Galactic Evolution of D and 3He

The most recent chemical evolution models for D and 3He are reviewed and their results compared with the available data.Models in agreement with the major galactic observational constraints predict deuterium depletion from the Big Bang to the present epoch smaller than a factor of 3 and therefore do not allow for D/H primordial abundances larger than 5 × 10-5. Models predicting higher D consumption do not seem to be able to reproduce other observed features of our galaxy (e.g. SFR, abundances, abundance ratios and/or gradients of heavier elements, metallicity distribution of G-dwarfs).Observational and theoretical 3He abundances can be reconciled with each other if the majority of low mass stars experience in the red giant phase a deep mixing allowing the consumption of most of the 3He produced during core-hydrogen burning.     

Tailored analyses of 24 Galactic WN stars

The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T_*=30–90,000 K), luminosity (log L_*/L=4.8–5.9), mass loss (M=0.9–12×10^–5 M yr^–1) and terminal velocity (v =630–3300 km s^–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10^–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage.     

The x-ray cluster baryon crisis

Nucleosynthesis in the standard hot big bang cosmology offers a successful account of the production of the light nuclides during the early evolution of the Universe. Consistency among the predicted and observed abundances of D,³He,⁴He and⁷Li leads to restrictive lower and upper bounds to the present density of nucleons. In particular, the upper bound ensures that nucleons cannot account for more than a small fraction (<0.06h ₅₀ ^–2 ) of the mass in a critical density (Einstein-de Sitter) Universe. In contrast, x-ray observations of rich clusters of galaxies suggest strongly that baryons (in hot gas) contribute a significant fraction of the total cluster mass (0.2h ₅₀ ^–3/2 ). If, indeed, clusters do provide a fair sample of the mass in the Universe, this crisis forces us to consider other ways of mitigating it, including the politically incorrect possibility that <1. The options, including magnetic or turbulent pressure, clumping and non-zero space curvature and/or cosmological constant, are discussed.     

Solar Energetic Particle Isotopic Composition

We discuss isotopic abundance measurements of heavy (6 ≤ Z ≤ 14) solar energetic particles with energies from ∼15 to 70 MeV/nucleon, focusing on new measurements made on SAMPEX during two large solar particle events in late 1992. These measurements are corrected for charge/mass dependent acceleration effects to obtain estimates of coronal isotopic abundances and are compared with terrestrial and solar wind isotope abundances. An example of new results from the Advanced Composition Explorer is included. This revised version was published online in June 2006 with corrections to the Cover Date.     

Abundances,Ionization States,Temperatures, and FIP in Solar Energetic Particles

The relative abundances of chemical elements and isotopes have been our most effective tool in identifying and understanding the physical processes that control populations of energetic particles. The early surprise in solar energetic particles (SEPs) was 1000-fold enhancements in \({}^{3}\mbox{He}/{}^{4}\mbox{He}\) from resonant wave-particle interactions in the small “impulsive” SEP events that emit electron beams that produce type III radio bursts. Further studies found enhancements in Fe/O, then extreme enhancements in element abundances that increase with mass-to-charge ratio \(A/Q\), rising by a factor of 1000 from He to Au or Pb arising in magnetic reconnection regions on open field lines in solar jets. In contrast, in the largest SEP events, the “gradual” events, acceleration occurs at shock waves driven out from the Sun by fast, wide coronal mass ejections (CMEs). Averaging many events provides a measure of solar coronal abundances, but \(A/Q\)-dependent scattering during transport causes variations with time; thus if Fe scatters less than O, Fe/O is enhanced early and depleted later. To complicate matters, shock waves often reaccelerate impulsive suprathermal ions left over or trapped above active regions that have spawned many impulsive events. Direct measurements of ionization states \(Q\) show coronal temperatures of 1–2 MK for most gradual events, but impulsive events often show stripping by matter traversal after acceleration. Direct measurements of \(Q\) are difficult and often unavailable. Since both impulsive and gradual SEP events have abundance enhancements that vary as powers of \(A/Q\), we can use abundances to deduce the probable \(Q\)-values and the source plasma temperatures during acceleration, ≈3 MK for impulsive SEPs. This new technique also allows multiple spacecraft to measure temperature variations across the face of a shock wave, measurements otherwise unavailable and provides a new understanding of abundance variations in the element He. Comparing coronal abundances from SEPs and from the slow solar wind as a function of the first ionization potential (FIP) of the elements, remaining differences are for the elements C, P, and S. The theory of the fractionation of ions by Alfvén waves shows that C, P, and S are suppressed because of wave resonances during chromospheric transport on closed magnetic loops but not on open magnetic fields that supply the solar wind. Shock waves can accelerate ions from closed coronal loops that easily escape as SEPs, while the solar wind must emerge on open fields.     

