1,231 research outputs found
Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)
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 NGC6266 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 ten more luminous GCs in the Milky Way. The
analysis of color-magnitude diagrams from multi-wavelength photometry reveals
that also NGC6266, similarly to other massive GCs, hosts a double main sequence
(MS), with the red and the blue component made up of the 79+-1% and the 21+-1%
of stars, respectively. The red MS is consistent with a stellar population with
primordial helium while the blue MS is highly helium-enhanced by Delta
Y=0.08+-0.01. Furthermore, the red MS exhibits an intrinsic broadening that can
not be attributed to photometric errors only and is consistent with a spread in
helium of ~0.025 dex. The comparison between NGC6266 and other GCs hosting
helium-enriched stellar populations supports the presence of a correlation
among helium variations, cluster mass, and HB extension.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22
M22 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, demonstrates that this Milky-Way satellite has experienced a
complex star-formation history. We have analysed FLAMES/UVES spectra for seven
stars covering a small color interval, on the reddest horizontal-branch (HB)
portion of this cluster and investigated possible relations between the
chemical composition of a star and its location along the HB. Our chemical
abundance analysis takes into account effects introduced by deviations from the
local-thermodynamic equilibrium (NLTE effects), that are significant for the
measured spectral lines in the atmospheric parameters range spanned by our
stars. We find that all the analysed stars are barium-poor and sodium-poor,
thus supporting the idea that the position of a star along the HB is strictly
related to the chemical composition, and that the HB-morphology is influenced
by the presence of different stellar populations.Comment: 21 pages, 3 figures, accepted for publication in Ap
Carbon and nitrogen abundances of stellar populations in the globular cluster M 2
We present CH and CN index analysis and C and N abundance calculations based
on the low-resolution blue spectra of red giant branch (RGB) stars in the
Galactic globular cluster NGC 7089 (M 2). Our main goal is to investigate the
C-N anticorrelation for this intermediate metallicity cluster. The data were
collected with DOLORES, the multiobject, low-resolution facility at the
Telescopio Nazionale Galileo. Spectroscopic data were coupled with UV
photometry obtained during the spectroscopic run. We found a considerable
star-to-star variation in both A(C) and A(N) at all luminosities for our sample
of 35 targets. These abundances appear to be anticorrelated, with a hint of
bimodality in the C content for stars with luminosities below the RBG bump
(V~15.7), while the range of variations in N abundances is very large and spans
almost ~ 2 dex. We find additional C depletion as the stars evolve off the RGB
bump, in fairly good agreement with theoretical predictions for metal-poor
stars in the course of normal stellar evolution. We isolated two groups with
N-rich and N-poor stars and found that N abundance variations correlate with
the (U-V) color in the DOLORES color-magnitude diagram (CMD). The V, (U-V) CMD
for this cluster shows an additional RGB sequence, located at the red of the
main RGB and amounting to a small fraction of the total giant population. We
identified two CH stars detected in previous studies in our U, V images. These
stars, which are both cluster members, fall on this redder sequence, suggesting
that the anomalous RGB should have a peculiar chemical pattern. Unfortunately,
no additional spectra were obtained for stars in this previously unknown RGB
branch.Comment: 15 pages, 14 figures; accepted for publication in A&
The oxygen vs. sodium (anti)correlation(s) in omega Cen
Recent exam of large samples of omega Cen giants shows that it shares with
mono-metallic globular clusters the presence of the sodium versus oxygen
anticorrelation, within each subset of stars with iron content in the range
-1.9<~[Fe/H]<~-1.3. These findings suggest that, while the second generation
formation history in omega Cen is more complex than that of mono-metallic
clusters, it shares some key steps with those simpler cluster. In addition, the
giants in the range -1.3<[Fe/H]<~-0.7 show a direct O--Na correlation, at
moderately low O, but Na up to 20 times solar. These peculiar Na abundances are
not shared by stars in other environments often assumed to undergo a similar
chemical evolution, such as in the field of the Sagittarius dwarf galaxy. These
O and Na abundances match well the yields of the massive asymptotic giant
branch stars (AGB) in the same range of metallicity, suggesting that the stars
at [Fe/H]>-1.3 in omega Cen are likely to have formed directly from the pure
ejecta of massive AGBs of the same metallicities. This is possible if the
massive AGBs of [Fe/H]>-1.3 in the progenitor system evolve when all the
pristine gas surrounding the cluster has been exhausted by the previous star
formation events, or the proto--cluster interaction with the Galaxy caused the
loss of a significant fraction of its mass, or of its dark matter halo, and the
supernova ejecta have been able to clear the gas out of the system. The absence
of dilution in the metal richer populations lends further support to a scenario
of the formation of second generation stars in cooling flows from massive AGB
progenitors. We suggest that the entire formation of omega Cen took place in a
few 10^8yr, and discuss the problem of a prompt formation of s--process
elements.Comment: The Astrophysical Journal, in pres
A Double Main Sequence in the Globular Cluster NGC 6397
High-precision multi-band HST photometry reveals that the main sequence (MS)
of the globular cluster NGC 6397 splits into two components, containing ~30%
and ~70% of the stars. This double sequence is consistent with the idea that
the cluster hosts two stellar populations: (i) a primordial population that has
a composition similar to field stars, and containing ~30% of the stars, and
(ii) a second generation with enhanced sodium and nitrogen, depleted carbon and
oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the
color difference between the two sequences across a variety of color baselines
and find that the second sequence is anomalously faint in m_F336W. Theoretical
isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap
A unique model for the variety of multiple populations formation(s) in globular clusters: a temporal sequence
We explain the multiple populations recently found in the 'prototype'
Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch
(AGB) scenario. The chemistry of the five -or more- populations is
approximately consistent with a sequence of star formation events, starting
after the supernovae type II epoch, lasting approximately until the time when
the third dredge up affects the AGB evolution (age ~90-120Myr), and ending when
the type Ia supernovae begin exploding in the cluster, eventually clearing it
from the gas. The formation of the different populations requires episodes of
star formation in AGB gas diluted with different amounts of pristine gas. In
the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV
Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in
the smallest mass AGB ejecta involved in the star formation of this population.
The possibly-iron-rich small population in NGC 2808 may be a result of
contamination by a single type Ia supernova. The NGC 2808 case is used to build
a general framework to understand the variety of 'second generation' stars
observed in GCs. Cluster-to-cluster variations are ascribed to differences in
the effects of the many processes and gas sources which may be involved in the
formation of the second generation. We discuss an evolutionary scheme, based on
pollution by delayed type II supernovae, which accounts for the properties of
s-Fe-anomalous clusters.Comment: 20 pages, 7 figures, in press on MNRA
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