Fundamental Mode RR Lyrae Stars Research Articles

Leveraging Deep Learning for Time-Series Extrinsic Regression in Predicting the Photometric Metallicity of Fundamental-Mode RR Lyrae Stars.

Astronomy is entering an unprecedented era of big-data science, driven by missions like the ESA's Gaia telescope, which aims to map the Milky Way in three dimensions. Gaia's vast dataset presents a monumental challenge for traditional analysis methods. The sheer scale of this data exceeds the capabilities of manual exploration, necessitating the utilization of advanced computational techniques. In response to this challenge, we developed a novel approach leveraging deep learning to estimate the metallicity of fundamental mode (ab-type) RR Lyrae stars from their light curves in the Gaia optical G-band. Our study explores applying deep-learning techniques, particularly advanced neural-network architectures, in predicting photometric metallicity from time-series data. Our deep-learning models demonstrated notable predictive performance, with a low mean absolute error (MAE) of 0.0565, the root mean square error (RMSE) of 0.0765, and a high R2 regression performance of 0.9401, measured by cross-validation. The weighted mean absolute error (wMAE) is 0.0563, while the weighted root mean square error (wRMSE) is 0.0763. These results showcase the effectiveness of our approach in accurately estimating metallicity values. Our work underscores the importance of deep learning in astronomical research, particularly with large datasets from missions like Gaia. By harnessing the power of deep-learning methods, we can provide precision in analyzing vast datasets, contributing to more precise and comprehensive insights into complex astronomical phenomena.

Homogeneous metallicities for 14 old LMC globular clusters based on their fundamental mode RR Lyrae stars

ABSTRACT We present the first internally consistent set of [Fe/H] values for 14 of the 15 old globular clusters in the Large Magellanic Cloud (LMC). We make use of periods and amplitudes of fundamental mode (ab-type) RR Lyrae stars in the vicinity of these LMC clusters, mostly taken from the Optical Gravitational Lensing Experiment (OGLE) data base, in order to calculate metallicities for the clusters. Our data set contains a total of 417 ab-type cluster RR Lyrae stars (289 from OGLE and the rest from the literature) and yields a set of homogeneous metallicities for 14 old LMC globular clusters all on the Zinn and West scale. No previously published work contains metallicities for as many of the old LMC globular clusters as presented herein. An analysis of a subset of our abundances compared with seven other metallicity studies in the peer-reviewed literature indicates very little overall agreement between these [Fe/H] values suggesting that significantly more effort needs to be devoted to measuring internally consistent [Fe/H]s for the old LMC globular clusters.

