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29th International Cosmic Ray Conference Pune (2005)  3, 153-156  Cosmic Ray Neon, Wolf-Rayet Stars, and the Origin of GCRs  W.R. Binnsa, M.E. Wiedenbeckb, M. Arnouldc, A.C. Cummingsd, J.S. Georged,  S. Gorielyc, M.H. Israela, R.A. Lesked, R.A. Mewaldtd, G. Meynete, L. M. Scotta,  E.C. Stoned and T.T. von Rosenvingef (a) Washington University, St. Louis, MO 63130 USA (b)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA (c)Institut d’Astronomie et d’Astrophysique, U.L.B., Bruxelles, Belgique (d)California Institute of Technology, Pasadena, CA 91125 USA  (e)Geneva Observatory, 1290 Sauverny, Switzerland (f)NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA Presenter: W.R. Binns (wrb@wuphys.wustl.edu), usa-binns-W-abs1-og12-oral  The abundances of neon and other refractory isotopes in the galactic cosmic rays (GCRs) have been measured by the Cosmic Ray Isotope Spectrometer (CRIS) aboard the ACE spacecraft. We have derived the 22Ne/20Ne ratio at the cosmic-ray source and obtain a ratio of 0.387 ± 0.007 (stat.) ± 0.022 (syst.), which corresponds to enhancement by a factor of 5.3±0.3 over that in the solar wind. We compare our ACE-CRIS data, and data from other experiments, with recent results from two-component Wolf-Rayet (WR) models. The three largest deviations of GCR isotope ratios from solar-system ratios predicted by these models, 12C/16O, 22Ne/20Ne, and 58Fe/56Fe, are observed in the GCRs. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that superbubbles are the likely source of at least a substantial fraction of GCRs.   1. Introduction  Several experiments have shown that the 22Ne/20Ne ratio at the GCR source is substantially greater than that in the solar wind [1-5]. A number of models have been proposed to explain the large 22Ne/20Ne ratio.  The most widely accepted mechanism for producing the neon ratio excess was first introduced by [6] and was studied in more detail by [7]. They suggested that the large 22Ne/20Ne ratio in GCRs could be due to Wolf-Rayet (WR) star ejecta mixed with material of solar-system composition. The WC phase [8] of WR stars is characterized by the wind enrichment of He-burning products, especially carbon and oxygen. Also, at the beginning of the He-burning phase, 22Ne is strongly enhanced as a result of 14N destruction [8,9] through the α-capture reactions 14N(α,γ)18F(e+ν)18O(α,γ)22Ne. An excess of the elemental Ne/He ratio in the winds of WC stars has been confirmed observationally [10], which is consistent with a large 22Ne excess, and gives support to the idea of Cassé and Paul [6]. The high velocity winds characteristic of WR stars can inject the surface material into regions where standing shocks, formed by those winds and the winds of the hot, young, precursor OB stars interacting with the interstellar medium (ISM), can pre-accelerate the WR material.   2. Measurements and Source Composition  The CRIS instrument [11] consists of four stacks of silicon detectors to measure dE/dx and total energy (Etot), and a scintillating-fiber hodoscope to measure trajectory. The dE/dx-Etot method is used to determine particle charge and mass. The geometrical factor of the instrument is ~250 cm2sr and the total vertical silicon thickness is 4.5 cm. The precision with which angle is measured by the hodoscope is ≤ 0.1°. The neon data were collected from 1997 Dec. 5 through 1999 Sept. 24 and are a selected, high-resolution data set. Events were selected with trajectory angle ≤25º relative to the detector normal and particles stopping within 750 µm of the single surface of each silicon wafer having a dead layer were excluded from analysis. Nuclei that interacted in CRIS were rejected by using the bottom silicon anticoincidence detector, by requiring  154                                                                              W.R. Binns et al.  consistency in charge estimates, and by rejecting particles with trajectories that exit through the side of a silicon stack. The average mass resolution for neon that we obtained is 0.15 amu (rms), which is sufficiently good that there is only a slight overlap of the particle distributions for adjacent masses. In Figure 1, the total number of neon events is ~4.6 × 104. This Figure also shows histograms of F and O isotopes that are used in a “tracer” propagation to obtain the 22Ne/20Ne ratio at the cosmic rays source.   