Presolar Nebula Research Articles

Galactic Evolution of D and 3He Including Stellar Production of 3He

New stellar models which track the production and destruction of {sup 3}He (and D) have been evolved for a range of stellar masses (0.65{le}{ital M}/{ital M}{sub {circle_dot}}{le}100), metallicities (0.01{le}{ital Z}/{ital Z}{sub {circle_dot}}{le}1), and initial (main-sequence) {sup 3}He mass fractions (10{sup {minus}5}{le}{ital X}{sub 3,MS}{le}10{sup {minus}3}). Armed with the {sup 3}He yields from these stellar models we have followed the evolution of D and {sup 3}He using a variety of chemical evolution models with and without infall of primordial or processed material. Production of new {sup 3}He by the lower mass stars overwhelms any reasonable primordial contributions and leads to predicted abundances in the presolar nebula and/or the present interstellar medium in excess of the observationally inferred values. This result, which obtains even for zero primordial D and {sup 3}He, and was anticipated by Rood, Steigman, & Tinsley is insensitive to the choice of chemical evolution model; it is driven by the large {sup 3}He yields from low-mass stars. In an attempt to ameliorate this problem we have considered a number of nonstandard models in which the yields from low-mass stars have been modified. Although several of these nonstandard models may be consistent with the {sup 3}He data, they may be inconsistentmore » with observations of {sup 12}C/{sup 13}C, {sup 18}O, and, most seriously the super-{sup 3}He rich planetary nebulae. Even using the most extreme of these nonstandard models we obtain a generous upper bound to pregalactic {sup 3}He:{ital X}{sub 3{ital P}}{le}3.2{times}10{sup {minus}5} which, nonetheless, leads to a stringent lower bound to the universal density of nucleons. {copyright} {ital 1996 The American Astronomical Society.}« less

Generic Evolution of Deuterium and 3He

The primordial abundances of deuterium and of helium-3, produced during big bang nucleosynthesis, depend sensitively on the baryon density. Thus, the observed abundances of D and \he may provide useful ``baryometers'' provided the evolution from primordial to present (or, presolar nebula) abundances is understood. Inevitably, the derivation of primordial from observed abundances requires the intervention of a model for galactic evolution and, so, the inferred primordial abundances are, necessarily, model dependent. Here, an analytic framework for the evolution of D and \he is presented which is ``generic'' in the sense that it should describe the results of any specific galactic evolution model. The ``effective \he survival fraction'', $\Gamma _3$, is the one free parameter which is model specific. Solar system and interstellar data are used to infer upper and lower bounds to the primordial deuterium mass fraction $(X_{2P})$ as a function of $\Gamma _3$ and, these bounds are used to constrain the present baryon-to-photon ratio $(\eta)$ and baryon density $(\Omega_B)$. For $\Gamma _3 \geq$ 1/4 it is found that (from D and \he alone): $3.1 \leq \eta_{10} \leq 9.0$; $0.045 \leq \Omega_B h^2_{50} \leq 0.133$ (where $H_0 = 50h_{50}$ km$\,$s$^{-1}$ Mpc$^{-1}$).

Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite

Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the α-amino acids and α-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula.

The luminosity of the collapsing presolar nebula

A new attempt has been made to model the presolar nebula and the protostellar sun in a consistent way. For this purpose, a fully implicit 2D-radiation-hydrodynamic code has been developed. As a first step the isothermal phase of collapse is reinvestigated. The influence of rotation on the luminosity of an isothermally collapsing protostellar fragment is studied.

