472 research outputs found
CN Bimodality at Low Metallicity: The Globular Cluster M53
We present low resolution UV-blue spectroscopic observations of red giant
stars in the globular cluster M53 ([Fe/H]=-1.84), obtained to study primordial
abundance variations and deep mixing via the CN and CH absorption bands. The
metallicity of M53 makes it an attractive target: a bimodal distribution of
3883 angstrom CN bandstrength is common in moderate- and high-metallicity
globular clusters ([Fe/H] > -1.6) but unusual in those of lower metallicity
([Fe/H] < -2.0). We find that M53 is an intermediate case, and has a broad but
not strongly bimodal distribution of CN bandstrength, with CN and CH
bandstrengths anticorrelated in the less-evolved stars. Like many other
globular clusters, M53 also exhibits a general decline in CH bandstrength and
[C/Fe] abundance with rising luminosity on the red giant branch.Comment: 8 pages including 11 figures and 1 table, accepted by PAS
Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance
We present [Ffe/H] abundance results that involve a sample of stars with a
wide range in luminosity from luminous giants to stars near the turnoff in a
globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more
luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous
than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II
lines in high dispersion spectra observed with HIRES at the W. M. Keck
Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines
agree with each other and with earlier determinations. Also the [Fe/H] obtained
from Fe I and Fe II lines is constant within the rather small uncertainties for
this group of stars over the full range in Teff and luminosity, suggesting that
NLTE effects are negligible in our iron abundance determination. In this
globular cluster, there is no difference among the mean [Fe/H] of giant stars
located at or above the RHB, RHB stars, giant stars located below the RHB and
stars near the turnoff.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 2 of a pair
C and N Abundances in Stars At the Base of the Red Giant Branch in M5
We present an analysis of a large sample of moderate resolution Keck LRIS
spectra of subgiant (V \sim 17.2) and fainter stars in the Galactic globular
cluster M5 (NGC 5904) with the goal of deriving C and N abundances.
Star-to-star stochastic variations with significant range in both [C/Fe] and
[N/Fe] are found at all luminosities extending to the bottom of the RGB at M_V
\sim +3. Similar variations in CH appear to be present in the main sequence
turnoff spectra. There is no sign of a change in the behavior of C and N with
evolutionary stage over the full range in luminosity of the RGB and SGB. The C
and N abundances appear strongly anti-correlated, as would be expected from the
CN-cycle processing of stellar material. Yet the present stars are considerably
fainter than the RGB bump, the point at which deep mixing is believed to set
in. On this basis, while the observed abundance pattern is consistent with
proton capture nucleosynthesis, we infer that the site of the reactions is
likely not within the present sample, but rather in a population of more
massive (2 -- 5 M(Sun)) now defunct stars. The range of variation of the N
abundances is very large and the sum of C+N increases as C decreases. To
reproduce this requires the incorporation not only of CN but also of
ON-processed material. Furthermore, the existence of this correlation is quite
difficult to reproduce with an external mechanism such as ``pollution'' with
material processed in a more massive AGB star, which mechanism is fundamentally
stochastic in nature. We therefore suggest that although the internal mixing
hypothesis has serious flaws,new theoretical insights are needed and it should
not be ruled out yet. (abridged)Comment: Slightly updated version to conform to that accepted by the A
The Production of Sodium and Aluminum in Globular Cluster Red Giant Stars
We study the production of Na and Al around the hydrogen shell of two
red-giant sequences of different metallicity in order to explain the abundance
variations seen in globular cluster stars in a mixing scenario. Using detailed
stellar models together with an extensive nuclear reaction network, we have
calculated the distribution of the various isotopic abundances around the
hydrogen shell at numerous points along the red-giant branch. These
calculations allow for the variation in both temperature and density in the
shell region as well as the timescale of the nuclear processing, as governed by
the outward movement of the hydrogen shell. The reaction network uses updated
rates over those of Caughlin \& Fowler (1988). We find evidence for the
production of Na and Al occurring in the NeNa and MgAl cycles. In particular,
Na is significantly enhanced throughout the region above the hydrogen shell.
The use of the newer reaction rates causes a substantial increase in the
production of Al above the hydrogen shell through heavy leakage from the
NeNa cycle and should have an important effect on the predicted surface
abundances. We also find that the nuclear processing is considerably more
extensive at lower metallicities.Comment: 4 pages with 4 EPS figures embedded, accepted by ApJL March 28, 199
Oxygen Abundances in Two Metal-Poor Subgiants from the Analysis of the 6300 A Forbidden O I Line
Recent LTE analyses (Israelian et al. 1998 and Bosegaard et al. 1999) of the
OH bands in the optical-ultraviolet spectra of nearby metal-poor subdwarfs
indicate that oxygen abundances are generally higher than those previously
determined. The difference increases with decreasing metallicity and reaches
delta([O/Fe]) ~ +0.6 dex as [Fe/H] approaches -3.0.
Employing high resolution (R = 50000), high S/N (~ 250) echelle spectra of
the two stars found by Israelian et al. (1998) to have the highest
[O/Fe]-ratios, viz, BD +23 3130 and BD +37 1458, we conducted abundance
analyses based on about 60 Fe I and 7-9 Fe II lines. We determined from Kurucz
LTE models the values of the stellar parameters, as well as abundances of Na,
Ni, and the traditional alpha-elements, independent of the calibration of color
vs scales. We determined oxygen abundances from spectral synthesis of
the stronger line (6300 A) of the [O I] doublet.
The syntheses of the [O I] line lead to smaller values of [O/Fe], consistent
with those found earlier among halo field and globular cluster giants. We
obtain [O/Fe] = +0.35 +/- 0.2 for BD +23 3130 and +0.50 +/- 0.2 for BD +37
1458. In the former, the [O I] line is very weak (~ 1 mA), so that the quoted
[O/Fe] value may in reality be an upper limit.
Therefore in these two stars a discrepancy exists between the [O/Fe]- ratios
derived from [O I] and the OH feature, and the origin of this difference
remains unclear. Until the matter is clarified, we suggest it is premature to
conclude that the ab initio oxygen abundances of old, metal-poor stars need to
be revised drastically upward.Comment: 38 pages, 5 tables, 14 figures To appear in July 1999 AJ Updated
April 16, 1999. Fixed typo
Linearly scaling direct method for accurately inverting sparse banded matrices
In many problems in Computational Physics and Chemistry, one finds a special
kind of sparse matrices, termed "banded matrices". These matrices, which are
defined as having non-zero entries only within a given distance from the main
diagonal, need often to be inverted in order to solve the associated linear
system of equations. In this work, we introduce a new O(n) algorithm for
solving such a system, being n X n the size of the matrix. We produce the
analytical recursive expressions that allow to directly obtain the solution, as
well as the pseudocode for its computer implementation. Moreover, we review the
different options for possibly parallelizing the method, we describe the
extension to deal with matrices that are banded plus a small number of non-zero
entries outside the band, and we use the same ideas to produce a method for
obtaining the full inverse matrix. Finally, we show that the New Algorithm is
competitive, both in accuracy and in numerical efficiency, when compared to a
standard method based in Gaussian elimination. We do this using sets of large
random banded matrices, as well as the ones that appear when one tries to solve
the 1D Poisson equation by finite differences.Comment: 24 pages, 5 figures, submitted to J. Comp. Phy
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