11 research outputs found
Multiple Stellar Populations outside the tidal radius of NGC1851 through Gaia DR3 XP Spectra
Ancient Galactic Globular Clusters (GCs) have long fascinated astronomers due
to their intriguing multiple stellar populations characterized by variations in
light-element abundances. Among these clusters, Type-II GCs stand out as they
exhibit stars with large differences in heavy-element chemical abundances.
These enigmatic clusters, comprising approximately 17\% of analyzed GCs with
MPs, have been hypothesized to be the remnants of accreted dwarf galaxies. We
focus on one of the most debated Type~II GCs, NGC1851, to investigate its MPs
across a wide spatial range of up to 50 arcmin from the cluster center. By
using Gaia DR3 low-resolution XP spectra, we generate synthetic photometry to
perform a comprehensive analysis of the spatial distribution and kinematics of
the canonical and anomalous populations within this GC. By using appropriate
CMDs from the synthetic photometry in the BVI bands and in the
band introduced in this work, we identify distinct stellar sequences associated
with different heavy-element chemical composition. Our results suggest that the
canonical and the anomalous populations reside both inside and outside the
tidal radius of NGC1851, up to a distance that exceeds by 3.5 times its tidal
radius. However, 80\% of stars outside the tidal radius are consistent
with belonging to the canonical population, emphasizing its dominance in the
cluster's outer regions. Remarkably, canonical stars exhibit a more circular
on-sky morphology, while the anomalous population displays an elliptical shape.
Furthermore, we delve into the kinematics of the multiple populations. Our
results reveal a flat/increasing velocity dispersion profile in the outer
regions and hints of a tangentially anisotropic motion in the outer regions,
indicating a preference for stars to escape on radial orbits.Comment: 13 pages, 10 figures, accepted for publication in A&
Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters
Binary stars play a crucial role in our understanding of the formation and
evolution of star clusters and their stellar populations. We use Gaia Data
Release 3 to homogeneously analyze 78 Galactic open clusters and the unresolved
binary systems they host, each composed of two main sequence (MS) stars. We
first investigated the structural parameters of these clusters, such as the
core radius and the central density, and determined the cluster mass function
(MF) and total mass by interpolating the density profile of each cluster. We
measured the fraction of binaries with a large mass ratio and the fraction of
blue straggler stars (BSSs), and finally investigated possible connections
between the populations of binary stars and BSSs with the main parameters of
the host cluster. {Remarkably, we find that the MFs of 78 analyzed open
clusters follow a similar trend and are well reproduced by two single power-law
functions, with a change in slope around masses of 1. The fraction
of binary stars ranges from 15\% to more than 60\% without
significant correlation with the mass and the age of the host cluster.
Moreover, we detect hints of a correlation between the total fraction of binary
stars and the central density of the host cluster. We compared the fraction of
binary stars with that of BSSs, finding that clusters with high and low central
density exhibit different trends. The fraction of binaries does not
significantly change with the mass of the primary star and the mass ratio. The
radial distribution of binary stars depends on cluster age. The binaries of
clusters younger than 800\,Myr typically show a flat radial distribution,
with some hints of a double peak. In contrast, the binaries of the remaining
clusters are more centrally concentrated than the single stars, which is
similar to what is observed in globular clusters.Comment: 27 pages, 27 figures, accepted for publication in Astronomy &
Astrophysic
Stroemgren - near-infrared photometry of the Baade's Window. I. The bulge globular cluster NGC6528 and the surrounding field
We present Stroemgren-NIR photometry of NGC6528 and its surroundings in the
Baade's Window. uvby images were collected with EFOSC2@NTT, while NIR catalogs
are based on VIRCAM@VISTA and SOFI@NTT data. The matching with HST photometry
allowed us to obtain proper-motion-cleaned samples of cluster and bulge stars.
