Search

We aim to construct an exceptionally deep (V ≲ 27) catalog of variable objects in selected Galactic and extragalactic fields visited multiple times by the Hubble Space Telescope (HST). While HST observations of some of these fields were searched for specific types of variables before (most notably, the extragalactic Cepheids), we attempt a systematic study of the population of variable objects of all types at the magnitude range not easily accessible with ground-based telescopes. The variability timescales that can be probed range from hours to years depending on how often a particular field has been visited. For source extraction and cross-matching of sources between visits we rely on the Hubble Source Catalog which includes 107 objects detected with WFPC2, ACS, and WFC3 HST instruments. The lightcurves extracted from the HSC are corrected for systematic effects by applying local zero-point corrections and are screened for bad measurements. For each lightcurve we compute variability indices sensitive to a broad range of variability types. The indices characterize the overall lightcurve scatter and smoothness. Candidate variables are selected as having variability index values significantly higher than expected for objects of similar brightness in the given set of observations. The Hubble Catalog of Variables will be released in 2018. [ABSTRACT FROM AUTHOR]

We present a comparative analysis of theoretical and observed light curves of Cepheid variables using Fourier decomposition. The theoretical light curves at multiple wavelengths are generated using stellar pulsation models for chemical compositions representative of Cepheids in the Galaxy and Magellanic Clouds. The observed light curves at optical (VI), near-infrared (JHKs) and mid-infrared (3.6 & 4.5-µm) bands are compiled from the literature. We discuss the variation of light curve parameters as a function of period, wavelength and metallicity. Theoretical and observed Fourier amplitude parameters decrease with increase in wavelength while the phase parameters increase with wavelength. We find that theoretical amplitude parameters obtained using canonical mass-luminosity levels exhibit a greater offset with respect to observations when compared to non-canonical relations. We also discuss the impact of variation in convective efficiency on the light curve structure of Cepheid variables. The increase in mixing length parameter results in a zero-point offset in bolometric mean magnitudes and reduces the systematic large difference in theoretical amplitudes with respect to observations. [ABSTRACT FROM AUTHOR]

SuperWASP is one of the largest ground-based surveys for transiting exoplanets. To date, it has observed over 31 million stars. Such an extensive database of time resolved photometry holds the potential for extensive searches of stellar variability, and provide solid candidates for the upcoming TESS mission. Previous work by e.g. [15], [5], [12] has shown that the WASP archive provides a wealth of pulsationally variable stars. In this talk I will provide an overview of the SuperWASP project, present some of the published results from the survey, and some of the on-going work to identify key targets for the TESS mission. [ABSTRACT FROM AUTHOR]

The Palomar Transient Factory (PTF) and its successor, the intermediate PTF (iPTF), are wide-field synoptic sky surveys aimed to detect transients. Even though the main science goal for PTF/iPTF is to detect various types of transients, the synoptic nature of the surveys can also be used for the study of variable stars. In this proceedings contribution, I will first give a brief introduction to PTF/iPTF, followed by the two pulsating stars studies using the PTF/iPTF data: the Ultra-Long Period Cepheids (ULPC) in M31 and the RR Lyrae in the Kepler field. For the formal study, we searched the M31's ULPC using PTF imaging data, and follow up the candidates with other telescopes. Our finding revealed that there are only two ULPC in M31. I will give a brief implication of our finding in distance scale studies. For the latter study, I will present our work on the derivation of metallicity-light curve relation in native PTF/iPTF R-band using the RRab stars in the Kepler field. [ABSTRACT FROM AUTHOR]

The Antarctic plateau is one of the best observing sites on the surface of the Earth thanks to its extremely cold, dry, stable and transparent atmosphere conditions. Various astronomical activities are underway there and the Chinese Center for Antarctic Astronomy (CCAA) is dedicated to developing Antarctic astronomy at the highest point, Dome A or the Chinese Kunlun station. So far a large number of images have been collected from a 14.5-cm quad-telescope called the Chinese Small Telescope ARray (CSTAR) and the first two of a trio of 50-cm Antarctic Survey Telescopes (AST3-1 and AST3-2). [ABSTRACT FROM AUTHOR]

