Jet Propulsion Laboratory

About: Jet Propulsion Laboratory is a facility organization based out in La Cañada Flintridge, California, United States. It is known for research contribution in the topics: Mars Exploration Program & Telescope. The organization has 8801 authors who have published 14333 publications receiving 548163 citations. The organization is also known as: JPL & NASA JPL.

The Magnetorotational Instability in Core-Collapse Supernova Explosions

Abstract: We investigate the action of the magnetorotational instability (MRI) in the context of iron-core collapse. Exponential growth of the field on the timescale Ω-1 by the MRI will dominate the linear growth process of field-line "wrapping" with the same characteristic time. We examine a variety of initial rotation states, with solid-body rotation or a gradient in rotational velocity, that correspond to models in the literature. A relatively modest value of the initial rotation, a period of ~10 s, will give a very rapidly rotating proto-neutron star and hence strong differential rotation with respect to the infalling matter. We assume conservation of angular momentum on spherical shells. Rotational distortion and the dynamic feedback of the magnetic field are neglected in the subsequent calculation of rotational velocities. In our rotating and collapsing conditions, a seed field is expected to be amplified by the MRI and to grow exponentially to a saturation field. Results are discussed for two examples of saturation fields, a fiducial field that corresponds to vA = rΩ and a field that corresponds to the maximum growing mode of the MRI. We find, as expected, that the shear is strong at the boundary of the newly formed proto-neutron star and, unexpectedly, that the region within the stalled shock can be subject to strong MHD activity. Modest initial rotation velocities of the iron core result in sub-Keplerian rotation and a sub-equipartition magnetic field that nevertheless produce substantial MHD luminosity and hoop stresses: saturation fields of order 1015-1016 G can develop ~300 ms after bounce with an associated MHD luminosity of ~1052 ergs s-1. Bipolar flows driven by this MHD power can affect or even cause the explosions associated with core-collapse supernovae.

The Evolution of Early-Type Galaxies in Distant Clusters

Abstract: We present results from an optical-IR photometric study of early-type galaxies in 19 galaxy clusters out to z=0.9. The galaxy sample is selected on the basis of morphologies determined from HST WFPC2 images, and is photometrically defined in the K-band to minimize redshift-dependent selection biases. The optical-IR colors of the early-type cluster galaxies become bluer with increasing redshift in a manner consistent with the passive evolution of an old stellar population formed at an early cosmic epoch. The degree of color evolution is similar for clusters at similar redshift, and does not depend strongly on the optical richness or X-ray luminosity of the cluster, suggesting that the history of early-type galaxies is relatively insensitive to environment. The slope of the color-magnitude relationship shows no significant change out to z=0.9, providing evidence that it arises from a correlation between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in the optical-IR colors is small and nearly constant with redshift, indicating that the majority of giant, early-type galaxies in clusters share a common star formation history, with little perturbation due to uncorrelated episodes of later star formation. Taken together, our results are consistent with models in which most early-type galaxies in rich clusters are old, formed the majority of their stars at high redshift in a well-synchronized fashion, and evolved quiescently thereafter.

Global simulations of ice nucleation and ice supersaturation with an improved cloud scheme in the Community Atmosphere Model

Abstract: [1] A process-based treatment of ice supersaturation and ice nucleation is implemented in the National Center for Atmospheric Research Community Atmosphere Model (CAM). The new scheme is designed to allow (1) supersaturation with respect to ice, (2) ice nucleation by aerosol particles, and (3) ice cloud cover consistent with ice microphysics. The scheme is implemented with a two-moment microphysics code and is used to evaluate ice cloud nucleation mechanisms and supersaturation in CAM. The new model is able to reproduce field observations of ice mass and mixed phase cloud occurrence better than previous versions. The model is able to reproduce observed patterns and frequency of ice supersaturation. Simulations indicate homogeneous freezing of sulfate and heterogeneous freezing on dust are both important ice nucleation mechanisms, in different regions. Simulated cloud forcing and climate is sensitive to different formulations of the ice microphysics. Arctic surface radiative fluxes are sensitive to the parameterization of ice clouds. These results indicate that ice clouds are potentially an important part of understanding cloud forcing and potential cloud feedbacks, particularly in the Arctic.

A quantum Rosetta stone for interferometry

Abstract: Heisenberg-limited measurement protocols can be used to gain an increase in measurement precision over classical protocols. Such measurements can be implemented using, for example, optical Mach—Zehnder interferometers and Ramsey spectroscopes. We address the formal equivalence between the Mach—Zehnder interferometer, the Ramsey spectroscope and a generic quantum logic circuit. Based on this equivalence we introduce the 'quantum Rosetta stone', and we describe a projective-measurement scheme for generating the desired correlations between the interferometric input states in order to achieve Heisenberg-limited sensitivity. The Rosetta stone then tells us that the same method should work in atom spectroscopy.