Protosolar nebula and composition of comets

Comets seem to be composed of matter, which is supposed to have the same molecular composition as protosolar nebula. Although there are no unbiased evidence that cometary nuclei retain the molecular composition inherited from the protosolar cloud, the observed properties of comets indicate that there is at least a resemblance between cometary composition and the material properties of dense interstellar clouds. Therefore the origin of comets could be searched in the cold stages of the protosolar nebula and molecular abundances of grain mantles in this nebula may be similar to those in the cometary dust. It is suggested that comets may contain pristine, virtually unaltered protosolar material and their study might be very relevant way to more information about processes in early stages of the solar nebula. Our knowledge about composition of the cometary nucleus is still relatively scarce, but we can partly deduce it from data obtained either by ground-based spectroscopy or by “in situ” mass spectrometry from space experiments. Most important were the discovery of fluffy CHON particles composed partly or even completely from compounds containing light elements. No consensus concerning the presence of interstellar pristine matter in comet has been reached from various approaches to determine the relationship between comets and interstellar grains. Most of these studies are based on infrared spectroscopy. Another method is the comparison on the chemical models of the protosolar nebula with the volatile compounds of the cometary nuclei. Both gas-phase and grain-surface chemistry are considered and initial gas-phase atomic abundances are assumed to be protosolar. The cometary matter is certainly not identical with the typical material of dense interstellar cool dense clouds, but it is closer to it than any other type of matter in solar system so far accessible to us. The data from comets combined with models of chemical evolution of matter in environment similar as prevailed the early stage of presolar nebula may at least impose constrains on the condition for comet formation. Here presented study is a preliminary contribution to such studies.

NH3, H2S, and the radio brightness temperature spectra of the giant planets

Recent radio interferometer observations of Neptune enable comparisons of the radio brightness temperature (TB) spectra of all four giant planets. This comparison reveals evidence for fundamental differences in the compositions of Uranus’ and Neptune’s upper tropospheres, particularly in their ammonia (NH3) and hydrogen sulfide (H2S) mixing ratios, despite those planets’ outward similarities. The tropospheric abundances of these constituents yield information about their deep abundances, and ultimately about the formation of the planets from the presolar nebula (Atreya et al., 1995).

Comets as a Reflection of Interstellar Medium Chemistry

A brief summary of the observed infrared and other properties of interstellar dust is given. Chemical, physical and morphological criteria are discussed concerning the degree to which there are constraints relating comets to interstellar dust chemistry representative of the presolar nebula. Results of theoretical modelling of dust and gas evolution in dense clouds are used to compare with observed dust composition. Sources of the distribution of simple as well as complex molecules in the coma are related to what is presently known about the volatile ices in interstellar dust and to processes leading to evaporation of organic “refractory” grain mantle material represented by laboratory residues produced by photoprocessing of ices. The criterion of preservation of interstellar volatiles in comets leads to the further criterion that the ice in comets is amorphous. Criteria for relating interstellar dust volatiles to asteroids are discussed.

Giordano Bruno, the June 1975 Meteoroid Storm, Encke, and Other Taurid Complex Objects

The Tunguska event on the Earth and a seismically detected meteoroid "storm" on the Moon occurred in late June, and the objects producing these events arrived at the Earth or Moon from the same direction, thus suggesting a common source. The arrival direction of the object that produced the lunar crater Giordano Bruno is poorly constrained, but a similar arrival direction and a common source for this object cannot be ruled out. Laser ranging of the Moon may confirm or refute the predicted direction of momentum transfer during the impact. Other Solar System objects which cross the Earth's orbit at the same time of the year and arrive from the same direction are the meteoroids of the beta-Taurid meteor stream, the comet, Encke, long considered to be related to Taurid meteors based on the similarity of their orbits, and the asteroids 2201 Oljata and DS 1978. Secondary or ejecta fragments from impacts of primary Taurid Complex objects have been detected. One group of these secondaries was probably detected by the HEOS-2 dust detector in June of 1973. Corvid meteors observed only in late June of 1937 may be secondaries from the Giordano Bruno impact in June of 1178. Objects that products meteorite falls, fireballs, airwaves, and flashes on the Moon do not show a preference for late June and, therefore, are not part of the Taurid Complex. Objects that make up the Taurid Complex are survivors of a long history of impacts and collisions that continuously reduce large objects to small ones. The apparent inconsistency of long lifetimes against collision or perturbation for Encke-like orbits together with short lifetimes against mass loss by evaporization and ejection of solid particles from Encke-like objects may be reconciled if the early ancestors of today's Taurid Complex have existed in the inner Solar System, inactive and undetected, since their constituents were condensed and accreted from the presolar nebula.