The huge color sensitivity of Stroemgren-NIR CMDs helped us in disentangling
age and metallicity effects. The RGB of NGC6528 is reproduced by scaled-solar
isochrones with solar abundance or alpha-enhanced isochrones with the same iron
content, and an age of t = 11+/-1 Gyr. These findings support literature age
estimates for NGC6528. We also performed a theoretical metallicity calibration
based on the Stroemgren index m1 and on visual-NIR colors for RGs, by adopting
scaled-solar and alpha-enhanced models. We applied the calibration to estimate
the metallicity of NGC6528, finding [Fe/H] = -0.04+/-0.02, with an intrinsic
dispersion of 0.27 dex (by averaging abundances based on the scaled-solar [m],
y - J and [m], y - K Metallicity-Index-Color relations), and of -0.11+/-0.01
(sig = 0.27 dex), by using the m1, y - J and m1, y - K relations. These
findings support the results of Zoccali et al. (2004) which give [Fe/H] =
-0.10+/-0.2, and a low alpha-enhancement, [alpha/Fe] = 0.1, and of Carretta et
al. (2001), that find [Fe/H] = 0.07+/-0.01, with [alpha/Fe] = 0.2. By applying
the scaled-solar MIC relations to Baade's window RGs, we find a metallicity
distribution extending from [Fe/H] ~ -1.0 to ~ 1 dex, with peaks at [Fe/H] ~
-0.2 and +0.55 ([m], y - J and [m], y - K relations), and [Fe/H] ~ -0.25 and
+0.4 (m1, y - J and m1, y - K relations). These findings are in good agreement
with the spectroscopic studies of Hill et al. (2011) for the Baade's window, of
Uttenthaler et al. (2012) for a region centered at (l,b) = (0, -10), and with
the results of the ARGOS survey (Ness et al. 2013a).Comment: 19 pages, 13 figures, 3 tables, accepted for publication on Astronomy
& Astrophysic
Multiple Stellar Populations in Metal-Poor Globular Clusters with JWST: a NIRCam view of M 92
Recent work on metal-intermediate globular clusters (GCs) with [Fe/H]=
and has illustrated the theoretical behavior of multiple populations in
photometric diagrams obtained with the James Webb Space Telescope (JWST). These
results are confirmed by observations of multiple populations among M-dwarfs of
47 Tucanae. Here, we explore the multiple populations in metal-poor GCs with
[Fe/H]=2.3. We take advantage of synthetic spectra and isochrones that
account for the chemical composition of multiple populations to identify
photometric diagrams that separate the distinct stellar populations of GCs. We
derived high-precision photometry and proper motion for main-sequence stars in
the metal-poor GC M 92 from JWST and Hubble Space Telescope (HST) images. We
identified a first generation (1G) and two main groups of second-generation
stars (2G and 2G) and investigated their kinematics and
chemical composition. We find isotropic motions with no differences among the
distinct populations. The comparison between the observed colors of M 92 stars
and the colors derived by synthetic spectra reveals that helium abundance of
2G and 2G stars are higher than that of the 1G by and , respectively. The vs. color-magnitude diagram shows that below the knee, MS
stars exhibit a wide color broadening due to multiple populations. We constrain
the amount of oxygen variation needed to reproduce the observed MS width, which
is consistent with results on red-giant branch stars. We conclude that multiple
populations with masses of 0.1-0.8 share similar chemical
compositions.Comment: 15 pages, 9 figures. Submitted to ApJ on April 6th, 202
Hubble Space Telescope survey of Magellanic Cloud star clusters: photometry and astrometry of 113 clusters and early results
In the past few years, we have undertaken an extensive investigation of star clusters and their stellar populations in the Large and Small Magellanic Clouds (LMC and SMC) based on archival images collected with the Hubble Space Telescope. We present photometry and astrometry of stars in 101 fields observed with the Wide Field Channel of the Advanced Camera for Surveys and the Ultraviolet and Visual Channel and the Near-Infrared Channel of Wide Field Camera 3. These fields comprise 113 star clusters. We provide differential-reddening maps for those clusters with significant reddening variations across the field of view. We illustrate various scientific outcomes that arise from the early inspection of the photometric catalogs. In particular, we provide new insights into the extended main-sequence turnoff (eMSTO) phenomenon: (i) We detected eMSTOs in two clusters, KMHK 361 and NGC 265, which had no previous evidence of multiple populations. This finding corroborates the conclusion that the eMSTO is a widespread phenomenon among clusters younger than ~2 Gyr. (ii) The homogeneous color-magnitude diagrams (CMDs) of 19 LMC clusters reveal that the distribution of stars along the eMSTO depends on cluster age. (iii) We discovered a new feature along the eMSTO of NGC 1783, which consists of a distinct group of stars on the red side of the eMSTO in CMDs composed of UV filters. Furthermore, we derived the