The production of circumstellar26Al by massive stars

We investigate the production of²⁶Al during hydrogen burning and its ejection by massive single and binary stars. Effects of convection and rotation are studied. We discuss the importance of RSGs, LBVs and WR stars to the total Galactic²⁶Al production, and the detection probability of the²⁶Al decay in individual objects as P Cygni, Velorum and Carinae.     

26Al production in the Vela and Orion regions

The allsky image of ²⁶Al radiation at 1.809 MeV with COMPTEL suggests that clusters of massive stars dominate the Galactic production of ²⁶Al. Studies of rather well-known localized regions are most promising to further this interpretation. In the Vela region, excessive emissivity is found compared to other Galactic regions. This may be due to few prominent foreground sources, such as the Vela SNR and RX J0852.0-4622 combined; but more plausibly the star forming activity along the Vela Molecular Ridge is enhanced in general, too. In the anticenter region, the detected ²⁶Al emission appears related to nucleosynthetic activity in the Orion OB1 association: It seems plausible that ejecta are being vented into the Eridanus bubble blown by earlier massive stars’ activity.     

26Al and the partial ionization of the solar nebula

Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of²⁶Al will result in strong coupling of the gas to magnetic fields. In the absence of²⁶Al,⁴⁰K will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.also Department of Astronomy.     

Spatial heterogeneity of 26Al/27Al and stable oxygen isotopes in the solar nebula

Abstract— The degree of isotopic spatial heterogeneity in the solar nebula has long been a puzzle, with different isotopic systems implying either large‐scale initial spatial homogeneity (e.g., ²⁶Al chronometry) or a significant amount of preserved heterogeneity (e.g., ratios of the three stable oxygen isotopes, ¹⁶O, ¹⁷O, and ¹⁸O). We show here that in a marginally gravitationally unstable (MGU) solar nebula, the efficiency of large‐scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of ?10% about the mean value of ²⁶Al/²⁷Al on time scales of thousands of years. A similar dispersion would be expected for ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios produced by ultraviolet photolysis of self‐shielded molecular CO gas at the surface of the outer solar nebula. In addition to preserving a chronological interpretation of initial ²⁶Al/²⁷Al ratios and the self‐shielding explanation for the oxygen isotope ratios, these solar nebula models offer a self‐consistent environment for achieving large‐scale mixing and transport of thermally annealed dust grains, shock‐wave processing of chondrules and refractory inclusions, and giant planet formation.     

The production of 26Al, 60Fe and 44Ti in massive stars of solar metallicity

We discuss the production of the three gamma ray emitters ²⁶Al, ⁶⁰Fe and ⁴⁴Ti in a 15 M_⊙ and a 25 M_⊙ star of solar metallicity.     

On the survivability of an enantiomeric excess of amino acids in comet nuclei during the decay of26Al and other radionuclides

We present a computer model calculation for the racemization of a possible excess of amino acids in the icy fraction of comet nuclei bring about by ionizing radiation released during the decay of²⁶Al,⁴⁰K,²³⁵U,²³⁸U and²³²Th. The model takes into account a total of 110 chemical reactions, of which 91 are needed to explain the radiation chemical processing of the major constituents of comet nuclei (Navarro-Gonzálezet al., 1992) and 19 are necessary to simulate the radiolysis of glycine/alanine mixtures in aqueous solutions (Navarro-Gonzálezet al., 1994 and 1996). It is predicted that an enantiomeric excess of alanine would not be destroyed by radioracemization during the decay of embedded radionuclides. Nevertheless, this enantiomeric excess could be attenuated by the formation of racemic amino acids in the interior of comet nuclei as a result of the radiation-induced polymerization of HCN.     

Nucleosynthesis of26Al in the early solar system and in cosmic rays

The nucleus²⁶Al(^1/2 = 7.4 × 10⁵ yr) has long been considered as a possible heat source during the formation of the solar system. Recent experimental work has found no evidence for²⁶Al at the time of final solidification of the meteorites. Due to the short half-life these measurements do not rule out the possibility that²⁶Al was a significant heat source a few million years prior to final solidification. For²⁶Al to be an effective heat source in the early solar system it is necessary for the ratio²⁶Al/Si to be 2×10^–7 at the time of a solidification. The nucleosynthetic yields of²⁶Al by silicon burning, carbon burning, and spallation are discussed. It is shown that²⁶Al can be synthesized in carbon and/or silicon-burning supernovae. However, time scales in the early solar system make it more likely that²⁶Al, if present in planets, was synthesized by a proton irradiation in the early solar system. An integrated proton flux >4×10¹⁸ cm^–2 is shown to be necessary in order for²⁶Al to be a significant heat source. No conclusive evidence has been observed for an irradiation of this magnitude. Therefore, unless such evidence is found, it should be assumed that²⁶Alwas not involved in the formation of the solar system. In addition, the production of²⁶Al in cosmic rays is discussed and it is shown that either resolution of the Al isotopes in the cosmic rays or accurate measurements of the variation with energy of the cosmic ray abundance ratios Al/Si and Mg/Si can be used to determine the age of the cosmic rays, _C.R.. Current abundance data are extremely uncertain; however, the trend tends toward values of _C.R. 10⁶ yr.Supported in part by the National Science Foundation [GP-19887, GP-28027, GP-27304].     

