Numerical simulations of the production of extinct short‐lived nuclides by magnetic flaring in the early solar system

Abstract Using the X‐ray flare observations of low‐mass protostars, we developed numerical simulations of thermal processing and irradiation of protoCAIs in the magnetic reconnection ring within the X‐wind formulation. Observed X‐ray flare luminosities have been used to model various simulation flare characteristics. Several approximations have been made regarding the thermal evolution that involve condensation, evaporation, and coagulation of protoCAIs. Ensembles of refractory cores with ferromagnesian mantles were evolved for irradiation production of the short‐lived nuclides ⁷Be, ¹⁰Be, ⁴¹Ca, ³⁶Cl, ²⁶Al, and ⁵³Mn. Three distinct grain‐size distributions of protoCAIs with refractory cores in the ranges of 32μ m‐20 mm, 125 μ m‐16 mm, and 500 μ m‐13 mm were thermally evolved for irradiation. The latter two size distributions were found to result in the accumulation of protoCAIs in the reconnection ring during an X‐wind cycle, and hence can account for the total inventory of ²⁶Al in the early solar system. The canonical value of ?5 × 10^‐5 for ²⁶Al/²⁷ Al can be inferred from the impulsive flare simulations by a suitable choice of simulation parameters. However, in most of the remaining simulations, the irradiation of protoCAIs by superflare(s) with Lx > 10³² ergs s^‐1 subsequent to their thermal processing in the reconnection ring would be required to explain the experimental abundances of the short‐lived nuclides. These superflares have never been reliably observed in young stellar objects. If they are real, they would be extremely rare. The paucity of these superflares could impose stringent constraints on the validity of the X‐wind irradiation scenario as the source of the short‐lived nuclides.     

Stellar nucleosynthetic contribution of extinct short‐lived nuclei in the early solar system and the associated isotopic effects

Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short‐lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short‐lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf‐Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of ²⁶Al/²⁷Al in the early solar system, almost all stellar sources except low‐mass AGB stars imply large isotopic anomalies in Ca‐Al‐rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short‐lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high‐injection mass cut is a possible scenario. There is a possibility of the contribution of short‐lived nuclides by a 1.5–3 M_⊙ AGB star as it implies the smallest shift in stable isotopes. A low‐mass AGB star of relatively low metallicity would be even a better source of short‐lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute ⁶⁰Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of ²⁶Al and ⁴¹Ca by irradiation in the early solar system. Significant production of ²⁶Al and ⁶⁰Fe can be explained if massive, rotating Wolf‐Rayet stars that evolved to supernovae Ib/c were involved.     

Recalculation of data for short‐lived radionuclide systems using less‐biased ratio estimation

Ratios determined from counting a subset of atoms in a sample are positively biased relative to the true ratio in the sample ( Ogliore et al. 2011 ). The relative magnitude of the bias is approximately equal to the inverse of the counts in the denominator of the ratio. SIMS studies of short‐lived radionuclides are particularly subject to the problem of ratio bias because the abundance of the daughter element is low, resulting in low count rates. In this paper, we discuss how ratio bias propagates through mass‐fractionation corrections into an isochron diagram, thereby affecting the inferred initial ratio of short‐lived radionuclides. The slope of the biased isochron can be either too high or too low, depending on how it is calculated. We then reanalyze a variety of previously published data sets and discuss the extent to which they were affected by ratio bias. New, more accurate, results are presented for each study. In some cases, such as for ⁵³Mn‐⁵³Cr in pallasite olivines and ⁶⁰Fe‐⁶⁰Ni in chondrite sulfides, the apparent excesses of radiogenic isotopes originally reported disappear completely. Many of the reported initial ⁶⁰Fe/⁵⁶Fe ratios for chondrules from ordinary chondrites are no longer resolved from zero, though not all of them. Data for ¹⁰Be‐¹⁰B in CAIs were only slightly affected by bias because of how they were reduced. Most of the data sets were recalculated using the ratio of the total counts, which increases the number of counts in the denominator isotope and reduces the bias. However, if the sum of counts is too low, the ratio may still be biased and a less‐biased estimator, such as Beale’s estimator, must be used. Ratio bias must be considered in designing the measurement protocol and reducing the data. One can still collect data in cycles to permit editing of the data and to monitor and correct for changes in ion‐beam intensity, even if total counts are used to calculate the final ratio. The cycle data also provide a more robust estimate of the uncertainties from temporal variations in the secondary ion signal.     

