142 research outputs found
Differential chemical abundance analysis of a 47 Tuc AGB star with respect to Arcturus
This study resolves a discrepancy in the abundance of Zr in the 47 Tucanae
asymptotic giant branch star Lee 2525. This star was observed using the echelle
spectrograph on the 2.3 m telescope at Siding Spring Observatory. The analysis
was undertaken by calibrating Lee 2525 with respect to the standard giant star
Arcturus. This work emphasises the importance of using a standard star with
stellar parameters comparable to the star under analysis rather than a
calibration with respect to the Sun (Koch & McWilliam 2008). Systematic errors
in the analysis process are then minimised due to the similarity in atmospheric
structure between the standard and programme stars. The abundances derived for
Lee 2525 were found to be in general agreement with the Brown & Wallerstein
(1992) values except for Zr. In this study Zr has a similar enhancement
([Zr/Fe] = +0.51 dex) to another light s-process element, Y ([Y/Fe] = +0.53
dex), which reflects current theory regarding the enrichment of s-process
elements by nuclear processes within AGB stars (Busso et al. 2001). This is
contrary to the results of Brown & Wallerstein (1992) where Zr was
under-abundant ([Zr/Fe] = +0.51 dex) and Y was over-abundant ([Y/Fe] = +0.50
dex) with respect to Fe.Comment: 11 pages, 5 figures Accepted for publication in MNRA
Beyond the Brim of the Hat: Kinematics of Globular Clusters out to Large Radius in the Sombrero Galaxy
We have obtained radial velocity measurements for 51 new globular clusters
around the Sombrero galaxy. These measurements were obtained using
spectroscopic observations from the AAOmega spectrograph on the
Anglo-Australian Telescope and the Hydra spectrograph at WIYN. Combined with
our own past measurements and velocity measurements obtained from the
literature we have constructed a large database of radial velocities that
contains a total of 360 confirmed globular clusters. Previous studies' analyses
of the kinematics and mass profile of the Sombrero globular cluster system have
been constrained to the inner ~9' (~24 kpc or ~5 effective radii), but our new
measurements have increased the radial coverage of the data, allowing us to
determine the kinematic properties of M104 out to ~15' (~41 kpc or ~9 effective
radii). We use our set of radial velocities to study the GC system kinematics
and to determine the mass profile and V-band mass-to-light profile of the
galaxy. We find that the V-band mass-to-light ratio increases from 4.5 at the
center to a value of 20.9 at 41 kpc (~9 effective radii or 15'), which implies
that the dark matter halo extends to the edge of our available data set. We
compare our mass profile at 20 kpc (~4 effective radii or ~7.4') to the mass
computed from x-ray data and find good agreement. We also use our data to look
for rotation in the globular cluster system as a whole, as well as in the red
and blue subpopulations. We find no evidence for significant rotation in any of
these samples.Comment: Accepted for publication in the Astronomical Journal; 23 pages, 14
figures, and 2 table
Sodium content as a predictor of the advanced evolution of globular cluster stars
The asymptotic giant branch (AGB) phase is the final stage of nuclear burning
for low-mass stars. Although Milky Way globular clusters are now known to
harbour (at least) two generations of stars they still provide relatively
homogeneous samples of stars that are used to constrain stellar evolution
theory. It is predicted by stellar models that the majority of cluster stars
with masses around the current turn-off mass (that is, the mass of the stars
that are currently leaving the main sequence phase) will evolve through the AGB
phase. Here we report that all of the second-generation stars in the globular
cluster NGC 6752 -- 70 per cent of the cluster population -- fail to reach the
AGB phase. Through spectroscopic abundance measurements, we found that every
AGB star in our sample has a low sodium abundance, indicating that they are
exclusively first-generation stars. This implies that many clusters cannot
reliably be used for star counts to test stellar evolution timescales if the
AGB population is included. We have no clear explanation for this observation.Comment: Published in Nature (online 29 May 2013, hard copy 13 June), 12
pages, 3 figures + supplementary information sectio
The Asymptotic Giant Branches of GCs: Selective Entry Only
The handful of available observations of AGB stars in Galactic Globular
Clusters suggest that the GC AGB populations are dominated by cyanogen-weak
stars. This contrasts strongly with the distributions in the RGB (and other)
populations, which generally show a 50:50 bimodality in CN band strength. If it
is true that the AGB populations show very different distributions then it
presents a serious problem for low mass stellar evolution theory, since such a
surface abundance change going from the RGB to AGB is not predicted by stellar
models. However this is only a tentative conclusion, since it is based on very
small AGB sample sizes. To test whether this problem really exists we have
carried out an observational campaign specifically targeting AGB stars in GCs.