Radiogenic heating of comets by 26Al and implications for their time of formation

The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii 1 km < R < 100 km was investigated. The radioisotopes considered were 26Al, 40K, 232Th, 235U, and 238U. Except for the 26Al abundance, which was varied, the other initial abundances were kept fixed, at values derived from those of chondritic meteorites and corresponding to a gas-to-dust ratio of 1. The initial models were homogeneous and isothermal (To = 10 K) amorphous ice spheres, in a circular orbit at 10(4) AU from the Sun. The main object of this study was to examine the conditions under which the transition temperature from amorphous into cubic ice (Ta = 137 K) would be reached. It was shown that the influence of the short-lived radionuclide 26Al dominates the effect of other radioactive species for bodies of radii up to approximately 50 km. Consequently, if we require comets to retain their ice in amorphous form, as suggested by observations, an upper limit of approximately 4 x 10(-9) is obtained for the initial 26Al abundance in comets, a factor of 100 lower than that of the inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of a coexistence of molten cometary cores and extended amorphous ice mantles is ruled out. Larger icy spheres (R > 100 km) reached Ta even in the absence of 26Al, due to the decay of the other radionuclides. As a result, a crystalline core formed whose relative size depended on the composition assumed. Thus the outermost icy satellites in the solar system, which might have been formed of ice in the amorphous state, have probably undergone crystallization and may have exhibited eruptive activity when the gas trapped in the amorphous ice was released (e.g., Miranda).     

Evolution of protoplanetary disk inferred from 26Al chronology of individual chondrules

Abstract– We review the ²⁶Al ages of chondrules in various type 3.0 chondrites. The ²⁶Al ages of chondrules are 1–3 Myr after calcium‐aluminum‐rich inclusion (CAI) for LL3.0, CO3.0, and Acfer 094 (Ungrouped C 3.0). Available data for chondrules in CR chondrites indicate that many chondrules are relatively younger (≥3 Myr), although data from chondrules in CR3.0 are not yet available to confirm their younger ages. The total ranges for the ²⁶Al ages of chondrules in a single chondrite group are more than 0.5–1 Myr. However, most chondrules show relatively narrow range of ages in a single chondrite group (0.2–0.4 Myr, 1 SD), which might be short enough to preserve the group‐specific chemical and isotope signatures against radial diffusion of solid in the disk. Distinct oxygen isotope reservoirs might exist in the protoplanetary disk simultaneously, which could be spatially separated.     

Mg isotopic heterogeneity,Al‐Mg isochrons,and canonical 26Al/27Al in the early solar system

There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of ²⁶Mg/²⁴Mg with ²⁷Al/²⁴Mg, yielding an isochron corresponding to an initial (²⁶Al/²⁷Al)_o = (5.27 ± 0.18) × 10^?5 and an initial (²⁶Mg/²⁴Mg)_o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same ²⁶Al/²⁷Al. No inference about uniformity/heterogeneity of ²⁶Al/²⁷Al on a macro scale can be based on the initial (²⁶Mg/²⁴Mg)_o values. Different values of ²⁶Al/²⁷Al for samples representing the same point in time would prove heterogeneity of ²⁶Al/²⁷Al. The important issue is whether the bulk solar inventory of ²⁶Al/²⁷Al was approximately 5 × 10^?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with ²⁶Al/²⁷Al from approximately 5 × 10^?5 to below 0.2 × 10^?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of ¹⁶O‐poor material; and the relationship to earlier formed ¹⁶O‐rich material in the disk. ²⁶Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.     

