Browsing by Author "Contreras Ramos, Rodrigo"
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- ItemHelium 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, SanziaThe 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....
- ItemMultiple 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, MárcioSince 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....
- ItemNear-IR period-luminosity relations for pulsating stars in ω Centauri (NGC 5139) (Corrigendum)(2017) Navarrete, C.; Catelan, Márcio; Contreras Ramos, Rodrigo; Alonso-García, J.; Gran, F.; Dékány, I.; Minniti, D.Erratum for: "An incorrect zero point for the empirical period-luminosity-metallicity (PL-Z) relations for RR Lyrae (RRL) stars in ω Centauri (NGC 5139) was inadvertently reported in Table 2 and Eqs. (4) and (5) of our original paper. The corrected table and equations are given below. The actual analysis that is described in the paper was based on the correct zero points, and so all of the results reported in our original publication remain unchanged".
- ItemNew type II Cepheids from VVV data towards the Galactic center(2019) Braga, V. F.; Contreras Ramos, Rodrigo; Minniti, D.; Ferreira Lopes, C. E.; Catelan, Márcio; Minniti, J. H.; Nikzat, F.; Zoccali, M.Context. The Galactic center (GC) is the densest region of the Milky Way. Variability surveys towards the GC potentially provide the largest number of variable stars per square degree within the Galaxy. However, high stellar density is also a drawback due to blending. Moreover, the GC is affected by extreme reddening, therefore near infrared observations are needed. Aims: We plan to detect new variable stars towards the GC, focusing on type II Cepheids (T2Cs) which have the advantage of being brighter than RR Lyrae stars. Methods: We perform parallel Lomb-Scargle and Generalized Lomb-Scargle periodogram analysis of the Ks-band time series of the VISTA variables in the Vía Láctea survey, to detect periodicities. We employ statistical parameters to clean our sample. We take account of periods, light amplitudes, distances, and proper motions to provide a classification of the candidate variables. Results: We detected 1019 periodic variable stars, of which 164 are T2Cs, 210 are Miras and 3 are classical Cepheids. We also found the first anomalous Cepheid in this region. We compare their photometric properties with overlapping catalogs and discuss their properties on the color-magnitude and Bailey diagrams. Conclusions: We present the most extensive catalog of T2Cs in the GC region to date. Offsets in E(J - Ks) and in the reddening law cause very large (∼1-2 kpc) uncertainties on distances in this region. We provide a catalog which will be the starting point for future spectroscopic surveys in the innermost regions of the Galaxy. Full Tables 2 and 3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A151....
- ItemPulsating stars in ω Centauri. Near-IR properties and period-luminosity relations(2017) Navarrete, Camila; Catelan, Marcio; Contreras Ramos, Rodrigo; Alonso-García, Javier; Gran, Felipe; Dékány, István; Minniti, Danteω Centauri (NGC 5139) contains many variable stars of different types, including the pulsating type II Cepheids, RR Lyrae and SX Phoenicis stars. We carried out a deep, wide-field, near-infrared (IR) variability survey of ω Cen, using the VISTA telescope. We assembled an unprecedented homogeneous and complete J and KS near-IR catalog of variable stars in the field of ω Cen. In this paper we compare optical and near-IR light curves of RR Lyrae stars, emphasizing the main differences. Moreover, we discuss the ability of near-IR observations to detect SX Phoenicis stars given the fact that the amplitudes are much smaller in these bands compared to the optical. Finally, we consider the case in which all the pulsating stars in the three different variability types follow a single period-luminosity relation in the near-IR bands....
- ItemRelative 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, ManuelaRecently, 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....