Chemistry in circumstellar disks - CS toward HL Tauri

High spatial and spectral resolution dust continuum and CS line-emission maps of the material immediately surrounding HL Tauri are presented. The 3.06-thermal dust map gives an upper limit to the source size of roughly 200 AU. When combined with previous millimeter continuum measurements, a beta value of 1.2 +/-0.3 is derived along with a disk mass of 0.1 solar mass, in agreement with previous disk radiative transfer models of the IR and submillimeter flux from HL Tau. The low dust emissivity index as compared to particles in the diffuse ISM is best explained by grain growth or composition evolution. The present observations of the CS and CO emission lines forming the molecular cloud surrounding HL Tauri are consistent with the 'canonical' temperatures, densities, and abundances derived in other Taurus cloud cores. In contrast, the CS emission in aperture synthesis maps at 650-AU spatial resolution is most consistent with CS/CO ratios at least 25-50 times lower in the gas immediately surrounding HL Tauri if the CS emission is optically thin. The continuum and spectral line results obtained are strong indicators of conditions thought to be similar to those in the presolar nebula some 4.5 AE ago.

Laboratory simulation of the photoprocessing and warm-up of cometary and pre-cometary ices: Production and analysis of complex organic molecules

The possibility that the organic molecules that have been found near comets could have formed by UV photolysis of interstellar ices was investigated by simulating this process in the laboratory. It is found that oxygen rich organics containing COH, CH and CO groups are readily produced in this way. These results indicate that part of the organic material in comets may have formed by UV irradiation of ices, either in the pre-solar nebula or in the interstellar phase.

Nature and origin of matrix in the unique type 3 chondrite, Kakangari

The mineralogy and textural characteristics of fine-grained matrix (<1 μm) in the unique type 3 chondrite, Kakangari, have been investigated by high resolution and analytical electron microscopy. The matrix contains a diverse primary mineralogy dominated by enstatite and olivine, with lesser amounts of albite, anorthite, Cr-spinel, troilite and Fe,Ni metal. Texturally, the matrix is characterized by irregular clusters or aggregates of crystals which constitute distinct structural units within the matrix, ~2–8 μm in diameter, and frequently have distinct mineralogies. Enstatite in these clusters is an intergrowth of monoclinic and orthorhombic polymorphs, with a microstructure indicative of an origin by high temperature inversion from protopyroxene during rapid cooling. Corroded inclusions of olivine frequently occur within enstatite, suggesting that enstatite formed by reaction of olivine with a silica polymorph at high temperature. The matrix mineralogy of Kakangari formed by high temperature annealing and reaction of fine-grained dust which probably consisted of amorphous or partially crystalline presolar or solar nebula condensates. The constituents of Kakangari matrix cannot, therefore, be considered as pristine, unprocessed solar system material. Annealing probably occurred in a thermal event similar to that which formed the chondrules, but the microstructures and textures of the matrix components suggest that melting did not occur. Chondrules in Kakangari were probably formed from identical precursors to the matrix, but reached temperatures sufficiently high to produce melting. Despite this high temperature processing, the mineralogical clusters in the matrix preserve mineralogical and, hence, chemical heterogeneities in the precursor materials similar to those observed in interplanetary dust particles.

The interstellar dust model of comet dust constrained by 3.4 μm and 10 μm emission

The bulk and microstructure of comet nuclei are derived from the morphological structure and chemical composition of submicron sized interstellar dust grains which have undergone cold aggregation in the pre-solar nebula. The density, size distribution and chemical composition of comet dust deduced from observations of masses, mass spectra, and infrared emission in the 3.4 μm, 10 μm and the continuum are compared with models of fluffy aggregates of interstellar dust. It is shown that the 10 μm emission of comet Halley is produced by predominantly interstellar amorphous silicates with a small (−5% admixture) of crystalline silicates. The organic refractory mantles on interstellar silicates are absolutely required to raise the emitting grain temperature high enough to make the 10 μm peak observable. Pure silicates would be too poor emitters even at submicron sizes without this extra heating. Furthermore it is shown that even silicate particles as small as 0.5 μm radius are still too large even with organic mantles to provide efficient 10 μm emission and that the most probable silicate core size is ∼0.05 μm with a mantle thickness > 0.02 μm. Finally, it is shown that the number of small cometary particles with masses ≲ 10 −g g must be substantially larger than has generally been assumed, and that they must be fluffy.

