2,454 research outputs found
Oxygen and Sodium Abundances in M13 (NGC 6205) Giants: Linking Globular Cluster Formation Scenarios, Deep Mixing, and Post-RGB Evolution
We present O, Na, and Fe abundances, as well as radial velocities, for 113
red giant branch (RGB) and asymptotic giant branch (AGB) stars in the globular
cluster M13. The abundances and velocities are based on spectra obtained with
the WIYN-Hydra spectrograph, and the observations range in luminosity from the
horizontal branch (HB) to RGB-tip. The results are examined in the context of
recent globular cluster formation scenarios. We find that M13 exhibits many key
characteristics that suggest its formation and chemical enrichment are
well-described by current models. Some of these observations include: the
central concentration of O-poor stars, the notable decrease in [O/Fe] (but
small increase in [Na/Fe]) with increasing luminosity that affects primarily
the "extreme" population, the small fraction of stars with halo-like
composition, and the paucity of O-poor AGB stars. In agreement with recent
work, we conclude that the most O-poor M13 giants are likely He-enriched and
that most (all?) O-poor RGB stars evolve to become extreme HB and AGB-manqu\'e
stars. In contrast, the "primordial" and "intermediate" population stars appear
to experience standard HB and AGB evolution.Comment: Accepted for publication in The Astrophysical Journal Letters. 18
pages; 3 figures; 1 tabl
Insights from the Outskirts: Chemical and Dynamical Properties in the outer Parts of the Fornax Dwarf Spheroidal Galaxy
We present radial velocities and [Fe/H] abundances for 340 stars in the
Fornax dwarf spheroidal from R~16,000 spectra. The targets have been obtained
in the outer parts of the galaxy, a region which has been poorly studied
before. Our sample shows a wide range in [Fe/H], between -0.5 and -3.0 dex, in
which we detect three subgroups. Removal of stars belonging to the most
metal-rich population produces a truncated metallicity distribution function
that is identical to Sculptor, indicating that these systems have shared a
similar early evolution, only that Fornax experienced a late, intense period of
star formation (SF). The derived age-metallicity relation shows a fast increase
in [Fe/H] at early ages, after which the enrichment flattens significantly for
stars younger than ~8 Gyr. Additionally, the data indicate a strong population
of stars around 4 Gyr, followed by a second rapid enrichment in [Fe/H]. A
leaky-box chemical enrichment model generally matches the observed relation but
does not predict a significant population of young stars nor the strong
enrichment at late times. The young population in Fornax may therefore
originate from an externally triggered SF event. Our dynamical analysis reveals
an increasing velocity dispersion with decreasing [Fe/H] from sigma_sys 7.5
km/s to >14 km/s, indicating an outside-in star formation history in a dark
matter dominated halo. The large velocity dispersion at low metallicities is
possibly the result of a non-Gaussian velocity distribution amongst stars older
than ~8 Gyr. Our sample also includes members from the Fornax GCs H2 and H5. In
agreement with past studies we find [Fe/H]=-2.04+-0.04 and a mean radial
velocity RV=59.36+-0.31 km/s for H2 and [Fe/H]=-2.02+-0.11 and RV=59.39+-0.44
km/s for H5. Overall, we find large complexity in the chemical and dynamical
properties, with signatures that additionally vary with galactocentric
distance.Comment: 21 pages, 18 figures, 4 tables, accepted for publication in A&
A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
Recent observations have shown that a growing number of the most massive
Galactic globular clusters contain multiple populations of stars with different
[Fe/H] and neutron-capture element abundances. NGC 6273 has only recently been
recognized as a member of this "iron-complex" cluster class, and we provide
here a chemical and kinematic analysis of > 300 red giant branch (RGB) and
asymptotic giant branch (AGB) member stars using high resolution spectra
obtained with the Magellan-M2FS and VLT-FLAMES instruments. Multiple lines of
evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that
ranges from about [Fe/H] = -2 to -1 dex, and may include at least three
populations with different [Fe/H] values. The three populations identified here
contain separate first (Na/Al-poor) and second (Na/Al-rich) generation stars,
but a Mg-Al anti-correlation may only be present in stars with [Fe/H] > -1.65.