GaiaData Release 3

Context.RR Lyrae stars are excellent tracers of the oldest stars (ages ≳ 9–10 Gyr) and standard candles for measuring the distance to stellar systems that are mainly composed of an old stellar population. TheGaiaThird Data Release (DR3) publishes a catalogue of full-sky RR Lyrae stars observed during the initial 34 months of science operations. They were processed through the Specific Object Study (SOS) pipeline, which was developed to validate and characterise Cepheids and RR Lyrae stars (SOS Cep&RRL) observed byGaia.Aims.The main steps of the SOS Cep&RRL pipeline are described in the documentation and papers accompanying previousGaiadata releases. For DR3, the pipeline was modified in its process: in addition to theGaiamultiband (G,GBP,GRP) time-series photometry, the epoch radial velocities measured for RR Lyrae and Cepheids with the Radial Velocity Spectrometer (RVS) on boardGaiawere also processed through the pipeline.Methods.The SOS Cep&RRL validation of DR3 candidate RR Lyrae stars relies on diagnostics tools that include the period versusG-amplitude diagram and the period versusϕ21andϕ31parameters of theGlight-curve Fourier decomposition, as defined by a reference sample of bona fide RR Lyrae stars known in the literature (that we named Gold Sample). Great care was devoted to building a large and pure Gold Sample comprising more than 200 000 RR Lyrae stars. The SOS processing led to an initial catalogue of 271779 RR Lyrae stars that are listed in thevari_rrlyraetable of the DR3 archive. A thorough cleaning procedure was then performed to produce a final catalogue of SOS-confirmed DR3 RR Lyrae stars by dropping sources that clearly are contaminants or have an uncertain classification.Results.Multiband time-series photometry and characterisation are published inGaiaDR3 for a clean, validated sample of 270 905 RR Lyrae stars (174 947 fundamental-mode, 93 952 first-overtone, and 2006 double-mode RR Lyrae) that were confirmed and fully characterised by the SOS Cep&RRL pipeline. They are distributed throughout the sky, including variables in 95 globular clusters and 25 Milky Way (MW) companions (the Magellanic Clouds, seven dwarf spheroidal galaxies, and 16 ultra-faint dwarf satellites of the MW). RVS time-series radial velocities are also published for 1096 RR Lyrae and 799 Cepheids of different types (classical, anomalous, and type II Cepheids). Of the 270 905 DR3 RR Lyrae stars, 200 294 are already known in the literature (Gold Sample) and 70 611 are new discoveries byGaia, to the best of our knowledge. An estimate of the interstellar absorption is published for 142 660 fundamental-mode RR Lyrae stars from a relation based on theG-band amplitude, the (G−GRP) colour and the pulsation period. Metallicities derived from the Fourier parameters of the light curves are also released for 133 559 RR Lyrae stars.Conclusions.The finalGaiaDR3 catalogue of confirmed RR Lyrae stars almost doubles the DR2 RR Lyrae catalogue. An increase in statistical significance, a better characterisation of the RR Lyrae pulsational and astrophysical parameters, and the improved astrometry published withGaiaEDR3 make the SOS Cep&RRL DR3 sample the largest, most homogeneous, and parameter-richest catalogue of all-sky RR Lyrae stars published so far in the magnitude range from ⟨G⟩=7.64 mag (the magnitude of RR Lyr itsef, the class prototype) to ⟨G⟩=21.14 mag (the faintest RR Lyrae in the catalogue).

Photometric Metallicity Prediction of Fundamental-mode RR Lyrae Stars in the Gaia Optical and K s Infrared Wave Bands by Deep Learning

RR Lyrae stars are useful chemical tracers thanks to the empirical relationship between their heavy-element abundance and the shape of their light curves. However, the consistent and accurate calibration of this relation across multiple photometric wave bands has been lacking. We have devised a new method for the metallicity estimation of fundamental-mode RR Lyrae stars in the Gaia optical G and near-infrared VISTA K s wave bands by deep learning. First, an existing metallicity prediction method is applied to large photometric data sets, which are then used to train long short-term memory recurrent neural networks for the regression of the [Fe/H] to the light curves in other wave bands. This approach allows an unbiased transfer of our accurate, spectroscopically calibrated I-band formula to additional bands at the expense of minimal additional noise. We achieve a low mean absolute error of 0.1 dex and high R 2 regression performance of 0.84 and 0.93 for the K s and G bands, respectively, measured by cross-validation. The resulting predictive models are deployed on the Gaia DR2 and VVV inner bulge RR Lyrae catalogs. We estimate mean metallicities of −1.35 dex for the inner bulge and −1.7 dex for the halo, which are significantly less than the values obtained by earlier photometric prediction methods. Using our results, we establish a public catalog of photometric metallicities of over 60,000 Galactic RR Lyrae stars and provide an all-sky map of the resulting RR Lyrae metallicity distribution. The software code used for training and deploying our recurrent neural networks is made publicly available in the open-source domain.

Metallicity of Galactic RR Lyrae from Optical and Infrared Light Curves. I. Period–Fourier–Metallicity Relations for Fundamental-mode RR Lyrae

Abstract We present newly calibrated period–ϕ 31–[Fe/H] relations for fundamental-mode RR Lyrae stars in the optical and, for the first time, mid-infrared. This work’s calibration data set provides the largest and most comprehensive span of parameter space to date, with homogeneous metallicities from −3 ≲ [Fe/H] ≲ 0.4 and accurate Fourier parameters derived from 1980 ASAS-SN (V band) and 1083 WISE (NEOWISE extension, W1 and W2 bands) RR Lyrae stars with well-sampled light curves. We compare our optical period–ϕ 31–[Fe/H] relation with those available in the literature and demonstrate that our relation minimizes systematic trends in the lower and higher metallicity range. Moreover, a direct comparison shows that our optical photometric metallicities are consistent with both those from high-resolution spectroscopy and globular clusters, supporting the good performance of our relation. We found an intrinsic scatter in the photometric metallicities (0.41 dex in the V band and 0.50 dex in the infrared) by utilizing large calibration data sets covering a broad metallicity range. This scatter becomes smaller when optical and infrared bands are used together (0.37 dex). Overall, the relations derived in this work have many potential applications, including large-area photometric surveys with James Webb Space Telescope in the infrared and LSST in the optical.

