4 pages, 1 figure. Poster presented at Wroclaw HELAS Workshop, "Interpretation of Astroseismic Data", 23 - 27 June 2008, Wroclaw, Poland. Shorter version will be published in Communications in Astroseismology
We have conducted a systematic search for multiperiodic pulsations in the RR Lyrae-type stars of the galactic globular cluster Omega Cen. Secondary periodicities close to the primary pulsation frequency have been detected in 17 out of 70 studied fundamental mode (RRab) pulsators and in 35 out of 81 overtone (RRc) pulsators. Because of the observed period ratios, these newly detected periodicities must correspond to nonradial modes. Their beating with the primary radial pulsation leads to a slow amplitude and phase modulation, commonly referred to as the Blazhko effect. The incidence rate of Blazhko modulation in Omega Cen RRab stars (24 +/- 5%) is similar to that observed in the Galactic Bulge. In the case of Omega Cen RRc stars, the incidence rate of Blazhko effect is exceptionally high (38 +/- 5%), more than 3 times higher than in any other studied population.
In addition to Blazhko variables, we have also identified two RR Lyr variables exhibiting first overtone/second overtone double-mode pulsations, and a triple-mode High Amplitude Delta Scuti variable.
This paper presents results of a fully automated search for multiperiodic RR Lyrae stars in the center of the Galaxy. The search was carried out on OGLE-II database created by means of image subtraction method. I found more than 2700 RR Lyrae stars with about 600 showing various kinds of multiperiodic behavior. Previous OGLE-I database contained only about 200 RR Lyrae stars in the Galactic Bulge, that is twelve times less than the current sample. There are two most interesting outcomes for Bulge: very high percentage of Blazhko stars and very low percentage of RRd stars. Blazhko effect was co…
We present the results of a systematic search for multiperiodic pulsators among the Galactic Bulge RR Lyrae stars of the OGLE-1 sample. We identify one "canonical" double-mode variable (RRd star) pulsating in two radial modes. In 38 stars we detect secondary periodicities very close to the primary pulsation frequency. This type of multiperiodic variables constitute ~23% of RRab and ~5% of RRc population of the Bulge. With the observed period ratios of 0.95-1.02 the secondary periods must correspond to nonradial modes of oscillation. Their beating with the primary (radial) pulsation leads to a …
The All Sky Automated Survey (ASAS) monitors bright stars (8 mag < V < 14 mag) south of declination +28 deg. The ASAS Catalogue of Variable Stars (ACVS) presently contains 50,099 objects; among them are 2212 objects classified as RR Lyrae pulsating variables. We use ASAS photometric V band data to search for multiperiodicity in those stars. We find that 73 of 1435 RRab stars and 49 of 756 RRc stars exhibit the Blazhko effect. We observe a deficiency of RRab Blazhko variables with main pulsation periods greater than 0.65 days. The Blazhko periods of RRc stars exhibit a strongly bimodal distribu…
A century of observations of omega Centauri allows us to accurately follow the period changes of a large sample of variable stars. Although the period changes are composed of monotone and irregular changes, period increases dominate in most of the RRab stars. The typical period increase rates are of the same order of magnitude as model calculations predict for the evolved, redward phase of horizontal branch evolution. For the 44 well observed RRab stars which show monotonic period changes, a mean rate of 0.15 d/Myr has been found. The period changes of the first overtone RRc type stars show a …
We present Near-Infrared (NIR) $J$ and $K_s$-band observations for 181 RR Lyrae stars in the Galactic Globular Cluster $ω$ Cen. The comparison between predicted and empirical slopes of NIR Period-Luminosity (PL) relations indicates a very good agreement. Cluster distance estimates based on NIR PL relations agree quite well with recent determinations based on different standard candles, giving a true mean distance modulus $μ= 13.71\pm0.05$, and $d=5.52\pm0.13$ kpc.