A model for planetesimal meltdown by 26Al and its implications for meteorite parent bodies

Abstract— The melting of planetesimals heated by ²⁶Al has been modelled using a new finite difference method that incorporates convection. As an example, we consider a planetesimal with a radius of 64 km, which accretes instantaneously at t = 0.75 Myr (after the formation of calcium‐aluminum‐rich inclusions) from cold (250 K) nebular dust with 50% porosity. At t = 0.9 Myr (T = 700 K), the planetesimal shrinks to a radius of 50 km due to sintering. At t = 1.2 Myr (T= 1425 K), the fully insulated interior, deeper than a few kilometers, starts to melt, and at t = 1.5 Myr (T = 1725 K), with 50% melting, convection starts. By t = 2 Myr, the planetesimal is a globe of molten, convecting slurry inside a thin residual crust. From about t = 2.5 Myr, the crust thickens rapidly as the power of ²⁶Al fades. Planetesimals probably melt in this manner when they accrete before t = 1.3 Myr and are large enough to insulate themselves (R >20 km for accretion at t = 0, rising to >80 km at t = 1.3 Myr). Melting behavior will also be affected by the level of ⁶⁰Fe in nebular dust, by the extent of devolatilization reactions and basalt segregation during heating, and by gradual accretion. The model suggests that a) the parent bodies of differentiated meteorites had accreted before about t = 1.5 to 2 Myr and before most chondritic parent bodies had formed, and b) that molten planetesimals may be a source for chondrule melt droplets.     

The role of exchange reactions in oxygen isotope fractionation during CAI and chondrule formation

Abstract— The role of oxygen isotope exchange during evaporation and condensation of silicate melt is quantitatively evaluated. Silicate dusts instantaneously heated above liquidus temperature are assumed to cool in gas and experience partial evaporation and subsequent recondensation. The results show that isotopic exchange effectively suppresses mass‐dependent O‐isotope fractionation even if the degree of evaporation is large, which is the fundamental difference from the case without isotopic exchange. The final composition of silicate melt strongly depends on the initial abundance of oxygen in the ambient gas relative to that in silicate dust, but not on the cooling rate of the system. The model was applied to O‐isotope evolution of silicate melts in isotopically distinct gas of the protoplanetary disk. It was found that deviation from a straight mixing line toward the δ¹⁸O‐rich side on the three‐oxygen isotope diagram is inevitable when mass‐dependent fractionation and isotopic exchange take place simultaneously; the degree of deviation depends on the abundance of oxygen in an ambient gas and isotopic exchange efficiency. The model is applied to explain O‐isotopic compositions of igneous CAIs and chondrules.     

Cross sections from 5 to 35 MeV for the reactions natMg(3He,x)26Al, 27Al(3He,x)26Al,natCa(3He,x)41Ca,and natCa(3He,x)36Cl: Implications for early irradiation in the solar system

Abstract– Cross sections were measured for the nuclear reactions ^natMg(³He,x)²⁶Al, ²⁷Al(³He,x)²⁶Al, ^natCa(³He,x)⁴¹Ca, and ^natCa(³He,x)³⁶Cl in the energy region from approximately 5–35 MeV. The rates of these reactions are important for studies of early solar system irradiation scenarios. The ²⁶Al, ³⁶Cl, and ⁴¹Ca were separated chemically, and the numbers of atoms produced in each reaction channel were measured using accelerator mass spectrometry (AMS). From these results, 26 cross sections were determined and compared with predictions of the TALYS code. Agreement is within 40% for most cross sections. Our measurements were used to model the production of ⁷Be, ¹⁰Be, ²⁶Al, and ⁴¹Ca in the early solar system. For projectiles ¹H, ³He, and ⁴He, we assumed energy spectra of the general form E^?α. For a wide range of parameterizations, the modeled ratios of ⁷Be/Be and ¹⁰Be/Be on the one hand and of ²⁶Al/²⁷Al and ⁴¹Ca/Ca on the other are coupled because the excitation functions for the relevant nuclear reactions have similar shapes. Modeling of a closed system with the constraint that ¹⁰Be/⁹Be = 0.001 fails to reproduce simultaneously the range of ⁷Be/⁹Be, ²⁶Al/²⁷Al, and ⁴¹Ca/Ca ratios inferred for the early solar system from studies of meteorites.     

Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources

Abstract— Numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with ²⁶Al and ⁶⁰Fe as the heat sources. Planetesimal accretion was started at chosen times up to 3 Ma after Ca‐Al‐rich inclusions (CAIs) were formed, and was continued for periods of 0.001–1 Ma. The planetesimals were initially porous, unconsolidated bodies at 250 K, but became sintered at around 700 K, ending up as compact bodies whose final radii were 20, 50, 100, or 270 km. With further heating, the planetesimals underwent melting and igneous differentiation. Two approaches to core segregation were tried. In the first, labelled A, the core grew gradually before silicate began to melt, and in the second, labelled B, the core segregated once the silicate had become 40% molten. In A, when the silicate had become 20% molten, the basaltic melt fraction began migrating upward to the surface, carrying ²⁶Al with it. The ⁶⁰Fe partitioned between core and mantle. The results show that the rate and timing of core and crust formation depend mainly on the time after CAIs when planetesimal accretion started. They imply significant melting where accretion was complete before 2 Ma, and a little melting in the deep interiors of planetesimals that accreted as late as 3 Ma. The latest melting would have occurred at <10 Ma. The effect on core and crust formation of the planetesimal's final size, the duration of accretion, and the choice of (⁶⁰Fe/⁵⁶Fe)_initial were also found to be important, particularly where accretion was late. The results are consistent with the isotopic ages of differentiated meteorites, and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 Ma after CAIs.     

Tritium chain of the hydrogen cycle of thermonuclear reactions in the solar interior

The tritium chain of the hydrogen cycle on the Sun including the reactions ³He(e⁻, ν _e) ³H(p, γ)⁴He is considered. The flux of tritium neutrinos at a distance of 1 AU is 8.1 × 10⁴ cm⁻² s⁻¹. It exceeds the neutrino flux from the (hep)-reaction by one order of magnitude. The radial distribution of the yield of ³H neutrinos inside the Sun and their energy spectrum, which has the form of a line at an energy of 2.5–3.0 keV, have also been calculated. The flux of thermal tritium neutrinos is accompanied by a very weak flux of antineutrinos (∼10³ cm⁻² yr⁻¹) with an energy below 18.6 keV. These antineutrinos are produced in the URCA processes ³He ⇆ ³H.     

Heterogeneous distribution of 26Al at the birth of the solar system: Evidence from refractory grains and inclusions

We review recent results on O‐ and Mg‐isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum‐rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in ¹⁶O to varying degrees relative to the standard mean ocean water value: the Δ¹⁷O_SMOW value ranges from approximately ?16‰ to ?35‰, and recorded heterogeneous distribution of ²⁶Al in their formation region: the inferred (²⁶Al/²⁷Al)₀ ranges from approximately 6.5 × 10^?5 to <2 × 10^?6. There is no correlation between O‐ and Mg‐isotope compositions of the refractory objects: ²⁶Al‐rich and ²⁶Al‐poor refractory objects have similar O‐isotope compositions. We suggest that ²⁶Al was injected into the ²⁶Al‐poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of ²⁶Al/²⁷Al ratio (approximately 5 × 10^?5). The ²⁶Al‐rich and ²⁶Al‐poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of ²⁶Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their ²⁶Al‐²⁶Mg systematics, and the canonical (²⁶Al/²⁷Al)₀ does not represent the initial abundance of ²⁶Al in the solar system; instead, it may or may not represent the average abundance of ²⁶Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical ²⁶Al/²⁷Al ratio relative to the timing of complete delivery of stellar ²⁶Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing ²⁶Al resulted in no observable changes in O‐isotope composition of the solar system. Instead, the variations in O‐isotope compositions between individual CAIs indicate that O‐isotope composition of the CAI‐forming region varied, because of coexisting of ¹⁶O‐rich and ¹⁶O‐poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O‐isotope compositions of these reservoirs with time, e.g., due to CO self‐shielding in the disk.     

Correlation between relative ages inferred from 26Al and bulk compositions of ferromagnesian chondrules in least equilibrated ordinary chondrites

Abstract— We have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial ²⁶Al/²⁷Al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1). The initial ²⁶Al/²⁷Al ratios of these chondrules were obtained by Kita et al. (2000) and Mostefaoui et al. (2002), corresponding to relative ages from 0.7 ± 0.2 to 2.4 ?0.4/+0.7 Myr after calcium‐aluminum‐rich inclusions (CAIs), by assuming a homogeneous distribution of ²⁶Al in the early solar system. The measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and, thus, the chondrules in this study are regarded as representatives of ferromagnesian chondrules. The relative ages of the chondrules appear to correlate with bulk abundances of Si and the volatile elements (Na, K, Mn, and Cr), but there seems to exist no correlation of relative ages neither with Fe nor with refractory elements. Younger chondrules tend to be richer in Si and volatile elements. Our result supports the result of Mostefaoui et al. (2002) who suggested that pyroxene‐rich chondrules are younger than olivine‐rich ones. The correlation provides an important constraint on chondrule formation in the early solar system. It is explained by chondrule formation in an open system, where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation.