with an updated nuclear physics, adopting the neutron capture cross sections of Beer, Voß, & Winters (1992) together with their temperature-dependence. Stellar models producing a mean neutron exposure of τ0 ≃ 0.30 mb−1 are able to reproduce the solar distribution of the s-abundances satisfactory, but the Ba isotopes show some overproduction. Such a strong indication suggests a revision of the Ba cross sections (see Gallino, Raiteri, & Busso 1992). Once that a suitable choice of σn, γ(Ba) is made, it is found that a r-contribution to solar Ba of the order of 10% can be expected.

Isotopic r-process abundances and nuclear structure far from stability - Implications for the r-process mechanism

Attempts to explain the source of r-process elements in nature by particular astrophysical sites face the entwined uncertainties, stemming from the extrapolation of nuclear properties far from stability, inconsistent sources of different properties (e.g., nuclear masses and half-lives), and the (poor) understanding of astrophysical conditions, which are hard to disentangle. We utilize the full isotopic r-process abundances in nature [especially in all of the three peaks (A≃80, 130, 195)] and a unified model for all nuclear properties involved (aided by recent experimental knowledge in the r-process path), to deduce uniquely the conditions necessary to produce such an abundance pattern

Cosmic rays with E > 1019eV : Origin and transport

Cosmic rays of energy > 1019eV are an interesting mystery of nature. Their origin is still unknown : classical acceleration mechanisms have great difficulty to produce them in all astrophysical sites considered. Because they are so susceptible to photo pion production and cannot fail to interact with the 2.7 K background, their age cannot exceed 3 × 106 years, so their sources cannot be too far from our galaxy. A breakthrough in the understanding of this phenomenon can be expected if more accurate data than presently available, of great statistical significance, can be obtained.

TEM studies of circumstellar dust

In recent years the development of special chemical separation techniques has permitted the isolation of material from primitive meteorites that can be unambiguously identified as pre-solar grains-that is, solids that condensed in the atmospheres of ancient stars and later became incorporated into the cloud of gas and dust which gave rise to our solar system. This identification is made possible in part by development of sophisticated mass spectrometry techniques for the analysis of ensembles of these grains and in part by ion microprobe techniques for determining the isotopic structures in single micron-sized grains. In particular, meteoritic grains which have isotopic compositions radically different from the solar system average can be assigned a pre-solar origin. In some cases, plausible astrophysical sites can be identified: for example, isotopic structures of Kr, Xe, Ba, and Nd in some grains resemble those expected to be produced by s-process nucleosynthesis in asymptotic giant branch (AGB) stars.

Contribution of gauge theories to baryon asymmetry

In this paper we examine a new paradigm for baryon asymmetry in which the fundamental Lagrangian is a major symmetry breaking factor. It is opined that gauge fields operating in various astrophysical sites resulted incp violating processes which led to the spontaneous baryon asymmetry. Existing quantum electrodynamic (QED) letters within the astrophysical environment arose from the mathematical formalism of the quantum chromodynamic (QCD) roots. Further implications of the result are also discussed.

The s-Process in AGB Stars

The evidence of an exponential distribution of neutron exposures for reproducing the solar-system s-isotopes between Zr and Pb, the main component, comes from a phenomenological analysis of the s-path in the σN versus A plot (see Käppeler, Beer and Wisshak, 1989). The resulting mean neutron exposure is τ0 = 0.30 ± 0.01 mb−1. The study of branchings with no or weak temperature dependence (95Zr; 147Nd, 147,148Pm; 185W, 186Re) allows one to derive an effective neutron density nn =(3.4±1.0)×108cm−3. On the other hand, an effective temperature is obtained from the branchings 134’135Cs, 151Sm, 154Eu and 176Lu: T8 = 3.4 ± 0.5 (Käppeler et al., 1990). Although the phenomenological approach is very useful because it is independent of stellar models, nonetheless it can only lead to effective physical conditions, symplifying the complexity of the astrophysical sites.

An analytical formulation of the double-pulse s-process model

view Abstract Citations (10) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Analytical Formulation of the Double-Pulse s-Process Model Beer, Hermann Abstract Recent studies of the astrophysical site of the s-process suggest a double-pulse s-process in the He-burning shell of low-mass asymptotic giant branch stars, where the C-13(alpha, n) and the Ne-22(alpha, n) neutron sources are active as well. Previous analyses have been performed in terms of a pulsed neutron s-process from intermediate-mass stars. The periodic neutron bursts were ascribed to the Ne-22(alpha, n) reaction only. The current effort is to reformulate the mathematics of the s-process to study the combined effect of repeated double pulses with different properties. Similar to the model of a single-pulse s-process, the effect of the pulses for the double-pulse model shows up at the branchings but, in contrast to the single-pulse model, also at the unbranched s-process path and at isomeric states populated by neutron capture. The new model contains the simpler models as special cases, and it is able to overcome difficulties associated with the single-pulse model. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170517 Bibcode: 1991ApJ...379..409B Keywords: Asymptotic Giant Branch Stars; Neutron Sources; Nuclear Fusion; Stellar Composition; Stellar Models; Helium; Neutron Distribution; Solar System; Stellar Mass; Astrophysics; ABUNDANCES; NUCLEAR REACTIONS; NUCLEOSYNTHESIS full text sources ADS |

A new site for the astrophysical gamma-process

The study suggests that the requisite thermodynamic conditions may occur when carbon-oxygen white dwarfs explode by deflagration or detonation. When these stars undergo such explosive disruption, there will be a region near the surface where the burning temperature lies in the 2.4-3.2 range. To examine this astrophysical site, calculations are performed for an s-process nucleosynthesis during helium shell flashes and the nuclear transmission taking place when such mass zones are heated by the deflagration or detonation wave, and the results are compared with the solar-system distribution of the p-isotopes. It is demonstrated that Type Ia supernovas provide a viable site for the gamma process, and that the same thermodynamic conditions would also exist in Type II-p powered supernovas, provided that they are powered by detonation.