From Interstellar Dust to Comet Dust and Interplanetary Particles

AbstractInterstellar dust is described as consisting predominantly (by mass) of tenth micron (mean size) silicate core-organic refractory mantle particles which have evolved over galactic time scales of the order of 5 billion years. These particles were incorporated into comets and asteroids in the presolar nebula 4.5 billion years ago. The fragmentation of those primitive bodies gives rise to solar system debris which shows up as comet dust, zodiacal light, IDP’s and meteorites. The chemical and morphological structure of comet dust is derived here as fluffy aggregates of interstellar dust. The chemical and morphological structure of the chondritic porous IDP’S are then derived from comet dust which has evolved in the solar system. Zodiacal light particles are interpreted as various stages between comet dust and IDP’s. Meteorites appear to be a side branch in the evolution from interstellar to solar system particles.

Interstellar dust as the source of organic molecules in Comet Halley

Interstellar dust is described as consisting predominantly (by mass) of tenth micron (mean size) silicate core-organic refractory mantle particles which have evolved over galactic time scales of the order of 5 billion years. These particles were incorporated into comets and asteroids in the presolar nebula 4.5 billion years ago. The fragmentation of those primitive bodies gives rise to solar system debris which shows up as comet dust, zodiacal light, IDP's and meteorites. The chemical and morphological structure of comet dust is derived here as fluffy aggregates of interstellar dust. The chemical and morphological structure of the chondritic porous IDP's are then derived from comet dust which has evolved in the solar system. Zodiacal light particles are interpreted as various stages between comet dust and IDP's. Meteorites appear to be a side branch in the evolution from interstellar to solar system particles.

A chemical and isotopic study of hibonite-rich refractory inclusions in primitive meteorites

The ion microprobe has been used to determine the concentrations of a number of trace elements (including all 14 stable rare earth elements) and the isotopic compositions of Ca and Mg in seven hibonite-rich refractory inclusions. Hibonite in every inclusion analyzed shows the effect of REE partitioning with other solid or liquid phases, in which hibonite is enriched in light over heavy REE compared to other coexisting phases. This partitioning causes Cl chondritenormalized enrichments to drop by factors of 4 to 22 from lanthanum to lutetium. Fractionations due to gas-solid partitioning of REE are sometimes superimposed on the pattern produced by partitioning between condensed phases. Two inclusions, HAL and DH-H1, show evidence for formation in extremely oxidizing environments and their Ca is mass fractionated in favor of the heavy isotopes by 7 and 12%./amu, respectively. Two corundum-hibonite inclusions show evidence of extremely high temperature gas-solid fractionation, such that the more volatile light REE are depleted as a group relative to the more refractory heavy REE. These inclusions both have Ca that is mass fractionated in favor of the light isotopes. Hibonite preserves the largest nuclear isotope anomalies in Ti found to date ( Fahey et al. 1985a; Hinton el al., 1987b), but there does not seem to be a simple relationship between chemical composition, isotopic mass fractionation and the sign and magnitude of nuclear isotopic anomalies in hibonite-rich inclusions. One possibility for producing a variety of chemical compositions and degrees of mass fractionation while preserving nuclear isotope anomalies is by multiple cycles of condensation and evaporation in a turbulent protoplanetary accretion disk ( Morfill, 1983; Morfill and R. N. Clayton, 1986a,b). These cycles could also occur in a presolar nebula with large non-axisymmetric temperature and pressure distributions ( Boss, 1988). Nuclear anomalies are most likely to be preserved in hibonite because hibonite is the most refractory major phase to occur commonly in meteorites.

