Fourier Decomposition of RR Lyrae light curves and the SX Phe population in the central region of NGC 3201

CCD time-series observations of the central region of the globular cluster NGC~3201 were obtained with the aim of performing the Fourier decomposition of the light curves of the RR~Lyrae stars present in that field. This procedure gave the mean values, for the metallicity, of [Fe/H]$_{ZW}=-1.483 \pm 0.006$ (statistical) $\pm 0.090$ (systematical), and for the distance, $5.000 \pm 0.001$~kpc (statistical) $\pm 0.220$ (systematical). The values found from two RRc stars are consistent with those derived previously. The differential reddening of the cluster was investigated and individual reddenings for the RR Lyrae stars were estimated from their $V-I$ curves. We found an average value of $E(B-V)= 0.23 \pm 0.02$. An investigation of the light curves of stars in the {\it blue stragglers} region led to the discovery of three new SX~Phe stars. The period-luminosity relation of the SX~Phe stars was used for an independent determination of the distance to the cluster and of the individual reddenings. We found a distance of 5.0 kpcComment: To appear in Revista Mexicana de Astronom\'ia y Astrof\'isica, Octuber 2014 issue, Vol 50. 17 pages, 10 figure

A super lithium-rich red-clump star in the open cluster Trumpler 5

Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation. Aims. To investigate the chemical abundance pattern of the old open cluster Trumpler\,5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum. Methods. One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416. Results. Star #3416 is super Li-rich with A(Li)=3.75\,dex. The lack of $^6$Li enrichment ($^6$Li/$^7$Li$<$2%), the low carbon isotopic ratio ($^{12}$C/$^{13}$C=14$\pm$3), and the lack of evidence for radial velocity variation or enhanced rotational velocity ($v\sin i = 2.8\,$\kms) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism. Conclusions. We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB.Comment: 7 pages, 2 figures. Accepted for publication in A&A Letter

The blue straggler population of the open clusters Trumpler 5, Trumpler 20, and NGC 2477

We present a study, based on Gaia DR2, of the population of blue straggler stars (BSS) in the open clusters Trumpler 5, Trumpler 20, and NGC 2477. All candidates were selected according to their position in the color-magnitude diagram, to their proper motion components, and to their parallax. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found that Trumpler 5 hosts a large BSS population, which allowed us to analyze their radial distribution as a probe of the cluster's dynamical status. The BSS distribution was compared with that of red giant branch stars (RGB) to evaluate mass segregation. Our results indicate that blue straggler stars are not more centrally concentrated than RGB stars in any of the clusters. The radial distribution of BSS in Trumpler 5 is flat. Additionally, using a multi-epoch radial velocity survey conducted with the high-resolution spectrograph FLAMES/GIRAFFE at VLT, we measured the radial velocities of a sample of stragglers, for the sake of comparison with the mean radial velocity and the velocity dispersion of the clusters. Based on the radial velocity variations for different epochs, we roughly classified these stars as possible close-or long-period binaries.Comment: 20 pages, 5 figures, accepted for publication in A

{BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters

We have initiated a long-term project, the BOAO photometric survey of open clusters, to enlarge our understanding of galactic structure using UBVI CCD photometry of open clusters which have been little studied before. This is the second paper of the project in which we present the photometry of 12 open clusters. We have determined the cluster parameters by fitting the Padova isochrones to the color-magnitude diagrams of the clusters. All the clusters except for Be 0 and NGC 1348 are found to be intermediate-age to old (0.2 - 4.0 Gyrs) open clusters with a mean metallicity of [Fe/H] = 0.0.Comment: 11 page

COVID-19 with uncertain phases: estimation issues with an illustration for Argentina

We use an approach to assess COVID-19 performance that starts from what we consider is the most likely set of hypotheses about the uncertain evolution of the pandemic, that envisage a sequence of different cycles with unknown duration and magnitude over 18-24 months. This pattern implies a research strategy where short-term time series forecasting of the evolution of observed cases and deaths play a central role in both detecting transitions from phase to phase of infections and the estimation of necessarily changing structural parameters and indicators of a SIRD model. We illustrate our approach with Buenos Aires City performance, which represents a significant share of the Argentine case with an early introduction of a lockdown followed by a second wave latter on. This approach can be extended to include measures of the intensity and compliance of lockdowns, as well as the heterogeneity across areas. We find that mobility (as a proxy for the effectiveness of the lockdown) has an impact on observed cases in Buenos Aires City with a lag of 8 days and deaths relate with new cases registered 16 to 19 days before. Mobility has a clear impact on the growth rate of cases and by extension deaths. Our approach and results have implications for policy dialogue issues

Mergers of close primordial binaries

We study the production of main sequence mergers of tidally-synchronized primordial short-period binaries. The principal ingredients of our calculation are the angular momentum loss rates inferred from the spindown of open cluster stars and the distribution of binary properties in young open clusters. We compare our results with the expected number of systems that experience mass transfer in post-main sequence phases of evolution and compute the uncertainties in the theoretical predictions. We estimate that main-sequence mergers can account for the observed number of single blue stragglers in M67. Applied to the blue straggler population, this implies that such mergers are responsible for about one quarter of the population of halo blue metal poor stars, and at least one third of the blue stragglers in open clusters for systems older than 1 Gyr. The observed trends as a function of age are consistent with a saturated angular momentum loss rate for rapidly rotating tidally synchronized systems. The predicted number of blue stragglers from main sequence mergers alone is comparable to the number observed in globular clusters, indicating that the net effect of dynamical interactions in dense stellar environments is to reduce rather than increase the blue straggler population. A population of subturnoff mergers of order 3-4% of the upper main sequence population is also predicted for stars older than 4 Gyr, which is roughly comparable to the small population of highly Li-depleted halo dwarfs. Other observational tests are discussed.Comment: number of pages depends on font, margins, columns etc (58 with given format), 14 figures, submitted to the Astrophysical Journa

Reducción de imágenes del CFHT

Presentamos resultados preliminares de las reducciones astrométrica y fotométrica de imágenes obtenidas con la Cámara MegaCam del CFHT (Mauna Kea, Hawaii). El objetivo final es confeccionar un catálogo profundo de la zona alrededor de la eclíptica por su particular interés para el estudio de objetos del Sistema Solar. Dicho catálogo contendrá posiciones astrométricas de alta precisión y fotometría multibanda. Los resultados muestran que los procedimientos desarrollados son apropiados para la construcción de un catálogo de estas características.We present preliminary results of astrometric and photometric reductions of images taken with the MegaCam at CFHT (Mauna Kea, Hawaii). The final purpose of the work is to build a deep catalog of the ecliptical zone, which will be particularly useful for studies of the Solar System. This catalog will provide high precision astrometry and multiband photometry. The results show that the adopted criteria and procedures are suitable for this purpose.http://www.astronomiaargentina.org.ar/uploads/docs/baaa57.pdfpublishedVersionFil: Bustos Fierro, Iván H. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina.Fil: Calderón, Jesús H. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina.Fil: Ahumada, Javier A. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina.Fil: Calderón, Jesús H. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Astronomía Teórica y Experimental; Argentina.Astronomía (incluye Astrofísica y Ciencias del Espacio