173 research outputs found
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
The Contributions of Interactive Binary Stars to Double Main Sequence Turn-Offs and Dual Red Clump of Intermediate-Age Star Clusters
Double or extended main-sequence turn-offs (DMSTOs) and dual red clump (RC)
were observed in intermediate-age clusters, such as in NGC 1846 and 419. the
DMSTOs are interpreted as that the cluster has two distinct stellar popula-
tions with differences in age of about 200-300 Myr but with the same
metallicity. The dual RC is interpreted as a result of a prolonged star
formation. Using a stellar population-synthesis method, we calculated the
evolutions of binary-star stellar population (BSP). We found that binary
interactions and merging can reproduce the dual RC in the color-magnitude
diagrams of an intermediate-age cluster, whereas in actuality only a single
population exists. Moreover, the binary interactions can lead to an extended
MSTO rather than DMSTOs. However, the rest of main sequence, subgiant branch
and first giant branch are hardly spread by the binary interactions. Part of
the observed dual RC and extended MSTO may be the results of binary
interactions and merger.Comment: 10 pages, 2 figures; accepted by ApJ
The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158-N159-N160 Star-forming Complex
We study the hierarchical stellar structures in a ∼1.5 deg2 area covering the 30 Doradus-N158-N159-N160 star-forming complex with the VISTA Survey of Magellanic Clouds. Based on the young upper main-sequence stars, we find that the surface densities cover a wide range of values, from log(pc2) ≲ -2.0 to log(pc2) ≳ 0.0. Their distributions are highly non-uniform, showing groups that frequently have subgroups inside. The sizes of the stellar groups do not exhibit characteristic values, and range continuously from several parsecs to more than 100 pc; the cumulative size distribution can be well described by a single power law, with the power-law index indicating a projected fractal dimension D2 = 1.6 ± 0.3. We suggest that the phenomena revealed here support a scenario of hierarchical star formation. Comparisons with other star-forming regions and galaxies are also discussed.Fil: Sun, Ning-Chen. Peking University; ChinaFil: Grijs, Richard De. Peking University; ChinaFil: Subramanian, Smitha. Peking University; ChinaFil: Cioni, Maria-Rosa L.. Universita Zu Berlin. Universita Postdam; AlemaniaFil: Rubele, Stefano. Università di Padova; ItaliaFil: Bekki, Kenji. University of Western Australia; AustraliaFil: Ivanov, Valentin D.. European Southern Observatory; ChileFil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Ripepi, Vincenzo. Osservatorio Astronomico di Capodimonte; Itali
The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations
Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t/yr) ≤ 9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio
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
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
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
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