Presolar Cloud Research Articles

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Chondrule Formation in Radiative Shock

Previous models of solid grain melting in solar nebula shocks have neglected gas cooling behind the shock front; i.e., they considered adiabatic shocks. The effect of gas cooling in the postshock region is studied in this article. It was found that shocked gas is cooled efficiently by dipole molecules and small dust particles, and this results in a sharp increase in gas density in the postshock region. Submillimeter and larger grains cross the region of cooling before being decelerated, and are heated by the drag (through the cooled and compressed postshock gas) more strongly than in an adiabatic shock. This effect opens the possibility of melting of millimeter-size dust aggregates (chondrule precursors) in that cooler region, even when the matter is transparent for the thermal radiation of the aggregates. The possibility of formation of chondrule precursors in the collapsing presolar cloud and of melting of the precursors in the radiative accretional shock during formation of the solar nebula is speculated. In such a shock, grains could be melted at heliocentric radii up to the inner part of asteroidal belt if the density of infalling gas were only few times larger than the average density of infalling gas corresponding to an accretion rate of 10 -5 M ⊙ year -1; the limiting radius for melting increases with density. Clumpy accretion and enhancement of density in the vicinity of the centrifugal radius of infalling gas provide the necessary densities of infalling gas. Enhancement of drag heating due to fast cooling of postshock gas is also significant for shocks of other origin in the solar nebula. At the central plane of a minimum-mass solar nebula, chondrule precursors could be melted in shocks with velocity ≥10 km sec -1. The duration of the molten state and the cooling rate of a grain are determined by the grain's kinetic energy and the rate of its decay (not the time scale of radiational cooling for the grain!). Millimeter-size compact grams have temperatures ≥1600 K for <10 3 sec, and ≥700 K for ∼10 4 sec, times that are appropriate for chondrules. These time scales could be larger in faster shocks when grains have larger kinetic energy, Chondrule precursors could be formed as fluffy aggregates before and during the collapse of presolar cloud. Later they must increase their density. The possibility of increasing the density of the aggregates by means of mutual collisions or as a result of surface tension when they began to melt is discussed possibility of increasing the density of the aggregates by means of mutual collisions or as a result of surface tension when they began to melt is discussed.

The origin of chromitic chondrules and the volatility of Cr under a range of nebular conditions

We characterize ten chromitic chondrules, two spinelian chondrules and one spinel-bearing chondrule and summarize data for 120 chromitic inclusions discovered in an extensive survey of ordinary chondrites. Compositional and petrographic evidence suggests that chromitic chondrules and inclusions are closely related. TheCr/(Cr + Al) ratios in the spinel of these objects range from 0.5 to 0.9 and bulk Al 2O 3 contents are uniformly high ( > 10 wt%, except for one with 8 wt%). No other elements having comparable solar abundances are so strongly enriched, and alkali feldspar and merrillite are more common than in normal chondrules. The Cr/Mg ratios in chromitic chondrules are 180–750 × the ratios in the bulk chondrite. With the possible exception of magnetic clumping of chromite in the presolar cloud, mechanical processes cannot account for this enrichment. Examination of nebular equilibrium processes shows that 50%-condensation temperatures of Cr atpH 2/pH 2O of 1500 are several tens of degrees below those of Mg as Mg 2SiO 4; the condensation of Cr is primarily as MgCr 2O 4 dissolved in MgAl 2O 4 at nebular pressures of 10 −4 atm or below. AtpH 2= 10 −3 atm condensation as Cr in Fe-Ni is favored. Making the nebula much more oxidizing reduces the difference in condensation temperatures but Mg remains more refractory. We conclude that nebular equilibrium processes are not responsible for the enhanced Cr/Mg ratios. We propose that both Cr and Al became enriched in residues formed by incomplete evaporation of presolar lumps. We suggest that spinels remained as solid phases when the bulk of the silicates were incorporated into the evaporating melt; vaporization of Al and Cr were inhibited by the slow kinetics of diffusion. Subsequent melting and crystallization of these residues fractionated Cr from Al. The resulting materials constituted major components in the precursors of chromitic chondrules. Our model implies that chromitic chondrules and inclusions preserve the Cr isotopic record of presolar sources.

A note on the prebiotic synthesis of organic acids in carbonaceous meteorites.

Strong similarities between monocarboxylic and hydroxycarboxylic acids in the Murchison meteorite suggest corresponding similarities in their origins. However, various lines of evidence apparently implicate quite different precursor compounds in the synthesis of the different acids. These seeming inconsistencies can be resolved by postulating that the apparent precursors also share a related origin. Pervasive D enrichment indicates that this origin was in a presolar molecular cloud. The organic acids themselves were probably synthesized [correction of synthesised] in an aqueous environment on an asteroidal parent body, the hydroxy (and amino) acids by means of the Strecker cyanohydrin reaction.