Effects of meteoroid shape on cosmogenic nuclide production processes

The shape of meteorites is one of the major factors influencing the production of cosmogenic nuclides. Numerical simulations using the Los Alamos Code System (LCS) particle production and transport codes were done to investigate particle fluxes and production rates of cosmogenic nuclides ¹⁰Be, ²⁶Al, and ⁶⁰Co in meteoroids of spherical, ellipsoidal, and cylindrical shapes. The calculations show that fluxes of nuclear active particles and also production rates of cosmogenic nuclides are sensitive to the shape of the irradiated parent body.     

Light noble gases and cosmogenic radionuclides in Estherville,Budulan, and other mesosiderites: Implications for exposure histories and production rates

Abstract— We report measurements of ²⁶AI, ¹⁰Be, ⁴¹Ca, and ³⁶Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are ²⁶Al/¹⁰Be = 0.77 ± 0.02; ³⁶Cl/¹⁰Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find ²⁶Al/¹⁰Be = 0.72 ± 0.01 and ³⁶Cl/¹⁰Be = 4.5 ± 0.2. The average ⁴¹Ca/³⁶Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The ⁴¹Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of ²⁶Al and ¹⁰Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the ³⁶Cl and ⁴¹Ca activities of the metal phases, matrix effects are unimportant for ¹⁰Be and minor for ²⁶Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of ³⁶Cl/³⁶Ar exposure ages for the metal phases agree well with published values. Neon‐21 production rates for mesosiderite silicates calculated from these ages and from measured ²¹Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the ²¹Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low‐energy secondary particles and hence the ²¹Ne production from Mg. As ¹⁰Be production is relatively insensitive to this matrix effect, ¹⁰Be/²¹Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard ²²Ne/²¹Ne based “shielding” corrections give fairly reliable ²¹Ne production rates in the mesosiderite silicates.     

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl‐³⁶Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl‐³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.     

New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are ⁵³Mn and ⁶⁰Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for ³He, ²¹Ne, and ³⁸Ar are in agreement. For ⁴He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled ³⁶Ar/³⁸Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm³STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply ^3,4He, ²¹Ne, or ^36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the ⁴¹K‐⁴⁰K‐and the ³⁶Cl‐³⁶Ar dating methods with the new model. Note that we distinguish between ³⁶Ar ages (calculated via ³⁶Ar production rates only) and ³⁶Cl‐³⁶Ar ages. Exposure ages for Grant and Carbo, calculated with the revised ⁴¹K‐⁴⁰K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using ³He, ²¹Ne, and ³⁸Ar but higher than the 3⁶Ar‐ and ³⁶Cl‐³⁶Ar ages by ?30%. For Carbo the ⁴¹K‐⁴⁰K age is ?40% lower than the ages obtained using ³He, ²¹Ne, and ³⁸Ar but equal to the ³⁶Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR.     

Short‐lived radioactivity in the early solar system: The Super‐AGB star hypothesis

Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., ²⁶Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to ⁶⁰Fe predicted from stars with initial mass in the range approximately 7–11 M_⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 10⁸ K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce ²⁶Al/²⁷Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the ²⁶Al/²⁷Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 10² to several 10³ of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ¹⁷O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides ⁴¹Ca and ⁶⁰Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as ¹⁰⁷Pd.     

The production of cosmogenic nuclides in stony meteoroids by galactic cosmic‐ray particles

Abstract— We present a purely physical model for the calculation of depth‐ and size‐dependent production rates of cosmogenic nuclides by galactic cosmic‐ray (GCR) particles. besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on only one free parameter—the integral number of gcr particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth‐ and size‐dependent production rates for ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, and ⁵³Mn as well as for the rare gas isotopes ³He, ²⁰Ne, ²¹Ne, ²²Ne, ³⁶Ar, and ³⁸Ar. The new data differ from semi‐empirical estimates by up to a factor of 4 but agree within ～20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter ²²Ne/²¹Ne and the correlations ²⁶Al vs. ¹⁰Be, ²⁶Al vs. ⁵³Mn, ¹⁰Be/²¹Ne vs. ²²Ne/²¹Ne, and ³⁶Ar vs. ³⁶Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.     

Calibrating a physical model based on Geant4 to calculate cosmogenic nuclide production rates on lunar surface

A physical model based on the open‐source toolkit Geant4 for production rates of cosmogenic nuclei on the lunar surface is proposed and calibrated. The fluxes of proton and neutron beneath the lunar surface are obtained by simulating the physical processes between the cosmic‐ray particles and the lunar surface material. By combining the experimental proton cross sections and the a posteriori neutron cross sections, we calculate the production rate depth profiles of long‐lived nuclei (¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, and ⁵³Mn). Through comparing experimental and theoretical data for these nuclei, we find that for all the selected nuclei, experimental and theoretical production rate depth profiles agree well with each other by introducing a single normalization factor. It means that the physical model based on Geant4 can also reproduce the depth profiles of cosmogenic nuclei, and that this model can be used by everyone worldwide. In addition, we predict the production rates of three stable nuclei (²¹Ne, ²²Ne, and ³⁸Ar).     