Kinematics of RR Lyrae stars in the Galactic bulge with OGLE-IV and Gaia DR2

ABSTRACT We analyse the kinematics and spatial distribution of 15 599 fundamental-mode RR Lyrae (RRL) stars in the Milky Way bulge by combining OGLE-IV photometric data and Gaia DR2 proper motions. We show that the longitudinal proper motions and the line-of-sight velocities can give similar results for the rotation in the Galactic central regions. The angular velocity of bulge RRLs is found to be around 35 km s−1 kpc−1, significantly smaller than that for the majority of bulge stars (50–60 km s−1 kpc−1); bulge RRLs have larger velocity dispersion (120–140 km s−1) than younger stars. The dependence of the kinematics of the bulge RRLs on their metallicities is shown by their rotation curves and spatial distributions. Metal-poor RRLs ([Fe/H]<−1) show a smaller bar angle than metal-rich ones. We also find clues suggesting that RRLs in the bulge are not dominated by halo stars. These results might explain some previous conflicting results over bulge RRLs and help understand the chemodynamical evolution of the Galactic bulge.

Near-infrared Search for Fundamental-mode RR Lyrae Stars toward the Inner Bulge by Deep Learning

Abstract Aiming to extend the census of RR Lyrae stars to highly reddened low-latitude regions of the central Milky Way, we performed a deep near-IR variability search using data from the VISTA Variables in the Vía Láctea (VVV) survey of the bulge, analyzing the photometric time series of over a hundred million point sources. In order to separate fundamental-mode RR Lyrae (RRab) stars from other periodically variable sources, we trained a deep bidirectional long short-term memory recurrent neural network (RNN) classifier using VVV survey data and catalogs of RRab stars discovered and classified by optical surveys. Our classifier attained a ∼99% precision and recall for light curves with signal-to-noise ratio above 60, and is comparable to the best-performing classifiers trained on accurate optical data. Using our RNN classifier, we identified over 4300 hitherto unknown bona fide RRab stars toward the inner bulge. We provide their photometric catalog and VVV photometric time series.

The Bulge Radial Velocity Assay for RR Lyrae Stars (BRAVA-RR) DR2: A Bimodal Bulge?

Abstract The radial velocities of 2768 fundamental-mode RR Lyrae stars (RRLs) toward the southern Galactic bulge are presented, spanning the southern bulge from −8° < l < + 8° and −3° < b < −6°. Distances derived from the pulsation properties of the RRLs are combined with Gaia proper motions to give constraints on the orbital motions of 1389 RRLs. The majority (∼75%) of the bulge RRLs have orbits consistent with these stars being permanently bound to <3.5 kpc from the Galactic Center, similar to the bar. However, unlike the bulge giants, the RRLs exhibit slower rotation and a higher velocity dispersion. The higher velocity dispersion arises almost exclusively from halo interlopers passing through the inner Galaxy. We present 82 stars with space velocities ≳500 km s−1 and find that the majority of these high-velocity stars are halo interlopers; it is unclear if a subsample of these stars with similar space velocities has a common origin. Once the 25% of the sample represented by halo interlopers is cleaned, we can clearly discern two populations of bulge RRLs in the inner Galaxy. The first population of RRLs is not as tightly bound to the Galaxy (but is still confined to the inner ∼3.5 kpc) and is both spatially and kinematically consistent with the barred bulge. The second population is more centrally concentrated and does not trace the bar. One possible interpretation is that this population was born prior to bar formation, as their spatial location, kinematics, and pulsation properties suggest, possibly from an accretion event at high redshift.

