66 research outputs found
Not-So-Simple Stellar Populations in the Intermediate-age Large Magellanic Cloud Star Clusters NGC 1831 and NGC 1868
Using a combination of high-resolution Hubble Space Telescope/WFPC2
observations, we explore the physical properties of the stellar populations in
two intermediate-age star clusters in the Large Magellanic Cloud, NGC 1831 and
NGC 1868, based on their color-magnitude diagrams. We show that both clusters
exhibit extended main-sequence turn-offs. To explain the observations, we
consider variations in helium abundance, binarity, age dispersions, and fast
rotation of the clusters' member stars. The observed narrow main sequence
excludes significant variations in helium abundance in both clusters. We first
establish the clusters' main-sequence binary fractions using the bulk of the
clusters' main-sequence stellar populations >1 mag below their turn-offs. The
extent of the turn-off regions in color--magnitude space, corrected for the
effects of binarity, implies that age spreads of order 300 Myr may be inferred
for both clusters if the stellar distributions in color--magnitude space were
entirely due to the presence of multiple populations characterized by an age
range. Invoking rapid rotation of the population of cluster members
characterized by a single age also allows us to match the observed data in
detail. However, when taking into account the extent of the red clump in
color--magnitude space, we encounter an apparent conflict for NGC 1831 between
the age dispersion derived from that based on the extent of the main-sequence
turn-off and that implied by the compact red clump. We therefore conclude that,
for this cluster, variations in stellar rotation rate are preferred over an age
dispersion. For NGC 1868, both models perform equally well.Comment: 34 pages, 19 figures, accepted in Ap
First Observational Signature of Rotational Deceleration in a Massive, Intermediate-age Star Cluster in the Magellanic Clouds
While the extended main-sequence turn-offs (eMSTOs) found in almost all 1--2
Gyr-old star clusters in the Magellanic Clouds are often explained by
postulating extended star-formation histories, the tight subgiant branches
(SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the
SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at
redder colors. We carefully assess and confirm the reality of this
observational trend. If we would assume that the widths of the features in
color--magnitude space were entirely owing to a range in stellar ages, the
star-formation histories of the eMSTO stars and the blue SGB region would be
significantly more prolonged than that of the red part of the SGB. This cannot
be explained by assuming an internal age spread. We show that rotational
deceleration of a population of rapidly rotating stars, a currently hotly
debated alternative scenario, naturally explains the observed trend along the
SGB. Our analysis shows that a `converging' SGB could be produced if the
cluster is mostly composed of rapidly rotating stars that slow down over time
owing to the conservation of angular momentum during their evolutionary
expansion from main-sequence turn-off stars to red giants.Comment: 11 pages, preprint format (uses aastex6.cls); ApJ Letters, in pres
Discovery of Extended Main Sequence Turn-offs in Four Young Massive Clusters in the Magellanic Clouds
An increasing number of young massive clusters (YMCs) in the Magellanic
Clouds have been found to exhibit bimodal or extended main sequences (MSs) in
their color--magnitude diagrams (CMDs). These features are usually interpreted
in terms of a coeval stellar population with different stellar rotational
rates, where the blue and red MS stars are populated by non- (or slowly) and
rapidly rotating stellar populations, respectively. However, some studies have
shown that an age spread of several million years is required to reproduce the
observed wide turn-off regions in some YMCs. Here we present the
ultraviolet--visual CMDs of four Large and Small Magellanic Cloud YMCs, NGC
330, NGC 1805, NGC 1818, and NGC 2164, based on high-precision Hubble Space
Telescope photometry. We show that they all exhibit extended main-sequence
turn-offs (MSTOs). The importance of age spreads and stellar rotation in
reproducing the observations is investigated. The observed extended MSTOs
cannot be explained by stellar rotation alone. Adopting an age spread of 35--50
Myr can alleviate this difficulty. We conclude that stars in these clusters are
characterized by ranges in both their ages and rotation properties, but the
origin of the age spread in these clusters remains unknown.Comment: 14 pages, 9 figures, ApJ accepte
The radial distributions of the two main-sequence components in the young massive star cluster NGC 1856
The recent discovery of double main sequences in the young, massive star
cluster NGC 1856 has caught significant attention. The observations can be
explained by invoking two stellar generations with different ages and
metallicities or by a single generation of stars composed of two populations
characterized by different rotation rates. We analyzed the number ratios of
stars belonging to both main-sequence components in NGC 1856 as a function of
radius. We found that their number ratios remain approximately unchanged from
the cluster's central region to its periphery, indicating that both components
are homogeneously distributed in space. Through a comparison of the loci of the
best-fitting isochrones with the ridge lines of both stellar components, we
found that both multiple stellar populations and rapid stellar rotation can
potentially explain the observed main-sequence bifurcation in NGC 1856.
However, if NGC1856 were a young representative of the old globular clusters,
then the multiple stellar populations model would not be able to explain the
observed homogeneity in the spatial distributions of these two components,
since all relevant scenarios would predict that the second stellar generation
should be formed in a more compact configuration than that of the first stellar
generation, while NGC 1856 is too young for both stellar generations to have
been fully mixed dynamically. We speculate that the rapid stellar rotation
scenario would be the favored explanation of the observed multiple stellar
sequences in NGC 1856.Comment: 11 pages, 6 figures, ApJ accepte
New insights into the formation of the blue main sequence in NGC 1850
Recent discoveries of bimodal main sequences (MSs) associated with young
clusters (with ages Gyr) in the Magellanic Clouds have drawn a lot
of attention. One of the prevailing formation scenarios attributes these split
MSs to a bimodal distribution in stellar rotation rates, with most stars
belonging to a rapidly rotating population. In this scenario, only a small
fraction of stars populating a secondary blue sequence are slowly or
non-rotating stars. Here, we focus on the blue MS in the young cluster NGC
1850. We compare the cumulative number fraction of the observed blue-MS stars
to that of the high-mass-ratio binary systems at different radii. The
cumulative distributions of both populations exhibit a clear anti-correlation,
characterized by a highly significant Pearson coefficient of . Our
observations are consistent with the possibility that blue-MS stars are
low-mass-ratio binaries, and therefore their dynamical disruption is still
ongoing. High-mass-ratio binaries, on the other hand, are more centrally
concentrated.Comment: 8 pages, 6 figures, accepted to Ap
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