Measurements of the properties of solar wind plasma relevant to studies of its coronal sources

Interplanetary measurements of the speeds, densities, abundances, and charge states of solar wind ions are diagnostic of conditions in the source region of the solar wind. The absolute values of the mass, momentum, and energy fluxes in the solar wind are not known to an accuracy of 20%. The principal limitations on the absolute accuracies of observations of solar wind protons and alpha particles arise from uncertain instrument calibrations, from the methods used to reduce the data, and from sampling biases. Sampling biases are very important in studies of alpha particles. Instrumental resolution and measurement ambiguities are additional major problems for the observation of ions heavier than helium. Progress in overcoming some of these measurement inadequacies is reviewed.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Properties of the interstellar gas inside the heliosphere

Methods and results of investigations of the interstellar gas inside the heliosphere are summarized and discussed. Flow parameters of H and He and the relative abundances of H, He, N, O, and Ne in the distant heliosphere are given. Charge exchange processes in front of the heliosphere affect the flow of hydrogen and oxygen through the heliopause. The speed of hydrogen is reduced by 6 km/s, and screening leads to a reduction of the O/He and H/He ratios in the neutral gas entering the heliosphere. When the screening effect and the acceleration processes leading to the anomalous cosmic rays (ACR) are sufficiently understood, abundances in the LIC can be derived from measurements inside the heliosphere. Since isotopic ratios are virtually not changed by screening or by EUV and solar wind ionisation, relative abundances of isotopes in the gaseous phase of the LIC can be determined with no or minor correction from investigations of the neutral gas, pickup ions and ACR particles.     

Elemental abundances in corotating interaction regions at high solar latitudes

Throughout 1993, as the Ulysses spacecraft traveled from 23° to 45° south heliolatitude, the HI-SCALE instrument on the spacecraft measured a recurrent series of enhanced particle fluxes with a recurrence period of 26.5 days. These particles are accelerated from a background seed population by the corotating interaction regions (CIRs) associated with a southern solar polar coronal hole. Using the Wart detector telescope of the HI-SCALE instrument, we have analyzed the elemental abundances of C, N, O, and Fe relative to He for 0.5–4.0 MeV/nucl ions and Ne, Mg, and Si for 1.0–4.0 MeV/nucl ions in the CIRs. We compare the relative abundances to some previous measurements reported from 1 A.U. as well as with solar photosphere abundances. We note that HI-SCALE measurements of the heliolatitude dependence of the oxygen abundance and spectrum as reported by Lanzerottiet al. (1994) suggest that a substantial fraction of the seed population for the CIR-accelerated oxygen is likely to be the anomalous oxygen component of the cosmic rays.     

Cosmic-ray Composition as Observed by Ulysses

The cosmic ray isotopic composition measurements from the High Energy Telescope (HET) on the Ulysses spacecraft are reviewed. The source isotopic composition of key elements is found to be surprisingly like the Solar system abundances with the notable exception of ²²Ne. The average density of interstellar material cosmic rays traverse is found to be 0.25 atom cm^–3, corresponding to a confinement time of 20 Myr. Vanadium isotopic abundances are shown to be consistent with weak cosmic-ray reacceleration. The implications of these measurements are discussed.     

