80,035 research outputs found
Formation of Magnetized Prestellar Cores with Ambipolar Diffusion and Turbulence
We investigate the roles of magnetic fields and ambipolar diffusion during
prestellar core formation in turbulent giant molecular clouds (GMCs), using
three-dimensional numerical simulations. Our simulations focus on the shocked
layer produced by a converging flow within a GMC, and survey varying ionization
and angle between the upstream flow and magnetic field. We also include ideal
magnetohydrodynamic (MHD) and hydrodynamic models. From our simulations, we
identify hundreds of self-gravitating cores that form within 1 Myr, with masses
M ~ 0.04 - 2.5 solar-mass and sizes L ~ 0.015 - 0.07 pc, consistent with
observations of the peak of the core mass function (CMF). Median values are M =
0.47 solar-mass and L = 0.03 pc. Core masses and sizes do not depend on either
the ionization or upstream magnetic field direction. In contrast, the
mass-to-magnetic flux ratio does increase with lower ionization, from twice to
four times the critical value. The higher mass-to-flux ratio for low ionization
is the result of enhanced transient ambipolar diffusion when the shocked layer
first forms. However, ambipolar diffusion is not necessary to form low-mass
supercritical cores. For ideal MHD, we find similar masses to other cases.
These masses are 1 - 2 orders of magnitude lower than the value that defines a
magnetically supercritical sphere under post-shock ambient conditions. This
discrepancy is the result of anisotropic contraction along field lines, which
is clearly evident in both ideal MHD and diffusive simulations. We interpret
our numerical findings using a simple scaling argument which suggests that
gravitationally critical core masses will depend on the sound speed and mean
turbulent pressure in a cloud, regardless of magnetic effects.Comment: 41 pages, 14 figures, 3 tables, accepted for publication in
Astrophysical Journa
Noise spectra of stochastic pulse sequences: application to large scale magnetization flips in the finite size 2D Ising model
We provide a general scheme to predict and derive the contribution to the
noise spectrum of a stochastic sequence of pulses from the distribution of
pulse parameters. An example is the magnetization noise spectra of a 2D Ising
system near its phase transition. At , the low frequency spectra is
dominated by magnetization flips of nearly the entire system. We find that both
the predicted and the analytically derived spectra fit those produced from
simulations. Subtracting this contribution leaves the high frequency spectra
which follow a power law set by the critical exponents.Comment: 4 pages, 5 figures. We improved text and included a predicted noise
curve in Figure 4. 2 examples from Figure 3 are remove
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Unfolding the impacts of transaction-specific investments: Moderation by out-of-thechannel-loop perceptions and achievement orientations
When distribution channel partners make specific investments, tailored to a particular supplier, it could prompt either opportunism or beneficial (e.g., extra-role) behaviors. The impact of the investment in turn may depend on whether the channel partner perceives that it is being left out of the channel loop by the supplier, as well as that partner’s achievement orientation. This study considers a sample of 155 IT professional service firms and finds that their knowledge-intensive, transaction-specific investments (TSIs) encourage distinct behavioral intentions. If they perceive that the supplier is leaving them out of the channel loop, the effects of the TSIs get amplified in relation to opportunistic and extra-role behavioral intentions. Furthermore, the firms’ achievement orientation moderates these influences. Suppliers thus should attend closely to achievement-oriented partners to ensure they do not perceive that they have been left out of the channel loop
Effect of electron-phonon scattering on shot noise in nanoscale junctions
We investigate the effect of electron-phonon inelastic scattering on shot
noise in nanoscale junctions in the regime of quasi-ballistic transport. We
predict that when the local temperature of the junction is larger than its
lowest vibrational mode energy , the inelastic contribution to shot noise
(conductance) increases (decreases) with bias as (). The
corresponding Fano factor thus increases as . We also show that the
inelastic contribution to the Fano factor saturates with increasing thermal
current exchanged between the junction and the bulk electrodes to a value
which, for , is independent of bias. A measurement of shot noise may
thus provide information about the local temperature and heat dissipation in
nanoscale conductors.Comment: 4 pages, 2 figure
Seebeck Coefficients in Nanoscale Junctions: Effects of Electron-vibration Scattering and Local Heating
We report first-principles calculations of inelastic Seebeck coefficients in
an aluminum monatomic junction. We compare the elastic and inelastic Seebeck
coefficients with and without local heating. In the low temperature regime, the
signature of normal modes in the profiles of the inelastic Seebeck effects is
salient. The inelastic Seebeck effects are enhanced by the normal modes, and
further magnified by local heating. In the high temperature regime, the
inelastic Seebeck effects are weakly suppressed due to the quasi-ballistic
transport.Comment: 3 Figure
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