397,065 research outputs found
New Keck Observations of Lithium in Very Metal-poor Stars
Lithium abundances have been determined in more than 100 metal-poor halo
stars both in the field and in clusters. From these data we find trends of Li
with both temperature and metallicity and a real dispersion in Li abundances in
the Spite Li plateau. We attribute this dispersion primarily to Li depletion
(presumably due to extra mixing induced by stellar rotation) and to Galactic
chemical evolution. We derive a primordial Li of 2.44 0.18 for A(Li) =
log N(Li/H) + 12.00. This agrees with the Li abundances predicted by the
results. For stars cooler than the Li plateau we have evidence that Li
depletion sets in at hotter temperatures for the higher metallicity stars than
for the low-metal stars. This is the opposite sense of predictions from stellar
models. The smooth transition of the Li content from the Li plateau stars to
the cool stars adds weight to the inference of Li depletion in the plateau
stars.Comment: Invited talk for IAU Symposium 228 "From Lithium to Uranium..." held
in Paris in May, 2005. 6 pages, 6 figure
The Synthesis and Characterization of New, Robust Titanium (IV) Scorpionate Complexes
Titanium complexes possessing sterically encumbered ligands have allowed for the preparation of reactive moieties (imido, alkylidene and alkylidyne species) relevant to reactions such as olefin polymerization and alkyne hydroamination. For this reason, we have targeted robust scorpionate ancillary ligands to support reactive titanium centers. Thus, a series of titanium complexes were synthesized using an achiral oxazoline-based scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate [To^M^]^-^ as well as the related chiral ligand, tris(4-isopropyl-2-oxazolinyl)phenyl borate [To^P^]^-^. The complex [Ti(κ^3^- To^M^)Cl~3~] was prepared in moderate yield (43%) by the rapid (<1 min at room temperature) reaction of Li[To^M^] and TiCl~4~ in methylene chloride; this new compound was characterized by ^1^H NMR spectroscopy as the expected C~3v~-symmetric species. One route to Ti (IV) alkyls involves salt metathesis; accordingly, syntheses of [To^M^]Ti alkyl complexes by interaction of [Ti(κ^3^-To^M^)Cl~3~] and one or three equivalents of alkylating agents, such as benzyl potassium (KCH~2~C~6~H~5~), trimethylsilylmethyl
lithium (LiCH~2~Si(CH~3~) ~3~), or neopentyl lithium (LiCH~2~C(CH~3~)~3~) are currently under investigation. The complexes [Ti(=NBut) (κ~3~-To^M^)(Cl)(Bu^t^py)] (Bu^t^py=4 tert-butylpyridine) and [Ti(=NBu^t^) (κ~3~-To^P^)(Cl)(Bu^t^py)] were synthesized by reaction of the known Ti imido [Ti(=NBu^t^)(Cl)~2~(Bu^t^py)~2~] with Li[To^M^] or Li[To^P^], respectively, by stirring overnight in methylene chloride at ambient temperature. The complexes were identified using ^1^H NMR spectroscopy, ^1^H-^13^C HMQC and ^1^H-^15^N HMBC correlation experiments
Beryllium in Ultra-Lithium-Deficient Halo Stars - The Blue Straggler Connection
There are nine metal-deficient stars that have Li abundances well below the
Li plateau that is defined by over 100 unevolved stars with temperatures above
5800 K and values of [Fe/H] 1.0. Abundances of Be have been determined
for most of these ultra-Li-deficient stars in order to investigate the cause of
the Li deficiencies. High-resolution and high signal-to-noise spectra have been
obtained in the Be II spectral region near 3130 \AA for six ultra-Li-deficient
stars with the Keck I telescope and its new uv-sensitive CCD on the upgraded
HIRES. The spectrum synthesis technique has been used to determine Be
abundances. All six stars are found to have Be deficiencies also. Two have
measurable - but reduced - Be and four have only upper limits on Be. These
results are consistent with the idea that these Li- and Be-deficient stars are
analogous to blue stragglers. The stars have undergone mass transfer events (or
mergers) which destroy or dilute both Li and Be. The findings cannot be matched
by the models that predict that the deficiencies are due to extra-mixing in a
subset of halo stars that were initially rapid rotators, with the possible
exception of one star, G 139-8. Because the ultra-Li-deficient stars are also
Be-deficient, they appear to be genuine outliers in population of halo stars
used to determine the value of primordial Li; they no longer have the Li in
their atmospheres that was produced in the Big Bang.Comment: 17 pages of text, 12 figures, 3 tables Submitted to Ap
Lithium abundances in extremely metal-poor turn-off stars
We discuss the current status of the sample of Lithium abundances in
extremely metal poor (EMP) turn-off (TO) stars collected by our group, and
compare it with the available literature results. In the last years, evidences
have accumulated of a progressive disruption of the Spite plateau in stars of
extremely low metallicity. What appears to be a flat, thin plateau above
[Fe/H]\sim-2.8 turns, at lower metallicities, into a broader distribution for
which the plateau level constitutes the upper limit, but more and more stars
show lower Li abundances. The sample we have collected currently counts
abundances or upper limits for 44 EMP TO stars between [Fe/H]=-2.5 and -3.5,
plus the ultra-metal poor star SDSS J102915+172927 at [Fe/H]=-4.9. The
"meltdown" of the Spite plateau is quite evident and, at the current status of
the sample, does not appear to be restricted to the cool end of the effective
temperature distribution. SDSS J102915+172927 displays an extreme Li depletion
that contrasts with its otherwise quite ordinary set of [X/Fe] ratios.Comment: 6 pages, 4 figures, proceedings of the "Lithium in the Cosmos"
conference, Paris, 27-29 February 201
Comparison of Body Composition Measurements using a New Caliper, Two Established Calipers, Hydrostatic Weighing, and BodPod
Purposes: (1) To compare the Lafayette Instruments (LI) skinfold caliper to the Lange (L) and Harpenden (H) calipers using a diverse subject population. (2) To determine the validity of the LI caliper in a subset of subjects by comparing body compositions from skinfold thicknesses to those measured by hydrostatic weighing (HW) and air displacement plethysmography (ADP). (3) To compare measurements obtained by experienced (EX) and inexperienced (IX) technicians using all three calipers. Methods: Skinfold measurements were performed by both EX and IX technicians using three different calipers on 21 younger (21.2 ± 1.5 yrs) and 20 older (59.2 ± 4 yrs) subjects. Body compositions were calculated using the Jackson-Pollock seven-site and three-site formulas. HW and ADP tests were performed on a subset of subjects (10 younger, 10 older). Results: No significant differences existed between LI and L or H when measurements were made by EX. Further, the LI-EX measurements were highly correlated to both H-EX and L-EX. No significant differences existed in the subgroup between LI-EX and HW or ADP. Skinfold determinations made by EX and IX were similar. Conclusions: Similar body compositions determined using LI, H, and L suggest that LI determines body composition as effectively as H and L. High correlations between the three calipers support this notion. Similar results between LI and HW/ADP subgroup suggest that the LI caliper may be a valid method of measuring body composition. Overall, performance by IX was similar to EX and suggests similar ease of use for all three calipers
Structural and optical properties of MOCVD AllnN epilayers
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Environmental advertising in China and the USA: the desire to go green
Book Review
Environmental advertising in China and the USA: the desire to go green, by Xinghua Li, London and New York, NY, Routledge, 2016, 158 pp., £90.00 (hardback), ISBN 978-415-74413-
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