120 research outputs found
Dynamical orbital evolution of asteroids and planetesimals across distinct chemical reservoirs due to accretion growth of planets in the early solar system
N-body numerical simulations code for the orbital motion of
asteroids/planetesimals within the asteroid belt under the gravitational
influence of the sun and the accreting planets has been developed. The aim is
to make qualitative, and to an extent a semi-quantitative argument, regarding
the possible extent of radial mixing and homogenization of planetesimal
reservoirs of the two observed distinct spectral types , viz., the S-type and
C-type, across the heliocentric distances due to their dynamical orbital
evolution, thereby, eventually leading to the possible accretion of asteroids
having chemically diverse constituents. The spectral S-type and C-type
asteroids are broadly considered as the parent bodies of the two observed major
meteoritic dichotomy classes, namely, the non-carbonaceous (NC) and
carbonaceous (CC) meteorites, respectively. The present analysis is performed
to understand the evolution of the observed dichotomy and its implications due
to the nebula and early planetary processes during the initial 10 Myrs (Million
years). The homogenization across the two classes is studied in context to the
accretion timescales of the planetesimals with respect to the half-life of the
potent planetary heat source, 26Al. The accretion over a timescale of ~1.5 Myr.
possibly resulted in the planetary-scale differentiation of planetesimals to
produce CC and NC achondrites and iron meteorite parent bodies, whereas, the
prolonged accretion over a timescale of 2-5 Myrs. resulted in the formation of
CC and NC chondrites. Our simulation results indicate a significant role of the
initial eccentricities and the masses of the accreting giant planets,
specifically, Jupiter and Saturn, in triggering the eccentricity churning of
the planetesimals across the radial distances......Comment: Accepte
Heterogeneous distribution of Al-26 at the birth of the Solar System
It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct
radionuclide, was uniformly distributed in the nascent Solar System with the
initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar
origin. However, the stellar source of Al-26 and the manner in which it was
injected into the solar system remain controversial: the Al-26 could have been
produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star
and injected either into the protosolar molecular cloud or protoplanetary disk.
Corundum (Al2O3) is thermodynamically predicted to be the first condensate from
a cooling gas of solar composition. Here we show that micron-sized corundum
condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition
recorded heterogeneous distribution of Al-26 at the birth of the solar system:
the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50%
of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich
refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich
calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite
crystals in CM chondrites, and FUN (fractionation and unidentified nuclear
isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the
apparently early and short duration (<0.3 Ma) of condensation of refractory
O-16-rich solids in the solar system, we infer that Al-26 was injected into the
collapsing protosolar molecular cloud and later homogenized in the
protoplanetary disk. The apparent lack of correlation between Al-26 abundance
and O-isotope compositions of corundum grains put important constraints on the
stellar source of Al-26 in the solar system.Comment: Astrophysical Journal Letters 733, L3
Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes with a Shock Wave. I. Varied Shock Speeds
The discovery of decay products of a short-lived radioisotope (SLRI) in the
Allende meteorite led to the hypothesis that a supernova shock wave transported
freshly synthesized SLRI to the presolar dense cloud core, triggered its
self-gravitational collapse, and injected the SLRI into the core. Previous
multidimensional numerical calculations of the shock-cloud collision process
showed that this hypothesis is plausible when the shock wave and dense cloud
core are assumed to remain isothermal at ~10 K, but not when compressional
heating to ~1000 K is assumed. Our two-dimensional models (Boss et al. 2008)
with the FLASH2.5 adaptive mesh refinement (AMR) hydrodynamics code have shown
that a 20 km/sec shock front can simultaneously trigger collapse of a 1 solar
mass core and inject shock wave material, provided that cooling by molecular
species such as H2O, CO, and H2 is included. Here we present the results for
similar calculations with shock speeds ranging from 1 km/sec to 100 km/sec. We
find that shock speeds in the range from 5 km/sec to 70 km/sec are able to
trigger the collapse of a 2.2 solar mass cloud while simultaneously injecting
shock wave material: lower speed shocks do not achieve injection, while higher
speed shocks do not trigger sustained collapse. The calculations continue to
support the shock-wave trigger hypothesis for the formation of the solar
system, though the injection efficiencies in the present models are lower than
desired.Comment: 39 pages, 14 figures. in press, Ap
Evolution of the Solar Nebula. IX. Gradients in the Spatial Heterogeneity of the Short-Lived Radioisotopes Fe and Al and the Stable Oxygen Isotopes
Short-lived radioisotopes (SLRI) such as Fe and Al were likely
injected into the solar nebula in a spatially and temporally heterogeneous
manner. Marginally gravitationally unstable (MGU) disks, of the type required
to form gas giant planets, are capable of rapid homogenization of isotopic
heterogeneity as well as of rapid radial transport of dust grains and gases
throughout a protoplanetary disk. Two different types of new models of a MGU
disk in orbit around a solar-mass protostar are presented. The first set has
variations in the number of terms in the spherical harmonic solution for the
gravitational potential, effectively studying the effect of varying the spatial
resolution of the gravitational torques responsible for MGU disk evolution. The
second set explores the effects of varying the initial minimum value of the
Toomre stability parameter, from values of 1.4 to 2.5, i.e., toward
increasingly less unstable disks. The new models show that the basic results
are largely independent of both sets of variations. MGU disk models robustly
result in rapid mixing of initially highly heterogeneous distributions of SLRIs
to levels of 10% in both the inner ( 10 AU) disk
regions, and to even lower levels ( 2%) in intermediate regions, where
gravitational torques are most effective at mixing. These gradients should have
cosmochemical implications for the distribution of SLRIs and stable oxygen
isotopes contained in planetesimals (e.g., comets) formed in the giant planet
region ( 5 to 10 AU) compared to those formed elsewhere.Comment: 37 pages, 1 table, 19 figures, ApJ accepte
Injection of Short-Lived Radionuclides into the Early Solar System from a Faint Supernova with Mixing-Fallback
Several short-lived radionuclides (SLRs) were present in the early solar
system, some of which should have formed just prior to or soon after the solar
system formation. Stellar nucleosynthesis has been proposed as the mechanism
for production of SLRs in the solar system, but no appropriate stellar source
has been found to explain the abundances of all solar system SLRs.
In this study, we propose a faint supernova with mixing and fallback as a
stellar source of SLRs with mean lives of <5 Myr (26Al, 41Ca, 53Mn, and 60Fe)
in the solar system. In such a supernova, the inner region of the exploding
star experiences mixing, a small fraction of mixed materials is ejected, and
the rest undergoes fallback onto the core. The modeled SLR abundances agree
well with their solar system abundances if mixing-fallback occurs within the
C/O-burning layer. In some cases, the initial solar system abundances of the
SLRs can be reproduced within a factor of 2. The dilution factor of supernova
ejecta to the solar system materials is ~10E-4 and the time interval between
the supernova explosion and the formation of oldest solid materials in the
solar system is ~1 Myr. If the dilution occurred due to spherically symmetric
expansion, a faint supernova should have occurred nearby the solar system
forming region in a star cluster.Comment: 14 pages, 4 figure
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