Production rates of cosmogenic helium‐3, neon‐21, and neon‐22 in ordinary chondrites and the lunar surface

Abstract— The production of ³He, ²¹Ne, and ²²Ne in meteoroids of various sizes and in the lunar surface was investigated. The LAHET code system, a purely physical model for calculating cosmic‐ray particle fluxes, was used to simulate cosmic‐ray particle interactions with extraterrestrial matter. We discuss the depth and size dependence of the shielding parameter ²²Ne/²¹Ne, which is used for reconstruction of pre‐atmospheric sizes, depth, and exposure histories. The ²²Ne/²¹Ne ratio decreases with increasing depth or pre‐atmospheric size but then increases with depth in very large objects. This increase with depth in the ²²Ne/²¹Ne ratio means that this ratio is a poor indicator of shielding in some large objects. The dependence of ³He/²¹Ne as function of ²²Ne/²¹Ne was also calculated, and differences between the calculations and the Bern line are discussed.     

Radar sporadic meteor rates and solar activity

The correlation of sporadic meteor rates from radar observations in January, August, and December non-show-er periods in 1958–2000, and relevant solar activity represented by the solar relative number, R, is investigated. Similar analysis of the December sporadic period was already presented by Simek 1999, and Pecina. Complete analysis indicates high correlation of both phenomena with sporadic meteor counts curve following that of solar activity after 1.5–2 years in the mean eleven year solar cycle with the correlation index exceeding 70%. This result supports the large volume of observing material of the Ondřejov meteor radar in the above mentioned span covering almost four solar cycles. This revised version was published online in July 2006 with corrections to the Cover Date.     

The production of cosmogenic nuclides by galactic cosmic‐ray particles for 2π exposure geometries

Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for ¹⁰Be, ²⁶Al, and ⁵³Mn to measured data from the Apollo 15 drill core. The J_0,GCR value of 4.54 cm^?2 s^?1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J_0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl and ⁵³Mn, as well as for the rare gas isotopes ^20,21,22Ne. In addition we present production rates for ^36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the ²²Ne/²¹Ne ratio as a shielding parameter is also discussed.     

Calibration of cosmogenic noble gas production based on 36Cl‐36Ar ages. Part 2. The 81Kr‐Kr dating technique

We calibrated the ⁸¹Kr‐Kr dating system for ordinary chondrites of different sizes using independent shielding‐corrected ³⁶Cl‐³⁶Ar ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q. The thus‐determined average cosmogenic ⁷⁸Kr/⁸³Kr, ⁸⁰Kr/⁸³Kr, ⁸²Kr/⁸³Kr, and ⁸⁴Kr/⁸³Kr ratiC(Lavielle and Marti 1988; Wieler 2002). The cosmogenic ⁷⁸Kr/⁸³Kr ratio is correlated with the cosmogenic ²²Ne/²¹Ne ratio, confirming that ⁷⁸Kr/⁸³Kr is a reliable shielding indicator. Previously, ⁸¹Kr‐Kr ages have been determined by assuming the cosmogenic production rate of ⁸¹Kr, P(⁸¹Kr)_c, to be 0.95 times the average of the cosmogenic production rates of ⁸⁰Kr and ⁸²Kr; the factor Y = 0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However, Y should be regarded as an empirical adjustment. For samples whose ⁸⁰Kr and ⁸²Kr concentrations may be affected by neutron‐capture reactions, the shielding‐dependent cosmogenic (⁷⁸Kr/⁸³Kr)_c ratio has been used instead to calculate P(⁸¹Kr)/P(⁸³Kr), as for some lunar samples, this ratio has been shown to linearly increase with (⁷⁸Kr/⁸³Kr)_c (Marti and Lugmair 1971). However, the ⁸¹Kr‐Kr ages of our samples calculated with these methods are on average ~30% higher than their ³⁶Cl‐³⁶Ar ages, indicating that most if not all the ⁸¹Kr‐Kr ages determined so far are significantly too high. We therefore re‐evaluated both methods to determine P(⁸¹Kr)_c/P(⁸³Kr)_c. Our new Y value of 0.70 ± 0.04 is more than 25% lower than the value of 0.95 used so far. Furthermore, together with literature data, our data indicate that for chondrites, P(⁸¹Kr)_c/P(⁸³Kr)_c is rather constant at 0.43 ± 0.02, at least for the shielding range covered by our samples ([⁷⁸Kr/⁸³Kr]_c = 0.119–0.185; [²²Ne/²¹Ne]_c = 1.083–1.144), in contrast to the observations on lunar samples. As expected considering the method used, ⁸¹Kr‐Kr ages calculated either directly with this new P(⁸¹Kr)_c/P(⁸³Kr)_c value or with our new Y value both agree with the corresponding ³⁶Cl‐³⁶Ar ages. However, the average deviation of 2% indicates the accuracy of both new ⁸¹Kr‐Kr dating methods and the precision of the new dating systems of ~10% is demonstrated by the low scatter in the data. Consequently, this study indicates that the ⁸¹Kr‐Kr ages published so far are up to 30% too high.     

