980 research outputs found
Dense Gas in the Milky Way
We present a study of dense gas emission in the Milky Way in order to serve
as a basis for comparison with extragalactic results. This study combines new
observations of HCN, CS, and CO in individual GMCs and in the Milky Way plane
with published studies of emission from these molecules in the inner 500 pc of
the Milky Way. We find a strong trend in the fraction of emission from dense
gas tracers as a function of location in the Milky Way: in the bulge,
I_{HCN}/I_{CO} = 0.081 \pm 0.004, in the plane, I_{HCN}/I_{CO} = 0.026 \pm
0.008 on average, and over the full extent of nearby GMCs, I_{HCN}/I_{CO} =
0.014 \pm 0.020. Similar trends are seen in I_{CS}/I_{CO}. The low intensities
of the HCN and CS emission in the plane suggests that these lines are produced
by gas at moderate densities; they are thus not like the emission produced by
the dense, pc-scale star forming cores in nearby GMCs. The contrast between the
bulge and disk ratios in the Milky Way is likely to be caused by a combination
of higher kinetic temperatures as well as a higher dense gas fraction in the
bulge of the Milky Way.Comment: 34 pages LaTeX, AASTEX macros, includes 11 postscript figures. To
appear in ApJ 478, March 199
Moving Mirrors and Thermodynamic Paradoxes
Quantum fields responding to "moving mirrors" have been predicted to give
rise to thermodynamic paradoxes. I show that the assumption in such work that
the mirror can be treated as an external field is invalid: the exotic
energy-transfer effects necessary to the paradoxes are well below the scales at
which the model is credible. For a first-quantized point-particle mirror, it
appears that exotic energy-transfers are lost in the quantum uncertainty in the
mirror's state. An accurate accounting of these energies will require a model
which recognizes the mirror's finite reflectivity, and almost certainly a model
which allows for the excitation of internal mirror modes, that is, a
second-quantized model.Comment: 7 pages, Revtex with Latex2
Red-shifts near black holes
A simple ordinary differential equation is derived governing the red-shifts
of wave-fronts propagating through a non-stationary spherically symmetric
space-time. Approach to an event horizon corresponds to approach to a fixed
point; in general, the phase portrait of the equation illuminates the
qualitative features of the geometry. In particular, the asymptotics of the
red-shift as a horizon is approached, a critical ingredient of Hawking's
prediction of radiation from black holes, are easily brought out. This
asympotic behavior has elements in common with the universal behavior near
phase transitions in statistical physics. The validity of the Unruh vacuum for
the Hawking process can be understood in terms of this universality. The
concept of surface gravity is extended to to non-stationary spherically
symmetric black holes. Finally, it is shown that in the non-stationary case,
Hawking's predicted flux of radiation from a black hole would be modified.Comment: 20 pages, plain Tex, IOP macros, 4 eps figures, accepted by CQ
The electromagnetic field near a dielectric half-space
We compute the expectations of the squares of the electric and magnetic
fields in the vacuum region outside a half-space filled with a uniform
non-dispersive dielectric. This gives predictions for the Casimir-Polder force
on an atom in the `retarded' regime near a dielectric. We also find a positive
energy density due to the electromagnetic field. This would lead, in the case
of two parallel dielectric half-spaces, to a positive, separation-independent
contribution to the energy density, besides the negative, separation-dependent
Casimir energy. Rough estimates suggest that for a very wide range of cases,
perhaps including all realizable ones, the total energy density between the
half-spaces is positive.Comment: Latex2e, IOP macros, 15 pages, 2 eps figure
Non-Gaussian curvature distribution of actin-propelled biomimetric colloid trajectories
We analyze the motion of colloids propelled by a comet-like tail of polymerizing actin filaments. The curvature of the particle trajectories deviates strongly from a Gaussian distribution, implying that the underlying microscopic processes are fluctuating in a non-independent manner. Trajectories for beads of different size all showed the same non-Gaussian behavior, while the mean curvature decreased weakly with size. A stochastic simulation that includes nucleation, force-dependent dissociation, growth, and capping of filaments, shows that the non-Gaussian curvature distribution can be explained by a positive feedback mechanism in which attached chains under higher tension are more likely to sna
Force generation of curved actin gels characterized by combined AFM-epifluorescence measurements
Polymerization of actin into branched filaments is the driving force behind active migration of eukaryotic cells and motility of intracellular organelles. The site-directed assembly of a polarized branched array forms an expanding gel that generates the force that pushes the membrane. Here, we use atomic force microscopy to understand the relation between actin polymerization and the produced force. Functionalized spherical colloidal probes of varying size and curvature are attached to the atomic force microscopy cantilever and initiate the formation of a polarized actin gel in a solution mimicking the in vivo context. The gel growth is recorded by epifluorescence microscopy both against the cantilever and in the perpendicular (lateral) nonconstrained direction. In this configuration, the gel growth stops simultaneously in both directions at the stall force, which corresponds to a pressure of 0.15 nN/μm(2). The results show that the growth of the gel is limited laterally, in the absence of external force, by internal mechanical stresses resulting from a combination of the curved geometry and the molecular mechanism of site-directed assembly of a cohesive branched filament array
Bank Directors: Heightened Expectations and Blurred Lines in a Changing Regulatory Framework, A Conversation from The Clearing House Annual Conference (Moderators: Lissa L. Broome and Derek M. Bush)
On November 21, 2014, the Center for Banking and Finance at the University of North Carolina School of Law hosted a dialogue on heightened expectations and blurred lines for bank directors in a changing regulatory environment at The Clearing House’s Annual Conference. Biographical information about the moderators and the panelists is set forth before the transcript of the dialogue begins
- …