W.R. Webber1, A. Lukasiak2, F.B. McDonald2 and P. Ferrando3
We have now analyzed 17 years of Voyager 1 and Voyager 2 data and derived isotopic composition ratios for C, N and O elements in cosmic rays with a statistical accuracy exceeding that of the sum of all previously published spacecraft measurements. We have also used newly measured cross sections in the propagation calculations to derive the source composition of these elements. We find; (1) The source 13C/12C ratio is 2.5 å lower than the solar ratio of 1.1%. (2) For N we find the source 14N/16O ratio to be 5.0% and the source 15N/16O ratio to be 0.97% - about one å larger than the ratio of 0.37% expected for a solar composition. (3) The source 18O/16O ratio is determined to be 0.24 ñ 0.17%, including both measurement and propagation errors, almost exactly the solar ratio of 0.23%. The implications of these results for cosmic ray origin theories are discussed.
1 Astronomy Department, New Mexico State University, Las Cruces, NM 88003 2 Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742 3 Service d' Astrophysique, C.E.N. Saclay, 91191, Gif- sur - Yvette, France
1. INTRODUCTION Determining the isotopic composition of C, N and O in cosmic rays is of crucial importance for understanding the sources of cosmic rays and the nucleosynthesis occurring therein. Already there have been numerous reports of differences in the isotopic composition of these elements with respect to the solar composition that may suggest, for example, that Wolf - Rayet stars contribute significantly to the cosmic ray population. Recent improved measurements from the Voyager spacecraft and other spacecraft as summarized by Lukasiak et al., 1994, suggest differences of 2 - 3 å or larger in the key cosmic ray and solar 13C/12C, 15N/16O and 18O/16O ratios.
In this paper we report new measurements of these ratios using 17 years of available Voyager 1 and 2 data - resulting in þ3 times more events than the previous analysis by Lukasiak et.al., 1994. This new data has been interpreted using new measurements of the key cross sections into 13C, 15N and 18O from the Transport (Chen et. al., 1995) and the New Mexico - French cross section collaborations (Webber et. al., 1995) that reduce the cross sectional errors by a proportional amount. In the light of this new data, significant compositional differences still exist between the cosmic ray sources and solar 13C, but not 15N or 18O. We further examine just what limits on the compositional differences of these elements can be set by future improved cosmic ray data, along with the limitations of the present cross section data base which is unlikely to be significantly improved in the future.
2. INSTRUMENTATION, DATA ANALYSIS AND RESULTS The data from the High Energy Telescope (HET) of the CRS experiment on both the Voyager 1 and 2 spacecraft have been used in this analysis. The experiment has been extensively described previously (Stone et al., 1977). The HET is a double-ended telescope consisting of a combination of solid-state detectors (see Figure 1). The cosmic-ray nuclei considered in this paper are events which have entered the B end and stop in either one of the first three C detectors. The HET data are from the low-gain mode, which allows the analysis of all events from He to Ni. The separation of different chemical species and isotopes in these telescopes is achieved by using the multiple dE/dx versus residual energy E technique.
The first step in the analysis of the experimental data is the removal of background events which is accomplished by using the two independent dE/dx analyses to reject events for which the two analyses are not consistent. We start with the charge assignment which relies on the well-known dE/dx versus E technique. Using an energy loss program we calculated the theoretical tracks for each element. These theoretical tracks are fitted to the data for the elements from carbon to silicon. For every event we have determined a charge value from the position of an event in respect to the closest tracks corresponding to two sequential charges as described by Ferrando et al., (1991). Elements from C to O were separated with a charge resolution of þ0.04 charge units.
In the next stage of data analysis we used these charge data to determine for each event two mass values, A1 and A2, corresponding to an analysis of B1 versus þC and B2 versus þC pulse height data.
For the mass analysis a mass value was determined for every event from the position of an event with respect to the closest mass track. A mass consistency criterion for the two determined A1 and A2 values was used to reject background events. We kept all the events with þA2 - A1 þ smaller than 3 times the standard deviation. The number of events rejected by using this mass consistency criterion was 8%. The simulation tracks were then further adjusted to fit the distribution of events corresponding to different isotopes and to optimize the mass resolution. The HET has four dead layers between the C counters and the events from the regions of these layers have mass values which are not well resolved. To improve the mass resolution further we removed all events stopping in these dead layers. Figure 2 shows the mass histograms obtained for the elements C, N and O in the combined Voyager 1 and 2 spacecraft cosmic-ray measurements. The inserts in this figure emphasize the separation of the 13C from 12C and 18O from 16O isotopes. The solid lines in Figure 2 correspond to a fit of a multi-gaussian function to the isotope distribution. The mass resolution of the HET telescope is å = 0.2 amu for C.
