896 research outputs found
Magnetic Fields Recorded by Chondrules Formed in Nebular Shocks
Recent laboratory efforts (Fu et al., 2014) have constrained the remanent
magnetizations of chondrules and the magnetic field strengths at which the
chondrules were exposed to as they cooled below their Curie points. An
outstanding question is whether the inferred paleofields represent the
background magnetic field of the solar nebula or were unique to the
chondrule-forming environment. We investigate the amplification of the magnetic
field above background values for two proposed chondrule formation mechanisms,
large-scale nebular shocks and planetary bow shocks. Behind large-scale shocks,
the magnetic field parallel to the shock front is amplified by factors , regardless of the magnetic diffusivity. Therefore, chondrules melted in
these shocks probably recorded an amplified magnetic field. Behind planetary
bow shocks, the field amplification is sensitive to the magnetic diffusivity.
We compute the gas properties behind a bow shock around a 3000 km-radius
planetary embryo, with and without atmospheres, using hydrodynamics models. We
calculate the ionization state of the hot, shocked gas, including thermionic
emission from dust, and thermal ionization of gas-phase potassium atoms, and
the magnetic diffusivity due to Ohmic dissipation and ambipolar diffusion. We
find that the diffusivity is sufficiently large that magnetic fields have
already relaxed to background values in the shock downstream where chondrules
acquire magnetizations, and that these locations are sufficiently far from the
planetary embryos that chondrules should not have recorded a significant
putative dynamo field generated on these bodies. We conclude that, if melted in
planetary bow shocks, chondrules probably recorded the background nebular
field.Comment: 17 pages, 11 figures, accepted for publication in Ap
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
The collapse of protoplanetary clumps formed through disc instability: 3D simulations of the pre-dissociation phase
We present 3D smoothed particle hydrodynamics simulations of the collapse of
clumps formed through gravitational instability in the outer part of a
protoplanetary disc. The initial conditions are taken directly from a global
disc simulation, and a realistic equation of state is used to follow the clumps
as they contract over several orders of magnitude in density, approaching the
molecular hydrogen dissociation stage. The effects of clump rotation,
asymmetries, and radiative cooling are studied. Rotation provides support
against fast collapse, but non-axisymmetric modes develop and efficiently
transport angular momentum outward, forming a circumplanetary disc. This
transport helps the clump reach the dynamical collapse phase, resulting from
molecular hydrogen dissociation, on a thousand-year timescale, which is smaller
than timescales predicted by some previous spherical 1D collapse models.
Extrapolation to the threshold of the runaway hydrogen dissociation indicates
that the collapse timescales can be shorter than inward migration timescales,
suggesting that clumps could survive tidal disruption and deliver a proto-gas
giant to distances of even a few AU from the central star.Comment: Accepted for publication in MNRA
Big Sky Documentary Film Festival Study
This report provides information on attendees of Missoula\u27s Big Sky Documentary Film Festival including where they were from, group aize, number of films attended, satisfaction with the event and basic demographics. In addition, spending by individual attendees from outside Missoula County is estimated
Geotourism in the Crown of the Continent
Geotourism is a new niche segment of sustainable tourism focused on sustaining and enhancing the geographical character of a place. The purpose of this study was to answer the question: to what degree do visitors in the \u27Crown of the Continent\u27 share the values of geotourism? Visitors to sites on National Geographic\u27s Crown of the Continent geotourism mapguide were intercepted and asked to fill out a survey regarding their travel values
Multi-stage Inspection of Laser Welding Defects using Machine Learning
As welding processes become faster and components consist of many more welds compared to previous applications, there is a need for fast but still precise quality inspection. The aim of this paper is to compare already existing approaches, namely single-sensor systems (SSS) and multi-sensor systems (MSS) with a proposed cascaded system (CS). The introduced CS is characterized by the fact that not all available data are analyzed, but only cleverly selected ones. The different approaches consisting of neural networks are compared in terms of their accuracy and computational effort. The data are recorded from scratch and include two common sensor systems for quality control, namely a photodiode (PD) and a high-speed camera (HSC). As a result, when the CS makes half of the final decisions based on a SSS with PD signals and the other half based on a SSS with HSC images, the estimated computational effort is reduced by almost 50% compared to the SSS with HSC images as input. At the same time, the accuracy decreases only by 0.25% to 95.96%. Additionally, based on the CS, a general cascaded system (GCS) for quality inspection is proposed
Gravitational instabilities in a protosolar-like disc - I. Dynamics and chemistry
MGE gratefully acknowledges a studentship from the European Research Council (ERC; project PALs 320620). JDI gratefully acknowledges funding from the European Union FP7-2011 under grant agreement no. 284405. ACB's contribution was supported, in part, by The University of British Columbia and the Canada Research Chairs program. PC and TWH acknowledge the financial support of the European Research Council (ERC; project PALs 320620).To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs.Publisher PDFPeer reviewe
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
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