136 research outputs found
Updated properties of the old open cluster Melotte 66: Searching for multiple stellar populations
Multiple generations of stars are routinely encountered in globular clusters
but no convincing evidence has been found in Galactic open clusters to date. In
this paper we use new photometric and spectroscopic data to search for multiple
stellar population signatures in the old, massive open cluster, Melotte~66. The
cluster is known to have a red giant branch wide in color, which could be an
indication of metallicity spread. Also the main sequence is wider than what is
expected from photometric errors only. This evidence might be associated with
either differential reddening or binaries. Both hypothesis have, however, to be
evaluated in detail before recurring to the presence of multiple stellar
populations. New, high-quality, CCD UBVI photometry have been acquired to this
aim with high-resolution spectroscopy of seven clump stars, that are
complemented with literature data. Our photometric study confirms that the
width of the main sequence close to the turn off point is entirely accounted
for by binary stars and differential reddening, with no need to advocate more
sofisticated scenarios, such as metallicity spread or multiple main sequences.
By constructing synthetic color-magnitude diagrams, we infer that the binary
fraction has to be as large as 30 and their mass ratio in the range 0.6-1.0.
As a by-product of our simulations, we provide new estimates of the cluster
fundamental parameters. We measure a reddening E(B-V)=0.150.02, and
confirm the presence of a marginal differential reddening. The distance to the
cluster is kpc and the age is 3.40.3 Gyr, which is
somewhat younger and better constrained than previous estimates. Our detailed
abundance analysis reveals that, overall, Melotte~66 looks like a typical
object of the old thin disk population.Comment: 14 pages, 18 eps figure, in press in Astronomy and Astrophysics.
Abstract shortened to fit arXiv constraint
ESO452-SC11: The lowest mass globular cluster with a potential chemical inhomogeneity
We present the largest spectroscopic investigation of one of the faintest and
least studied stellar clusters of the Milky Way, ESO452-SC11. Using the
Anglo-Australian Telescope AAOmega and Keck HIRES spectrographs we have
identified 11 members of the cluster and found indications of star-to-star
light element abundance variation, primarily using the blue cyanogen (CN)
absorption features. From a stellar density profile, we estimate a total
cluster mass of solar masses. This would make
ESO452-SC11 the lowest mass cluster with evidence for multiple populations.
These data were also used to measure the radial velocity of the cluster
( km s) and confirm that ESO452-SC11 is relatively
metal-rich for a globular cluster ([Fe/H]). All known massive
clusters studied in detail show multiple populations of stars each with a
different chemical composition, but many low-mass globular clusters appear to
be chemically homogeneous. ESO452-SC11 sets a lower mass limit for the multiple
stellar population phenomenon.Comment: 13 pages, 11 figures. Accepted for publication in MNRA
First Asteroseismic Analysis of the Globular Cluster M80: Multiple Populations and Stellar Mass Loss
Asteroseismology provides a new avenue for accurately measuring the masses of
evolved globular cluster (GC) stars through the detection of their solar-like
oscillations. We present the first detections of solar-like oscillations in 47
red giant branch (RGB) and early asymptotic giant branch (EAGB) stars in the
metal-poor GC M80; only the second ever with measured seismic masses. We
investigate two major areas of stellar evolution and GC science; the multiple
populations and stellar mass-loss. We detected a distinct bimodality in the
EAGB mass distribution. We showed that this is likely due to sub-population
membership. If confirmed, it would be the first direct measurement of a mass
difference between sub-populations. A mass difference was not detected between
the sub-populations in our RGB sample. We instead measured an average RGB mass
of 0.782\pm0.009~\msun, which we interpret as the average between the
sub-populations. Differing mass-loss rates on the RGB has been proposed as the
second parameter that could explain the horizontal branch (HB) morphology
variations between GCs. We calculated an integrated RGB mass-loss separately
for each sub-population: 0.12\pm0.02~\msun (SP1) and 0.25\pm0.02~\msun
(SP2). Thus, SP2 stars have greatly enhanced mass-loss on the RGB. Mass-loss is
thought to scale with metallicity, which we confirm by comparing our results to
a higher metallicity GC, M4. We also find that M80 stars have insignificant
mass-loss on the HB. This is different to M4, suggesting that there is a
metallicity and temperature dependence in the HB mass-loss. Finally, our study
shows the robustness of the -independent mass scaling relation in
the low-metallicity (and low-surface gravity) regime.Comment: 20 pages, 11 figure
The Parent Populations of 6 groups identified from Chemical Tagging in the Solar neighborhood
We estimate the size and distribution of the parent populations for the 6