The impact of semi-convection and overshooting on the surface abundances in massive stars

We analyse the effects of semi-convection and overshooting on the predicted surface abundances after the first and second dredge-ups in 15 and 20 M Pop. I stars. Overshooting is applied either to the core boundary or to the boundaries of all convective zones. It is shown that the surface abundances are sensitive to the mixing scheme adopted in the interior. The models including semi-convection lead to lower¹²C/¹³C ratios than the other mixing schemes, while models with overshooting predict higher enhancements of sodium at the surface.     

Helium-3: Status and Prospects

We report on our continuing efforts to determine 3He abundances in H II regions and planetary nebulae. Our detections of 3He in some PNe show that some stars produce large amounts of 3He. However the H II region abundances show no evidence for this production. From our sample of > 40 H II regions, the subsample which should yield the most reliable abundances has 3He/H abundances which scatter between 1-2 × 10-5. There is no trend with either galactocentric distance or metallicity. Even if we do not understand the underlying mechanisms, we see empirically that stars neither produce nor destroy 3He in a major way. We thus suggest that the level of the "3He Plateau" (3He/H = 1.5 _-0.5 ^+1.0 × 10^-5) is a reasonable estimate for the primordial 3He.     

Isotopic Fractionation by Plasma Processes

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gy ago from the protosolar nebula. The solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. Data from space missions and theoretical models indicate that the isotopes of the volatile elements N, O, Ne, and Ar are fractionated by at most a few percent per atomic mass unit in different solar wind regimes. In contrast, isotopic abundances of solar and heliospheric energetic particles at energies larger than about 100 keV/amu are observed to strongly vary relative to solar abundances. Processes such as resonant acceleration or pre-acceleration by plasma waves, first-order Fermi acceleration, or propagation in the interplanetary plasma are discussed as causes for charge-to-mass dependent fractionation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature Fluctuations,H II Region Abundances,and Primordial Helium

There is evidence for temperature fluctuations in Planetary Nebulae and in some Galactic H II regions. If such fluctuations occur in the low metallicity, extragalactic H II regions used to probe the primordial helium abundance, the derived 4He mass fraction, YP, could be systematically different from the true primordial value. Although this effect could be large, there are no data which allow us to estimate the size of the temperature fluctuations for the extragalactic H II regions. Therefore, we have explored this effect via Monte Carlo simulations of the data in which the abundances derived from a fiducial data set are modified by T chosen from a distribution with 0 T Tmax where Tmax is varied from 500 K to 4000 K.     

Processes affecting abundances in the solar wind

Data on composition in the solar wind are summarized and compared with best estimates of abundances in the outer convective zone of the Sun. Several mechanisms of element and isotope fractionation are discussed in relation to observed abundances and their variations.The evidence available so far indicates that in addition to ion fractionation in the corona there is a separation mechanism operating at low solar altitude that affects solar wind composition. It is suggested that the systematic depletion of helium observed in the solar wind is in part caused by ion-neutral separation in the chromosphere-transition zone. Conditions for this mechanism to be effective are discussed. It is shown that ion-neutral separation is much more pronounced than ion-ion separation under these conditions. Therefore, this mechanism should fractionate elements according to the rate at which first ionization occurs. This implies that isotope fractionation by this mechanism is minor.Ion-neutral separation may be responsible for the general depletion that is observed in the slow interstream solar wind as well as in the fast streams coming out of coronal holes. However, the occurrences of very low He/H ratios are probably caused in the corona.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