Blazhko effect in the Galactic bulge fundamental mode RR Lyrae stars – II. Modulation shapes, amplitudes, and periods

ABSTRACT The number of stars observed by the Optical Gravitational Lensing Experiment (OGLE) project in the Galactic bulge offers an invaluable chance to study RR Lyrae stars in a statistical manner. We used data of 3141 fundamental-mode RR Lyrae stars showing the Blazhko effect observed in OGLE-IV to investigate a possible connection between modulation amplitudes and periods, light curve, and pulsation characteristics. We found that there is no simple monotonic correlation between any combination of two parameters concerning the Blazhko and pulsation amplitudes, periods, and the shape of the light curves. There are only systematic limits. There is a bottom limit of the modulation period with respect to the pulsation period. We also found that the possible range of modulation amplitudes decreases with increasing pulsation period, which could point towards that the Blazhko effect is suppressed in cooler, larger, more luminous, and less metal abundant bulge RR Lyrae stars. Our investigation revealed that the distribution of the modulation periods can be described with two populations of stars with the mean modulation periods of 48 and 186 d. There is a certain region with a low density of the modulated stars, which we call the Blazhko valley, in the pulsation period–modulation period plane. Based on the similarity of the modulation envelopes, basically every star can be assigned to one of six morphological classes. The double modulation was found in 25 per cent of the studied stars. Only 6.3 per cent of modulated stars belong to the Oosterhoff group II.

Humps and bumps: the effects of shocks on the optical light curves of fundamental-mode RR Lyrae stars

We present the most extended and homogeneous study carried out so far of the main and early shocks in 1485 RR Lyrae stars in the Galactic bulge observed by the Optical Gravitational Lensing Experiment. We selected nonmodulated fundamental-mode RR Lyrae stars with good-quality photometry. Using a self-developed method, we determined the centers and strengths of main and early shock features in the phased light curves. We found that the positions of both humps and bumps are highly correlated with the pulsation properties of the studied variables. Pulsators with a pronounced main shock are concentrated in the low-amplitude regime of the period–amplitude diagram, while stars with a strong early shock have average and above-average pulsation amplitudes. A connection between the main and early shocks and the Fourier coefficients is also observed. In the color–magnitude diagram, we see a separation between stars with strong and weak shocks. Variables with a pronounced main shock cluster close to the fundamental red edge of the instability strip (IS), while stars with a strong early shock tend to clump in the center and near the fundamental blue edge of the IS. The appearance of shocks and their properties appear to be independent of the direction of evolution estimated from the period change rate of the studied stars. In addition, the differences in the period change rate between the two main Oosterhoff groups found in the Galactic bulge suggest that stars of Oosterhoff type I are located close to the zero-age horizontal branch while Oosterhoff type II variables are on their way toward the fundamental red edge of the instability strip, and have therefore already left the zero-age horizontal branch.

Mapping the Interstellar Reddening and Extinction toward Baade’s Window Using Minimum Light Colors of ab-type RR Lyrae Stars: Revelations from the De-reddened Color–Magnitude Diagrams

Abstract We have obtained repeated images of six fields toward the Galactic bulge in five passbands (u, g, r, i, z) with the DECam imager on the Blanco 4 m telescope at CTIO. From more than 1.6 billion individual photometric measurements in the field centered on Baade’s window, we have detected 4877 putative variable stars. A total of 474 of these have been confirmed as fundamental mode RR Lyrae stars, whose colors at minimum light yield line-of-sight reddening determinations, as well as a reddenning law toward the Galactic Bulge, which differs significantly from the standard R V = 3.1 formulation. Assuming that the stellar mix is invariant over the 3 square-degree field, we are able to derive a line-of-sight reddening map with sub-arcminute resolution, enabling us to obtain de-reddened and extinction corrected color–magnitude diagrams (CMDs) of this bulge field using up to 2.5 million well-measured stars. The corrected CMDs show unprecedented detail and expose sparsely populated sequences: for example, delineation of the very wide red giant branch, structure within the red giant clump, the full extent of the horizontal branch, and a surprising bright feature that is likely due to stars with ages younger than 1 Gyr. We use the RR Lyrae stars to trace the spatial structure of the ancient stars and find an exponential decline in density with Galactocentric distance. We discuss ways in which our data products can be used to explore the age and metallicity properties of the bulge, and how our larger list of all variables is useful for learning to interpret future LSST alerts.