To obtain the 22Ne/20Ne abundance ratio at the comic-ray source from the ratio observed, we have used the “tracer method” [12], which utilizes observed abundances of isotopes that are almost entirely secondary to infer the secondary contribution to isotopes like 22Ne, for which the observed fluxes are a mixture of primary and secondary nuclei.  21Ne, 19F, and 17O are such  “tracer” isotopes. We have used cross-sections from accelerator measurements to estimate cross-sections for some of the most important reactions in the propagation. For all other reactions, the Silberberg et al. cross-sections [13], scaled to available measured data, were used [14]. This analysis, combining the results derived using the three tracer isotopes gives an overall 22Ne/20Ne ratio of 0.387 ± 0.007 (stat.) ± 0.022 (syst.). This corresponds to a ratio relative to solar system (22Ne/20Ne)GCRS/(22Ne/20Ne)SW of 5.3 ± 0.3.   4. Superbubble and Wolf-Rayet Models of Galactic Cosmic Ray Origin Most core-collapse supernovae (SNe) in our galaxy (~90%) are believed to occur in OB associations that form superbubbles within giant molecular clouds [15]. Additionally, most WR stars are located in OB associations and many of their O and B star constituents evolve into WR’s. These massive stars have short lifetimes, typically a few million years, and the WR phase is typically a few hundred thousand years [16]. It seems almost certain that pre-supernova WR wind material will be accelerated either by the SN shock from the evolved WR star that initially ejected the material or by nearby SNe from short-lived O and B stars. 19 20 21 22 2301x1032x1033x1034x10317 18 19 20 2102x1024x1026x1028x10215 16 17 18 1902x1044x1046x104   Neon(a)  Counts per bin  Fluorine(b)  Mass (amu) Oxygen CountsOxygen Counts x 10(c)Figure 1-Mass histograms of neon, fluorine, and oxygen isotopes The mass of the neon isotopes synthesized and ejected in superbubbles by massive stars in their WR and core-collapse SN phases has been calculated by [15]. They estimated that a mass fraction, 18±5%, of WR plus SN ejecta must be mixed with ISM material of solar-system composition in the superbubble core to obtain the 22Ne/20Ne ratio reported in an earlier analysis of the CRIS results [17]. They conclude that “the 22Ne abundance in the GCRs is not anomalous but is a natural consequence of the superbubble origin of GCRs in which the bulk of GCRs are accelerated by SN shocks in the high-metallicity, WR wind and SN ejecta enriched, interiors of superbubbles”. Additionally, they suggest that the measured value of the 22Ne/20Ne ratio provides evidence for a superbubble origin of GCRs.  We have examined other isotope ratios at the cosmic-ray source, inferred from our CRIS observations and others, as a further test of the superbubble model of the origin of cosmic rays. In Figure 2 we show these ratios and compare them with modeling calculations of WR outflow [16] for metallicity Z=0.02 and initial rotational equatorial velocities at the stellar surface of either 0 or 300 km/s. For each WR model star, the mixture (by mass) of WR star outflow with material of solar-system (solar-wind) composition required to give the 22Ne/20Ne ratio measured by CRIS for the GCR source was calculated.  Table 1 shows the mass  Cosmic Ray Neon, Wolf-Rayet Stars …                                                       155  fraction (p) of the total cosmic ray source material that is required from the WR star since its initial formation for each case. The mixing fractions in Table 1 are similar to the value of 0.18±0.05% derived by [15]. The high p-values predicted for the M ≥ 85 M0 stars do not present a problem since these very massive stars are much rarer than the lower mass stars.  The closed circles in Figure 2 are CRIS results [18,19] for elements heavier than neon. Ulysses Mg and Si data (not plotted; [4]) are in good agreement with our CRIS results, while their 58Fe/56Fe ratio [20] is significantly lower than the CRIS value. The lighter elements are plotted as solid diamonds and are mean values of GCR source abundances, relative to solar system [23], obtained from Ulysses [4], ISEE-3 [1,21], Voyager [2] and HEAO-C2 [22]. The error bars are based on weighted means from these experiments.   