1,068 research outputs found
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 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
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
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
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.
Vibration Serviceability Assessment of a Historic Suspension Footbridge
Experimental and numerical studies for the structural and vibration serviceability assessment of a historic suspension footbridge adopting non-invasive surveys and low-cost equipment are presented. Field surveys have been carried out to determine geometric properties, ambient vibration tests have been performed to estimate the dynamic properties, and the dynamic response of the footbridge under the action of a single crossing pedestrian has been recorded. Based on field surveys, a 3D Finite Element model was built and was then calibrated against ambient vibration test results. The experimentally-measured maximum acceleration under the action of one crossing pedestrian is compared with the ones obtained numerically and analytically. Furthermore, vibration serviceability assessment under multi-pedestrian loading is carried out, adopting the simplified procedure recommended by a recent guideline. Results show that low-cost non-invasive dynamic testing is suitable to correctly identify the footbridge vertical natural frequencies and mode shapes, including higher-order ones, and to draw considerations about the state of degradation of the structure. Moreover, the level of vibration under the action of a single pedestrian can be estimated with sufficient accuracy using a simplified loading model, provided that the modal damping ratio is properly tuned
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