Gaia Data Release 2

Context. The Gaia second Data Release (DR2) presents a first mapping of full-sky RR Lyrae stars and Cepheids observed by the spacecraft during the initial 22 months of science operations. Aims. The Specific Objects Study (SOS) pipeline, developed to validate and fully characterise Cepheids and RR Lyrae stars (SOS Cep&RRL) observed by Gaia, has been presented in the documentation and papers accompanying the Gaia first Data Release. Here we describe how the SOS pipeline was modified to allow for processing the Gaia multi-band (G, GBP, and GRP) time-series photometry of all-sky candidate variables and produce specific results for confirmed RR Lyrae stars and Cepheids that are published in the DR2 catalogue. Methods. The SOS Cep&RRL processing uses tools such as the period–amplitude and the period–luminosity relations in the G band. For the analysis of the Gaia DR2 candidates we also used tools based on the GBP and GRP photometry, such as the period–Wesenheit relation in (G, GRP). Results. Multi-band time-series photometry and characterisation by the SOS Cep&RRL pipeline are published in Gaia DR2 for 150 359 such variables (9575 classified as Cepheids and 140 784 as RR Lyrae stars) distributed throughout the sky. The sample includes variables in 87 globular clusters and 14 dwarf galaxies (the Magellanic Clouds, 5 classical and 7 ultra-faint dwarfs). To the best of our knowledge, as of 25 April 2018, the variability of 50 570 of these sources (350 Cepheids and 50 220 RR Lyrae stars) has not been reported before in the literature, therefore they are likely new discoveries by Gaia. An estimate of the interstellar absorption is published for 54 272 fundamental-mode RR Lyrae stars from a relation based on the G-band amplitude and the pulsation period. Metallicities derived from the Fourier parameters of the light curves are also released for 64 932 RR Lyrae stars and 3738 fundamental-mode classical Cepheids with periods shorter than 6.3 days.

Confirmation of a New Metal-poor Globular Cluster in the Galactic Bulge∗

Abstract We use deep near-IR photometry of the VISTA Variables in the Vía Láctea (VVV) Survey and deep DECam Plane Survey (DECaPS) optical photometry to confirm the physical reality of the candidate globular cluster (GC) Minni 22, which is located in the Galactic bulge. This object, which was detected as a high density region in our maps of bulge red giants, is now confirmed as a real GC based on the optical and near-IR color–magnitude diagrams. We also recover three known fundamental mode (ab type) RR Lyrae stars within 2 arcmin of the cluster center. The presence of RR Lyrae stars also seems to confirm Minni 22 as a bonafide old and metal-poor GC. We estimate a cluster reddening mag and determine its heliocentric distance D = 7.4 ± 0.3 kpc. The optical and near-IR color–magnitude diagrams reveal well-defined red giant branches in all cases, including a red giant branch bump at Ks = 13.30 ± 0.05 mag. The comparison with theoretical isochrones yields a mean metallicity of [Fe/H] = −1.3 ± 0.3 dex, and age of t ∼ 11.2 Gyr. This is a good example of a new low-luminosity (M V = −6.2 mag) GC found in the central bulge of the Milky Way. After discussing the different ways to confirm the existence of bulge GC candidates, we find that one of the best methods is to use the CMDs from the combination of the DECaPS + VVV photometries.

Blazhko Effect in the first overtone RR Lyrae stars of the OGLE Galactic bulge collection

We present the analysis of the Blazhko effect - quasi-periodic modulation of pulsation amplitude and/or phase - in the Galactic bulge first overtone RR Lyrae stars (RRc). We used the data gathered during the fourth phase of the Optical Gravitational Lensing Experiment (OGLE). Out of 10 826 analyzed RRc stars, Blazhko effect was detected in 607 stars which constitute 5.6 percent of the sample. It is the largest and most homogeneous sample of modulated RRc stars analyzed so far. Modulation periods cover a wide range, from slightly above 2 d to nearly 3000 d. Multiperiodic modulation was detected in 47 stars. The appearance of modulation in the frequency domain was studied in detail. Modulation manifests either as close doublets or as equidistant triplets and multiplets centered on radial mode frequency and its harmonics. In a significant fraction (29 percent) of stars, we have detected the modulation frequency itself, which corresponds to the modulation of the mean stellar brightness. Our search for period doubling effect, that was discovered recently in modulated fundamental mode RR Lyrae stars, and triggered development of new model behind the Blazhko modulation, yielded negative result. In 104 stars we detected additional signals that could correspond to both radial and non-radial modes. Statistical properties of modulated stars were analyzed in detail and confronted with properties of non-modulated stars and of modulated fundamental mode RR Lyrae stars. Our analysis provides constraints for the models to explain the Blazhko phenomenon, which still remains a puzzle more than hundred years after its discovery.

