Variable stars in the VVV globular clusters

Indexación: Scopus.The VVV survey observed some of the most crowded and most obscured regions in the inner Milky Way during the last years. A significant sample of the less known globular clusters in our galaxy lie there. Combining the high-resolution, wide-field, near infrared capabilities of the survey camera, the use of 5 different filters, and multi-epoch observations, we are able to overcome many of the previous challenges that prevented a proper study of these objects. Particularly, the identification of the RR Lyrae stars in these globular clusters is proving to be a fundamental tool to establish accurately their distances and reddenings, and to infer information about the Oosterhoff dichotomy that Galactic globular clusters seem to follow. © The Authors, published by EDP Sciences, 2017.https://www.epj-conferences.org/articles/epjconf/pdf/2017/21/epjconf_puls2017_01022.pd

New RR Lyrae variables in binary systems

Despite their importance, very few RR Lyrae (RRL) stars have been known to reside in binary systems. We report on a search for binary RRL in the OGLE-III Galactic bulge data. Our approach consists in the search for evidence of the light-travel time effect in so-called observed minus calculated ($O-C$) diagrams. Analysis of 1952 well-observed fundamental-mode RRL in the OGLE-III data revealed an initial sample of 29 candidates. We used the recently released OGLE-IV data to extend the baselines up to 17 years, leading to a final sample of 12 firm binary candidates. We provide $O-C$ diagrams and binary parameters for this final sample, and also discuss the properties of 8 additional candidate binaries whose parameters cannot be firmly determined at present. We also estimate that $\gtrsim 4$ per cent of the RRL reside in binary systems.Comment: MNRAS Letters, in pres

Bulge RR Lyrae stars in the VVV tile b201\textit{b201}

The VISTA Variables in the V\'ia L\'actea (VVV) Survey is one of the six ESO public surveys currently ongoing at the VISTA telescope on Cerro Paranal, Chile. VVV uses near-IR ($ZYJHK_{\rm s}$) filters that at present provide photometry to a depth of $K_{\rm s} \sim 17.0$ mag in up to 36 epochs spanning over four years, and aim at discovering more than 10$^6$ variable sources as well as trace the structure of the Galactic bulge and part of the southern disk. A variability search was performed to find RR Lyrae variable stars. The low stellar density of the VVV tile $\textit{b201}$, which is centered at ($\ell, b$) $\sim$ ($-9^\circ, -9^\circ$), makes it suitable to search for variable stars. Previous studies have identified some RR Lyrae stars using optical bands that served to test our search procedure. The main goal is to measure the reddening, interstellar extinction, and distances of the RR Lyrae stars and to study their distribution on the Milky Way bulge. A total of 1.5 sq deg were analyzed, and we found 39 RR Lyrae stars, 27 of which belong to the ab-type and 12 to the c-type. Our analysis recovers all the previously identified RR Lyrae variables in the field and discovers 29 new RR Lyrae stars. The reddening and extinction toward all the RRab stars in this tile were derived, and distance estimations were obtained through the period--luminosity relation. Despite the limited amount of RR Lyrae stars studied, our results are consistent with a spheroidal or central distribution around $\sim 8.1$ and $\sim 8.5$ kpc. for either the Cardelli or Nishiyama extinction law.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and Astrophysic

The WFCAM Multi-wavelength Variable Star Catalog

Stellar variability in the near-infrared (NIR) remains largely unexplored. The exploitation of public science archives with data-mining methods offers a perspective for the time-domain exploration of the NIR sky. We perform a comprehensive search for stellar variability using the optical-NIR multi-band photometric data in the public Calibration Database of the WFCAM Science Archive (WSA), with the aim of contributing to the general census of variable stars, and to extend the current scarce inventory of accurate NIR light curves for a number of variable star classes. We introduce new variability indices designed for multi-band data with correlated sampling, and apply them for pre-selecting variable star candidates, i.e., light curves that are dominated by correlated variations, from noise-dominated ones. Pre-selection criteria are established by robust numerical tests for evaluating the response of variability indices to colored noise characteristic to the data. We find 275 periodic variable stars and an additional 44 objects with suspected variability with uncertain periods or apparently aperiodic variation. Only 44 of these objects had been previously known, including 11 RR~Lyrae stars in the outskirts of the globular cluster M3 (NGC~5272). We provide a preliminary classification of the new variable stars that have well-measured light curves, but the variability types of a large number of objects remain ambiguous. We classify most of the new variables as contact binary stars, but we also find several pulsating stars, among which 34 are probably new field RR~Lyrae and 3 are likely Cepheids. We also identify 32 highly reddened variable objects close to previously known dark nebulae, suggesting that these are embedded young stellar objects. We publish our results and all light-curve data as the WFCAM Variable Star Catalog.Comment: 21 pages, 11 figure

The VVV Survey reveals classical Cepheids tracing a young and thin stellar disk across the Galaxy's bulge

Solid insight into the physics of the inner Milky Way is key to understanding our Galaxy's evolution, but extreme dust obscuration has historically hindered efforts to map the area along the Galactic mid-plane. New comprehensive near-infrared time-series photometry from the VVV Survey has revealed 35 classical Cepheids, tracing a previously unobserved component of the inner Galaxy, namely a ubiquitous inner thin disk of young stars along the Galactic mid-plane, traversing across the bulge. The discovered period (age) spread of these classical Cepheids implies a continuous supply of newly formed stars in the central region of the Galaxy over the last 100 million years.Comment: Accepted for publication in The Astrophysical Journal Letters. Near-IR finding charts of the Cepheids are available at the following URL: http://www.astro.puc.cl/~idekany/pub/inner_disk_ceph_fc.pn

Blazhko modulation in the infrared

We present first direct evidence of modulation in the $K$-band of Blazhko-type RR Lyrae stars that are identified by their secular modulations in the I-band data of OGLE-IV. A method has been developed to decompose the $K$-band light variation into two parts originating from the temperature and the radius changes using synthetic data of atmosphere-model grids. The amplitudes of the temperature and the radius variations derived from the method for non-Blazhko RRab stars are in very good agreement with the results of the Baade-Wesselink analysis of RRab stars in the M3 globular cluster confirming the applicability and correctness of the method. It has been found that the Blazhko modulation is primarily driven by the change in the temperature variation. The radius variation plays a marginal part, moreover it has an opposite sign as if the Blazhko effect was caused by the radii variations. This result reinforces the previous finding based on the Baade-Wesselink analysis of M3 (NGC 5272) RR Lyrae, that significant modulation of the radius variations can only be detected in radial-velocity measurements, which relies on spectral lines that form in the uppermost atmospheric layers. Our result gives the first insight into the energetics and dynamics of the Blazhko phenomenon, hence it puts strong constraints on its possible physical explanations.Comment: 16 pages, accepted for publication in MNRA