2,896 research outputs found
Achieving the Potential of Health Care Performance Measures: Timely Analysis of Immediate Health Policy issues
The United States is on the cusp of a new era, with greater demand for performance information, greater data availability, and a greater willingness to integrate performance information into public policy. This era has immense promise to deliver a learning health care system that encourages collaborative improvements in systems-based care, improves accountability, helps consumers make important choices, and improves quality at an acceptable cost. However, to curtail the possibility of unintended adverse consequences, it is important that we invest in developing sound measures, understand quality measures' strengths and limitations, study the science of quality measurement, and reduce inaccurate inferences about provider performance
The Maximum Flux of Star-Forming Galaxies
The importance of radiation pressure feedback in galaxy formation has been
extensively debated over the last decade. The regime of greatest uncertainty is
in the most actively star-forming galaxies, where large dust columns can
potentially produce a dust-reprocessed infrared radiation field with enough
pressure to drive turbulence or eject material. Here we derive the conditions
under which a self-gravitating, mixed gas-star disc can remain hydrostatic
despite trapped radiation pressure. Consistently taking into account the
self-gravity of the medium, the star- and dust-to-gas ratios, and the effects
of turbulent motions not driven by radiation, we show that galaxies can achieve
a maximum Eddington-limited star formation rate per unit area
pc Myr,
corresponding to a critical flux of
kpc similar to previous estimates; higher fluxes eject mass in bulk,
halting further star formation. Conversely, we show that in galaxies below this
limit, our one-dimensional models imply simple vertical hydrostatic equilibrium
and that radiation pressure is ineffective at driving turbulence or ejecting
matter. Because the vast majority of star-forming galaxies lie below the
maximum limit for typical dust-to-gas ratios, we conclude that infrared
radiation pressure is likely unimportant for all but the most extreme systems
on galaxy-wide scales. Thus, while radiation pressure does not explain the
Kennicutt-Schmidt relation, it does impose an upper truncation on it. Our
predicted truncation is in good agreement with the highest observed gas and
star formation rate surface densities found both locally and at high redshift.Comment: Version accepted for publication in MNRAS. 12 pages, 8 figures. New
appendix on photon tirin
On the Role of Disks in the Formation of Stellar Systems: A Numerical Parameter Study of Rapid Accretion
We study rapidly accreting, gravitationally unstable disks with a series of
global, three dimensional, numerical experiments using the code ORION. In this
paper we conduct a numerical parameter study focused on protostellar disks, and
show that one can predict disk behavior and the multiplicity of the accreting
star system as a function of two dimensionless parameters which compare the
disk's accretion rate to its sound speed and orbital period. Although
gravitational instabilities become strong, we find that fragmentation into
binary or multiple systems occurs only when material falls in several times
more rapidly than the canonical isothermal limit. The disk-to-star accretion
rate is proportional to the infall rate, and governed by gravitational torques
generated by low-m spiral modes. We also confirm the existence of a maximum
stable disk mass: disks that exceed ~50% of the total system mass are subject
to fragmentation and the subsequent formation of binary companions.Comment: 16 pages, 12 figures, submitte
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