850 research outputs found
High-Temperature Processing of Solids Through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles
A fundamental, unsolved problem in Solar System formation is explaining the
melting and crystallization of chondrules found in chondritic meteorites.
Theoretical models of chondrule melting in nebular shocks has been shown to be
consistent with many aspects of thermal histories inferred for chondrules from
laboratory experiments; but, the mechanism driving these shocks is unknown.
Planetesimals and planetary embryos on eccentric orbits can produce bow shocks
as they move supersonically through the disk gas, and are one possible source
of chondrule-melting shocks. We investigate chondrule formation in bow shocks
around planetoids through 3D radiation hydrodynamics simulations. A new
radiation transport algorithm that combines elements of flux-limited diffusion
and Monte Carlo methods is used to capture the complexity of radiative
transport around bow shocks. An equation of state that includes the rotational,
vibrational, and dissociation modes of H is also used. Solids are followed
directly in the simulations and their thermal histories are recorded. Adiabatic
expansion creates rapid cooling of the gas, and tail shocks behind the embryo
can cause secondary heating events. Radiative transport is efficient, and bow
shocks around planetoids can have luminosities few
L. While barred and radial chondrule textures could be produced in
the radiative shocks explored here, porphyritic chondrules may only be possible
in the adiabatic limit. We present a series of predicted cooling curves that
merit investigation in laboratory experiments to determine whether the solids
produced by bow shocks are represented in the meteoritic record by chondrules
or other solids.Comment: Accepted for publication in ApJ. Images have been resized to conform
to arXiv limits, but are all readable upon adjusting the zoom. Changes from
v1: Corrected typos discovered in proofs. Most changes are in the appendi
Chondrule Formation in Bow Shocks around Eccentric Planetary Embryos
Recent isotopic studies of Martian meteorites by Dauphas & Pourmond (2011)
have established that large (~ 3000 km radius) planetary embryos existed in the
solar nebula at the same time that chondrules - millimeter-sized igneous
inclusions found in meteorites - were forming. We model the formation of
chondrules by passage through bow shocks around such a planetary embryo on an
eccentric orbit. We numerically model the hydrodynamics of the flow, and find
that such large bodies retain an atmosphere, with Kelvin-Helmholtz
instabilities allowing mixing of this atmosphere with the gas and particles
flowing past the embryo. We calculate the trajectories of chondrules flowing
past the body, and find that they are not accreted by the protoplanet, but may
instead flow through volatiles outgassed from the planet's magma ocean. In
contrast, chondrules are accreted onto smaller planetesimals. We calculate the
thermal histories of chondrules passing through the bow shock. We find that
peak temperatures and cooling rates are consistent with the formation of the
dominant, porphyritic texture of most chondrules, assuming a modest enhancement
above the likely solar nebula average value of chondrule densities (by a factor
of 10), attributable to settling of chondrule precursors to the midplane of the
disk or turbulent concentration. We calculate the rate at which a planetary
embryo's eccentricity is damped and conclude that a single planetary embryo
scattered into an eccentric orbit can, over ~ 10e5 years, produce ~ 10e24 g of
chondrules. In principle, a small number (1-10) of eccentric planetary embryos
can melt the observed mass of chondrules in a manner consistent with all known
constraints.Comment: Accepted for publication in The Astrophysical Journa
Chemistry in a gravitationally unstable protoplanetary disc
Until now, axisymmetric, alpha-disc models have been adopted for calculations
of the chemical composition of protoplanetary discs. While this approach is
reasonable for many discs, it is not appropriate when self-gravity is
important. In this case, spiral waves and shocks cause temperature and density
variations that affect the chemistry. We have adopted a dynamical model of a
solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc,
similar to those that may be found around Class 0 and early Class I protostars,
in a study of disc chemistry. We find that for each of a number of species,
e.g. H2O, adsorption and desorption dominate the changes in the gas-phase
fractional abundance; because the desorption rates are very sensitive to
temperature, maps of the emissions from such species should reveal the
locations of shocks of varying strengths. The gas-phase fractional abundances
of some other species, e.g. CS, are also affected by gas-phase reactions,
particularly in warm shocked regions. We conclude that the dynamics of massive
discs have a strong impact on how they appear when imaged in the emission lines
of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA
Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs
The formation and earliest stages of protoplanetary discs remain poorly
constrained by observations. ALMA will soon revolutionise this field.
Therefore, it is important to provide predictions which will be valuable for
the interpretation of future high sensitivity and high angular resolution
observations. Here we present simulated ALMA observations based on radiative
transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc
embedded in a 10 M_solar dense core, with structure similar to the pre-stellar
core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2,
OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and
kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA
The properties of pre-stellar discs in isolated and multiple pre-stellar systems
We present high-resolution 3D smoothed particle hydrodynamics simulations of the formation and evolution of protostellar discs in a turbulent molecular cloud. Using a piecewise polytropic equation of state, we perform two sets of simulations. In both cases, we find that isolated systems undergo a fundamentally different evolution than members of binary or multiple systems. When formed, isolated systems must accrete mass and increase their specific angular momentum, leading to the formation of massive, extended discs, which undergo strong gravitational instabilities and are susceptible to disc fragmentation. Fragments with initial masses of 5.5, 7.4 and 12 Mjup are produced in our simulations. In binaries and small clusters, we observe that due to competition for material from the parent core, members do not accrete significant amounts of high specific angular momentum gas relative to isolated systems. We find that discs in multiple systems are strongly self-gravitating but that they are stable against fragmentation due to disc truncation and mass profile steeping by tides, accretion of high specific angular momentum gas by other members and angular momentum being redirected into members' orbits. In general, we expect disc fragmentation to be less likely in clusters and to be more a feature of isolated system
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