155 research outputs found
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
Abundances in giant stars of the globular cluster NGC 6752
Recent theoretical yields and chemical evolution models demonstrate that
intermediate-mass AGB stars cannot reproduce the observed abundance
distributions of O, Na, Mg, and Al. As a further observational test of this
finding, we present elemental abundance ratios [X/Fe] for 20 elements in 38
bright giants of the globular cluster NGC 6752. Our mean abundance ratios
[X/Fe] are in good agreement with previous studies of this cluster and are also
consistent with other globular clusters and field stars at the same
metallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibit values, in
agreement with field stars at the same metallicity, that lie approximately
midway between the pure r-process and the solar (s-process + r-process) mix,
indicating that AGB stars have played a role in the chemical evolution of the
proto-cluster gas.
For the first time, we find possible evidence for an abundance variation for
elements heavier than Al in this cluster. We find a correlation between [Si/Fe]
and [Al/Fe] which is consistent with the abundance anomalies being synthesized
via proton captures at high temperatures. Leakage from the Mg-Al chain into
28Si may explain the Si excess in stars with the highest [Al/Fe]. We identify
correlations between [Y/Fe] and [Al/Fe], [Zr/Fe] and [Al/Fe], and [Ba/Fe] and
[Al/Fe] suggesting that Y, Zr, and Ba abundances may increase by about 0.1 dex
as Al increases by about 1.3 dex. While the correlations are statistically
significant, the amplitudes of the variations are small. If the small
variations in Y, Zr, and Ba are indeed real, then the synthesis of the Al
anomalies must have taken place within an unknown class of stars that also ran
the s-process. [Abridged]Comment: Accepted for publication in A&
The Dynamical Equilibrium of Galaxy Clusters
If a galaxy cluster is effectively in dynamical equilibrium then all galaxy
populations within the cluster must have distributions in velocity and position
that individually reflect the same underlying mass distribution, although the
derived virial masses can be quite different. Specifically, within the CNOC
cluster sample the virial radius of the red galaxy population is, on the
average, a factor of smaller than that of the blue population.
The red galaxies also have a smaller RMS velocity dispersion, a factor of within our sample. Consequently, the virial mass calculated from the
blue galaxies is times larger than from the red galaxies.
However, applying the Jeans equation of stellar-hydrodynamical equilibrium to
the red and blue subsamples separately give statistically identical cluster
mass profiles. This is strong evidence that these clusters are effectively
equilibrium systems, and therefore empirically demonstrates that the masses in
the virialized region are reliably estimated using dynamical techniques.Comment: Submitted for publication in ApJLetts. 12 pages as a uufile, also
available at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/br/br.ps.g
Mg isotopic ratios in giant stars of the globular cluster NGC 6752
Mg isotopic abundance ratios are measured in 20 bright red giants in globular
cluster NGC 6752 based on very high-resolution, high signal-to-noise spectra
obtained with UVES on the VLT. There is a considerable spread in the ratio
24Mg:25Mg:26Mg with values ranging from 53:9:39 to 83:10:7. We measured the
abundances of O, Na, Mg, Al, and Fe combining our sample with 21 RGB bump stars
(Grundahl et al. 2002). The abundances of the samples are consistent and
exhibit the usual anticorrelations between O-Na and Mg-Al. A positive
correlation is found between 26Mg and Al, a mild anticorrelation is found
between 24Mg and Al, while no correlation is found between 25Mg and Al. None of
the elemental or isotopic abundances show a dependence on evolutionary status
and, as shown by Gratton et al. (2001), the abundance variations exist even in
main sequence stars. This strongly suggests that the star-to-star abundance
variations are a result of varying degrees of pollution with intermediate mass
AGB stars being likely polluters. Even for the least polluted stars, the
abundances of 25Mg and 26Mg relative to 24Mg are considerably higher than
predicted for ejecta from Z=0 supernovae. Zero metallicity AGB stars may be
responsible for these higher abundances. Our measured Mg isotopic ratios reveal
another layer to the globular cluster star-to-star abundance variations that
demands extensions of our present theoretical knowledge of stellar
nucleosynthesis by giant stars.Comment: Accepted for publication in A&
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
The Distribution Of Heavy Elements In Spiral And Elliptical Galaxies
This review recaps significant results as they apply to non-dwarf galaxies,
including the Milky Way, spiral disks and bulges, and elliptical and lenticular
galaxies. Conclusions that span the galaxy types treated here are as follows.
All galaxies, on average, have heavy element abundances (metallicities) that
systematically decrease outward from their galactic centers while their global
metallicities increase with galaxy mass. Abundance gradients are steepest in
normal spirals and are seen to be progressively flatter going in order from
barred spirals, lenticulars, and ellipticals. For spiral galaxies, local
metallicity appears to be correlated with total (disk plus bulge) surface
density. Observed abundance patterns indicate that N production is dominated by
primary processes at low metallicity and secondary processes at high
metallicity; C production increases with increasing metallicity; and O, Ne, S,
and Ar are produced in lockstep independent of metallicity. In elliptical
galaxies, nuclear abundances are in the range [Z/H] = 0.0 to 0.4, but the
element mixture is not scaled-solar. In large elliptical galaxies [Mg/Fe] is in
the range 0.3 to 0.5, decreasing to ~0 in smaller elliptical galaxies. Other
light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity
dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the
cause of the connection is unclear.Comment: 55-page manuscript plus 16 figures. Invited review to appear in the
Publications Of The Astronomical Society Of The Pacifi
BVRI Light Curves for 29 Type Ia Supernovae
BVRI light curves are presented for 27 Type Ia supernovae discovered during
the course of the Calan/Tololo Survey and for two other SNe Ia observed during
the same period. Estimates of the maximum light magnitudes in the B, V, and I
bands and the initial decline rate parameter m15(B) are also given.Comment: 17 pages, figures and tables are not included (contact first author
if needed), to appear in the Astronomical Journa
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
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