Chemical inhomogeneities amongst first population stars in globular clusters Evidence for He variations

Spreads in light element abundances among stars (also known as multiple populations) are observed in nearly all globular clusters. One way to map such chemical variations using high-precision photometry is to employ a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters (called the “chromosome map”), to maximise the separation between the different multiple populations. For each individual cluster its chromosome map separates the first population (with metal abundance patterns typical of field halo stars) from the second population (which displays distinctive abundance variations among a specific group of light elements). Surprisingly, the distribution of first population stars in chromosome maps of several but not all clusters has been found to be more extended than expected from purely observational errors, suggesting a chemically inhomogeneous origin. We consider here three clusters with similar metallicity ([Fe/H] ~ −1.3) and different chromosome maps, namely NGC 288, M 3, and NGC 2808, and argue that the first population extended distribution (as observed in two of these clusters) is due to spreads of the initial helium abundance and possibly a small range of nitrogen abundances as well. The presence of a range of initial He and N abundances amongst stars traditionally thought to have homogeneous composition, and that these spreads appear only in some clusters, challenges the scenarios put forward so far to explain the multiple population phenomenon

High-precision abundances of first population stars in NGC 2808: confirmation of a metallicity spread

Photometric investigations have revealed that Galactic globular clusters exhibit internal metallicity variations amongst the so-called first-population stars, until now considered to have a homogeneous initial chemical composition. This is not fully supported by the sparse spectroscopic evidence, which so far gives conflicting results. Here, we present a high-resolution re-analysis of five stars in the Galactic globular cluster NGC 2808 taken from the literature. Target stars are bright red giants with nearly identical atmospheric parameters belonging to the first population according to their identification in the chromosome map of the cluster, and we have measured precise differential abundances for Fe, Si, Ca, Ti, and Ni to the ~0.03 dex level. Thanks to the very small uncertainties associated to the differential atmospheric parameters and abundance measurements, we find that target stars span a range of iron abundance equal to 0.25 +/- 0.06 dex. The individual elemental abundances are highly correlated with the position of the star along the extended sequence described by first population objects in the cluster chromosome map: bluer stars have a lower iron content. This agrees with inferences from the photometric analysis. The differential abundances for all other elements also show statistically significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti, and Ni variations are highly correlated with iron variations and the total abundance spreads for all elements are consistent within the error bars. This suggests a scenario in which short-lived massive stars exploding as supernovae contributed to the self-enrichment of the gas in the natal cloud while star formation was still ongoing.Comment: 9 pages, 9 figures. Accepted for publication in A&

Semantic Web Technologies Meet BIM for Accessing and Understanding Cultural Heritage

Within the EU funded project INCEPTION – Inclusive Cultural Heritage in Europe through 3D semantic modelling, the key-targeted achievement is the development of a specific cloud based platform, in order to accomplish the main objectives of accessing, understanding and strengthening European Cultural Heritage by means of enriched 3D models. The whole INCEPTION project is based on the close connection between state-of-the-art architectural modeling technologies (BIM, Building Information Modeling) and the latest cutting-edge web technologies. The platform is grounded on semantic web technologies and makes extensive use of WebGL and RESTful APIs, in order to enrich heritage 3D models by using Semantic Web standards. The INCEPTION platform will be a space for interchange of information and for the dialogue among professionals, students, scholars, curators, non-expert users, etc. Furthermore, the Semantic Web structure interlinks the platform with external Cultural Heritage available linked data and makes it gradually enhanced by specific flexible data structures provided as project specific ontologies. The paper will describe solutions based on the match between BIM, Cloud and Semantic Web

High-precision abundances of first-population stars in NGC 2808: confirmation of a metallicity spread

Photometric investigations have revealed that Galactic globular clusters (GCs) exhibit internal metallicity variations amongst the so- called first-population stars, which until now were considered to have a homogeneous initial chemical composition. This is not fully supported by the sparse spectroscopic evidence, which so far gives conflicting results. Here, we present a high-resolution re-analysis of five stars in the Galactic GC NGC 2808 taken from the literature. Target stars are bright red giants with nearly identical atmospheric parameters belonging to the first population according to their identification in the chromosome map of the cluster, and we measured precise differential abundances for Fe, Si, Ca, Ti, and Ni to the ∼0.03 dex level. Thanks to the very small uncertainties associated with the differential atmospheric parameters and abundance measurements, we find that target stars span a range of iron abundance equal to 0.25 ± 0.06 dex. The individual elemental abundances are highly correlated with the positions of the stars along the extended sequence described by first-population objects in the cluster chromosome map: bluer stars have a lower iron content. This agrees with inferences from the photometric analysis. The differential abundances of all other elements also show statistically significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti, and Ni variations are highly correlated with iron variations and the total abundance spreads for all elements are consistent within the error bars. This suggests a scenario in which short-lived massive stars exploding as supernovae contributed to the self-enrichment of the gas in the natal cloud while star formation was still ongoing

Expanding the Time Domain of Multiple Populations: Evidence of Nitrogen Variations in the ~1.5 Gyr Old Star Cluster NGC 1783