We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega Cen obtained with FLAMES. The well-known metallicity spread is visible among the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~ -1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~ -1.7), in good agreement with the corresponding zero-age horizontal branch models with cosmological helium abundance Y = 0.246. This result conflicts with the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to the anomalous blue main-sequence stars, which share a s…
We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster Omega Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over Delta-S and other techniques in determining the metallicity of RR Lyrae stars, and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the Delta-S observations of Butler et al. (1978) and Gratton et al. (1986), we have found that our metallicities, [Fe/H](hk), deviate systematically from th…
We present a theoretical approach to infer information about RR Lyrae variables from the morphology of their light curves. The method, already successfully applied to the field first overtone variable U Comae, is now tested on a RR$_{\mathrm ab}$ variable in the globular cluster $ω$ Cen. We show that, with this method, it could be possible to give an estimate of the distance modulus of the cluster with an uncertainty not larger than $\pm$0.1 mag. The predicted variable luminosity and mass are well within the range of theoretical evolutionary expectations giving the ``feeling'' of a full compat…
We present wide-field and high-precision BV and Ca & Stromgren by photometry of omega Centauri and its RR Lyrae stars, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of omega Cen are well discriminated from foreground Galactic disk stars from the different distributions in the hk [=(Ca-b)-(b-y)] vs. b-y diagram. The color-magnitude diagrams show the presence of several distinct red-giant branches with a red, metal-rich, sequence clearly separated from other bluer metal-poor ones. Comparison with our population models suggests the most …
New photometry of RRab and RRc stars in Omega Cen is used to calibrate their absolute magnitudes M_V as a function of: a) metallicity; and b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the CASE project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables we obtained a relation of M_V=(0.26\pm 0.08)[Fe/H]+(0.91\pm 0.13). A dereddened distance modulus to the LMC based on that formula is $μ_{0}=18.56\pm 0.14$ mag. The second calibration of M_V, which is b…
We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variabl…
We discuss the results of a systematic search for multiperiodic pulsations in Galactic Bulge Delta Scuti and RR Lyrae stars. Six "normal" double-mode variables pulsating in two radial modes have been identified (5 Delta Scuti-type and 1 RR Lyrae-type). In 37 RR Lyrae stars secondary periodicities very close to the primary pulsation frequency have been detected. These periodicities correspond to nonradial modes of oscillation. They are found in \~23% of RRab and in ~3% of RRc variables of our sample.
Aim: The aim of this work was to search for double mode pulsators among RR Lyr variables of globular cluster Omega Cen. Methods: We conducted a systematic frequency analysis of CASE photometry of Omega Cen RR Lyr stars. We searched for periodicities using Fourier and ANOVA periodograms, combined with consecutive prewhitening technique. Results: We discovered six double mode pulsators, with the first overtone and a secondary mode of higher frequency simultaneously excited. These are the first double mode RR Lyr stars identified in Omega Cen. In variable V10 period ratio of the two modes is 0.80…
We present precise optical and near-infrared ground-based photometry of two Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.84+/-0.05, Omega Cen; m_I(TRGB)=9.46+/-0.06, 47 Tuc--and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with each other within one sigma. Absolute TRGB and RR Lyrae distance…
CCD observations obtained by the OGLE team for 128 RR Lyrae variables in Omega Cen have been analysed. The P-L and P-A plots indicate that, in addition to fundamental (RRab) and first overtone (RRc) pulsators, the Omega Cen RR Lyrae population seems to include second overtone (RRe) and possibly third overtone pulsators. The mean periods derived for the RRab and RRc stars are typical values for an Oosterhoff type II (OoII) cluster. Nevertheless, the P-A plot plot also shows that some of the RR Lyrae variables have `Oosterhoff type I' (OoI) characteristics. Relative luminosities derived for the …
