3,332 research outputs found
The Chemical Compositions of the SRd Variable Stars-- II. WY Andromedae, VW Eridani, and UW Librae
Chemical compositions are derived from high-resolution spectra for three
stars classed as SRd variables in the General Catalogue of Variable Stars.
These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0,
VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions
are identical to within the measurement errors with the compositions of
subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at
the tip of the first giant branch or in the early stages of evolution along the
asymptotic giant branch (AGB). There is no convincing evidence that these SRd
variables are experiencing thermal pulsing and the third dredge-up on the AGB.
The SRds appear to be the cool limit of the sequence of RV Tauri variables.Comment: 14 pages, 1 figure, 4 table
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
Star-to-star Na and O abundance variations along the red giant branch in NGC 2808
We report for the first time Na and O abundances from high-resolution, high
S/N echelle spectra of 20 red giants in NGC 2808, taken as part of the Science
Verification program of the FLAMES multi-object spectrograph at the ESO VLT. In
these stars, spanning about 3 mag from the red giant branch (RGB) tip, large
variations are detected in the abundances of oxygen and sodium, anticorrelated
with each other; this is a well known evidence of proton-capture reactions at
high temperatures in the ON and NeNa cycles. One star appears super O-poor; if
the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O
depletion levels as large as those of M 13. This result confirms our previous
findings based on lower resolution spectra (Carretta et al. 2003) of a large
star-to-star scatter in proton capture elements at all positions along the RGB
in NGC 2808, with no significant evolutionary contribution. Finally, the
average metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from
19 stars.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
Beryllium in turnoff stars of NGC6397: early Galaxy spallation, cosmochronology and cluster formation
We present the first detection of beryllium in two turnoff stars of the old,
metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected
and we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. The
beryllium abundance is very similar to that of field stars of similar Fe
content. We interpret the beryllium abundance observed as the result of primary
spallation of cosmic rays acting on a Galactic scale, showing that beryllium
can be used as a powerful cosmochronometer for the first stellar generations.
With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the
onset of star formation in the Galaxy, in excellent agreement with the age
derived from main sequence fitting. From the same spectra we also find low O
(noticeably different for the two stars) and high N abundances, suggesting that
the original gas was enriched in CNO processed material. Our beryllium results,
together with the N, O, and Li abundances, provide insights on the formation of
this globular cluster, showing that any CNO processing of the gas must have
occurred in the protocluster cloud before the formation of the stars we observe
now. We encounter, however, difficulties in giving a fully consistent picture
of the cluster formation, able to explain the complex overall abundance
pattern.Comment: to appear in A&
OFFO minimization algorithms for second-order optimality and their complexity
An Adagrad-inspired class of algorithms for smooth unconstrained optimization
is presented in which the objective function is never evaluated and yet the
gradient norms decrease at least as fast as \calO(1/\sqrt{k+1}) while
second-order optimality measures converge to zero at least as fast as
\calO(1/(k+1)^{1/3}). This latter rate of convergence is shown to be
essentially sharp and is identical to that known for more standard algorithms
(like trust-region or adaptive-regularization methods) using both function and
derivatives' evaluations. A related "divergent stepsize" method is also
described, whose essentially sharp rate of convergence is slighly inferior. It
is finally discussed how to obtain weaker second-order optimality guarantees at
a (much) reduced computional cost
The First Detailed Abundances for M giants in Baade's Window from Infrared Spectroscopy
We report the first abundance analysis of 14 M giant stars in the Galactic
bulge, based on R=25,000 infrared spectroscopy (1.5-1.8um) using NIRSPEC at the
Keck II telescope. Because some of the bulge M giants reach high luminosities
and have very late spectral type, it has been suggested that they are the
progeny of only the most metal rich bulge stars, or possibly members of a
younger bulge population. We find the iron abundance and composition of the M
giants are similar to those of the K giants that have abundances determined
from optical high resolution spectroscopy: =-0.190 +/- 0.020 with a
1-sigma dispersion of 0.08 +/- 0.015. Comparing our bulge M giants to a control
sample of local disk M giants in the Solar vicinity, we find the bulge stars
are enhanced in alpha elements at the level of +0.3 dex relative to the Solar
composition stars, consistent with other studies of bulge globular clusters and
field stars. This small sample shows no dependence of spectral type on
metallicity, nor is there any indication that the M giants are the evolved
members of a subset of the bulge population endowed with special
characteristics such as relative youth or high metallicity. We also find low
12C/13C < 10, confirming the prsence of extra-mixing processes during the red
gaint phase of evolutionComment: 19 pages, 7 figures, accepted for publication in the Astrophysical
Journa
2-MethylÂpyrazine 1,4-dioxide
The title compound, C5H6N2O2, was prepared from 2-methylÂpyrazine, acetic acid and hydrogen peroxide. In the crystal, ÏâÏ stacking interÂactions between neighboring molÂecules are observed, with a centroidâcentroid distance of 3.7370â
Ă
, an interÂplanar distance of 3.167â
Ă
, and a slippage of 1.984â
Ă
. Each molÂecule is linked to four neighbors through CâHâŻO hydrogen-bonding interÂactions, forming one-dimensional ribbons
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