For nuclei heavier than neon, the WR models are in reasonable agreement with data, with the exception of the high-mass (85 and 120 solar masses) rotating star models that predict a deficiency in the 25Mg/24Mg ratio, which is not observed. The observed enhancement of 58Fe/56Fe is roughly consistent with the enhancement of this ratio predicted by the models. For elements lighter than neon, there is usually only a single isotope for which source abundances can be obtained with sufficient precision to constrain the models. Therefore the ratios compared are for different elements. This makes comparisons more complicated since atomic fractionation effects may be important for some ratios. We see in Figure 2 that the measured 12C/16O source ratio is much larger than in the Solar System and is in qualitative agreement with WR models for non-rotating stars, and rotating stars with initial masses of 40 and 60 M0.  It is in disagreement with models of the heavier rotating stars. However, the factor of two than foexplanation is elemensupport to a model ioxygen is highly volain silicates [23]. Thushould be preferentiarefractory in its elemeprimarily as a gas. Sostrictly valid compari0.210.21 No Rotation64Ni/58 Ni62Ni/58 Ni61Ni/58 Ni60Ni/58 Ni58Fe/56Fe12C/16O13C/12C14N/16O22Ne/20Ne23Na/24Mg25Mg/24Mg26Mg/24Mg29Si/28Si30Si/28Si34S/32S54Fe/56Fe57Fe/56FeN/Nea) 40 solar masses 60 solar masses 85 solar masses 120 solar masses Ratio Relative to Solar System Abundances CRIS Data CRIS Data Volatility Adjusted Combined Data Combined Data Volatility AdjustedInitial Equitorial Rotation          300 km/sb)Figure 2--Comparison of CRIS and other data with WR calculationsWR Initial Mass (M) N40 60 85 120 Table o-Rot. WR Fract. (p) Rot. WR Fract. (p) --- 0.22 0.20 0.16 0.12 0.44 0.16 0.37  experimental 14N/16O ratio is smaller by more than a r the model calculations and for the Solar System. It is likely that at least part of the tal and mass fractionation of the GCR source material. Recent work [24,25] has given n which GCR fractionation is governed by volatility. Although atomic or molecular tile, ~23% of the oxygen in the ISM is believed to exist in refractory compounds, e.g. s in the Meyer et al. [24] and Ellison et al. [25] models, that fraction of the oxygen lly injected into the GCRs. Additionally, a significant fraction of carbon, which is ntal form, exists in the ISM as a volatile in molecules such as CO [24]. Nitrogen exists  both the 12C/16O and the 14N/16O GCR ratios should be corrected for this effect to have son. We can make a rough adjustment to the 14N/16O ratio since the fraction of 16O that  156                                                                             W.R. Binns et al.  exists in the solid state in the pre-solar nebula has been estimated.  Meyer et al. [24] show that the GCR source to solar-system abundance ratio for the refractory elements is roughly a factor of 13 larger than for nitrogen. They also point out that, even for volatile elements there is a systematic enhancement in the abundance of heavy volatiles compared to light volatiles. They estimate the mass dependence of this enhancement as A0.8±0.2, where A is the atomic mass. The ratio adjusted for this fractionation is plotted as an open diamond in Figure 2. The 12C/16O ratio is more difficult to correct since the fraction of carbon that is in the volatile state in the ISM is poorly known. Therefore we have not attempted this adjustment. We have adjusted the N/Ne ratio for the mass dependent enhancement (open diamond) and the 22Ne/20Ne ratio has similarly been adjusted for mass dependence (open circle). The adjusted 14N/16O and N/Ne ratios are in much better agreement with both the solar system and WR modeling results. These adjusted ratios should be regarded as approximate values showing that ratios previously thought to be inconsistent with solar-system abundances may be consistent if GCRs are properly fractionated on the basis of volatility and mass.  We see that after adjustments for elemental fractionation, the CRIS data and data from other experiments show an isotopic composition similar to the one obtained by mixing about 20% of WR wind material with about 80% of material of solar-system composition. The largest ratios predicted by the WR models (including fractionation adjustments), 12C/16O, 22Ne/20Ne, and 58Fe/56Fe are in fact observed. We take this agreement as evidence that WR star ejecta is likely an important component of the cosmic-ray source material. Since most WR stars reside in superbubbles, as do most core-collapse supernovae, superbubbles must be the predominant site of injection of WR material into the GCR source material. 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Cosmic Ray Neon, Wolf-Rayet Stars, and the Origin of GCRs

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