5,022 research outputs found
Galactic Evolution of Nitrogen
We present detailed spectroscopic analysis of nitrogen abundances in 31
unevolved metal-poor stars analysed by spectral synthesis of the near-UV NH
band at 3360 A observed at high resolution with various telescopes. We found
that [N/Fe] scales with that of iron in the metallicity range -3.1 < [Fe/H] <0
with the slope 0.01+-0.02. Furthermore, we derive uniform and accurate (N/O)
ratios using oxygen abundances from near-UV OH lines obtained in our previous
studies. We find that a primary component of nitrogen is required to explain
the observations. The NH lines are discovered in the VLT/UVES spectra of the
very metal-poor subdwarfs G64-12 and LP815-43 indicating that these stars are N
rich. The results are compared with theoretical models and observations of
extragalactic HII regions and Damped Ly systems. This is the first
direct comparison of the (N/O) ratios in these objects with those in Galactic
stars.Comment: 10 pages, 6 figures, to appear in Astronomy and Astrophysic
The Galactic evolution of phosphorus
As a galaxy evolves, its chemical composition changes and the abundance
ratios of different elements are powerful probes of the underlying evolutionary
processes. Phosphorous is an element whose evolution has remained quite elusive
until now, because it is difficult to detect in cool stars. The infrared weak P
I lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators
of the presence of phosphorus. The availability of CRIRES at VLT has permitted
access to this wavelength range in stellar spectra.We attempt to measure the
phosphorus abundance of twenty cool stars in the Galactic disk. The spectra are
analysed with one-dimensional model-atmospheres computed in Local Thermodynamic
Equilibrium (LTE). The line formation computations are performed assuming LTE.
The ratio of phosphorus to iron behaves similarly to sulphur, increasing
towards lower metallicity stars. Its ratio with respect to sulphur is roughly
constant and slightly larger than solar, [P/S]=0.10+- 0.10. We succeed in
taking an important step towards the understanding of the chemical evolution of
phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance
ratio is steeper than predicted by Galactic chemical evolution model model
developed by Kobayashi and collaborators. Phosphorus appears to evolve
differently from the light odd-Z elements sodium and aluminium. The constant
value of [P/S] with metallicity implies that P production is insensitive to the
neutron excess, thus processes other than neutron captures operate. We suggest
that proton captures on 30Si and alpha captures on $27Al are possibilities to
investigate. We see no clear distinction between our results for stars with
planets and stars without any detected planet.Comment: To be published on A&
Early Galactic Evolution of Carbon, Nitrogen and Oxygen
We present results on carbon, nitrogen, and oxygen abundances for a sample of
unevolved metal-poor stars with metallicities in the range -0.3< [Fe/H]< -3.
Oxygen abundances derived from different indicators are compared showing
consistently that in the range 0.3 >[Fe/H]>-3.0, the [O/Fe] ratio increases
from approximately 0 to 1. We find a good agreement between abundances based on
the forbidden line, the OH and IR triplet lines when gravities based on
Hipparcos} parallaxes are considered for the sample stars. Gravities derived
from LTE ionization balance in metal-poor stars with [Fe/H]< -1 are likely too
low, and could be responsible for an underestimation of the oxygen abundances
derived using the [OI] line. [C/Fe] and [N/Fe] ratios appear to be constant,
independently of metallicity, in the same range. However, they show larger
scatter than oxygen at a given metallicity, which could reflect the larger
variety of stellar production sites for these other elements.Comment: 10 pages, 3 figures, To appear in the proceedings of the conference
"The Chemical Evolution of The Milky Way: Stars versus Clusters", eds. F.
Matteucci and F. Giovannelli, Vulcano, Italy, September 20-24 199
On the Galactic Evolution of and
The determined abundances of primordial and provide a basis
with which to test the standard model of big bang nucleosynthesis in
conjunction with the other two light element isotopes and , also
produced in the big bang. Overall, consistency in the standard big bang
nucleosynthesis model is best achieved for a baryon-to-photon ratio of
typically for which the primordial value of is five
times greater than the present observed abundance and about three times greater
than the pre-solar value. We consider various models for the chemical evolution
of the Galaxy to test the feasibility for the destruction of D without the
overproduction of and overall metallicity. Models which are capable of
achieving this goal include ones with a star formation rate proportional to the
gas mass fraction or an exponentially decreasing star formation rate. We
discuss the effect of parameters that govern the initial mass function and of
surviving fractions of in stars between one and three solar masses.Comment: 25 pages, LaTeX, UMN-TH-1206/9
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