proper motions of stars in the fields of view of clusters with multi-epoch images. Proper motions allowed us to separate the bulk of bright field stars from cluster members and investigate the internal kinematics of stellar populations in various LMC and SMC fields. As an example, we analyze the field around NGC 346 to disentangle the motions of its stellar populations, including NGC 364 and BS 90, young and pre-main-sequence stars in the star-forming region associated with NGC 346, and young and old field stellar populations of the SMC. Based on these results and the fields around five additional clusters, we find that young SMC stars exhibit elongated proper-motion distributions that point toward the LMC, thus providing new evidence for a kinematic connection between the LMC and SMC.We thank the anonymous referee for various suggestions that improved the quality of the manuscript. This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research innovation programme (Grant Agreement ERC-StG
2016, No. 716082 ’GALFOR’, PI: Milone, http://progetti.dfa.unipd. it/GALFOR) and from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No. 101034319 and from the European Union – NextGenerationEU, beneficiary: Ziliotto. A.P.M., M.T., and E.D. acknowledge support from MIUR through the FARE project R164RM93XW SEMPLICE (PI: Milone). A.P.M. and M.T. have been supported by MIUR under PRIN program 2017Z2HSMF (PI: Bedin). This research was supported in part by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) through project number CE170100013. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.Peer ReviewedPostprint (published version
The Role of Cluster Mass in the Multiple Populations of Galactic and Extragalactic Globular Clusters
Studies based on Hubble Space Telescope (HST) photometry in F275W, F336W, and F438W have shown that the incidence and complexity of multiple populations (MPs) in globular clusters (GCs) depend on cluster mass. This result, however, is based on nearby, low-reddening Galactic GCs, for which we can obtain accurate F275W photometry. In this work, we exploit HST photometry in F336W, F438W, and F814W to characterize MPs in 68 Galactic and extragalactic GCs by using the pseudocolor C-F336W,C-F438W,C-F814W, which is more efficient in terms of observation time than the pseudocolor C-F275W,C-F336W,C-F438W adopted in previous works. We first analyze the Galactic GCs alone and find that the red giant branch (RGB) color width strongly correlates with [Fe/H]. After removing the dependence from metallicity, we obtain a strong correlation with cluster mass, thus confirming previous findings. We then exploit the RGB width to compare MPs in Galactic and extragalactic GCs. Similarly to Galactic GCs, the RGB width of Magellanic Cloud (MC) GCs correlates with cluster mass after removing the dependence from metallicity. This fact demonstrates that cluster mass is the main factor affecting the properties of MPs. The MC clusters exhibit, on average, narrower RGBs than Galactic GCs with similar mass and metallicity. We suggest that this difference is a signature of stellar mass loss in Galactic GCs. As an alternative, the mass?RGB?width relation would depend on the host galaxy. Finally, we use ground-based photometry to investigate Terzan 7 through the C-U,C-B,C-I pseudocolor, ground-based analogs of C-F336W,C-F438W,C-F814W and find that this cluster is consistent with a simple population
NGC1818 unveils the origin of the extended main-sequence turn-off in young Magellanic Clouds clusters
The nature of young star clusters in the Magellanic Clouds is debated. Here, the authors show an alternative approach that exploits data to exclude the presence of age differences greater than a few million years among cluster stars in a very young cluster.The origin of young star clusters represents a major challenge for modern stellar astrophysics. While stellar rotation partially explains the colour spread observed along main-sequence turn-offs, i.e. where stars leave the main-sequence after the exhaustion of hydrogen in their core, and the multiple main sequences in the colour-magnitude diagrams of stellar systems younger than approximately 2 Gyr, it appears that an age difference may still be required to fulfill the observational constraints. Here we introduce an alternative approach that exploits the main-sequence turn-on, i.e. the point alongside the colour-magnitude diagram where pre-main-sequence stars join the main-sequence, to disentangle between the effects of stellar rotation and age to assess the presence, or lack thereof, of prolonged star formation in the approximately 40-Myr-old cluster NGC1818. Our results provide evidence for a fast star formation, confined within 8 Myr, thus excluding age differences as responsible for the extended main-sequence turn-offs, and leading the way to alternative observational perspectives in the exploration of stellar populations in young clusters