88 research outputs found
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
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
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. X. The radial distribution of stellar populations in NGC 2808
Due to their extreme helium abundance, the multiple stellar populations of
the globular cluster NGC 2808 have been widely investigated from a photometric,
spectroscopic, and kinematic perspective. The most striking feature of the
color-magnitude diagram of NGC 2808 is the triple main sequence (MS), with the
red MS corresponding to a stellar population with primordial helium, and the
middle and the blue MS being enhanced in helium up to Y0.32 and
0.38, respectively. A recent study has revealed that this massive cluster
hosts at least five distinct stellar populations (A, B, C, D, and E). Among
them populations A, B, and C correspond to the red MS, while populations C and
D are connected to the middle and the blue MS. In this paper we exploit
Hubble-Space-Telescope photometry to investigate the radial distribution of the
red, the middle and the blue MS from the cluster center out to about 8.5
arcmin. Our analysis shows that the radial distribution of each of the three
MSs is different. In particular, as predicted from multiple-population
formation models, both the blue MS and the middle MS appears to be more
concentrated than the red MS with a significance level for this result wich is
above 3{\sigma}.Comment: Accepted for publication in MNRA
The link between chemical anomalies along the red giant branch and the horizontal branch extension in globular clusters
We find a strong correlation between the extension of the Na-O
anticorrelation observed in red giant branch (RGB) stars and the high
temperature extension of the horizontal branch (HB) blue tails of Galactic
globular clusters (GCs). The longer is the O-depleted tail of the Na-O
anticorrelation observed in the RGB stars, the higher is the maximum
temperature reached by the bluest HB stars in the GC. This result provides a
clear, empirical evidence of a link between the extension of the HB and the
presence of star-to-star abundance variations of proton-capture elements in GC
stars. We discuss the possible interpretation of this correlation.Comment: Comments: 6 pages, 1 figure, uses emulateapj.cls; accepted for
publication in the Astrophysical Journal Letter
Rotation of Hot Horizontal Branch Stars in the Globular Clusters NGC 1904, NGC 2808, NGC 6093 and NGC 7078
We present high resolution UVES+VLT spectroscopic observations of 56 stars in
the extended horizontal branch (EHB) of the Galactic globular clusters NGC
1904, NGC 2808, NGC 6093, and NGC 7078. Our data reveal for the first time the
presence in NGC 1904 of a sizable population of fast (vsini > 20 km/s)
horizontal branch (HB) rotators, confined to the cool end of the EHB, similar
to that found in M13. We also confirm the fast rotators already observed in NGC
7078. The cooler stars (Teff < 11,500 K) in these three clusters show a range
of rotation rates, with a group of stars rotating at ~ 15 km/s or less, and a
fast rotating group at ~ 30 km/s. Apparently, the fast rotators are relatively
more abundant in NGC 1904 and M13, than in NGC 7078. No fast rotators have been
identified in NGC 2808 and NGC 6093. All the stars hotter than Teff ~ 11,500 K
have projected rotational velocities vsini < 12 km/s, but less than 20% have
vsini < 2 km/s. The connection between photometric gaps in the HB and the
change in the projected rotational velocities is not confirmed by the new data.
However, our data are consistent with a relation between this discontinuity and
the HB jump. We discuss a number of possibilities for the origin of the stellar
rotation distribution along the HB. We conclude that none of them can yet
provide a satisfactory explanation of the observations.Comment: 12 pages, 3 figures, ApJ Letter, accepte
Recommended from our members
The ACS Survey Of Globular Clusters. V. Generating A Comprehensive Star Catalog For Each Cluster
The ACS Survey of Globular Clusters has used Hubble Space Telescope's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of similar to 10(5) tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.Astronom
New evidence for the complex struscture of the RGB of Omega Cen
We report on the complex structure of the red giant branch (RGB) of omega
Cen, based on a new wide field and wide color baseline B and I photometry. Our
color magnitude diagram (CMD) shows the presence of multiple populations along
this branch, in particular we discovered an anomalous branch (RGB-a), which
appears to be well separated from the bulk of the RGB stars. On the basis of
our CMD and from the previous literature we conclude that (1) these stars,
clearly identified as a separate population in our CMD, represent the extreme
metal rich extension ([Ca/H]>-0.3) of the stellar content of omega Cen, and
show anomalous abundances of s-process elements (as Ba and Zr) as well; (2)
they are physical members of the omega Cen system; (3) they comprise ~ 5% of
the stars of the whole system; (4) this component and the metal-intermediate
one (-0.4>[Ca/H]>-1) have been found to share the same spatial distribution,
both of them differing significantly from the most metal poor one ([Ca/H]<-1).