On the Organic Refractory Component of Cometary Dust

Cometary nuclei consist of ices intermixed with dust grains and are thought to be the least modified solar system bodies remaining from the time of planetary formation. Flyby missions to Comet P/Halley in 1986 showed that cometary dust is extremely rich in organics (∼50% by mass). However, this proportion appears to be variable among different comets. In comparison with the CI-chondritic abundances, the volatile elements H, C, and N are enriched in cometary dust indicating that cometary solid material is more primitive than CI-chondrites. Relative to dust in dense molecular clouds, bulk cometary dust preserves the abundances of C and N, but exhibits depletions in O and H. In most cases, the carbonaceous component of cometary particles can be characterized as a multi-component mixture of carbon phases and organic compounds. Cluster analysis identified a few basic types of compounds, such as elemental carbon, hydrocarbons, polymers of carbon suboxide and of cyanopolyynes. In smaller amounts, polymers of formaldehyde, of hydrogen cyanide and various unsaturated nitriles also are present. These compositionally simple types, probably, are essential "building blocks", which in various combinations give rise to the variety of involatile cometary organics. This revised version was published online in June 2006 with corrections to the Cover Date.     

Galileo Probe Nephelometer experiment

The objective of the Nephelometer Experient aboard the Probe of the Galileo mission is to explore the vertical structure and microphysical properties of the clouds and hazes in the atmosphere of Jupiter along the descent trajectory of the Probe (nominally from 0.1 to > 10 bars). The measurements, to be obtained at least every kilometer of the Probe descent, will provide the bases for inferences of mean particle sizes, particle number densities (and hence, opacities, mass densities, and columnar mass loading) and, for non-highly absorbing particles, for distinguishing between solid and liquid particles. These quantities, especially the location of the cloud bases, together with other quantities derived from this and other experiments aboard the Probe, will not only yield strong evidence for the composition of the particles, but, using thermochemical models, for species abundances as well. The measurements in the upper troposphere will provide ground truth data for correlation with remote sensing instruments aboard the Galileo Orbiter vehicle. The instrument is carefully designed and calibrated to measure the light scattering properties of the particulate clouds and hazes at scattering angles of 5.8°, 16°, 40°, 70°, and 178°. The measurement sensitivity and accuracy is such that useful estimates of mean particle radii in the range from about 0.2 to 20 can be inferred. The instrument will detect the presence of typical cloud particles with radii of about 1.0 , or larger, at concentrations of less than 1 cm³.Deceased.     

Galactic Evolution of the Light Elements: A New Set of B Observations

The boron 2500 spectral region has been observed with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) in a new set of metal-poor stars and analyzed by spectrum synthesis technique, adopting the most recent model atmospheres. By taking into account the Li and Be abundances available from the literature for this same set of objects, the resulting patterns of their light elements abundances cannot be easily justified with the currently known stellar structure scenarios. The finding of real differences in the B content between stars with very similar stellar characteristics suggest that also production effects, rather than depletion and/or mixing only, should be taken into account as a possible and valuable explanation.     

The Origin of Primary Cosmic Rays: Constraints from ACE Elemental and Isotopic Composition Observations

Cosmic-ray isotope observations from NASAs Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to ⁵⁶Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes ⁴⁸Ca and ⁶⁴Ni. Although the source abundances range over a factor 10⁴, most of the ratios to ⁵⁶Fe are consistent with solar-system values to within 20%. However, there are some notable differences, the most significant being an excess of (70±30)% relative to the solar system for the cosmic-ray source ratio ⁵⁸Fe/⁵⁶Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.     

Solar Energetic Particles: Sampling Coronal Abundances

In the large solar energetic particle (SEP) events, coronal mass ejections (CMEs) drive shock waves out through the corona that accelerate elements of the ambient material to MeV energies in a fairly democratic, temperature-independent manner. These events provide the most complete source of information on element abundances in the corona. Relative abundances of 22 elements from H through Zn display the well-known dependence on the first ionization potential (FIP) that distinguishes coronal and photospheric material. For most elements, the main abundance variations depend upon the gyrofrequency, and hence on the charge-to-mass ratio, Q/A, of the ion. Abundance variations in the dominant species, H and He, are not Q/A dependent, presumably because of non-linear wave-particle interactions of H and He during acceleration. Impulsive flares provide a different sample of material that confirms the Ne:Mg:Si and He/C abundances in the corona. This revised version was published online in June 2006 with corrections to the Cover Date.