Are all RR Lyrae stars modulated?

We analyzed 151 variables previously classified as fundamental mode RR Lyrae stars from Campaigns 01–04 of the Kepler two-wheel (K2) archive. By employing a method based on the application of systematics filtering with the aid of co-trending light curves in the presence of a large amplitude signal component, we searched for additional Fourier signals in the close neighborhood of the fundamental period. We found only 13 stars without such components, yielding the highest rate of modulated (Blazhko) stars detected so far (91 %). A detection efficiency test suggests that this occurrence rate likely implies a 100 % underlying rate. Furthermore, the same test performed on a subset of the Large Magellanic Cloud RR Lyrae stars from the MACHO archive shows that the conjecture of high true occurrence rate fits well to the low observed rate derived from this database.

Variable Stars and Stellar Populations in Andromeda XXVII. IV. An Off-centered, Disrupted Galaxy*

Abstract We present B and V time-series photometry of the M31 satellite galaxy Andromeda XXVII (And XXVII) that we observed with the Large Binocular Cameras of the Large Binocular Telescope. In the field of And XXVII we have discovered a total of 90 variables: 89 RR Lyrae stars and 1 Anomalous Cepheid. The average period of the fundamental mode RR Lyrae stars (RRab) (σ = 0.05 day) and the period–amplitude diagram place And XXVII in the class of Oosterhoff I/Intermediate objects. Combining information from the color–magnitude diagram (CMD) and the variable stars, we find evidence for a single old and metal-poor stellar population with [Fe/H] ∼ −1.8 dex and t ∼ 13 Gyr in And XXVII. The spatial distributions of RR Lyrae and red giant branch (RGB) stars give clear indication that And XXVII is a completely disrupted system. This is also supported by the spread observed along the line of sight in the distance to the RR Lyrae stars. The highest concentration of RGB and RR Lyrae stars is found in a circular area of 4 arcmin in radius, centered about 0.°2 in the southeast direction from Richardson et al.’s center coordinates of And XXVII. The CMD of this region is well-defined, with a prominent RGB and 15 RR Lyrae stars (out of the 18 found in the region) tracing a very tight horizontal branch at σ = 0.06 mag (average over 15 stars). We show that And XXVII is a strong candidate building block of the M31 halo.

Absolute Magnitudes and Colors of RR Lyrae Stars in DECam Passbands from Photometry of the Globular Cluster M5

Abstract We characterize the absolute magnitudes and colors of RR Lyrae stars in the globular cluster M5 in the ugriz filter system of the Dark Energy Camera (DECam). We provide empirical period–luminosity (P–L) relationships in all five bands based on 47 RR Lyrae stars of the type ab and 14 stars of the type c. The P–L relationships were found to be better constrained for the fundamental-mode RR Lyrae stars in the riz passbands, with dispersions of 0.03, 0.02 and 0.02 mag, respectively. The dispersion of the color at minimum light was found to be small, supporting the use of this parameter as a means to obtain accurate interstellar extinctions along the line of sight up to the distance of the RR Lyrae star. We found a trend of color at minimum light with a pulsational period that, if taken into account, brings the dispersion in color at minimum light to mag for the , , and colors. These calibrations will be very useful for using RR Lyrae stars from DECam observations as both standard candles for distance determinations and color standards for reddening measurements.

