156 research outputs found
Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy
Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from
HST/ACS data, reveal a well-delineated secondary clump located below the
classical compact red clump typical of intermediate-age populations. We
demonstrate that this feature belongs to the cluster itself, rather than to the
underlying SMC field. Then, we use synthetic CMDs to show that it corresponds
very well to the secondary clump predicted to appear as a result of He-ignition
in stars just massive enough to avoid electron-degeneracy settling in their
H-exhausted cores. The main red clump instead is made of the slightly less
massive stars which passed through electron-degeneracy and ignited He at the
tip of the RGB. In other words, NGC419 is the rare snapshot of a cluster while
undergoing the fast transition from classical to degenerate H-exhausted cores.
At this particular moment of a cluster's life, the colour distance between the
main sequence turn-off and the red clump(s) depends sensitively on the amount
of convective core overshooting, Lambda_c. By coupling measurements of this
colour separation with fits to the red clump morphology, we are able to
estimate simultaneously the cluster mean age (1.35(-0.04,+0.11) Gyr) and
overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like
NGC419 may constitute important marks in the age scale of intermediate-age
populations. After eye inspection of other CMDs derived from HST/ACS data, we
suggest that the same secondary clump may also be present in the LMC clusters
NGC1751, 1783, 1806, 1846, 1852, and 1917.Comment: To appear in MNRAS Letters (www.blackwell-synergy.com). Better
printed in colou
Can rotation explain the multiple main sequence turn-offs of Magellanic Cloud star clusters?
Many intermediate age star clusters in the Magellanic Clouds present multiple
main sequence turn-offs (MMSTO), which challenge the classical idea that star
formation in such objects took place over short timescales. It has been
recently suggested that the presence of fast rotators among main sequence stars
could be the cause of such features (Bastian & de Mink 2009), hence relaxing
the need for extended periods of star formation. In this letter, we compute
evolutionary tracks and isochrones of models with and without rotation. We find
that, for the same age and input physics, both kinds of models present
turn-offs with an almost identical position in the colour-magnitude diagrams.
As a consequence, a dispersion of rotational velocities in coeval ensembles of
stars could not explain the presence of MMSTOs. We construct several synthetic
colour-magnitude diagrams for the different kinds of tracks and combinations of
them. The models that best reproduce the morphology of observed MMSTOs are
clearly those assuming a significant spread in the stellar ages - as long as
~400 Myr - added to a moderate amount of convective core overshooting. Only
these models produce the detailed "golf club" shape of observed MMSTOs. A
spread in rotational velocities alone cannot do anything similar. We also
discuss models involving a mixture of stars with and without overshooting, as
an additional scenario to producing MMSTOs with coeval populations. We find
that they produce turn-offs with a varying extension in the CMD direction
perpendicular to the lower main sequence, which are clearly not present in
observed MMSTOs.Comment: To appear in MNRAS Letters. Figs. 2 and 3 are in colou
A new interpretation of the period-luminosity sequences of long-period variables
Period-luminosity (PL) sequences of long period variables (LPVs) are commonly
interpreted as different pulsation modes, but there is disagreement on the
modal assignment. Here, we re-examine the observed PL sequences in the Large
Magellanic Cloud, including the sequence of long secondary periods (LSPs), and
their associated pulsation modes. Firstly, we theoretically model the sequences
using linear, radial, non-adiabatic pulsation models and a population synthesis
model of the LMC red giants. Then, we use a semi-empirical approach to assign
modes to the pulsation sequences by exploiting observed multi-mode pulsators.
As a result of the combined approaches, we consistently find that sequences B
and C both correspond to first overtone pulsation, although there
are some fundamental mode pulsators at low luminosities on both sequences. The
masses of these fundamental mode pulsators are larger at a given luminosity
than the mass of the first overtone pulsators. These two sequences B and
C are separated by a small period interval in which large amplitude
pulsation in a long secondary period (sequence D variability) occurs, meaning
that the first overtone pulsation is not seen as the primary mode of pulsation.
Observationally, this leads to the splitting of the first overtone pulsation
sequence into the two observed sequences B and C. Our two
independent examinations also show that sequences A, A and C
correspond to third overtone, second overtone and fundamental mode pulsation,
respectively.Comment: 10 pages, 7 figures, accepted for publication in Ap
Star clusters with dual red clumps
A few star clusters in the Magellanic Clouds exhibit composite structures in
the red-clump region of their colour-magnitude diagrams. The most striking case
is NGC419 in the SMC, where the red clump is composed of a main blob as well as
a distinct secondary feature. This structure is demonstrated to be real and
corresponds to the simultaneous presence of stars which passed through electron
degeneracy after central-hydrogen exhaustion and those that did not. This rare
occurrence in a single cluster allows us to set stringent constraints on its
age and on the efficiency of convective-core overshooting during main-sequence
evolution. We present a more detailed analysis of NGC419, together with a first
look at other populous LMC clusters which are apparently in the same phase:
NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these
Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We
emphasise the extraordinary potential of these clusters as absolute calibration
marks on the age scale of stellar populations.Comment: contributed talk at IAUS 266 'Star clusters: basic galactic building
blocks', eds R. de Grijs and J. Lepine. A high resolution version of Fig. 1
is available in http://stev.oapd.inaf.it/~lgirardi/NGC419_hr.pd
The AGB bump: a calibrator for the core mixing
The efficiency of convection in stars affects many aspects of their evolution
and remains one of the key-open questions in stellar modelling. In particular,
the size of the mixed core in core-He-burning low-mass stars is still uncertain
and impacts the lifetime of this evolutionary phase and, e.g., the C/O profile
in white dwarfs. One of the known observables related to the Horizontal Branch
(HB) and Asymptotic Giant Branch (AGB) evolution is the AGB bump. Its
luminosity depends on the position in mass of the helium-burning shell at its
first ignition, that is affected by the extension of the central mixed region.