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history,exposure history,and pre‐atmospheric size

Abstract— We measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of ¹⁰Be, ²⁶Al, and ⁴¹Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the ²²Ne/²¹Ne ratio at very high shielding. The ²¹Ne/¹⁰Be and ²¹Ne/²⁶Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of ²¹Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture ³⁶Ar in most samples are consistent with measured concentrations of neutron‐capture ³⁶Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture ³⁶Ar were found in those samples with an excess of ²¹Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced ²¹Ne and neutron‐capture‐produced ³⁶Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic ⁴He and ⁴⁰Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.     

Short-lived radionuclides in the early solar system — A meteoritic perspective of the solar system formation

This paper reviews the evidence for short-lived radionuclides in the early solar system and evaluates the models of their origin. The stellar model requires that some freshly-nucleosynthesized radionuclides were injected into the proto-solar cloud shortly before it began to collapse. The spallation theory suggests that these nuclides were the products of interaction between energetic particles and gas/dust in the proto-solar cloud or solar nebula. A brief discussion is given to a new theory for the X-wind model of solar system formation.     

The nature of the solar activity and its relation to solar cosmic-ray production

The long-term variation of the solar activity and its nature are statistically analysed by using the data on the relative sunspot numbers for the last two hundred years. Based on this analysis, the occurrence frequency of large solar flares in the past is estimated to find a clue to deduce the production rate of high-energy particles from the Sun in the past. According to the results from the present study, during the period that the solar activity was much higher than that observed during solar cycle No. 19, this production rate must have been very much higher than that being currently observed. Such a period might have been hazardous to life on Earth because of the destruction of the ozone layer by the bombardment of solar cosmic rays.     

The short‐period low‐mass binary system CC Com revisited

In this study we determined precise orbital and physical parameters of the very short‐period low‐mass contact binary system CC Com. The parameters are obtained by analysis of new CCD data combined with archival spectroscopic data. The physical parameters of the cool and hot components are derived as M_c = 0.717(14) M_⊙, M_h = 0.378(8) M_⊙, R_c = 0.708(12) R_⊙, R_h = 0.530(10) R_⊙, L_c = 0.138(12) L_⊙, and L_h = 0.085(7) L_⊙, respectively, and the distance of the system is estimated as 64(4) pc. The times of minima obtained in this study and with those published before enable us to calculate the mass transfer rate between the components which is 1.6 × 10^–8 M_⊙ yr^–1. Finally, we discuss the possible evolutionary scenario of CC Com (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Laser‐induced melting experiments: Simulation of short‐term high‐temperature impact processes

This study introduces an experimental approach using direct laser irradiation to simulate the virtually instantaneous melting of target rocks during meteorite impacts. We aim at investigating the melting and mixing processes of projectile (iron meteorite; steel) and target material (sandstone) under idealized conditions. The laser experiments (LE) were able to produce features very similar to those of impactites from meteorite craters and cratering experiments, i.e., formation of lechatelierite, partial to complete melting of sandstone, and injection of projectile droplets into target melts. The target and projectile melts have experienced significant chemical modifications during interaction of these coexisting melts. Emulsion textures, observed within projectile‐contaminated target melts, indicate phase separation of silicate melts with different chemical compositions during quenching. Reaction times of 0.6 to 1.4 s could be derived for element partitioning and phase‐separation processes by measuring time‐depended temperature profiles with a bolometric detector. Our LE allow (i) separate melting at high temperatures to constrain primary melt heterogeneities before mixing of projectile and target, (ii) quantification of element partitioning processes between coexisting projectile and target melts, (iii) determination of cooling rates, and (iv) estimation of reaction times. Moreover, we used a thermodynamic approach to calculate the entropy gain during laser melting. The entropy changes for laser‐melting of sandstone and iron meteorite correspond to shock pressures and particle velocities produced during the impact of an iron projectile striking a quartz target at a minimum impact velocity of ~6 km s^?1, inducing peak shock pressures of ~100 GPa in the target.