This last evidence supports the hypothesis that metal rich components could
belong to an independent (proto?) stellar system captured in the past by omega
Cen.Comment: 5 pages, 3 figures, accepted for publication as ApJ Lette
The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy
We present new Hubble Space Telescope photometry of the massive globular
cluster M54 (NGC 6715) and the superposed core of the tidally disrupted
Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular
Clusters. Our deep (F606W~26.5), high-precision photometry yields an
unprecedentedly detailed color-magnitude diagram showing the extended blue
horizontal branch and multiple main sequences of the M54+Sgr system. The
distance and reddening to M54 are revised usingboth isochrone and main-sequence
fitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds the
M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the
globular cluster. Multiple turnoffs indicate the presence of at least two
intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4
to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgr
population of near-solar abundance. A trace population of even younger (0.1-0.8
Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgr
age-metallicity relation is consistent with a closed-box model and multiple
(4-5) star formation bursts over the entire life of the satellite, including
the time since Sgr began disrupting.Comment: Accepted to ApJ Letter; 11 pages, 2 figures; figure 1 uploaded as
jpg; paper in ApJ format with full-resolution figures available at:
http://www.astro.ufl.edu/~ata/public_hstgc/paperIV/paperIV.p
Multiple stellar populations in Magellanic Cloud clusters - V. The split main sequence of the young cluster NGC 1866
One of the most unexpected results in the field of stellar populations of the last few years is the discovery that some Magellanic Cloud globular clusters younger than ~400 Myr exhibit bimodal main sequences (MSs) in their colour-magnitude diagrams (CMDs). Moreover, these young clusters host an extended main-sequence turn-off(eMSTO) in close analogy with what is observed in most ~1-2 Gyr old clusters of both Magellanic Clouds. We use high-precision Hubble Space Telescope photometry to study the young star cluster NGC 1866 in the Large Magellanic Cloud. We discover an eMSTO and a split MS. The analysis of the CMD reveals that (i) the blue MS is the less populous one, hosting about one-third of the total number of MS stars; (ii) red MS stars are more centrally concentrated than blue MS stars; (iii) the fraction of blue MS stars with respect to the total number of MS stars drops by a factor of ~2 in the upper MS with mF814W âČ 19.7. The comparison between the observed CMDs and stellar models reveals that the observations are consistent with ~200 Myr old highly rotating stars on the red MS, with rotation close to critical value, plus a non-rotating stellar population spanning an age interval between ~140 and 220 Myr, on the blue MS. Noticeable, neither stellar populations with different ages only, nor coeval stellar models with different rotation rates, properly reproduce the observed split MS and eMSTO. We discuss these results in the context of the eMSTO and multiple MS phenomenonAPM, AFM, and HJ acknowledge support by the Australian Research Council through Discovery Early Career Researcher Awards DE150101816 and DE160100851 and Discovery project DP150100862
Chemical Abundances along the 1G Sequence of the Chromosome Maps: The Globular Cluster NGC 3201
The Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs) has investigated
multiple stellar populations by means of the âchromosome mapâ (ChM) diagnostic tool that maximizes the
separation between stars with different chemical compositions. One of the most challenging features revealed by
ChM analysis is the apparent inhomogeneity among stars belonging to the first population, a phenomenon largely
attributed to He variations. However, this explanation is not supported by uniformity in the p-capture elements of
these stars. The HST survey has revealed that the GC NGC 3201 shows exceptionally wide coverage in the
DF275W,F814W parameter of the ChM. We present a chemical abundance analysis of 24 elements in 18 giants
belonging to the first population of this GC and having a wide range in DF275W,F814W. As far as the p-capture
elements are concerned, the chemical abundances are typical of first-generation (1G) stars, as expected from
the location of our targets in the ChM. Based on radial velocities and chemical abundance arguments, we find that
the three stars with the lowest DF275W,F814W values are binary candidates. This suggests that at least those stars
could be explained with binarity. These results are consistent with evidence inferred from multiband photometry
that evolved blue stragglers (BSs) populate the bluest part of the 1G sequence in the ChM. The remaining 15
spectroscopic targets show a small range in the overall metallicity by âŒ0.10 dex, with stars at higher DF275W,F814W
values having higher absolute abundances. We suggest that a small variation in metals and binarity governs the
color spread of the 1G in the ChM and that evolved BSs contribute to the bluest tail of the 1G sequence.This work has received funding from the
European Research Council (ERC) under the European Unionâs
Horizon 2020 research innovation program (grant agreement
ERC-StG 2016, No. 716082 âGALFOR,â PI: Milone) and the
European Unionâs Horizon 2020 research and innovation
program under Marie SkĆodowska-Curie grant agreement No.
797100. A.P.M. and M.T. acknowledge support from MIUR
through the FARE project R164RM93XW âSEMPLICE.â H.J.
acknowledges support by the Australian Research Council
through the Discovery Project DP150100862
- âŠ