In this preliminary work we show how various assumptions on near-core mixing
and on the thermal stratification in the overshooting region affect the
luminosity of the AGB bump, as well as the period spacing of gravity modes in
core-He-burning models.Comment: Submitted to EPJ Web of Conferences, to appear in the Proceedings of
the 3rd CoRoT Symposium, Kepler KASC7 joint meeting; 2 pages, 2 figure
A synthetic sample of short-cadence solar-like oscillators for TESS
NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year
survey of most of the sky, which will include lightcurves for thousands of
solar-like oscillators sampled at a cadence of two minutes. To prepare for this
steady stream of data, we present a mock catalogue of lightcurves, designed to
realistically mimic the properties of the TESS sample. In the process, we also
present the first public release of the asteroFLAG Artificial Dataset
Generator, which simulates lightcurves of solar-like oscillators based on input
mode properties. The targets are drawn from a simulation of the Milky Way's
populations and are selected in the same way as TESS's true Asteroseismic
Target List. The lightcurves are produced by combining stellar models,
pulsation calculations and semi-empirical models of solar-like oscillators. We
describe the details of the catalogue and provide several examples. We provide
pristine lightcurves to which noise can be added easily. This mock catalogue
will be valuable in testing asteroseismology pipelines for TESS and our methods
can be applied in preparation and planning for other observatories and
observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives
containing the mock catalogue are available at
https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at
https://github.com/warrickball/s4tess . The first public release of the
asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at
https://warrickball.github.io/AADG3
The star formation history of the Large Magellanic Cloud star cluster NGC1751
The HST/ACS colour-magnitude diagrams (CMD) of the populous LMC star cluster
NGC1751 present both a broad main sequence turn-off and a dual clump of red
giants. We show that the latter feature is real and associate it to the first
appearance of electron-degeneracy in the H-exhausted cores of the cluster
stars. We then apply to the NGC1751 data the classical method of star formation
history (SFH) recovery via CMD reconstruction, for different radii
corresponding to the cluster centre, the cluster outskirts, and the underlying
LMC field. The mean SFH derived from the LMC field is taken into account during
the stage of SFH-recovery in the cluster regions, in a novel approach which is
shown to significantly improve the quality of the SFH results. For the cluster
centre, we find a best-fitting solution corresponding to prolonged star
formation for a for a timespan of 460 Myr, instead of the two peaks separated
by 200 Myr favoured by a previous work based on isochrone fitting. Remarkably,
our global best-fitting solution provides an excellent fit to the data - with
chi^2 and residuals close to the theoretical minimum - reproducing all the CMD
features including the dual red clump. The results for a larger ring region
around the centre indicate even longer star formation, but in this case the
results are of lower quality, probably because of the differential extinction
detected in the area. Therefore, the presence of age gradients in NGC1751 could
not be probed. Together with our previous findings for the SMC cluster NGC419,
the present results for the NGC1751 centre argue in favour of multiple star
formation episodes (or continued star formation) being at the origin of the
multiple main sequence turn-offs in Magellanic Cloud clusters with ages around
1.5 Gyr.Comment: To appear soon in MNRAS. 12 pages, better printed in colou
The star formation history of the SMC star cluster NGC419
The rich SMC star cluster NGC419 has recently been found to present both a
broad main sequence turn-off and a dual red clump of giants, in the sharp
colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the
Advanced Camera for Surveys on board the Hubble Space Telescope. In this work,
we apply to the NGC419 data the classical method of star formation history
(SFH) recovery via CMD reconstruction, deriving for the first time this
function for a star cluster with multiple turn-offs. The values for the cluster
metallicity, reddening, distance and binary fraction, were varied within the
limits allowed by present observations. The global best-fitting solution is an
excellent fit to the data, reproducing all the CMD features with striking
accuracy. The corresponding star formation rate is provided together with
estimates of its random and systematic errors. Star formation is found to last
for at least 700 Myr, and to have a marked peak at the middle of this interval,
for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation
episodes (or continued star formation) being at the origin of the multiple main
sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It
remains to be tested whether alternative hypotheses, such as a main sequence
spread caused by rotation, could produce similarly good fits to the data.Comment: 10 pages, MNRAS in pres
Modelling Long-Period Variables -- II. Fundamental mode pulsation in the nonlinear regime
Long-period variability in luminous red giants has several promising
applications, all of which require models able to accurately predict pulsation
periods. Linear pulsation models have proven successful in reproducing the
observed periods of overtone modes in evolved red giants, but they fail to
accurately predict their fundamental mode periods. Here, we use a 1D
hydrodynamic code to investigate the long-period variability of M-type
asymptotic giant branch stars in the nonlinear regime. We examine the period
and stability of low-order radial pulsation modes as a function of mass and
radius, and find overtone mode periods in complete agreement with predictions
from linear pulsation models. In contrast, nonlinear models predict an earlier
onset of dominant fundamental mode pulsation, and shorter periods at large
radii. Both features lead to a substantially better agreement with
observations, that we verify against OGLE and Gaia data for the Magellanic
Clouds. We provide simple analytic relations describing the nonlinear
fundamental mode period-mass-radius relation. Differences with respect to
linear predictions originate from the readjustment of the envelope structure
induced by large-amplitude pulsation. We investigate the impact of turbulent
viscosity on linear and nonlinear pulsation, and probe possible effects of
varying metallicity and carbon abundance.Comment: 18 pages, 17 figures; accepted for publication in MNRA
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