We present the result of a detailed analysis of Hubble Space Telescope UV and optical deep images of the massive and young (~1.5 Gyr) stellar cluster NGC 1783 in the Large Magellanic Cloud. This system does not show evidence of multiple populations (MPs) along the red giant branch (RGB) stars. However, we find that the cluster main sequence (MS) shows evidence of a significant broadening (50% larger than what is expected from photometric errors) along with hints of possible bimodality in the MP sensitive (m F343N - m F438W, m F438W) color-magnitude diagram (CMD). Such an effect is observed in all color combinations including the m F343N filter, while it is not found in the optical CMDs. This observational evidence suggests we might have found light-element chemical abundance variations along the MS of NGC 1783, which represents the first detection of MPs in a system younger than 2 Gyr. A comparison with isochrones including MP-like abundances shows that the observed broadening is compatible with a N abundance enhancement of ?([N/Fe]) ~ 0.3. Our analysis also confirms previous results about the lack of MPs along the cluster RGB. However, we find that the apparent disagreement between the results found on the MS and the RGB is compatible with the mixing effects linked to the first dredge up. This study provides new key information about the MP phenomenon and suggests that star clusters form in a similar way at any cosmic age

No evidence for younger stellar generations within the intermediate-age massive clusters NGC 1783, NGC 1806 and NGC 411

Recently, Li et al. claimed to have found evidence for multiple generations of stars in the intermediate-age clusters NGC 1783, NGC 1806 and NGC 411 in the Large and Small Magellanic Clouds. Here we show that these young stellar populations are present in the field regions around these clusters and are not likely associated with the clusters themselves. Using the same data sets, we find that the background subtraction method adopted by the authors does not adequately remove contaminating stars in the small number Poisson limit. Hence, we conclude that their results do not provide evidence of young generations of stars within these clusters

High-precision abundances of first-population stars in NGC 2808: Confirmation of a metallicity spread

Photometric investigations have revealed that Galactic globular clusters (GCs) exhibit internal metallicity variations amongst the so-called first-population stars, which until now were considered to have a homogeneous initial chemical composition. This is not fully supported by the sparse spectroscopic evidence, which so far gives conflicting results. Here, we present a high-resolution re-analysis of five stars in the Galactic GC NGC 2808 taken from the literature. Target stars are bright red giants with nearly identical atmospheric parameters belonging to the first population according to their identification in the chromosome map of the cluster, and we measured precise differential abundances for Fe, Si, Ca, Ti, and Ni to the ∼0.03 dex level. Thanks to the very small uncertainties associated with the differential atmospheric parameters and abundance measurements, we find that target stars span a range of iron abundance equal to 0.25 ± 0.06 dex. The individual elemental abundances are highly correlated with the positions of the stars along the extended sequence described by first-population objects in the cluster chromosome map: Bluer stars have a lower iron content. This agrees with inferences from the photometric analysis. The differential abundances of all other elements also show statistically significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti, and Ni variations are highly correlated with iron variations and the total abundance spreads for all elements are consistent within the error bars. This suggests a scenario in which short-lived massive stars exploding as supernovae contributed to the self-enrichment of the gas in the natal cloud while star formation was still ongoing

Expanding the Time Domain of Multiple Populations: Evidence of Nitrogen Variations in the similar to 1.5 Gyr Old Star Cluster NGC 1783

We present the result of a detailed analysis of Hubble Space Telescope UV and optical deep images of the massive and young (∼1.5 Gyr) stellar cluster NGC 1783 in the Large Magellanic Cloud. This system does not show evidence of multiple populations (MPs) along the red giant branch (RGB) stars. However, we find that the cluster main sequence (MS) shows evidence of a significant broadening (50% larger than what is expected from photometric errors) along with hints of possible bimodality in the MP sensitive (mF343N − mF438W, mF438W) color–magnitude diagram (CMD). Such an effect is observed in all color combinations including the mF343N filter, while it is not found in the optical CMDs. This observational evidence suggests we might have found light-element chemical abundance variations along the MS of NGC 1783, which represents the first detection of MPs in a system younger than 2 Gyr. A comparison with isochrones including MP-like abundances shows that the observed broadening is compatible with a N abundance enhancement of Δ([N/Fe]) ∼ 0.3. Our analysis also confirms previous results about the lack of MPs along the cluster RGB. However, we find that the apparent disagreement between the results found on the MS and the RGB is compatible with the mixing effects linked to the first dredge up. This study provides new key information about the MP phenomenon and suggests that star clusters form in a similar way at any cosmic age

The Search for Multiple Populations in Magellanic Cloud Clusters II: The Detection of Multiple Populations in Three Intermediate-Age SMC Clusters

This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45% whereas it is ~25% in NGC 339 and ~36% in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way

The search for multiple populations in Magellanic Cloud Clusters IV: Coeval multiple stellar populations in the young star cluster NGC 1978

We have recently shown that the $\sim2$ Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the sub-giant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Due to its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star-formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star-formation epochs have occurred within NGC 1978. First, we use UV CMDs to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of $1\pm20$ Myr between them. This is in tension with predictions from the AGB scenario for the origin of multiple populations. Second, we estimate the broadness of the main sequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of $\sim$65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate age clusters, while it fully supports predictions from the stellar rotation model