The energy interval for each isotope is mass-dependant and the energy limits for different isotopes are given in Table 1. In the calculations of isotopic ratios we corrected for these differences in the energy intervals by using a spectral index þ = 0.8 for all charges. In addition, corrections of 5.6% and 3.5% are made for 14N and 16O respectively, for the contribution of the anomalous component in the lowest energy channels. The results from this study for the isotopic composition are given in Table 1. They correspond to the level of solar modulation è = 500 MV which corresponds to the average for 17 years using the same interstellar He spectrum and modulation model as in Ferrando et al., (1991). The values of è for each year as determined from the measured yearly He spectrum and the weighted average for all 17 years are shown in Figure 3.
3. INTERPRETATION OF THE RESULTS To use our results to determine the source composition of the C, N and O isotopes we have used the "leaky box" model with an exponential distribution of path lengths. While this model provides a very simplified physical interpretation of the propagation of cosmic rays it can be used with accurate cross sections to predict a "reference" propagation. This reference propagation may be used, for example, to fit the well measured B/C ratio thus giving a measure of the net effect of galactic propagation since B is known to be absent in the cosmic ray sources. Thus, in effect, the production of any secondary isotope during galactic propagation can be predicted to essentially the same accuracy with which the measured B/C ratio can be predicted (ñ3%) as discussed by Webber and Soutoul, 1989. The fit to the B/C ratio made incorporating the latest cross sections is shown in Figure 4. It is important to note that this fit must be made at all energies and in particular at the low energies where the C, N and O isotopic measurements are made. For this reason we have derived values of the B/C ratio from the same Voyager data set as the C, N and O isotopes and these values are shown in Table 1 and Figure 4 along with the measurement at low energies of comparable accuracy from Wiedenbeck et. al., 1981. The best fit path length is þþ = 31.6 þR-0.60g/cm2 for rigidities > 4.7GV and þþ = 12.5þ = gcm-2 for lower rigidities, (for a modulation parameter í = 500MV corresponding to the 17 year average). This path length is slightly different than our earlier value discussed in Lukasiak et. al., 1994, and arises for two reasons; (1) We require a fit to the low energy data points as well as the higher energy HEAO-3 data of Engelmann et. al., 1990. (2) We have used updated cross sections including new measurements of the French - New Mexico and Transport Collaborations (Webber et. al., 1995, Chen et. al., 1995) in addition to the earlier set of measurements and semi-empirical calculations of Webber, Kish, and Schrier, 1990, for hydrogen and Ferrando et. al., 1988, for helium. The new cross sections incorporated in this calculation include those obtained using 11B, 12C, 14N, 15N, 16O, 19F, 20Ne and 22Ne beams, all relevant to the production of B, C, N, and O secondaries. In fact, for the isotopes in question direct measurements of cross sections now exist for þ95% of the total secondary production.
In the propagation calculations the input spectra of all species are assumed to be the same with exponent = -2.30 and the interstellar medium is assumed to be 90% hydrogen and 10% helium. Additional energy loss due to the large amount of ionized hydrogen in the disk is also included in the calculations (Soutoul, Ferrando and Webber, 1990). As described in Lukasiak et. al., 1994, we consider three sources of error in the determined source abundances. The first is just the statistical fitting error in the measured isotopic ratios as shown in column 6 of Table 1. The second includes the cross section uncertainties for the secondary production of the relevant isotopes. The third, which is combined with the second in column 7 of Table 1, relates to the uncertainties in the B cross sections and their effect on the chosen value for þþ.
We illustrate the total cross sectional uncertainties in Table 2 which shows the total production of 18O by heavier elements. In this calculation we show the estimated cross section and its uncertainty. This when multiplied by the relative abundance of the progenitor isotope (a full propagation calculation is used) gives the relative contribution of the particular isotope. So for example, 24Mg, 20Ne, 19F, 28Si and 22Ne produce þ75% of all 18O and all of these cross sections have now been measured.
The direct error in secondary production is then added in quadriture with the error introduced by fitting the B/C ratio to get the final result of 1.30ñ0.06% for the 18O/16O production ratio shown in column 7 of Table 1. Similarly the secondary production ratios are found for the other isotopes and then subtracted from the measured ratios to get the source ratios in column 8.
