891 research outputs found
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 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 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
Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419
Recent work revealed that both the helium variation within globular clusters
(GCs) and the relative numbers of first and second-generation stars (1G, 2G)
depend on the mass of the host cluster. Precise determination of the internal
helium variations and of the fraction of 1G stars are crucial constraints to
the formation scenarios of multiple populations (MPs). We exploit multi-band
Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one
of the most-massive and distant GCs of the Galaxy, almost isolated from its
tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and
identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which
comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the
observed colors of these four populations with the colors derived from
appropriate synthetic spectra to infer the relative helium abundances. We find
that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY
~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium
(Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most
of the current scenarios for MPs. Furthermore, the relatively larger fraction
of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting
literature results, we find that the fractions of 1G stars of GCs with large
perigalactic distance are typically higher than in the other GCs with similar
masses. This suggests that NGC 2419, similarly to other distant GCs, lost a
lower fraction of 1G stars.Comment: 10 pages, 8 figures, submitted to MNRAS January 22n
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 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.
Extended main sequence turnoff as a common feature of Milky Way open clusters
We present photometric analysis of twelve Galactic open clusters and show
that the same multiple-population phenomenon observed in Magellanic Clouds
(MCs) is present in nearby open clusters. Nearly all the clusters younger than
2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and
all the cluster younger than 700 Myr show broadened/split main sequences
(MSs). High-resolution spectroscopy has revealed that these clusters host stars
with a large spread in the observed projected rotations. In addition to
rotation, internal age variation is indicated as a possible responsible for the
eMSTOs, making these systems the possible young counterparts of globular
clusters with multiple populations. Recent work has shown that the
eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric
features have been discovered in a few Galactic open clusters, challenging the
idea that the color-magnitude diagrams (CMDs) of these systems are similar to
single isochrones and opening new windows to explore the eMSTO phenomenon. We
exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open
clusters younger than 1.5 Gyr. Our analysis suggests that: (i) twelve
open clusters show eMSTOs and/or broadened MSs, that cannot be due neither to
field contamination, nor binaries; (ii) split/broadened MSs are observed in
clusters younger than 700 Myr, while older objects display only an eMSTO,
similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age
spread, increases with age, following the relation observed in MCs clusters and
demonstrating that rotation is the responsible for this phenomenon.Comment: 17 pages, 42 figures, 1 table, accepted for publication in ApJ
(31/10/2018
Evaluating GAIA performances on eclipsing binaries. III. Orbits and stellar parameters for UW LMi, V432 Aur and CN Lyn
The orbits and physical parameters of three detached F and G-type eclipsing
binaries have been derived combining Hipparcos H(P) photometry with 8480-8740
Ang ground-based spectroscopy, simulating the photometric+spectroscopic
observations that the GAIA mission will obtain. Tycho B(T) and V(T) light
curves are too noisy to be modeled for the three targets, and only mean Tycho
colors are retained to constrain the temperature. No previous combined
photometric+spectroscopic solution exists in literature for any of the three
targets. Quite remarkably, CN Lyn turned out to be an equal masses F5 triple
system. Distances from the orbital solutions agree within the astrometric error
with the Hipparcos parallaxes.Comment: A&A, accepted in pres
Evaluating GAIA performances on eclipsing binaries. II. Orbits and stellar parameters for V781 Tau, UV Leo and GK Dra
The orbits and physical parameters of three close, double-lined G0 eclipsing
binaries have been derived combining H_P, V_T, B_T photometry from the
Hipparcos/Tycho mission with 8480-8740 \AA ground-based spectroscopy. The setup
is mimicking the photometric and spectroscopic observations that should be
obtained by GAIA. The binaries considered here are all of G0 spectral type, but
each with its own complications: V781 Tau is an overcontact system with
components of unequal temperature, UV Leo shows occasional surface spots and GK
Dra contains a delta-Scuti variable. Such peculiarities will be common among
binaries to be discovered by GAIA. We find that the values of masses, radii and
temperatures for such stars can be derived with a 1-2% accuracy using the
adopted GAIA-like observing mode.Comment: accepted for publication in Astronomy and Astrophysics, contains 5
Tables and 3 Figures; correction of an important typ
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