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 $\sim$2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and all the cluster younger than $\sim$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 $\sim$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 $\sim$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