103 research outputs found
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
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
The Formation of Globular Clusters as a Case of Overcooling
Driven by recent observational findings, we select massive interactive
binaries as the most suitable among the existing candidates for producing the
chemical patterns typical of multiple populations of Galactic globular
clusters. Still, to avoid supernova contamination we are further driven to
endorse the notion that above a critical mass stars fail to produce supernova
events, but rather eventually sink into black holes without ejecting much
energy and heavy metals. This assumption has the attractive implication of
suppressing star formation feedback for some 5--10 million years, in practice
leading to runaway star formation, analog to {\it overcooling} that in absence
of feedback would have turned most baryons into stars in the early Universe.
Under such conditions, multiple episodes of stars formation, incorporating
binary star ejecta from previous episodes, appear to be unavoidable, thus
accounting for the ubiquity of the multiple population phenomenon in globular
clusters.Comment: 7 pages. Submitted December 13, 2021, in original form, accepter
April 2, 2022. To appear on MNRA
Anomalous extinction towards NGC 1938
Intrigued by the extended red-giant clump (RC) stretching across the
colour-magnitude diagram of the stars in a 50x50 pc^2 region of the Large
Magellanic Cloud (LMC) containing the clusters NGC 1938 and NGC 1939, we have
studied the stellar populations to learn about the properties of the
interstellar medium (ISM) in this area. The extended RC is caused by a large
and uneven amount of extinction across the field. Its slope reveals anomalous
extinction properties, with Av/E(B-V)=4.3, indicating the presence of an
additional grey component in the optical contributing about 30% of the total
extinction in the field and requiring big grains to be about twice as abundant
as in the diffuse ISM. This appears to be consistent with the amount of big
grains injected into the surrounding ISM by the about 70 SNII explosions
estimated to have occurred during the lifetime of the ~120 Myr old NGC 1938.
Although this cluster appears today relatively small and would be hard to
detect beyond the distance of M 31, with an estimated initial mass of ~4800
Msun NGC 1938 appears to have seriously altered the extinction properties in a
wide area. This has important implications for the interpretation of
luminosities and masses of star-forming galaxies, both nearby and in the early
universe.Comment: 8 pages, 4 figures, accepted for publication in The Astrophysical
Journa
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
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
Multiple stellar populations in Galactic globular clusters: observational evidence
An increasing number of both photometric and spectroscopic observations over
the last years have shown the existence of distinct sub-populations in many
Galactic globular clusters and shattered the paradigm of globulars hosting
single, simple stellar populations.
These multiple populations manifest themselves in a split of different
evolutionary sequences in the cluster color-magnitude diagrams and in
star-to-star abundance variations. In this paper we will summarize the
observational scenario.Comment: 6 pages, 3 figures, Proceedings SF2A 201
Extended main-sequence turnoffs in the double cluster and Persei: The complex role of stellar rotation
Using {\sl Gaia} Data Release 2 photometry, we report the detection of
extended main-sequence turnoff (eMSTO) regions in the color--magnitude diagrams
(CMDs) of the Myr-old double clusters and Persei (NGC 869
and NGC 884). We find that stars with masses below 1.3 in
both and Persei populate narrow main sequences (MSs), while more
massive stars define the eMSTO, closely mimicking observations of young
Galactic and Magellanic Cloud clusters (with ages older than 30 Myr).
Previous studies based on clusters older than 30 Myr find that rapidly
rotating MS stars are redder than slow rotators of similar luminosity,
suggesting that stellar rotation may be the main driver of the eMSTO. By
combining photometry and projected rotational velocities from the literature of
stars in and Persei, we find no obvious relation between the
rotational velocities and colors of non-emission-line eMSTO stars, in contrast
with what is observed in older clusters. Similarly to what is observed in
Magellanic Cloud clusters, most of the extremely rapidly rotating stars,
identified by their strong H emission lines, are located in the red
part of the eMSTOs. This indicates that stellar rotation plays a role in the
color and magnitude distribution of MSTO stars. By comparing the observations
with simulated CMDs, we find that a simple population composed of coeval stars
that span a wide range of rotation rates is unable to reproduce the color
spread of the clusters' MSs. We suggest that variable stars, binary
interactions, and stellar rotation affect the eMSTO morphology of these very
young clusters.Comment: 14 pages, 12 figures, ApJ accepte
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