9,516 research outputs found
Stellar Chemical Abundances: In Pursuit of the Highest Achievable Precision
The achievable level of precision on photospheric abundances of stars is a
major limiting factor on investigations of exoplanet host star characteristics,
the chemical histories of star clusters, and the evolution of the Milky Way and
other galaxies. While model-induced errors can be minimized through the
differential analysis of spectrally similar stars, the maximum achievable
precision of this technique has been debated. As a test, we derive differential
abundances of 19 elements from high-quality asteroid-reflected solar spectra
taken using a variety of instruments and conditions. We treat the solar spectra
as being from unknown stars and use the resulting differential abundances,
which are expected to be zero, as a diagnostic of the error in our
measurements. Our results indicate that the relative resolution of the target
and reference spectra is a major consideration, with use of different
instruments to obtain the two spectra leading to errors up to 0.04 dex. Use of
the same instrument at different epochs for the two spectra has a much smaller
effect (~0.007 dex). The asteroid used to obtain the solar standard also has a
negligible effect (~0.006 dex). Assuming that systematic errors from the
stellar model atmospheres have been minimized, as in the case of solar twins,
we confirm that differential chemical abundances can be obtained at sub-0.01
dex precision with due care in the observations, data reduction and abundance
analysis.Comment: Accepted for publication in ApJ; 13 pages, 6 figures, 7 table
Age-related increase of kynurenic acid in human cerebrospinal fluid-IgG and beta(2)-microglobulin changes
Kynurenic acid (KYNA) is an endogenous metabolite in the kynurenine pathway of tryptophan degradation and is an antagonist at the glycine site of the N-methyl-D-aspartate as well as at the alpha 7 nicotinic cholinergic receptors. In the brain tissue KYNA is synthesised from L-kynurenine by kynurenine aminotransferases (KAT) I and II. A host of immune mediators influence tryptophan degradation. In the present study, the levels of KYNA in cerebrospinal fluid (CSF) and serum in a group of human subjects aged between 25 and 74 years were determined by using a high performance liquid chromatography method. In CSF and serum KAT I and II activities were investigated by radioenzymatic assay, and the levels of Ī²2-microglobulin, a marker for cellular immune activation, were determined by ELISA. The correlations between neurochemical and biological parameters were evaluated. Two subject groups with significantly different ages, i.e. 50 years, p < 0.001, showed statistically significantly different CSF KYNA levels, i.e. 2.84 Ā± 0.16 fmol/Ī¼l vs. 4.09 Ā± 0.14 fmol/Ī¼l, p < 0.001, respectively; but this difference was not seen in serum samples. Interestingly, KYNA is synthesised in CSF principally by KAT I and not KAT II, however no relationship was found between enzyme activity and ageing. A positive relationship between CSF KYNA levels and age of subjects indicates a 95% probability of elevated CSF KYNA with ageing (R = 0.6639, p = 0.0001). KYNA levels significantly correlated with IgG and Ī²2-microglobulin levels (R = 0.5244, p = 0.0049; R = 0.4253, p = 0.043, respectively). No correlation was found between other biological parameters in CSF or serum. In summary, a positive relationship between the CSF KYNA level and ageing was found, and the data would suggest age-dependent increase of kynurenine metabolism in the CNS. An enhancement of CSF IgG and Ī²2-microglobulin levels would suggest an activation of the immune system during ageing. Increased KYNA metabolism may be involved in the hypofunction of the glutamatergic and/or nicotinic cholinergic neurotransmission in the ageing CNS
Bounds on the electromagnetic interactions of excited spin-3/2 leptons
We discuss possible deviations from QED produced by a virtual excited
spin-3/2 lepton in the reaction . Data recorded
by the OPAL Collaboration at a c.m. energy are used to
establish bounds on the nonstandard-lepton mass and coupling strengths.Comment: Latex, 5 pages, 7 ps figures. To be published in Phys. Rev.
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