The strong correlation between [La/Eu] and [Fe/H] suggests that the s-process
must have dominated the heavy element enrichment at higher metallicities. A
small group of stars with low [alpha/Fe] is identified and may have been
accreted from a former surrounding field star population. The cluster's large
abundance variations are coupled with a complex, extended, and multimodal blue
horizontal branch (HB). The HB morphology and chemical abundances suggest that
NGC 6273 may have an origin that is similar to omega Cen and M 54.Comment: Accepted for Publication in The Astrophysical Journal; 50 pages; 18
figures; 8 tables; higher resolution figures are available upon request or in
the published journal articl
Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569
Photometric and spectroscopic analyses have shown that the Galactic bulge
cluster Terzan 5 hosts several populations with different metallicities and
ages that manifest as a double red horizontal branch (HB). A recent
investigation of the massive bulge cluster NGC 6569 revealed a similar, though
less extended, HB luminosity split, but little is known about the cluster's
detailed chemical composition. Therefore, we have used high-resolution spectra
from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical
compositions and radial velocity distributions of red giant branch and HB stars
in NGC 6569. We found the cluster to have a mean heliocentric radial velocity
of -48.8 km/s (sigma = 5.3 km/s; 148 stars) and a mean [Fe/H] =-0.87 dex (19
stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant
metallicity spread. NGC 6569 exhibits light- and heavy-element distributions
that are common among old bulge/inner Galaxy globular clusters, including clear
(anti)correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data
suggest that NGC 6569 may be composed of at least two distinct populations, and
the cluster's low mean [La/Eu] = -0.11 dex indicates significant pollution with
r-process material. We confirm that both HBs contain cluster members, but
metallicity and light-element variations are largely ruled out as sources for
the luminosity difference. However, He mass fraction differences as small as
delta Y ~ 0.02 cannot be ruled out and may be sufficient to reproduce the
double HB.Comment: 72 pages, 14 figures, 8 tables; published in The Astronomical
Journal; electronic versions of all tables are available in the published
versio
Detailed Abundances for a Large Sample of Giant Stars in the Globular Cluster 47 Tucanae (NGC 104)
47 Tuc is an ideal target to study chemical evolution and GC formation in
massive more metal-rich GCs since is the closest, massive GC. We present
chemical abundances for O, Na, Al, Si, Ca, Ti, Fe, Ni, La, and Eu in 164 red
giant branch (RGB) stars in the massive globular cluster 47 Tuc using spectra
obtained with both the Hydra multi-fiber spectrograph at the Blanco 4-m
telescope and the FLAMES multi-object spectrograph at the Very Large Telescope.
We find an average [Fe/H]=--0.790.09 dex, consistent with literature
values, as well as over-abundances of alpha-elements
([\alpha/\mbox{Fe}]\sim0.3 dex). The n-capture process elements indicate that
47 Tuc is r-process dominated ([Eu/La]=+0.24), and the light elements O, Na,
and Al exhibit star-to-star variations. The Na-O anti-correlation, a signature
typically seen in Galactic globular clusters, is present in 47 Tuc, and extends
to include a small number of stars with [O/Fe] \,--0.5. Additionally, the
[O/Na] ratios of our sample reveal that the cluster stars can be separated into
three distinct populations. A KS-test demonstrates that the O-poor/Na-rich
stars are more centrally concentrated than the O-rich/Na-poor stars. The
observed number and radial distribution of 47 Tuc's stellar populations, as
distinguished by their light element composition, agrees closely with the
results obtained from photometric data. We do not find evidence supporting a
strong Na-Al correlation in 47 Tuc, which is consistent with current models of
AGB nucleosynthesis yields.Comment: Accepted for publication in The Astrophysical Journa
NGC 7789: An Open Cluster Case Study
We have obtained high-resolution spectra of 32 giants in the open cluster NGC
7789 using the Wisconsin-Indiana-Yale-NOAO Hydra spectrograph. We explore
differences in atmospheric parameters and elemental abundances caused by the
use of the linelist developed for the Gaia-ESO Survey (GES) compared to one
based on Arcturus used in our previous work. [Fe/H] values decrease when using
the GES linelist instead of the Arcturus-based linelist; these differences are
probably driven by systematically lower (~ -0.1 dex) GES surface gravities.
Using the GES linelist we determine abundances for 10 elements - Fe, Mg, Si,
Ca, Ti, Na, Ni, Zr, Ba, and La. We find the cluster's average metallicity
[Fe/H] = 0.03 +/- 0.07 dex, in good agreement with literature values, and a
lower [Mg/Fe] abundance than has been reported before for this cluster (0.11
+/- 0.05 dex). We also find the neutron-capture element barium to be highly
enhanced - [Ba/Fe] = +0.48 +/- 0.08 - and disparate from cluster measurements
of neutron-capture elements La and Zr (-0.08 +/- 0.05 and 0.08 +/- 0.08,
respectively). This is in accordance with recent discoveries of supersolar Ba
enhancement in young clusters along with more modest enhancement of other
neutron-capture elements formed in similar environments.Comment: 15 pages, 9 figures, Table 1 typo fixe
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