302 research outputs found
Galactic Globular Cluster Metallicity Scale from the Calcium Triplet. II. Rankings, Comparisons and Puzzles
We compare our compilation of the W' calcium index for 71 Galactic globular
clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and
to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from
high-dispersion spectra analyzed with Kurucz (1992, private communication)
model atmospheres. We find our calcium ranking to be tightly correlated with
each comparison set, in a non-linear and a linear fashion, respectively. By
combining our calcium index information with the Zinn and West ranking, we are
able to rank the globular clusters in our sample with a typical precision of
+/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters more
metal rich than this, the ranking is less precise. The significant differences
between these metallicity scales raise important questions about our
understanding of Galactic formation and chemical enrichment processes.
Furthermore, in spite of the apparent improvement in metallicity ranking for
the Galactic globular clusters that results from our addition of information
from the Ca II triplet lines to the potpourri of other metallicity indicators,
caution -- perhaps considerable -- may be advisable when using W' as a
surrogate for metallicity, especially for systems where ranges in age and
metallicity are likely.Comment: To appear in the August 1997 issue of PASP Also available at
http://www.hia.nrc.ca/eprints.htm
Light Element Abundance Inhomogeneities in Globular Clusters: Probing Star Formation and Evolution in the Early Milky Way
Abundance patterns of the elements C, N, and O are sensitive probes of
stellar nucleosynthesis processes and, in addition, O abundances are an
important input for stellar age determinations. Understanding the nature of the
observed distribution of these elements is key to constraining protogalactic
star formation history. Patterns deduced from low-resolution spectroscopy of
the CN, CH, NH, and CO molecules for low-mass stars in their core-hydrogen or
first shell-hydrogen burning phases in the oldest ensembles known, the Galactic
globular star clusters, are reviewed. New results for faint stars in NGC 104
(47 Tuc, C0021-723) reveal that the bimodal, anticorrelated pattern of CN and
CH strengths found among luminous evolved stars is also present in stars
nearing the end of their main-sequence lifetimes. In the absence of known
mechanisms to mix newly synthesized elements from the interior to the
observable surface layers of such unevolved stars, those particular
inhomogeneities imply that the original material from which the stars formed
some 15 billion years ago was chemically inhomogeneous in the C and N elements.
However, in other clusters, observations of abundance ratios and C isotope
ratios suggest that alterations to surface chemical compositions are produced
as stars evolve from the main sequence through the red giant branch. Thus, the
current observed distributions of C, N, and O among the brightest stars (those
also observed most often) may not reflect the true distribution from which the
protocluster cloud formed. The picture which is emerging of the C, N and O
abundance patterns within globular clusters may be one whichComment: 12 pages in uuencoded compressed postscript (including figures), to
appear in the Canadian Journal of Physics (Special Issue in Honor of G.
Herzberg
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
The Age Dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump
Color-magnitude diagrams of globular clusters often exhibit a prominent
horizontal branch (HB) and may also show features such as the red giant branch
(RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theory
predicts that the luminosities of these features will depend on the metallicity
and age of the cluster. We calculate theoretical lines of 2 to 12 Gyr constant
age RGB-bumps and AGB-bumps in the V(HB-Bump)--[Fe/H] diagram, which shows the
brightness difference between the bump and the HB as a function of metallicity.
In order to test the predictions, we identify giant branch bumps in new Hubble
Space Telescope color-magnitude diagrams for 8 SMC clusters. First, we conclude
that the SMC cluster bumps are RGB-bumps. The data for clusters younger than ~6
Gyr are in fair agreement the relative age dependent luminosities of the HB and
RGB-bump. The V(HB-Bump)--[Fe/H] data for clusters older then ~6 Gyr
demonstrate a less satisfactory agreement with our calculations. We conclude
that ~6 Gyr is a lower bound to the age of clusters for which the Galactic
globular cluster, age independent V(HB-Bump)--[Fe/H] calibration is valid.
Application of the V(HB-bump)--[Fe/H] diagram to stellar population studies is
discussed.Comment: Accepted for publication in the Astrophysical Journal, 30 pages,
Latex aaspp4.sty, including 7 postscript figure
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Sixteen years of bathymetry and waves at San Diego beaches.
Sustained, quantitative observations of nearshore waves and sand levels are essential for testing beach evolution models, but comprehensive datasets are relatively rare. We document beach profiles and concurrent waves monitored at three southern California beaches during 2001-2016. The beaches include offshore reefs, lagoon mouths, hard substrates, and cobble and sandy (medium-grained) sediments. The data span two energetic El Niño winters and four beach nourishments. Quarterly surveys of 165 total cross-shore transects (all sites) at 100 m alongshore spacing were made from the backbeach to 8 m depth. Monthly surveys of the subaerial beach were obtained at alongshore-oriented transects. The resulting dataset consists of (1) raw sand elevation data, (2) gridded elevations, (3) interpolated elevation maps with error estimates, (4) beach widths, subaerial and total sand volumes, (5) locations of hard substrate and beach nourishments, (6) water levels from a NOAA tide gauge (7) wave conditions from a buoy-driven regional wave model, and (8) time periods and reaches with alongshore uniform bathymetry, suitable for testing 1-dimensional beach profile change models
Revisiting the Rigidly Rotating Magnetosphere model for sigma Ori E. I. Observations and Data Analysis
We have obtained 18 new high-resolution spectropolarimetric observations of
the B2Vp star sigma Ori E with both the Narval and ESPaDOnS
spectropolarimeters. The aim of these observations is to test, with modern
data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of
Townsend & Owocki (2005), applied to the specific case of sigma Ori E by
Townsend et al. (2005). This model includes a substantially offset dipole
magnetic field configuration, and approximately reproduces previous
observational variations in longitudinal field strength, photometric
brightness, and Halpha emission. We analyze new spectroscopy, including H I, He
I, C II, Si III and Fe III lines, confirming the diversity of variability in
photospheric lines, as well as the double S-wave variation of circumstellar
hydrogen. Using the multiline analysis method of Least-Squares Deconvolution
(LSD), new, more precise longitudinal magnetic field measurements reveal a
substantial variance between the shapes of the observed and RRM model
time-varying field. The phase resolved Stokes V profiles of He I 5876 A and
6678 A lines are fit poorly by synthetic profiles computed from the magnetic
topology assumed by Townsend et al. (2005). These results challenge the offset
dipole field configuration assumed in the application of the RRM model to sigma
Ori E, and indicate that future models of its magnetic field should also
include complex, higher-order components.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
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