Browsing by Author "Valcarce, Aldo A. R."

Now showing 1 - 11 of 11
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    Disentangling multiple stellar populations in globular clusters using the Strömgren system
    (2015) Alonso-García, J.; Catelan, Márcio; Amigo, P.; Cortés, C.; Kuehn, C. A.; Grundahl, F.; López, G.; Salinas, R.; Smith, H. A.; Stetson, P. B.; Sweigart, Allen V.; Valcarce, Aldo A. R.; Zoccali, M.
    An increasing amount of spectroscopic and photometric evidence is showing that the stellar populations of globular clusters are not as simple as they have been considered for many years. The presence of at least two different populations of stars is being discovered in a growing number of globular clusters, both in our Galaxy and in others. We have started a series of observations of Galactic globular clusters using the Strömgren photometric system in order to find the signatures of these multiple populations and establish their presence in a more complete sample of globular clusters in the Milky Way, and to study their radial distributions and extensions. We present here the first results of our survey....
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    Effects of Helium Enrichment in Globular Cluster Populations
    (2011) Valcarce, Aldo A. R.; Catelan, Márcio; Sweigart, Allen V.
    Recently, the understanding of globular clusters (GCs) has drastically changed owing to the improvement in spectroscopic and photometric observations, which have shown that not all GCs could be considered simple stellar populations. Whilst spectroscopic studies have shown variations of some light elements in different degrees in all observed GCs (e.g., O-Na anticorrelation, Carretta et al. 2009), photometric studies have shown splits in some phases of color-magnitude diagrams (CMDs), where the triple main sequence (MS) detected in NGC 2808 has been attributed to differences in the helium abundance (ΔY, Piotto et al. 2007), which cannot be measured with enough precision in GCs. In this contribution, we show the other effects that must be observed in CMDs of GCs if ΔY is real....
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    Fundamental properties of nearby stars and the consequences on ΔY/ΔZ
    (2013) Valcarce, Aldo A. R.; Catelan, Márcio; De Medeiros, J. R.
    Context. One of the greatest difficulties in astrophysics is the determination of the fundamental stellar parameters, one of which is the initial mass fraction of helium (Y). However, given that Y can be measured spectroscopically in only a small percentage of stars, a linear relationship is assumed between Y and the mass fraction of metals (Z) from a canonical perspective of the chemical evolution of the galaxies. This Y-Z relation is generally represented as Y = Yp + ΔY/ΔZ × Z, with the value of the helium-to-metal enrichment ratio (ΔY/ΔZ) assumed as a constant. However, there is no fundamental reason for every star to have a Y value on a linear scale with Z. Indeed, different ΔY/ΔZ values may be expected in different populations which have undergone different chemical enrichment histories. Aims: In this paper a new method for determining the fundamental stellar parameters of nearby stars is presented that uses at the same time Mbol, Teff, and log 1. One of these parameters is Y, which is used to determine the validity of the Y-Z relation. Methods: A new set of evolutionary tracks is created using the PGPUC stellar evolution code, which includes 7 masses (0.5 ≤ M/M⊙ ≤ 1.1), 7 helium abundances (0.230 ≤ Y ≤ 0.370), and 12 metallicities (1.6 × 10-4 ≤ Z ≤ 6.0 × 10-2) for solar-scaled chemical compositions ([α/Fe] = 0.0). The suggested method is tested using two different spectroscopic databases of nearby main sequence stars with precise parallaxes, and spectroscopic measurements of [Fe/H], Teff and 1. Results: The proposed method is compared to other techniques used to determine the fundamental stellar parameters, where one assumes an age of 5 Gyr for all nearby stars. This comparison demonstrates that the hypothesis regarding constant age leads to an underestimation of the Y value, especially for low metallicities. In addition, the suggested method is limited to masses above 0.60 M⊙ and requires high-precision measurements of spectroscopic surface gravities in order to obtain reliable results. Finally, estimating masses and Ages assuming a Y-Z relation rather than a free Y value may induce average errors of approximately 0.02 M⊙ and 2 Gyr, respectively....