Self-organization of cosmic radiation pressure instability

The instability caused by radiation pressure from uniformly distributed emitters in a continuous dissipative absorbing medium is analyzed in the absence of other astrophysical forces. Three analytical descriptions of this idealized system are outlined: mock gravity, linear instability theory, and a symmetric bubble model. These models are used to describe the behavior of an infinite, statistically homogeneous unstable expanding absorbing medium after the typical structures become both nonlinear and optically thick. It is argued that the system displays nonlinear self-organizing behavior: the absorbers evolve toward an autocatalytic, temporally self-similar steady state which is statistically independent of initial conditions. In this saturated state the absorbers are concentrated in thin walls around empty bubbles; as the instability develops the big bubbles get bigger and the small ones get crushed and disappear. A linear analysis shows that to first order the thin walls are indeed stable structures. It is speculated that this instability may play a role in forming cosmic large-scale structure. Other possible astrophysical sites for the nonlinear instability are briefly summarized.

High resolution gamma-ray astronomy: Observations and predictions of line shapes

The shapes of gamma-ray lines carry unique information about the physical processes and conditions in astrophysical sites. Galactic center and SN 1987A lines have been observationally resolved allowing their shapes to be studied. There are also significant new theoretical results concerning line shapes from Type I supernovae, supernova remnants and the interstellar medium. New work is presented on a simple treatment of line profiles for rotating disks and spherical shells.

The cosmic D/H ratio

Abstract The purpose of this review is to recall the different approaches that have been used to evaluate the cosmic deuterium abundance. From the various difficulties inherent to the observational techniques used, the most reasonable present day abundance value is shown to be of the order of 10 −5 . The comparison of this estimate with other light elements abundance evaluations seems to represent a spectacular confirmation of the standard Big Bang nucleosynthesis. However it is shown that the situation is still unclear and that observations in new astrophysical sites are needed in order either to confirm this model or to favour alternative scenarios.

New insights into the astrophysical r-process

Elements heavier than iron are known to be made partly by rapid neutron capture—the astrophysical r-process. New data from nuclear experiments and astronomical observations may allow some of the proposed astrophysical sites for the r-process to be ruled out and suggest that these elements are mostly produced in type II supernovae.

Insights on Dust Grain Formation and Destruction Provided by Gas-Phase Element Abundances

The abundances of free atoms and ions relative to hydrogen in the interstellar medium indicate how thoroughly different elements have condensed into solid form. Large contrasts in these depletions disclose differences in how tightly various elements are bound to the grains. While most grain nuclei probably form within astrophysical sites where densities are large, a substantial amount of the accretion of heavy elements must occur within interstellar clouds. Interstellar shocks created by supernova explosions or perhaps collisions of clouds destroy or significantly erode the grains. This viewpoint on the formation and destruction of grains is supported by the decrease in the severity of depletions in regions of lower than normal density or parcels of gas moving at high velocity. While depletions in gas away from the plane of the galaxy generally imitate the behavior of low density regions in the plane, mild anomalies for some elements may exist.

S-process krypton of variable isotopic composition in the Murchison meteorite.

Current theories on the origin of the chemical elements explain the abundance of medium-heavy and heavy nuclides to be due to the capture by pre-existing lighter nuclides of free neutrons on either a slow timescale (s-process) or a rapid timescale (r-process). Experimental evidence in support of these theories comes from the analysis of carbonaceous chondrites. In acid-resistant residues of these meteorites a kind of xenon has been found, the isotopic composition of which matches almost perfectly that predicted for s-process xenon. We report data that allow us, for the first time, to derive with reasonable precision the full isotopic spectrum of s-process krypton as well. We show that this s-Kr in a residue from Murchison meteorite did not originate in one single s-process but rather is a mixture of contributions from stellar environments where the density of free neutrons was not the same. The astrophysical conditions under which this Krypton has been produced were distinct from those that have been invoked to explain the Solar System s-process abundance. Similar to the 13C-rich carbon component in an aliquot of the same residue, the s-process Kr from different astrophysical sites has retained its identity during the accumulation and subsequent history of the meteorite.

X-RAYS FROM SPACE

X-rays from outer space are prevented by the atmosphere from reaching the ground. Therefore, X-rays astronomy developed as a new branch of astrophysics only with the advent of spatial technologies. It was born 25 years ago, on the 12 of June 1962, with the first observation of a remote X-ray source (Scorpius X-1). Whereas the pioneering experiments (UHURU, ...) brought to light the very existence of X-ray sources, the most recent ones (such as the EINSTEIN and EXOSAT satellites) were versatile and sophisticated observatories which performed spectroscopic studies as well as imagery. These rocket and satellite experiments have revealed that X-rays are emitted in most astrophysical sites : stars, binary stellar systems, interstellar medium, active galaxies and quasars, galaxy clusters, intercluster medium. Since X-rays originate in highly energetic processes (thermal emission of very hot plasmas, synchrotron emission of relativistic particles, ...) they shed some light on cataclysmic phenomena such as stellar explosions and propagation of the resulting shock wave, huge matter infalls onto the surface of black holes, ... X-ray astronomy may have not yet truly come of age : drastic improvements are expected from a new generation of instruments, such as the high resolution spectrometers which are to be implemented on board the future ESA satellite XMM. The interpretation of the wealth of data they will provide will require an ever increasing collaboration with specialists in spectroscopy and atomic processes.