Isotopic variations in the rock-forming elements in meteorites

Variations in isotopic abundances of the major rock-forming elements can be used as tracers for chemical processes in the solar nebula, and may also provide links to the presolar cloud from which the solar nebula was derived. Emphasis in this paper is placed on the correlation of isotopic variations between pairs of elements, both for mass-dependent fractionation effects and for nucleosynthetic effects. Variations in oxygen isotope abundances, which are ubiquitous in all Solar System matter, are decoupled from those in other elements, probably because of the effect of a large oxygen reservoir in the nebular gas. Among the metallic rock-forming elements (Mg, Si, Ca, Ti, Cr, Fe, Ni) isotopic variations are small to immeasurable in ordinary chondrites and achondrites. Large variations are observed in refractory inclusions in carbonaceous chondrites in the elements Mg, Si, Ca and Ti. Fractionation effects result from evaporation and condensation at high temperatures. The dominant nucleosynthetic effects are seen as excesses and deficiencies of the neutron-rich isotopes in the region of the iron abundance peak: 48 Ca, 49 Ti, 50 Ti, 54 Cr, 64 Ni. These effects result from mixing in different proportions of the products of different regions of a supernova. The rock-forming elements also show isotopic variations due to extinct radioactivities: 26 Mg, 53 Cr and 41 K. A local source is likely, as is heterogeneous distribution within the solar nebula.

A Striking Nitrogen Isotope Anomaly in the Bencubbin and Weatherford Meteorites

The stony-iron meteorites Bencubbin and Weatherford contain nitrogen with a ratio of nitrogen-15 to nitrogen-14 larger than normal by as much as a factor of 2. The excess nitrogen-15 may be due either to a nucleosynthetic origin or to extreme isotopic fractionation. In the former case, it may reflect failure to homogenize nitrogen-15 produced in nova explosions. In the latter case, it may reflect chemical processing at temperatures below 40 K in a presolar molecular cloud.

Evolution of grains in a turbulent solar nebula

In accretion disk models of the solar nebula, turbulence is driven by convective instability. This mechanism requires high opacity, which must be provided by solid grains. Evolution of the grain size distribution in a turbulent disk is computed numerically, using realistic collisional outcomes and strengths of grain aggregates, rather than an arbitrary “sticking efficiency.” The presence of turbulence greatly increases the rate of grain collisions; the coagulation rate is initially much greater than in a nonturbulent disk. Aggregates quickly reach sizes ∼0.1–1 cm, but erosion and breakup in collisions prevent growth of larger bodies for plausible aggregate impact strengths. These aggregates are too small to settle to the plane of the disk, and planetesimal formation is impossible as long as the turbulence persists. However, the opacity of the disk is reduced by aggregate formation; some combinations of opacity law and surface density produce an optically thin disk, cutting off turbulent convection. The disk may experience alternating periods of turbulence and quiescence, as grains are depleted by coagulation and replenished by infall from the presolar cloud. Planetesimals can form only during the quiescent intervals; it is argued that such episodes were rare during the lifetime of the accretion disk.

On the origin of the solar system and the exceptional position of the Sun in the Galaxy

The solar system's position in the Galaxy is an exclusive one, since the Sun is close to the corotation circle, which is the place where the angular velocity of the galactic differential rotation is equal to that of density waves displaying as spiral arms. Each galaxy contains only one corotation circle; therefore, it is an exceptional place. In the Galaxy, the deviation of the Sun from the corotation is very small — it is equal to ΔR/R⊙≈0.03, where ΔR=R c −R⊙,R c is the corotation distance from the galactic center andR⊙ is the Sun's distance from the galactic center. The special conditions of the Sun's position in the Galaxy explain the origin of the fundamental cosmogony timescalesT1≈4.6×109 yr,T2≃108 yr,T3≃106 yr detected by the radioactive decay of various nuclides. The timescaleT1 (the solar system's ‘lifetime’) is the protosolar cloud lifetime in a space between the galactic spiral arms. The timescaleT2 is the presolar cloud lifetime in a spiral arm.T3 is a timescale of hydrodynamical processes of a cloud-wave interaction. The possibility of the natural explanation of the cosmogony timescales by the unified process (on condition that the Sun is near the state of corotation) can become an argument in favour of the fact that the nearness to the corotation is necessary for the formation of systems similar to the Solar system. If the special position of the Sun is not incidental, then the corotation circles of our Galaxy, as well as those of other galaxies, are just regions where situations similar to ours are likely to be found.

A new approach to nucleocosmochronology

A new model of nucleocosmochronology based on discrete addition of r-process elements to the presolar cloud is proposed. It is suggested that the solar system accreted its r-process (and presumably all other) elements during its passage through the galactic arms, which has been treated as a discrete event. The number of times the presolar cloud passed through the galactic arm before contraction is calculated to be 50 +- 10.If one uses only the /sup 238/U//sup 232/Th ratio, the accretion (nucleosynthesis) period is calculated to be (8(+2, -1)) x 10/sup 9/ yr. The free decay period (also the transit time between the galactic arms) has been calculated to be (140 +- 10) x 10/sup 6/ yr, if one uses the /sup 129/I//sup 127/I ratio only.The model has been applied to predict the other two isotopic ratios, viz., /sup 244/Pu//sup 238/U and /sup 235/U//sup 238/U, in meteorites. The calculated values (0.0128 for /sup 244/Pu//sup 238/U and 0.292 for /sup 235/U//sup 238/U) are in remarkably good agreement with the measured values (0.0127 +- 0.0026 and 0.015 +- 0.0014 for /sup 244/Pu//sup 238/U, and 0.313 +- 0.026 for /sup 235/U//sup 238/U) in meteorites. This is the first time that a nucleochronology model has beenmore » put to such a test and has been so successful.The cosmoradiogenic fraction of /sup 1/87Os and /sup 87/Sr has been calculated to be 6.6% and 5.7%, respectively.« less