79 research outputs found
Mitigating the mass dependence in the scaling relation of red-giant stars
The masses and radii of solar-like oscillators can be estimated through the
asteroseismic scaling relations. These relations provide a direct link between
observables, i.e. effective temperature and characteristics of the oscillation
spectra, and stellar properties, i.e. mean density and surface gravity (thus
mass and radius). These scaling relations are commonly used to characterize
large samples of stars. Usually, the Sun is used as a reference from which the
structure is scaled. However, for stars that do not have a similar structure as
the Sun, using the Sun as a reference introduces systematic errors as large as
10\% in mass and 5\% in radius. Several alternatives for the reference of the
scaling relation involving the large frequency separation (typical frequency
difference between modes of the same degree and consecutive radial order) have
been suggested in the literature. In a previous paper, we presented a reference
function with a dependence on both effective temperature and metallicity. The
accuracy of predicted masses and radii improved considerably when using
reference values calculated from our reference function. However, the residuals
indicated that stars on the red-giant branch possess a mass dependence that was
not accounted for. Here, we present a reference function for the scaling
relation involving the large frequency separation that includes the mass
dependence. This new reference function improves the derived masses and radii
significantly by removing the systematic differences and mitigates the trend
with (frequency of maximum oscillation power) that exists when
using the solar value as a reference.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Diagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters
We conduct a series of comparisons between spectroscopic and photometric
observations of globular clusters and stellar models to examine their
predictive power. Data from medium-to-high resolution spectroscopic surveys of
lithium allow us to investigate first dredge-up and extra mixing in two
clusters well separated in metallicity. Abundances at first dredge-up are
satisfactorily reproduced but there is preliminary evidence to suggest that the
models overestimate the luminosity at which the surface composition first
changes in the lowest-metallicity system. Our models also begin extra mixing at
luminosities that are too high, demonstrating a significant discrepancy with
observations at low metallicity. We model the abundance changes during extra
mixing as a thermohaline process and determine that the usual diffusive form of
this mechanism cannot simultaneously reproduce both the carbon and lithium
observations. Hubble Space Telescope photometry provides turnoff and bump
magnitudes in a large number of globular clusters and offers the opportunity to
better test stellar modelling as function of metallicity. We directly compare
the predicted main-sequence turn-off and bump magnitudes as well as the
distance-independent parameter . We
require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some
clusters and cannot match the bump in low-metallicity systems. Changes to the
distance modulus, metallicity scale and bolometric corrections may impact on
the direct comparisons but , which is
also underestimated from the models, can only be improved through changes to
the input physics. Overshooting at the base of the convective envelope with an
efficiency that is metallicity dependent is required to reproduce the
empirically determined value of .Comment: 20 pages, 11 Figures, 4 Tables, Accepted for publication in MNRA
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
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
Fluorine variations in the globular cluster NGC 6656 (M22): implications for internal enrichment timescales
Observed chemical (anti)correlations in proton-capture elements among
globular cluster stars are presently recognized as the signature of
self-enrichment from now extinct, previous generations of stars. This defines
the multiple population scenario. Since fluorine is also affected by proton
captures, determining its abundance in globular clusters provides new and
complementary clues regarding the nature of these previous generations, and
supplies strong observational constraints to the chemical enrichment
timescales. In this paper we present our results on near-infrared CRIRES
spectroscopic observations of six cool giant stars in NGC 6656 (M22): the main
objective is to derive the F content and its internal variation in this
peculiar cluster, which exhibits significant changes in both light and heavy
element abundances. We detected F variations across our sample beyond the
measurement uncertainties and found that the F abundances are positively
correlated with O and anticorrelated with Na, as expected according to the
multiple population framework. Furthermore, our observations reveal an increase
in the F content between the two different sub-groups, s-process rich and
s-process poor, hosted within M22. The comparison with theoretical models
suggests that asymptotic giant stars with masses between 4 and 5 Msun are
responsible for the observed chemical pattern, confirming evidence from
previous works: the difference in age between the two sub-components in M22
must be not larger than a few hundreds Myr.Comment: Accepted for publication in The Astrophysical Journal, 10 pages, 10
figure
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
On the Numerical Treatment and Dependence of Thermohaline Mixing in Red Giants
In recent years much interest has been shown in the process of thermohaline
mixing in red giants. In low and intermediate mass stars this mechanism first
activates at the position of the bump in the luminosity function, and has been
identified as a likely candidate for driving the slow mixing inferred to occur
in these stars. One particularly important consequence of this process, which
is driven by a molecular weight inversion, is the destruction of lithium. We
show that the degree of lithium destruction, or in some cases production, is
extremely sensitive to the numerical details of the stellar models. Within the
standard 1D diffusion approximation to thermohaline mixing, we find that
dfferent evolution codes, with their default numerical schemes, can produce
lithium abundances that differ from one another by many orders of magnitude.
This disagreement is worse for faster mixing. We perform experiments with four
independent stellar evolution codes, and derive conditions for the spatial and
temporal resolution required for a converged numerical solution. The results
are extremely sensitive to the timesteps used. We find that predicted lithium
abundances published in the literature until now should be treated with
caution.Comment: 19 pages, 18 figures, accepted for publication by MNRAS; revised
version has corrected Fig 1
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