2,058 research outputs found
Super-Eddington Atmospheres that Don't Blow Away
We show that magnetized, radiation dominated atmospheres can support steady
state patterns of density inhomogeneity that enable them to radiate at far
above the Eddington limit, without suffering mass loss. The inhomogeneities
consist of periodic shock fronts bounding narrow, high-density regions,
interspersed with much broader regions of low density. The radiation flux
avoids the regions of high density, which are therefore weighed down by
gravity, while gas in the low-density regions is slammed upward into the shock
fronts by radiation force. As the wave pattern moves through the atmosphere,
each parcel of matter alternately experiences upward and downward forces, which
balance on average. Magnetic tension shares the competing forces between
regions of different densities, preventing the atmosphere from blowing apart.
We calculate the density structure and phase speed of the wave pattern, and
relate these to the wavelength, the density contrast, and the factor by which
the net radiation flux exceeds the Eddington limit. In principle, this factor
can be as large as the ratio of magnetic pressure to mean gas pressure, or the
ratio of radiation pressure to gas pressure, whichever is smaller. Although the
magnetic pressure must be large compared to the mean gas pressure in order to
support a large density contrast, it need not be large compared to the
radiation pressure. These highly inhomogeneous flows could represent the
nonlinear development of the "photon bubble" instability discovered by Gammie.
We briefly discuss the applicability of these solutions to astrophysical
systems.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journa
Births to Teens Older and Younger Than 17 Years in San Bernardino County and California: Variables Associated with Infant Mortality and Survival
Objective: The purpose of this cohort, descriptive study was to attempt to understand the variables associated with discordant infant mortality among teenagers 17-19 years old whose infants demonstrated higher mortality than infants of teenagers who were younger than 17 years old in San Bernardino County, California. The intent was to elicit further research and/or define appropriate interventions for teen mothers within the age range 17-19 years. Methods: Data was abstracted from an electronic infant mortality data set, the State of California Birth Cohort File in which birth records from San Bernardino County for the period 1989 through 1993 were matched with mortality records. Results: The data showed that infants of white teens within the 17-19 age groups were more likely to have higher infant mortality rates when compared to their younger peers. Infant mortality rates among offspring of Hispanic and black teenage mothers showed no discrepancy between the two groups nor between county and state rates. Conclusions: Further study is needed to answer why infants of white teen mothers in the 17-19 age groups have higher mortality rates. There is also a need to review the services rendered to pregnant and parenting adolescents in San Bernardino County. In addition, very low birth weight infants were much more likely to die when born to older teens than when born to younger teens
Corotating light cylinders and Alfv\'en waves
Exact relativistic force free fields with cylindrical symmetry are explored.
Such fields are generated in the interstellar gas via their connection to
pulsar magnetospheres both inside and outside their light cylinders. The
possibility of much enhanced interstellar fields wound on cylinders of Solar
system dimensions is discussed but these are most likely unstable.Comment: 6 pages, 6 figures, accepted by MNRA
Reactivity of the Indenyl Radical (C9 H7 ) with Acetylene (C2 H2 ) and Vinylacetylene (C4 H4 ).
The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C9 H7 . ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5 . ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol-1 ) and slow, contrary to the exoergic (-38 kJ mol-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces
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