205 research outputs found
Helium and multiple populations in the massive globular cluster NGC 6266 (M 62)
Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC 6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an extended HB and is among the 10 more luminous GCs in the Milky Way. The analysis of colour–magnitude diagrams from multiwavelength photometry reveals that also NGC 6266, similarly to other massive GCs, hosts a double main sequence (MS), with the red and the blue component made up of the 79 ± 1 per cent and the 21 ± 1 per cent of stars, respectively. The red MS is consistent with a stellar population with primordial helium while the blue MS is highly helium-enhanced by ΔY = 0.08 ± 0.01. Furthermore, the red MS exhibits an intrinsic broadening that cannot be attributed to photometric errors only and is consistent with a spread in helium of ∼0.025 dex. The comparison between NGC 6266 and other GCs hosting helium-enriched stellar populations supports the presence of a correlation among helium variations, cluster mass, and HB extension
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
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
Updated properties of the old open cluster Melotte 66: Searching for multiple stellar populations
Multiple generations of stars are routinely encountered in globular clusters
but no convincing evidence has been found in Galactic open clusters to date. In
this paper we use new photometric and spectroscopic data to search for multiple
stellar population signatures in the old, massive open cluster, Melotte~66. The
cluster is known to have a red giant branch wide in color, which could be an
indication of metallicity spread. Also the main sequence is wider than what is
expected from photometric errors only. This evidence might be associated with
either differential reddening or binaries. Both hypothesis have, however, to be
evaluated in detail before recurring to the presence of multiple stellar
populations. New, high-quality, CCD UBVI photometry have been acquired to this
aim with high-resolution spectroscopy of seven clump stars, that are
complemented with literature data. Our photometric study confirms that the
width of the main sequence close to the turn off point is entirely accounted
for by binary stars and differential reddening, with no need to advocate more
sofisticated scenarios, such as metallicity spread or multiple main sequences.
By constructing synthetic color-magnitude diagrams, we infer that the binary
fraction has to be as large as 30 and their mass ratio in the range 0.6-1.0.
As a by-product of our simulations, we provide new estimates of the cluster
fundamental parameters. We measure a reddening E(B-V)=0.150.02, and
confirm the presence of a marginal differential reddening. The distance to the
cluster is kpc and the age is 3.40.3 Gyr, which is
somewhat younger and better constrained than previous estimates. Our detailed
abundance analysis reveals that, overall, Melotte~66 looks like a typical
object of the old thin disk population.Comment: 14 pages, 18 eps figure, in press in Astronomy and Astrophysics.
Abstract shortened to fit arXiv constraint
Horizontal branch morphology and multiple stellar populations in the anomalous globular cluster M 22
M 22 is an anomalous globular cluster that hosts two groups of stars with different metallicity and s-element abundance. The star-to-star light-element variations in both groups, with the presence of individual Na-O and C-N anticorrelations, demonstratesWe are grateful to Peter Stetson for providing photometry of M22. A.P.M. acknowledges the financial support from the Australian Research Council through Discovery Project grant DP120100475. Support for this work has been provided by the IAC (grant 310394), and the Education and Science Ministry of Spain (grants AYA2007-3E3506 and AYA2010-16717)
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
First evidence of multiple populations along the AGB from Str\"omgren photometry
Spectroscopic studies have demonstrated that nearly all Galactic globular
clusters (GCs) harbour multiple stellar populations with different chemical
compositions. Moreover, colour-magnitude diagrams based exclusively on
Str\"omgrem photometry have allowed us to identify and characterise multiple
populations along the RGB of a large number of clusters. In this paper we show
for the first time that Str\"omgren photometry is also very effcient at
identifying multiple populations along the AGB, and demonstrate that the AGB of
M3, M92, NGC362, NGC1851, and NGC6752 are not consistent with a single stellar
population. We also provide a catalogue of RGB and AGB stars photometrically
identified in these clusters for further spectroscopic follow-up studies.We
combined photometry and elemental abundances from the literature for RGB and
AGB stars in NGC6752 where the presence of multiple populations along the AGB
has been widely debated. We find that, while the MS, SGB, and RGB host three
stellar populations with different helium and light element abundances, only
two populations of AGB stars are present in the cluster. These results are
consistent with standard evolutionary theory.Comment: 9 pages, 3 figures, 1 table in the main article, 3 tables in the
appendix of which 2 tables containing coordinates and photometry of
photometrically identified RGB and AGB star
Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4
We report new metallicities for stars of Galactic globular cluster M4 using
the largest number of stars ever observed at high spectral resolution in any
cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT
FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a
small wavelength range, and often have very low signal-to-noise ratios. We
attacked this dataset by reconsidering the whole method of abundance analysis
of large stellar samples from beginning to end. We developed a new algorithm
that automatically determines the atmospheric parameters of a star. Nearly all
data preparation steps for spectroscopic analyses are processed on the
syntheses, not the observed spectra. For 322 Red Giant Branch stars with we obtain a nearly constant metallicity, ( = 0.02). No difference in the metallicity at the level of
is observed between the two RGB sequences identified by
\cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars we
obtain ( = 0.09) after fixing the
microturbulent velocity. These values are consistent with previous studies that
have performed detailed analyses of brighter RGB stars at higher spectroscopic
resolution and wavelength coverage. It is not clear if the small mean
metallicity difference between brighter and fainter M4 members is real or is
the result of the low signal-to-noise characteristics of the fainter stars. The
strength of our approach is shown by recovering a metallicity close to a single
value for more than two thousand stars, using a dataset that is non-optimal for
atmospheric analyses. This technique is particularly suitable for noisy data
taken in difficult observing conditions.Comment: 17 pages, 20 figures, 3 tables. Accepted for publication in The
Astronomical Journa
Stars caught in the braking stage in young Magellanic Clouds clusters
The color-magnitude diagrams of many Magellanic Cloud clusters (with ages up
to 2 billion years) display extended turnoff regions where the stars leave the
main sequence, suggesting the presence of multiple stellar populations with
ages which may differ even by hundreds million years (Mackey et al. 2008,
Milone et al. 2009, Girardi et al. 2011). A strongly debated question is
whether such an extended turnoff is instead due to populations with different
stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al.
2013, Li et al. 2014). The recent discovery of a `split' main sequence in some
younger clusters (about 80--400Myr) added another piece to this puzzle. The
blue (red) side of the main sequence is consistent with slowly (rapidly)
rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et
al. 2017, Milone et al 2016), but a complete theoretical characterization of
the observed color-magnitude diagram appeared to require also an age spread
(Correnti et al. 2017). We show here that, in three clusters so far analyzed,
if the blue main sequence stars are interpreted with models that have been
always slowly rotating, they must be about 30% younger than the rest of the
cluster. If they are instead interpreted as stars initially rapidly rotating,
but that have later slowed down, the age difference disappears, and "braking"
also helps to explain the apparent age differences of the extended turnoff. The
age spreads in Magellanic Cloud clusters are a manifestation of rotational
stellar evolution. Observational tests are suggested.Comment: Accepted for publication and in state of Advance Online Publication
(from 24 July 2017) on Nature Astronom
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