1,186 research outputs found
Evidence of the inhomogeneity of the stellar population in the differentially reddened globular cluster NGC 3201
We report on evidence of the inhomogeneity (multiplicity) of the stellar
population in the Galactic globular cluster (GC) NGC 3201, which is irregularly
reddened across its face. We carried out a more detailed and careful analysis
of our recently published new multi-color photometry in a wide field of the
cluster with particular emphasis on the U band. Using the photometric data
corrected for differential reddening, we found for the first time two key signs
of the inhomogeneity in the cluster's stellar population and of its radial
variation in the GC. These are (1) an obvious trend in the color-position
diagram, based on the (U-B) color-index, of red giant branch (RGB) stars, which
shows that the farther from the cluster's center, the bluer on average the
(U-B) color of the stars is; and (2) the dependence of the radial distribution
of sub-giant branch (SGB) stars in the cluster on their U magnitude, where
brighter stars are less centrally concentrated than their fainter counterparts
at a confidence level varying between 99.2% and 99.9% depending on the
color-index used to select the stars. The same effects were recently found by
us in the GC NGC 1261. However, contrary to NGC 1261, we are not able to
unambiguously suggest which of the sub-populations of SGB/RGB stars can be the
progenitor of blue and red horizontal branch stars of the cluster. Apart from
M4, NGC 3201 is another GC very probably with an inhomogeneous stellar
population, which has essentially lower mass than the most massive Galactic GCs
where multiple stellar populations were unambiguously detected for the first
timeComment: 5 pages, 4 figure
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
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
The helium spread in the Globular cluster 47 Tuc
Spectroscopy has shown the presence of the CN band dicothomy and the Na-O
anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc,
otherwise considered a "normal" prototype of high metallicity clusters from the
photometric analysis. Very recently, the re-analysis of a large number of
archival HST data of the cluster core has been able to put into evidence the
presence of structures in the Sub Giant Branch: it has a brighter component
with a spread in magnitude by 0.06 mag and a second one, made of about
10% of stars, a little fainter (by 0.05 mag). These data also show that
the Main Sequence of the cluster has an intrinsic spread in color which, if
interpreted as due to a small spread in helium abundance, suggests
Y0.027. In this work we examine in detail whether the Horizontal
Branch morphology and the Sub Giant structure provide further independent
indications that a real --although very small-helium spread is present in the
cluster. We re--analyze the HST archival data for the Horizontal Branch of 47
Tuc, obtaining a sample of 500 stars with very small photometric errors,
and build population synthesis based on new models to show that its particular
morphology can be better explained by taking into account a spread in helium
abundance of 2% in mass. The same variation in helium is able to explain the
spread in luminosity of the Sub Giant Branch, while a small part of the second
generation is characterized by a small C+N+O increase and provides an
explanation for the fainter Sub Giant Branch. We conclude that three
photometric features concur to form the paradigm that a small but real helium
spread is present in a cluster that has no spectacular evidence for multiple
populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010
May 19; in original form 2010 February 9. 7 pages and 3 figures. No table
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
The Near Infrared NaI Doublet Feature in M Stars
The NaI near-infrared doublet has been used to indicate the dwarf/giant
population in composite systems, but its interpretation is still a contentious
issue. In order to understand the behaviour of this controversial feature, we
study the observed and synthetic spectra of cool stars. We conclude that the
NaI infrared feature can be used as a dwarf/giant discriminator. We propose a
modified definition of the NaI index by locating the red continuum at 8234
angstrons and by measuring the equivalent width in the range 8172-8197
angstrons, avoiding the region at lambda > 8197 angstrons, which contains VI,
ZrI, FeI and TiO lines. We also study the dependence of this feature on stellar
atmospheric parameters.Comment: 9 pages, (TeX file) + 7 Figures in Postscript format. Accepted for
publication in The Astrophysical Journa
Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence
We present a spectroscopic analysis of main sequence (MS) stars in the young
globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our
photometric survey on Magellanic Clouds clusters has revealed that NGC1818,
similarly to the other young objects with age 600 Myrs, displays not only an
extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2
Gyrs), but also a split MS. The most straightforward interpretation of the
double MS is the presence of two stellar populations: a sequence of
slowly-rotating stars lying on the blue-MS and a sequence of fast rotators,
with rotation close to the breaking speed, defining a red-MS. We report the
first direct spectroscopic measurements of projected rotational velocities
vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of
line profiles includes non-LTE effects, required for correctly deriving v sini
values. Our results suggest that: (i) the mean rotation for blue- and red-MS
stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91
km/s), respectively; (ii) eMSTO stars have different vsini, which are generally
lower than those inferred for red-MS stars, and (iii) as expected, Be stars
display the highest vsini values. This analyis supports the idea that distinct
rotational velocities play an important role in the appearence of multiple
stellar populations in the color-magnitude diagrams of young clusters, and
poses new constraints to the current scenarios.Comment: 16 pages, 1 table, 9 figures. Accepted for publication in AJ
(11/07/2018
The extended Main Sequence Turn Off cluster NGC1856: rotational evolution in a coeval stellar ensemble
Multiple or extended turnoffs in young clusters in the Magellanic Clouds have
recently received large attention. A number of studies have shown that they may
be interpreted as the result of a significant age spread (several 10^8yr in
clusters aged 1--2 Gyr), while others attribute them to a spread in stellar
rotation. We focus on the cluster NGC 1856, showing a splitting in the upper
part of the main sequence, well visible in the color m_{F336W}-m_{F555W}$, and
a very wide turnoff region. Using population synthesis available from the
Geneva stellar models, we show that the cluster data can be interpreted as
superposition of two main populations having the same age (~350Myr), composed
for 2/3 of very rapidly rotating stars, defining the upper turnoff region and
the redder main sequence, and for 1/3 of slowly/non-rotating stars. Since rapid
rotation is a common property of the B-A type stars, the main question raised
by this model concerns the origin of the slowly/non-rotating component. Binary
synchronization is a possible process behind the slowly/non-rotating
population; in this case, many slowly/non-rotating stars should still be part
of binary systems with orbital periods in the range from 4 to 500 days. Such
periods imply that Roche lobe overflow occurs, during the evolution of the
primary off the main sequence, so most primaries may not be able to ignite core
helium burning, consistently why the lack of a red clump progeny of the slowly
rotating population.Comment: 8 pages 4 figures, accepted for publication on Monthly Notices of the
R.A.
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