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    Globular Cluster UVIT Legacy Survey (GlobULeS). III. Omega Centauri in Far-ultraviolet
    (2022) Prabhu, Deepthi S.; Subramaniam, Annapurni; Sahu, Snehalata; Chung, Chul; Leigh, Nathan W. C.; Dalessandro, Emanuele; Chatterjee, Sourav; Rao, N. Kameswara; Shara, Michael; Cote, Patrick; Choudhury, Samyaday; Pandey, Gajendra; Valcarce, Aldo A. R.; Singh, Gaurav; Postma, Joesph E.; Rani, Sharmila; Bandyopadhyay, Avrajit; Geller, Aaron M.; Hutchings, John; Puzia, Thomas; Simunovic, Mirko; Sohn, Young-Jong; Thirupathi, Sivarani; Yadav, Ramakant Singh
    We present the first comprehensive study of the most massive globular cluster, Omega Centauri, in the far-ultraviolet (FUV), extending from the center to similar to 28% of the tidal radius using the Ultraviolet Imaging Telescope on board AstroSat. A comparison of the FUV-optical color-magnitude diagrams with available canonical models reveals that horizontal branch (HB) stars bluer than the knee (hHBs) and the white dwarfs (WDs) are fainter in the FUV by similar to 0.5 mag than model predictions. They are also fainter than their counterparts in M13, another massive cluster. We simulated HB with at least five subpopulations, including three He-rich populations with a substantial He enrichment of Y up to 0.43 dex, to reproduce the observed FUV distribution. We find the He-rich younger subpopulations to be radially more segregated than the He-normal older ones, suggesting an in situ enrichment from older generations. The omega Cen hHBs span the same T (eff) range as their M13 counterparts, but some have smaller radii and lower luminosities. This may suggest that a fraction of omega Cen hHBs are less massive than those of M13, similar to the result derived from earlier spectroscopic studies of outer extreme HB stars. The WDs in omega Cen and M13 have similar luminosity-radius-T (eff) parameters, and 0.44-0.46 M (circle dot) He-core WD model tracks evolving from progenitors with Y = 0.4 dex are found to fit the majority of these. This study provides constraints on the formation models of omega Cen based on the estimated range in age, [Fe/H], and Y (in particular) for the HB stars.
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    Globular clusters as laboratories for stellar evolution
    (2010) Catelan, Márcio; Valcarce, Aldo A. R.; Sweigart, Allen V.
    Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and—crucially- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium-abundance indicators, and present an overview of available constraints on the helium abundance in globular clusters....
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    Helium enhancement among horizontal-branch stars and the near-UV color-magnitude diagrams
    (2015) Valcarce, Aldo A. R.; Catelan, Márcio; Alonso-García, Javier; Contreras Ramos, Rodrigo; Alves, Sanzia
    The distribution of horizontal branch stars in globular cluster color-magnitude diagrams is a sensitive probe of their original helium abundances. In this sense, they have been extensively used recently as indicators of possible variations in the helium content Y among the different populations of stars within individual globular clusters. However, recent analyses based on visual and near-ultraviolet color-magnitude diagrams have provided conflicting results. In order to clarify the situation, in this presentation we discuss the optimum ranges of applicability of this “HB Y test” in different filter systems, in terms of the temperature range covered by the horizontal branch stars. In particular, we discuss the case of M3 (NGC 5272), based on multi-band photometry (HST, Strömgren, Johnson), thus setting new and tight constraints on the internal range in Y within this cluster. We also provide an application of this "HB Y test" to other selected clusters....
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    Multiple populations in Galactic globular clusters from a Strömgren perspective
    (2014) Alonso-García, J.; Catelan, Márcio; Amigo, P.; Cortés, C.; Kuehn, C. A.; Grundahl, F.; López, G.; Salinas, R.; Smith, H. A.; Stetson, P. B.; Sweigart, Allen V.; Valcarce, Aldo A. R.; Zoccali, M.
    Our view of the stellar populations in globular clusters has radically changed in the last years. Now we are coming to believe that all globular clusters host at least two distinct stellar populations. We present in this contribution preliminary results of a photometric survey we are currently conducting to disentangle photometrically the several populations in a significant number of Galactic globular clusters, using the Strömgren system, aiming to characterize their relative ratios and their radial distributions....
