787 research outputs found
A Small Step on the Long Road to Understanding the R-Stars: CNO Cycling in Candidate R-Star Progenitors
Recent work has proposed that a merger event between a red-giant and a He
white dwarf may be responsible for the production of R-stars (Izzard et al,
2007). We investigate the proposed evolution and nucleosynthesis of such a
model. We simulate the hypothesized late ignition of the core flash by
increasing the neutrino losses until the ignition occurs sufficiently far from
the centre that the subsequent evolution produces dredge-up of carbon to the
extent that the post-flash object is a carbon star. Detailed nucleosynthesis is
performed within this approximation, and we show that the overall properties
are broadly consistent with the observations. Details will depend on the
dynamics of the merger event.Comment: 8 pages, 6 figures, to be published in PAS
The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
We investigate the suggestion that there are stellar populations in some
globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the
primordial value. We assume that a previous generation of massive Asymptotic
Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB
yields are used to follow the evolution of helium and CNO using a Salpeter
initial mass function (IMF) and two top-heavy IMFs. In no case are we able to
produce the postulated large Y ~ 0.35 without violating the observational
constraint that the CNO content is nearly constant.Comment: accepted for publication in Ap
Lithium abundances in globular cluster giants: NGC 1904, NGC 2808, and NGC 362
The presence of multiple populations in globular clusters has been well
established thanks to high-resolution spectroscopy. It is widely accepted that
distinct populations are a consequence of different stellar generations:
intra-cluster pollution episodes are required to produce the peculiar chemistry
observed in almost all clusters. Unfortunately, the progenitors responsible
have left an ambiguous signature and their nature remains unresolved. To
constrain the candidate polluters, we have measured lithium and aluminium
abundances in more than 180 giants across three systems: NGC~1904, NGC~2808,
and NGC~362. The present investigation along with our previous analysis of M12
and M5 affords us the largest database of simultaneous determinations of Li and
Al abundances. Our results indicate that Li production has occurred in each of
the three clusters. In NGC~362 we detected an M12-like behaviour, with first
and second-generation stars sharing very similar Li abundances favouring a
progenitor that is able to produce Li, such as AGB stars. Multiple progenitor
types are possible in NGC~1904 and NGC~2808, as they possess both an
intermediate population comparable in lithium to the first generation stars and
also an extreme population, that is enriched in Al but depleted in Li. A simple
dilution model fails in reproducing this complex pattern. Finally, the internal
Li variation seems to suggest that the production efficiency of this element is
a function of the cluster's mass and metallicity - low-mass or relatively
metal-rich clusters are more adept at producing Li.Comment: Accepted for publication in MNRAS. 10 pages, 8 figure
Component release after exposure of Staphylococcus aureus cells to pulsed electric fields
The objective of this work was to get further insights on the mechanism of inactivation of bacterial cells by pulsed electric fields (PEF) through the study of the release of intracellular components after exposing Staphylococcus aureus cells in McIvlaine buffer (pH 7.0, 2 mS/cm) to PEF treatments of different intensity (18 and 25 kV/cm) and treatment times (from 20 to 400 mu s). Release of most compounds, except proteins, was almost immediate after the treatment, but the relative amount released depended on the molecule studied. A good correlation between the release of the smallest components studied (particularly ions) and membrane permeabilization (as measured by NaCl sensitization and PI entry) was observed. On the other hand, results obtained suggested that S. aureus inactivation by PEF would be related to the exit of cytoplasmic proteins of a molecular weight higher than 6 kDa. Results obtained in this work indicated that increasing PEF treatment time would reduce the capability of S. aureus cells to repair the electropores formed and suggested that this might be due to the formation of pores of a larger size, which S. aureus cells would be unable to reseal in a situation of homeostasis loss. Industrial relevance: Results reported here can help to design more effective treatments for microbial inactivation using PEF on food, and therefore facilitate its industrial implementation
Lithium abundances in globular cluster giants: NGC 6218 (M12) and NGC 5904 (M5)
Convergent lines of evidence suggest that globular clusters host multiple
stellar populations. It appears that they experience at least two episodes of
star formation whereby a fraction of first-generation stars contribute astrated
ejecta to form the second generation(s). To identify the polluting progenitors
we require distinguishing chemical signatures such as that provided by lithium.
Theoretical models predict that lithium can be synthesised in AGB stars,
whereas no net Li production is expected from other candidates. It has been
shown that in order to reproduce the abundance pattern found in M4, Li
production must occur within the polluters, favouring the AGB scenario. Here we
present Li and Al abundances for a large sample of RGB stars in M12 and M5.
These clusters have a very similar metallicity, whilst demonstrating
differences in several cluster properties. Our results indicate that the
first-generation and second-generation stars share the same Li content in M12;
we recover an abundance pattern similar to that observed in M4. In M5 we find a
higher degree of complexity and a simple dilution model fails in reproducing
the majority of the stellar population. In both clusters we require Li
production across the different stellar generations, but production seems to
have occurred to different extents. We suggest that such a difference might be
related to the cluster mass with the Li production being more efficient in
less-massive clusters. This is the first time a statistically significant
correlation between the Li spread within a GC and its luminosity has been
demonstrated. Finally, although Li-producing polluters are required to account
for the observed pattern, other mechanisms, such as MS depletion, might have
played a role in contributing to the Li internal variation, though at
relatively low level.Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 14
figure
An extreme paucity of second population AGB stars in the normal globular cluster M4
Galactic Globular clusters (GCs) are now known to harbour multiple stellar
populations, which are chemically distinct in many light element abundances. It
is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs
show different abundance distributions in light elements compared to those in
the red giant branch (RGB) and other phases, skewing toward more primordial,
field-star-like abundances, which we refer to as subpopulation one (SP1). As
part of a larger program targeting giants in GCs, we obtained high-resolution
spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using
the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we
report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which
contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB
abundance distribution is consistent with all AGB stars being from SP1. This
result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor)
avoid the AGB phase. This is an unexpected result given M4's horizontal branch
morphology -- it does not have an extended blue horizontal branch. This is the
first abundance study to be performed utilising the HERMES spectrograph.Comment: 5 pages, 2 figures, 4 tables (full Table 1 online). Accepted for
publication in MNRAS Letter
On the serendipitous discovery of a Li-rich giant in the globular cluster NGC 362
We have serendipitously identified the first lithium-rich giant star located
close to the red giant branch bump in a globular cluster. Through
intermediate-resolution FLAMES spectra we derived a lithium abundance of
A(Li)=2.55 (assuming local thermodynamical equilibrium), which is extremely
high considering the star's evolutionary stage. Kinematic and photometric
analysis confirm the object as a member of the globular cluster NGC 362. This
is the fourth Li-rich giant discovered in a globular cluster but the only one
known to exist at a luminosity close to the bump magnitude. The three previous
detections are clearly more evolved, located close to, or beyond the tip of
their red giant branch. Our observations are able to discard the accretion of
planets/brown dwarfs, as well as an enhanced mass-loss mechanism as a formation
channel for this rare object. Whilst the star sits just above the cluster bump
luminosity, its temperature places it towards the blue side of the giant branch
in the colour-magnitude diagram. We require further dedicated observations to
unambiguously identify the star as a red giant: we are currently unable to
confirm whether Li production has occurred at the bump of the luminosity
function or if the star is on the pre zero-age horizontal branch. The latter
scenario provides the opportunity for the star to have synthesised Li rapidly
during the core helium flash or gradually during its red giant branch ascent
via some extra mixing process.Comment: Accepted for publication in The Astrophysical Journal Letter
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