1,513 research outputs found
Searching for Dust around Hyper Metal-Poor Stars
We examine the mid-infrared fluxes and spectral energy distributions for
metal-poor stars with iron abundances [Fe/H] , as well as two
CEMP-no stars, to eliminate the possibility that their low metallicities are
related to the depletion of elements onto dust grains in the formation of a
debris disk. Six out of seven stars examined here show no mid-IR excess. These
non-detections rule out many types of circumstellar disks, e.g. a warm debris
disk ( K), or debris disks with inner radii AU, such as
those associated with the chemically peculiar post-AGB spectroscopic binaries
and RV Tau variables. However, we cannot rule out cooler debris disks, nor
those with lower flux ratios to their host stars due to, e.g. a smaller disk
mass, a larger inner disk radius, an absence of small grains, or even a
multicomponent structure, as often found with the chemically peculiar Lambda
Bootis stars. The only exception is HE0107-5240, for which a small mid-IR
excess near 10 microns is detected at the 2- level; if the excess is
real and associated with this star, it may indicate the presence of (recent)
dust-gas winnowing or a binary system.Comment: Accepted for publication in Ap
Partial nonlinear reciprocity breaking through ultrafast dynamics in a random photonic medium
We demonstrate that ultrafast nonlinear dynamics gives rise to reciprocity
breaking in a random photonic medium. Reciprocity breaking is observed via the
suppression of coherent backscattering, a manifestation of weak localization of
light. The effect is observed in a pump-probe configuration where the pump
induces an ultrafast step-change of the refractive index during the dwell time
of the probe light in the material. The dynamical suppression of coherent
backscattering is reproduced well by a multiple scattering Monte Carlo
simulation. Ultrafast reciprocity breaking provides a distinct mechanism in
nonlinear optical media which opens up avenues for the active manipulation of
mesoscopic transport, random lasers, and photon localization.Comment: 5 pages, 4 figure
The abundance of boron in evolved A- and B-type stars
Boron abundances in A- and B-type stars may be a successful way to track evolutionary effects in these hot stars. The light elements -- Li, Be, and B -- are tracers of exposure to temperatures more moderate than those in which the H-burning CN-cycle operates. Thus, any exposure of surface stellar layers to deeper layers will affect these light element abundances. Li and Be are used in this role in investigations of evolutionary processes in cool stars, but are not observable in hotter stars. An investigation of boron, however, is possible through the B II 1362=C5 resonance line. We have gathered high resolution spectra from the IUE database of A- and B-type stars near 10~M_\odot for which nitrogen abundances have been determined (by Gies & Lambert, 1992, and Venn 1995). The B II 1362=C5 line is blended throughout the temperature range of this program, requiring spectrum syntheses to recover the boron abundances. For no star could we synthesize the 1362=C5 region using the meteoritic/solar boron abundance of log(B) =3D 2.88 (Anders & Grevesse 1989); a lower boron abundance was necessary which may reflect evolutionary effects (e.g., mass loss or mixing near the main-sequence), the natal composition of the star forming regions, or a systematic error in the analyses (e.g., non-LTE effects). Regardless of the initial boron abundance, and despite the possibility of non-LTE effects, it seems clear that boron is severely depleted in some stars. It may be that the nitrogen and boron abundances are anticorrelated, as would be expected from mixing between the H-burning and outer stellar layers. If, as we suspect, a residue of boron is present in the A-type supergiants, we may exclude a scenario in which mixing occurs continuously between the surface and the dee
Greater than the sum of the parts? unpacking ethics of care within a community supported agriculture scheme
Book synopsis: Reconnecting so-called alternative food geographies back to the mainstream food system - especially in light of the discursive and material 'transgressions' currently happening between alternative and conventional food networks, this volume critically interrogates and evaluates what stands for 'food politics' in these spaces of transgression now and in the near future and addresses questions such as:
What constitutes 'alternative' food politics specifically and food politics more generally when organic and other 'quality' foods have become mainstreamed?
What has been the contribution so far of an 'alternative food movement' and its potential to leverage further progressive change and/or make further inroads into conventional systems?
What are the empirical and theoretical bases for understanding the established and growing 'transgressions' between conventional and alternative food networks?
Offering a better understanding of the evolving position of the corporate food system vis a vis alternative food networks, this book considers the prospects for economic, social, cultural and material transformations led by an increasingly powerful and legitimated alternative food network
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