largest (at least 20 stars in the Solar neighborhood) chemical groups
identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars
in the abundance groups tend to lie near a boundary in angular momentum versus
eccentricity space where the probability is highest for a star to be found in
the Solar neighborhood and where orbits have apocenter approximately equal to
the Sun's galactocentric radius. Assuming that the parent populations are
uniformly distributed at all azimuthal angles in the Galaxy, we estimate that
the parent populations of these abundance groups contain at least 200,000
members. The spread in angular momentum of the groups implies that the
assumption of a uniform azimuthal distribution only fails for the two youngest
groups and only for the highest angular momentum stars in them. The parent
populations of three thin disk groups have narrow angular momentum
distributions, but tails in the eccentricity and angular momentum distributions
suggest that only a small fraction of stars have migrated and increased in
eccentricity. In contrast, the parent populations of the thick disk groups
exhibit both wide angular momentum and eccentricity distributions implying that
both heating and radial migration has taken place.Comment: accepted for publication in MNRA
Principal Component Analysis on Chemical Abundances Spaces
In preparation for the HERMES chemical tagging survey of about a million
Galactic FGK stars, we estimate the number of independent dimensions of the
space defined by the stellar chemical element abundances [X/Fe]. [...] We
explore abundances in several environments, including solar neighbourhood
thin/thick disk stars, halo metal-poor stars, globular clusters, open clusters,
the Large Magellanic Cloud and the Fornax dwarf spheroidal galaxy. [...] We
find that, especially at low metallicity, the production of r-process elements
is likely to be associated with the production of alpha-elements. This may
support the core-collapse supernovae as the r-process site. We also verify the
over-abundances of light s-process elements at low metallicity, and find that
the relative contribution decreases at higher metallicity, which suggests that
this lighter elements primary process may be associated with massive stars.
[...] Our analysis reveals two types of core-collapse supernovae: one produces
mainly alpha-elements, the other produces both alpha-elements and Fe-peak
elements with a large enhancement of heavy Fe-peak elements which may be the
contribution from hypernovae. [...] The extra contribution from low mass AGB
stars at high metallicity compensates the dimension loss due to the
homogenization of the core-collapse supernovae ejecta. [...] the number of
independent dimensions of the [X/Fe]+[Fe/H] chemical space in the solar
neighbourhood for HERMES is about 8 to 9. Comparing fainter galaxies and the
solar neighbourhood, we find that the chemical space for fainter galaxies such
as Fornax and the Large Magellanic Cloud has a higher dimensionality. This is
consistent with the slower star formation history of fainter galaxies. [...]Comment: 28 pages, 25 figures, 3 tables, MNRAS (Accepted for publication- 2011
December 14
AGB subpopulations in the nearby globular cluster NGC6397
It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used.Based in part on data acquired through the AAO, via programme
15A/21 (PI Campbell). Part of this work was supported by the
DAAD (PPP project 57219117) with funds from the German
Federal Ministry of Education and Research (BMBF). BTM
acknowledges the financial support of his Australian Postgraduate
Award scholarship. SWC acknowledges federal funding from
the Australian Research Council though the Future Fellowship
grant entitled ‘Where are the Convective Boundaries in Stars?’
(FT160100046). VD acknowledges support from the AAO distinguished visitor programme 2016. LC gratefully acknowledges support from the Australian Research Council (grants
DP150100250, FT160100402)
High-resolution elemental abundance analysis of the open cluster IC 4756
We present detailed elemental abundances of 12 subgiants in the open cluster
IC 4756 including Na, Al, Mg, Si, Ca, Ti, Cr, Ni, Fe, Zn and Ba. We measure the
cluster to have [Fe/H] = -0.01 +/- 0.10. Most of the measured star-to-star
[X/H] abundance variation is below sigma < 0.03, as expected from a coeval
stellar population preserving natal abundance patterns, supporting the use of
elemental abundances as a probe to reconstruct dispersed clusters. We find
discrepancies between Cr I and Cr II abundances as well as between Ti I and Ti
II abundances, where the ionized abundances are larger by about 0.2 dex. This
follows other such studies which demonstrate the effects of overionization in
cool stars. IC 4756 are supersolar in Mg, Si, Na and Al, but are solar in the
other elements. The fact that IC 4756 is supersolar in some alpha-elements (Mg,
Si) but solar in the others (Ca, Ti) suggests that the production of
alpha-elements is not simply one dimensional and could be exploited for
chemical tagging.Comment: 13 pages, 13 figures, 10 tables, MNRAS (Accepted for publication-
2012 August 25
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