We now discuss these source ratios in more detail. 13C/12C For this source ratio we find 0.10ñ0.39%. This error is now þ0.6x the previous error given by Lukasiak et. al., 1994, which was the best yet reported up to 1994 - and the 13C/12C ratio itself is slightly smaller. This result is 2.5 å smaller than the solar ratio of 1.1%. In this case the largest error is the measurement error itself so improved cosmic ray data will help to elucidate this difference. It should be noted that all other high quality measurements of this ratio also give a lower than solar value as summarized by Lukasiak et. al., 1994. In fact the summary of all of the other measurements of the 13C/12C given in the above paper is -0.2ñ0.6%. We believe that a stronger case can now be made for a deficiency of 13C in the CRS relative to solar. 14N/16O Our measurement of this source ratio is 4.96ñ0.46%. This is almost the same as our earlier measurement but with þ0.5x the error. So taken with other earlier measurements a strong case can be made for a 14N/16O ratio at the CRS of þ5% or about 0.4 of the solar ratio of 12%.
15N/16O For this source ratio we find 0.97ñ0.62%. This is slightly smaller than our earlier value (Lukasiak et. al., 1994) - but now with only þ0.55x the error. Other measurements of this ratio described in Lukasiak et. al., 1994, give a smaller value consistent with the solar ratio of 0.37%. Our measurement is þ one å above the solar ratio. And now þ95% of the cross sections into 15N have been measured. Overall, we must conclude that there is no significant evidence for a source abundance of 15N different from the solar ratio.
It is interesting to note that in this case the cross section measurement errors and the propagation errors are about the same. Further improved measurements of this ratio in cosmic rays may thus still be limited in their interpretation by cross sectional uncertainties - particularly for potential source abundance ratios ó1%.
18O/16O Our value for this source ratio is 0.24ñ0.17%. This value is slightly smaller than our earlier value but with only 0.6x the error. This smaller ratio is mainly due to the larger production of 18O from the new cross sections, such as 22Ne used in this new calculation. (The net effect of the new cross sections at a fixed level of modulation is an increase of 8% in secondary production of 18O). This new source ratio is now completely consistent with the solar ratio of 0.23% as are the other earlier spacecraft measurements summarized in Lukasiak et. al., 1994.
The 17O abundance is also of interest since its solar source abundance is essentially zero (0.04%) and therefore it can be used as a tracer for secondary production calculations assuming the cosmic ray source ratio is also solar. Our value for this cosmic ray source ratio is 0.32ñ0.19% - consistent with zero.
4. SUMMARY AND DISCUSSION New Voyager measurements with improved statistics along with newly measured cross sections are used to derive more accurate source abundance ratios for the very important cosmic ray isotopes 13C, 14N, 15N and 18O. These measurements are particularly difficult because of the small values of these ratios - thus requiring both accurate data and cross sections to derive meaningful source ratios. But it is crucial to determine these ratios because of their important role in stellar nucleosynthesis in the source of cosmic rays themselves, and also because significant differences between solar and galactic source composition for all of these isotopes have been reported at one time or another in the past.
The low 13C/12C ratio in the CRS we find here could be evidence for an enrichment of 12C in these sources rather than a depletion of 13C. A contribution to cosmic ray 12C from Wolf-Rayet stars in which 12C is enhanced in the winds of WR-type receives support from these measurements. In fact a fractional contribution from these stars sufficient to produce the already observed enhancement of 22Ne/20Ne in the cosmic ray source (eg Prantzos et. al., 1986, Webber et. al., 1990) would give a reduced ratio of 13C/12C þ0.4% in the CRS, consistent with what we and others measure.
The fact that no significant enhancement in the 18O/16O ratio is observed in our data is relevant to this discussion since simulations of WR star evolution indicate a large 18O/16O excess during core He burning although the total injected output that eventually becomes cosmic rays may not contain an excess (Prantzos et. at., 1986). Measurements of both the 13C/12C and 18O/16O ratios for the CRS could in principle be used as an additional diagnostic of the WR contribution to the sources of cosmic rays. To fully exploit this approach better estimates are needed of the elemental and isotopic composition of WR ejecta over their lifetime.
Acknowledgements: The authors wish to thank the Voyager project office at JPL for their support through contract #959213 at the University of Maryland and contract #959160 at New Mexico State University.
References