Blazhko effect in the Galactic bulge fundamental mode RR Lyrae stars – I. Incidence rate and differences between modulated and non-modulated stars

We present the first paper of a series focused on the Blazhko effect in RR Lyrae type stars pulsating in the fundamental mode, that are located in the Galactic bulge. A~comprehensive overview about the incidence rate and light-curve characteristics of the Blazhko stars is given. We analysed 8\,282 stars having the best quality data in the OGLE-IV survey, and found that at least $40.3$\,\% of stars show modulation of their light curves. The number of Blazhko stars we identified is 3\,341, which is the largest sample ever studied implying the most relevant statistical results currently available. Using combined data sets with OGLE-III observations, we found that 50\,\% of stars that show unresolved close peaks to the main component in OGLE-IV are actually Blazhko stars with extremely long periods. Blazhko stars with modulation occur preferentially among RR Lyrae stars with shorter pulsation periods in the Galactic bulge. Fourier amplitude and phase coefficients based on the mean light curves appear to be substantially lower for Blazhko stars than for stars with unmodulated light curve in average. We derived new relations for the compatibility parameter $D_{m}$ in $I$ passband and relations that allow for differentiating modulated and non-modulated stars easily on the basis of $R_{31}$, $\phi_{21}$ and $\phi_{31}$. Photometric metallicities, intrinsic colours and absolute magnitudes computed using empirical relations are the same for Blazhko and non-modulated stars in the Galactic bulge suggesting no correlation between the occurrence of the Blazhko effect and these parameters.

PERIOD–COLOR AND AMPLITUDE–COLOR RELATIONS AT MAXIMUM AND MINIMUM LIGHT FOR RR LYRAE STARS IN THE SDSS STRIPE 82 REGION

ABSTRACT Investigation of period–color (PC) and amplitude–color (AC) relations at the maximum and minimum light can be used to probe the interaction of the hydrogen ionization front (HIF) with the photosphere and the radiation hydrodynamics of the outer envelopes of Cepheids and RR Lyraes. For example, theoretical calculations indicated that such interactions would occur at minimum light for RR Lyrae and result in a flatter PC relation. In the past, the PC and AC relations have been investigated by using either the (V − R)MACHO or (V − I) colors. In this work, we extend previous work to other bands by analyzing the RR Lyraes in the Sloan Digital Sky Survey Stripe 82 Region. Multi-epoch data are available for RR Lyraes located within the footprint of the Stripe 82 Region in five (ugriz) bands. We present the PC and AC relations at maximum and minimum light in four colors: (u − g)0, (g − r)0, (r − i)0, and (i − z)0, after they are corrected for extinction. We found that the PC and AC relations for this sample of RR Lyraes show a complex nature in the form of flat, linear or quadratic relations. Furthermore, the PC relations at minimum light for fundamental mode RR Lyrae stars are separated according to the Oosterhoff type, especially in the (g − r)0 and (r − i)0 colors. If only considering the results from linear regressions, our results are quantitatively consistent with the theory of HIF-photosphere interaction for both fundamental and first overtone RR Lyraes.

Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection – II. Short-period stars with a dominant radial fundamental mode

We report the discovery of a new group of double-periodic stars in the OGLE Galactic bulge photometry. In 38 stars identified as fundamental-mode RR Lyrae and four classified as first-overtone RR Lyrae, we detected an additional shorter periodicity. The periods of the dominant variability in the newly discovered group are 0.28 < PD < 0.41 d. Period ratios (0.68–0.72) are smaller than the period ratios of the Galactic bulge RRd stars. The typical amplitude ratio (of the additional to the dominant periodicity) is 20 per cent for the stars identified as fundamental-mode RR Lyrae and 50 per cent for stars classified as first-overtone RR Lyrae. 10 stars from our sample exhibit equidistant peaks in the frequency spectrum, which suggests the Blazhko-type modulation of the main pulsation frequency and/or additional periodicity. The Fourier coefficients R21 and R31 are some of the lowest among fundamental-mode RR Lyrae stars, but among the highest for the first-overtone pulsators. For the phase Fourier coefficients φ21 and φ31, our stars lie between RRab and RRc stars. The stars discussed were compared with radial linear pulsation models. Their position in the Petersen diagram cannot be reproduced by assuming that two radial modes are excited and their physical parameters are like those characteristic of RR Lyrae stars. The non-radial-mode scenario also faces difficulties. We conclude that the dominant variability is most likely due to pulsation in the radial fundamental mode, which applies to stars classified as first-overtone mode pulsators. At this point, we cannot explain the nature of the additional periodicity. Even more, the classification of the stars as RR Lyrae should be treated as tentative.