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    Multiple populations in Galactic globular clusters: a survey in the Strömgren system
    (2013) Alonso-García, J.; Catelan, Márcio; Amigo, P.; Cortés, C.; Kuehn, C. A.; Grundahl, F.; López, G.; Salinas, R.; Smith, H. A.; Stetson, P. B.; Sweigart, Allen V.; Valcarce, Aldo A. R.; Zoccali, M.
    We are coming to believe that stellar populations in globular clusters are not as simple as they were once thought to be. A growing amount of photometric and spectroscopic evidence shows that globular clusters host at least two different stellar populations. In our contribution to these proceedings we present the first results of a survey we are conducting to look for the presence of multiple populations in a significant number of Galactic globular clusters, using the Strömgren system. We intend to photometrically separate these populations and characterize their radial distributions and extensions....
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    Multiple populations in M3: chemical abundances in AGB stars
    (2015) Alves, Sanzia; Contreras Ramos, Rodrigo; Amigo, Pia; Valcarce, Aldo A. R.; Cortés, Cristian; Hajdu, Gergely; Jurcsik, Johanna; Zoccali, Manuela; Catelan, Márcio
    Since the discovery of multiple populations in globular clusters, it is widely accepted that most globulars have at least two populations that can be classified in terms of their Na abundance, with the Na-poor stars belonging to the first stellar population, and the Na-rich stars to a second population. In the case of NGC 6752, it has been found that all the Na-rich stars fail to reach the asymptotic giant branch (AGB) phase, thus suggesting that the second-generation stars in this cluster all become so-called AGB-manqué stars after a hot horizontal branch (HB) phase. In contrast, stars belonging to both populations are present on the AGB phase in 47 Tucanae, which is consistent with the lack of a sizeable population of very hot HB stars in this cluster. In this contribution, we use high-resolution spectra obtained with UVES@VLT to analyze the chemical behavior of AGB stars in M3 (NGC 5272), a globular cluster with an HB morphology intermediate between those of NGC 6752 and 47 Tuc, to check whether second-generation stars in this cluster undergo an AGB-manqué phase as in NGC 6752, or whether instead they are able to successfully reach the AGB, as seen in 47 Tuc....
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    Relative ratios and radial distributions of the multiple populations in the Galactic globular clusters
    (2015) Alonso-Garcia, Javier; Catelan, Marcio; Amigo, Pía; Contreras Ramos, Rodrigo; Cortés, Cristián; Kuehn, Charles; Grundahl, Frank; López, Graciela; Salinas, Ricardo; Smith, Horace; Stetson, Peter; Sweigart, Allen; Valcarce, Aldo A. R.; Zoccali, Manuela
    Recently, the long-standing paradigm that globular clusters are close approximations to simple stellar populations, with all stars formed at precisely the same time from populations having precisely the same chemical composition, has been shattered by a series of photometric and spectroscopic observations that reveal that these objects are more complex than we once thought . In this poster, we present the first results of a survey we are conducting among the Galactic globular clusters using the Strömgren photometric system. We show that the bluest Strömgren filters reveal broadenings or splits in the stellar sequences in the color-magnitude diagrams of the sampled clusters. These features allow us to disentangle successfully the different stellar populations in a given globular cluster, which let us measure their relative ratios, and explore their radial trends and gradients from the cluster center out to its tidal radius....
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    Study of the Helium Enrichment in Globular Clusters
    (2010) Valcarce, Aldo A. R.; Catelan, Marcio
    Globular clusters (GCs) are spheroidal concentrations typically containing of the order of 105 to 106, predominantly old, stars. Historically, they have been considered as the closest counterparts of the idealized concept of “simple stellar populations.” However, some recent observations suggest than, at least in some GCs, some stars are present that have been formed with material processed by a previous generation of stars. In this sense, it has also been suggested that such material might be enriched in helium, and that blue horizontal branch stars in some GCs should accordingly be the natural progeny of such helium-enhanced stars. In this contribution we show that, at least in the case of M3 (NGC 5272), the suggested level of helium enrichment is not supported by the available, high-precision observations....