283 research outputs found
The carbon-to-oxygen ratio in stars with planets
In some recent works, the C/O abundance ratio in high-metallicity stars with
planets is found to vary from 0.4 to about 1.0. This has led to discussions
about the existence of terrestrial planets with a carbon-dominated composition
that is very different from the composition of the Earth. The C/O values were
obtained by determining carbon abundances from high-excitation CI lines and
oxygen abundances from the forbidden [OI] line at 6300 A. This weak line is,
however, strongly affected by a nickel blend at high metallicities. Aiming for
more precise C/O ratios, oxygen abundances in this paper are derived from the
high-excitation OI triplet at 7774 A and carbon abundances from the CI lines at
5052 and 5380 A using MARCS model atmospheres and including non-LTE
corrections. The results do not confirm the high C/O ratios previously found.
C/O shows a tight, slightly increasing dependence on metallicity from C/O=0.58
at [Fe/H]=0.0 to C/O=0.70 at [Fe/H] =0.4 with an rms scatter of only 0.06.
Assuming that the composition of a proto-planetary disk is the same as that of
the host star, the C/O values found in this paper lend no support to the
existence of carbon-rich planets. The small scatter of C/O among thin-disk
stars suggests that the nucleosynthesis products of Type II supernovae and low-
to intermediate-mass stars are well mixed in the interstellar medium.Comment: 10 pages, 11 figures, accepted for publication in A&
High-precision abundances of elements in solar twin stars: Trends with stellar age and elemental condensation temperature
HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine
very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al,
Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is
correlated with elemental condensation temperature, Tc. In addition, precise
(sigma < 0.8 Gyr) stellar ages are obtained by interpolating between
Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the
ratio between refractory and volatile elements is lower in the Sun than in most
of the solar twins, but for many stars, the relation between [X/Fe] and Tc is
not well defined. For several elements there is, instead, an astonishingly
tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex
over an age interval of 8 Gyr in contrast to the lack of correlation between
[Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium
shows the opposite behavior so that [Y/Mg] can be used as a sensitive
chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated
with stellar age, but define a tight Ni-Na relation similar to that previously
found for more metal-poor stars. These results provide new constraints on
supernovae yields and Galactic evolution. Furthermore, it is found that the C/O
ratio evolves very little with time, which is of interest for discussions of
the composition of exoplanets.Comment: 13 pages, 14 figures, and 2 on-line tables. To appear in A&
Digesting New Elements in Peptide Transport
In this issue of Structure, Beale et al. (2015) define structurally and functionally a large extracellular domain unique to mammalian peptide transporters and its implications for the transport of basic di- and tri-peptides (Beale et al., 2015)
Initiating Heavy-atom Based Phasing by Multi-Dimensional Molecular Replacement
To obtain an electron-density map from a macromolecular crystal the
phase-problem needs to be solved, which often involves the use of heavy-atom
derivative crystals and concomitantly the determination of the heavy atom
substructure. This is customarily done by direct methods or Patterson-based
approaches, which however may fail when only poorly diffracting derivative
crystals are available, as often the case for e.g. membrane proteins. Here we
present an approach for heavy atom site identification based on a Molecular
Replacement Parameter Matrix (MRPM) search. It involves an n-dimensional search
to test a wide spectrum of molecular replacement parameters, such as clusters
of different conformations. The result is scored by the ability to identify
heavy-atom positions, from anomalous difference Fourier maps, that allow
meaningful phases to be determined. The strategy was successfully applied in
the determination of a membrane protein structure, the CopA Cu+-ATPase, when
other methods had failed to resolve the heavy atom substructure. MRPM is
particularly suited for proteins undergoing large conformational changes where
multiple search models should be generated, and it enables the identification
of weak but correct molecular replacement solutions with maximum contrast to
prime experimental phasing efforts.Comment: 19 pages total, main paper: 6 pages (2 figures), supplementary
material: 13 pages (2 figures, 9 tabels
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