- ItemStellar Variability in the VVV Survey: Highlights and Current Status(2015) Catelan, Marcio; Dékány, István; Minniti, Dante; Alonso-Garcia, Javier; Contreras Ramos, Rodrigo; Saito, Roberto; Hajdu, Gergely; Lucas, Phil; Kerins, Eamonn; Hempel, MarenThe Vista Variables in the Via Lactea (VVV) ESO Public Survey consists in a near-infrared time-series survey of the Galactic bulge and inner disk, covering 562 square degrees of the sky, over a total timespan of more than 5 years. In this contribution, we discuss some of our latest results, and provide an updated account of the current status of the survey, especially in the context of stellar variability studies....
- ItemVariable stars in the VVV globular clusters. II. NGC 6441, NGC 6569, NGC 6626 (M 28), NGC 6656 (M 22), 2MASS-GC 02, and Terzan 10(2021) Alonso-García, Javier; Smith, Leigh C.; Catelan, Márcio; Minniti, Dante; Navarrete, Camila; Borissova, Jura; Carballo-Bello, Julio A.; Contreras Ramos, Rodrigo; Fernández-Trincado, José G.; Ferreira Lopes, Carlos E.; Gran, Felipe; Garro, Elisa R.; Geisler, Doug; Guo, Zhen; Hempel, Maren; Kerins, Eamonn; Lucas, Philip W.; Palma, Tali; Peña Ramírez, Karla; Ramírez Alegría, Sebastián; Saito, Roberto K.Context. The Galactic globular clusters (GGCs) located in the inner regions of the Milky Way suffer from high extinction that makes their observation challenging. High densities of field stars in their surroundings complicate their study even more. The VISTA Variables in the Via Lactea (VVV) survey provides a way to explore these GGCs in the near-infrared where extinction effects are highly diminished. Aims: We conduct a search for variable stars in several inner GGCs, taking advantage of the unique multi-epoch, wide-field, near-infrared photometry provided by the VVV survey. We are especially interested in detecting classical pulsators that will help us constrain the physical parameters of these GGCs. In this paper, the second of a series, we focus on NGC 6656 (M 22), NGC 6626 (M 28), NGC 6569, and NGC 6441; these four massive GGCs have known variable sources, but quite different metallicities. We also revisit 2MASS-GC 02 and Terzan 10, the two GGCs studied in the first paper of this series. Methods: We present an improved method and a new parameter that efficiently identify variable candidates in the GGCs. We also use the proper motions of those detected variable candidates and their positions in the sky and in the color-magnitude diagrams to assign membership to the GGCs. Results: We identify and parametrize in the near-infrared numerous variable sources in the studied GGCs, cataloging tens of previously undetected variable stars. We recover many known classical pulsators in these clusters, including the vast majority of their fundamental mode RR Lyrae. We use these pulsators to obtain distances and extinctions toward these objects. Recalibrated period-luminosity-metallicity relations for the RR Lyrae bring the distances to these GGCs to a closer agreement with those reported by Gaia, except for NGC 6441, which is an uncommon Oosterhoff III GGC. Recovered proper motions for these GGCs also agree with those reported by Gaia, except for 2MASS-GC 02, the most reddened GGC in our sample, where the VVV near-infrared measurements provide a more accurate determination of its proper motions. Lightcurves and full Tables 2, 5, 6, 7, 8, and 9 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A47...
- ItemVariable Stars in VVV Globular Clusters(2015) Contreras Ramos, Rodrigo; Alonso-Garcia, Javier; Minniti, Dante; Catelan, Márcio; Zoccali, Manuela; Gran, Felipe; Dekany, IstvanThe Vista Variables in the Via Lactea (VVV) is an ongoing variability survey observing the central regions of our Galaxy in the near-infrared by means of the VISTA telescope. Numerous (36) inner Galactic globular clusters fall inside the area covered by the VVV survey. Most of these clusters, especially the faintest ones, have been poorly studied in the past due to the difficulties caused by the presence of an elevated extinction and high field stellar densities in their lines of sight. We will present the first results of a search for variable stars we are conducting in some of these clusters. Our main aim is to obtain a better determination of the physical parameters of these globular clusters and put constrains on the formation scenario of our galaxy using the information provided by their variables stars.