We have obtained medium resolution spectra for about 250 AGB stars across 9
Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We
present some of the preliminary findings of the study for the second parameter
trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there
is a deficiency of stars with strong CN bands on the AGB. To confirm that this
phenomenon is robust and not just confined to CN band strengths and their
vagaries, we have made observations using FLAMES/VLT to measure elemental
abundances for NGC 6752.We present some initial results from this study also.
Our sodium abundance results show conclusively that only a subset of stars in
GCs experience the AGB phase of evolution. This is the first direct, concrete
confirmation of the phenomenon.Comment: 4 pages, to appear in conference proceedings of "Reading the book of
globular clusters with the lens of stellar evolution", Rome, 26-28 November
201
s- and r-process element abundances in the CMD of 47 Tucanae using the Robert Stobie Spectrograph on SALT
A recent study by Wylie et al 2006 has revealed that s-process element
abundances are enhanced relative to iron in both red giant branch and
asymptotic giant branch stars of 47 Tucanae. A more detailed investigation into
s-process element abundances throughout the colour-magnitude diagram of 47
Tucanae is vital in order to determine whether the observed enhancements are
intrinsic to the cluster. This paper explores this possibility through
observational and theoretical means. The visibility of s- and r-process element
lines in synthetic spectra of giant and dwarf stars throughout the colour
magnitude diagram of 47 Tucanae has been explored. It was determined that a
resolving power of 10 000 was sufficient to observe s-process element abundance
variations in globular cluster giant branch stars. These synthetic results were
compared with the spectra of eleven 47 Tucanae giant branch stars observed
during the performance verification of the Robert Stobie Spectrograph on the
Southern African Large Telescope. Three s-process elements, Zr, Ba, Nd, and one
r-process element, Eu, were investigated. No abundance variations were found
such that [X/Fe] = 0.0 +/- 0.5 dex. It was concluded that this resolving power,
R ~ 5000, was not sufficient to obtain exact abundances but upper limits on the
s-process element abundances could be determined.Comment: 7 pages, 11 figure
M 54 + Sagittarius = omega Centauri
We derive homogeneous abundances of Fe, O, Na and alpha-elements from high
resolution FLAMES spectra for 76 red giant stars in NGC 6715 (M 54) and for 25
red giants in the surrounding nucleus of the Sagittarius (Sgr) dwarf galaxy.
Our main findings are that: (i) we confirm that M 54 shows intrinsic
metallicity dispersion, ~0.19 dex r.m.s.; (ii) when the stars of the Sgr
nucleus are included, the metallicity distribution strongly resembles that in
omega Cen; the relative contribution of the most metal-rich stars is however
different in these two objects; (iii) in both GCs there is a very extended Na-O
anticorrelation, signature of different stellar generations born within the
cluster, and (iv) the metal-poor and metal-rich components in M 54 (and omega
Cen) show clearly distinct extension of the Na-O anticorrelation, the most
heavily polluted stars being those of the metal-rich component. We propose a
tentative scenario for cluster formation that could explain these features.
Finally, similarities and differences found in the two most massive GCs in our
Galaxy can be easily explained if they are similar objects (nuclear clusters in
dwarf galaxies) observed at different stages of their dynamical evolution.Comment: Uses emulateapj, 8 pages, 3 figures, accepted for publication on ApJ
Letter
The origin of the split red clump in the Galactic bulge of the Milky Way
Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the
red giant clump stars are split into two components along the line of sight. We
investigate this split using the three fields from the ARGOS survey that lie on
the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is
evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not
in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split.
We compare the spatial distribution and kinematics of the clump stars with
predictions from an evolutionary N-body model of a bulge that grew from a disk
via bar-related instabilities. The density distribution of the peanut-shaped
model is depressed near its minor axis. This produces a bimodal distribution of
stars along the line of sight through the bulge near its minor axis, very much
as seen in our observations. The observed and modelled kinematics of the two
groups of stars are also similar. We conclude that the split red clump of the
bulge is probably a generic feature of boxy/peanut bulges that grew from disks,
and that the disk from which the bulge grew had relatively few stars with
[Fe/H] < -0.5Comment: 12 pages, 9 figures, accepted for publication in Ap
Argos - III. Stellar populations in the galactic bulge of the milky way
We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Λ cold dark matter theory, or from disc instabilities,
Cyanogen in NGC 1851 red giant branch and asymptotic giant branch stars: Quadrimodal distributions
The Galactic globular cluster NGC 1851 has raised much interest since Hubble Space Telescope photometry revealed that it hosts a double subgiant branch. Here we report on our homogeneous study into the cyanogen (CN) band strengths in the red giant branc
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