46 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
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
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 Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
It is now widely accepted that globular cluster red giant branch stars owe
their strange abundance patterns to a combination of pollution from progenitor
stars and in situ extra mixing. In this hybrid theory a first generation of
stars imprint abundance patterns into the gas from which a second generation
forms. The hybrid theory suggests that extra mixing is operating in both
populations and we use the variation of [C/Fe] with luminosity to examine how
efficient this mixing is. We investigate the observed red giant branches of M3,
M13, M92, M15 and NGC 5466 as a means to test a theory of thermohaline mixing.
The second parameter pair M3 and M13 are of intermediate metallicity and our
models are able to account for the evolution of carbon along the RGB in both
clusters. Although, in order to fit the most carbon-depleted main-sequence
stars in M13 we require a model whose initial [C/Fe] abundance leads to a
carbon abundance lower than is observed. Furthermore our results suggest that
stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3).
In the metal-poor regime only NGC 5466 can be tentatively explained by
thermohaline mixing operating in multiple populations. We find thermohaline
mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15.
It appears as if extra mixing is occurring before the luminosity function bump
in these clusters. To reconcile the data with the models would require first
dredge-up to be deeper than found in extant models.Comment: 13 Pages, 3 figures. Accepted for publication in the Astrophysical
Journa
Thermohaline Mixing and its Role in the Evolution of Carbon and Nitrogen Abundances in Globular Cluster Red Giants: The Test Case of Messier 3
We review the observational evidence for extra mixing in stars on the red
giant branch (RGB) and discuss why thermohaline mixing is a strong candidate
mechanism. We recall the simple phenomenological description of thermohaline
mixing, and aspects of mixing in stars in general. We use observations of M3 to
constrain the form of the thermohaline diffusion coefficient and any associated
free parameters. This is done by matching [C/Fe] and [N/Fe] along the RGB of
M3. After taking into account a presumed initial primordial bimodality of
[C/Fe] in the CN-weak and CN-strong stars our thermohaline mixing models can
explain the full spread of [C/Fe]. Thermohaline mixing can produce a
significant change in [N/Fe] as a function of absolute magnitude on the RGB for
initially CN-weak stars, but not for initially CN-strong stars, which have so
much nitrogen to begin with that any extra mixing does not significantly affect
the surface nitrogen composition.Comment: 33 Pages, 10 Figures. Accepted for publication in The Astrophysical
Journa
On the impact of the structural surface effect on global stellar properties and asteroseismic analyses
In a series of papers, we have recently demonstrated that it is possible to
construct stellar structure models that robustly mimic the stratification of
multi-dimensional radiative magneto-hydrodynamic simulations at every time-step
of the computed evolution. The resulting models offer a more realistic
depiction of the near-surface layers of stars with convective envelopes than
parameterizations, such as mixing length theory, do. In this paper, we explore
how this model improvement impacts on seismic and non-seismic properties of
stellar models across the Hertzsprung-Russell diagram. We show that the
improved description of the outer boundary layers alters the predicted global
stellar properties at different evolutionary stages. In a hare and hound
exercise, we show that this plays a key role for asteroseismic analyses, as it,
for instance, often shifts the inferred stellar age estimates by more than 10
per cent. Improper boundary conditions may thus introduce systematic errors
that exceed the required accuracy of the PLATO space mission. Moreover, we
discuss different approximations for how to compute stellar oscillation
frequencies. We demonstrate that the so-called gas approximation
performs reasonably well for all main-sequence stars. Using a Monte Carlo
approach, we show that the model frequencies of our hybrid solar models are
consistent with observations within the uncertainties of the global solar
parameters when using the so-called reduced approximation.Comment: Submitted to MNRA