The compilation of the metallicity values of $ω$~Centauri stars led to detection of spatial asymmetry in the metallicity distribution of the bright giants. It was found that the most and least metal-poor objects are separated along galactic latitude. Statistical tests show that the chance occurrence of the phenomenon is very low. The subgiant and the RR~Lyrae samples do not show, however, similar segregation of the different metallicity objects. The physical parameters of the variables calculated by using empirical formulae indicate that the bulk of the RRab stars comprises a very homogeneous …
We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, omega Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B-V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre spectrograph at the Anglo Australian Telescope, and cover the approximate range lambda~3840-4940 Angstroem. We measure the radial velocities, effective temperatures, metallicities and surface gravities by fitting A…
CCD observations for 10 (6 previously known and 4 newly discovered) of the 11 RR Lyrae variables in NGC 5897 have been analysed. The period-luminosity and period-amplitude plots indicate that the population of RR Lyrae variables in NGC 5897 includes 3 fundamental mode, 4 first-overtone and 4 second-overtone variables with mean periods 0.828, 0.459 and 0.343 days respectively. The variables have properties that are similar to some of the Oosterhoff type II variables in Omega Centauri. Two of the new variables (V11 and V13) were previously considered to be possible non-variables that lie within …
We present an analysis and interpretation of oscillation spectra for all 69 SX Phoenicis stars discovered in the field of the cluster. For most of the stars we have reliable absolute magnitudes and colors. Except of one, or perhaps two, objects, the stars are cluster members. Their pulsational behaviour is very diversified. Multiperiodic variability with at least part of the excited modes being nonradial is most common but there are also many cases of high amplitude, presumably radial mode, pulsators. In a number of such cases we have evidence for two radial modes being excited. Parameters of …
We present a variable star catalog of an extensive ground-based wide-field variability survey in the globular cluster omega Centauri. Using the ANU 40-inch (1m) telescope at Siding Spring Observatory, the cluster was observed with a 52'x52' (0.75 deg^2) field for 25 nights. A total of 187 variable stars were identified in the field, 81 of which are new discoveries. This work comprises the widest field variability survey yet undertaken for this cluster. Here we present the V+R lightcurves and preliminary analysis of the detected variable stars, comprising 58 eclipsing binaries, 69 RR Lyrae star…
Using the 2dF multi-fibre instrument at the AAT, we have carried out a spectroscopic survey of over 1500 stars brighter than B=16.6 mag, which have been confirmed by the proper motion study of van Leeuwen et al. (2000) to be members of the massive galactic globular cluster, omega Centauri. The survey aimed at sampling the Hertzsprung-Russell Diagram in a uniform way, thus avoiding a bias towards the dominant stellar population and allowing the study of stars with less common properties such as particularly metal-rich or metal-poor stars. Exceptionally, though, emphasis has been given to includ…
We have constructed synthetic color-magnitude diagrams (CMDs) for omega Cen and other massive globular clusters with apparently peculiar CMD morphology. Our population models, which adopt the most up-to-date input physics and parameters, show that the observed CMD of omega Cen can be reproduced by adopting (1) multimodal metallicity distribution function as derived from the observed color distribution of red-giant-branch (RGB) stars, and (2) an internal age-metallicty relation among the populations therein. Similar results were obtained for other massive globular clusters with bimodal horizont…
Fourier coefficents have been derived for the $V$ and $R$ light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. The $ϕ_{31}$ and $R_{21}$ coefficients have been compared with those of the first overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, $ω$ Centauri and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5 and the Oosterhoff type I variables in $ω$ Cen, but they are different from the Oosterhoff type II variables in $ω$ Cen. Equations derived from hydrodynam…
By using the full span of multicolor data on a representative sample of first overtone RR Lyrae stars in the Large Magellanic Cloud (LMC) we revisit the problem of the incidence rate of the amplitude/phase-modulated (Blazhko) stars. Multicolor data, obtained by the MAssive Compact Halo Objects (MACHO) project, are utilized through a periodogram averaging method. This enables us to increase the number of detected multiperiodic variables by 18% relative to the number obtained by the analysis of the best single color data. We also test the maximum modulation period detectable in the present datas…