High Temperatures in the Early Solar Nebula

One fundamental controversy about terrestrial planet and asteroid formation is the discrepancy between meteoritical evidence for high temperatures (1500 K to 2000 K) in the inner solar nebula, and much lower theoretical temperature predictions on the basis of models of viscous accretion disks that neglect compressional heating of infalling gas. It is shown here that rigorous numerical calculations of the collapse of a rotating, three-dimensional presolar nebula are capable of producing temperatures on the order of 1500 K in the asteroid region (2.5 astronomical units), in either nearly axisymmetric or strongly nonaxisymmetric nebula models. The latter models may permit significant thermal cycling of solid components in the early inner solar nebula.

Protostellar formation in rotating interstellar clouds. V - Nonisothermal collapse and fragmentation

Numerical calculations are presented for rigorous models spanning a four-dimensional parameter space of initial conditions of the three-dimensional collapse of rotating protostellar clouds, encompassing radiative transfer in the Eddington approximation and detailed thermodynamical relations. It is found that protostellar formation may involve a few stages of hierarchical fragmentation terminated by increased thermal pressure in the nonisothermal regime, that high thermal energy clouds remain nearly axisymmetric during the first dynamic collapse phase, and that very slowly rotating clouds can fragment. The presolar nebula was probably formed from a cloud with very little initial rotation.

Interstellar and cometary molecules: Prebiological connections

The cometary modelling of Greenberg (1982), Delsemme (1984) and others suggests a close relationship between interstellar gases and grains, and comets. Cometary nuclei, assembled from primordial dust and ices, are the most "pristine" and thus the most interesting art i facts of the presolar nebula. Excluding H, He, and Ne as too volatile to condense, the elemental abundances of comets are found to approximate solar abundances. But, a four-fold depletion of carbon has yet to be satisfactorily explained. Delsemme has considered loss of volatile methane, formation of graphite grains and cold temperature shift of the CO+3H2=CHh+H20 equilibrium toward CO (Delsemme, 1977). Another possib i l i ty is a large mass fraction (0.19 of heavy organic compounds (Greenberg, 1982).

Three-dimensional calculations of the formation of the presolar nebula from a slowly rotating cloud

The presolar nebula may have formed from the collapse of a very slowly rotating interstellar cloud. The first three-dimensional, hydrodynamical calculations of the collapse of such clouds are presented. The models include radiative transfer in the Eddington approximation, as well as detailed equations of state appropriate for the nonisothermal regime of protostellar evolution. Very slowly rotating clouds, i.e., those with initial ratios of rotational energy to gravitational energy of 10 −3 or less, avoid fragmentation and instead collapse to form single central objects, containing quasistatic cores with densities of about 10 −10 g cm −3. These cores are, however, surrounded by significantly nonaxisymmetric regions, such that the presolar nebula would have been bar-like over the scale of the present solar system. This nonaxisymmetry, coupled with differential rotation, results in gravitational torques that produce rapid outward transfer of angular momentum. The center of the presolar nebula should then be able to contract and collapse to pre-main-sequence densities without suffering fission or fragmentation.

Angular momentum transfer by gravitational torques and the evolution of binary protostars

The efficiency of angular momentum transport by gravitational torques is investigated semi-analytically for two idealized models. The first model, a rotating ellipsoid embedded within another ellipsoid, is compared with numerical results for the fission instability of a rapidly-rotating polytrope. The fission instability is aborted by the rapid transfer of angular momentum outward by gravitational torques. The global rates of angular momentum transfer by gravitational torques in rotating gas clouds such as the presolar nebula are shown to be comparable to the rates assumed to be appropriate for transfer by turbulent stresses. The second model is a binary system embedded within a rotating ellipsoid. The binary orbital angular momentum decreases significantly when the phase angle with the ellipsoid is constant; the binary separation may then decrease by a factor of 102 within about an orbital period. For a variable phase angle, little secular loss of orbital angular momentum occurs. Binaries which form in the isothermal regime of the theory of hierarchical fragmentation will not undergo orbital decay, whereas very close binaries composed of nonisothermal fragments may decay and merge into single objects.