The variable star population in the Large Magellanic Cloud outer-halo globular cluster NGC 2257 has been reinvestigated using photometry (to ∼20th mag) of over 400 new B, V CCD images taken with the CTIO 0.9-m telescope on 14 nights in Dec’07 and Jan’08. The derived periods of most of the stars are consistent with the pulsation periods derived previously, and where there were discrepancies these have been resolved. Accurate Fourier coefficients and parameters were computed for the B and V light curves. Six new variable stars were discovered (V45–V50), including a bright candidate long-period variable star showing secondary oscillations (V45). Examination of archival HST images and previously-published photometry shows that the excess brightness of two bright RR Lyrae stars (V48, V50) is due to contamination from close red giant branch stars. Among the previously known variable stars three double-mode (RRd) RR Lyrae stars (V8, V16 and V34) and several Blazhko variables were discovered. The total number of cluster variable stars now stands at forty-seven: 23 RRab stars, four of which show Blazhko light-curve variations; 20 RRc stars, one showing clear Blazhko variations and another showing possible Blazhko variations; the three RRd stars, all having the dominant period ∼0.36 day and period ratios P1/P0∼0.7450, and the LPV located near the tip of the red giant branch. A comparison with similar stars in other environments shows that the RRd stars in NGC 2257 are most similar to those in IC 4499. [ABSTRACT FROM AUTHOR]

In this paper we report on new RR Lyrae stars discovered in the first long run fields (LRc01 and LRa01) of the CoRoT mission. CoRoT frequency analysis of the high quality light curve with an unprecedented photometric accuracy and uninterrupted coverage over long period (∼150 days) are shown. The CoRoT results reveal four of the seven stars affected with the Blazhko effect and showing a very high fraction of the Blazhko stars ever obtained. The CoRoT RRLyrae stars show pulsation periods strongly nonlinear and complex multiplet structures, such as the triplet, quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet frequency structures. Additional frequencies are detected as well with a linear combination with the fundamental radial mode, showing a strong evidence for direct detections of nonradial pulsations. We put into evidence new classes of RR Lyrae stars showing the Blazhko double mode RR Lyrae stars (fundamental and second overtone), nonradial and radial (multimodes) pulsations and new behaviour of the Blazhko effect, such as the Blazhko modulation in both mixing modes (second Blazhko effect), the CoRoT Blazhko results are consistent with Stothers’explanation of the Blazhko effect with period and amplitude changes. However, we stress for developing it in further theoretical investigation, with the aim of finding the physical mechanism behind the Blazhko enigma. [ABSTRACT FROM AUTHOR]

The bump, a manifestation of a shock wave on the descending branch or around minimum, is a very remarkable feature of certain monoperiodic RR Lyrae stars. Excellent theoretical identification, as one of the three shock waves, is given by Fokin et al. [5]. The CoRoT Space telescope supports us with incredibly high precision datasets. They allow us to carry out investigations that have never been possible before. Using a 152-day long CoRoT dataset we are able to divide it not only into shorter subsets (check for short-term variability) but into segments according to the remarkable features of the light curve. EN2_STAR_MON_0101370131 is a monoperiodic, non-Blazhko CoRoT RR Lyrae star of the first long run with a very definite bump on the lower part of the descending branch. The light curve is nicely fitted by the pulsation period and its 33 harmonics. The harmonics are highly needed to describe the descending branch. A definite question was addressed to the dataset: can we connect certain harmonics to the special features of the light curve? Our result shows that the 4th and 7th harmonics have close connection to the bump. Our observational results are compared to the results of the theoretical light curve selected from a grid of RR Lyrae models. Investigation of a radial dependence of the shock waves’ characteristic timescale needs a theoretical model with properly zoned outer layers. [ABSTRACT FROM AUTHOR]

The SuperMACHO Project is a five-year survey toward the Large Magellanic Cloud (LMC) aimed at understanding the nature of the populations of lenses responsible for the excess microlensing rates observed by the MACHO project [1]. Survey observations were completed in 2006. A rich side-product of this survey is a catalog of variable sources down to a depth of VR 23, including many classes of pulsating variables such as δ-Scuti and RR Lyrae. Through their position in the Period-Luminosity diagram and their light curve characteristics we have identified 2323 high amplitude δ-Scuti (HADS) having high quality light curves. Using Fourier decomposition of the HADS light curves, we find that the period-luminosity (PL) relation defined by the first-overtone (FO) pulsators does not show a clear separation from the PL-relation defined by the fundamental (F) pulsators. This differs from other instability strip pulsators such as type c RR Lyrae. We also present evidence for a larger amplitude, subluminous population of HADS similar to that observed in Fornax [2]. [ABSTRACT FROM AUTHOR]

The ZZ Ceti star KUV 02464+3239 was observed over a whole season at the mountain station of Konkoly Observatory. A rigorous frequency analysis revealed 6 certain periods between 619 and 1250 seconds, with no shorter period modes present. We use the observed periods, published effective temperature and surface gravity, along with the model grid code of Bischoff-Kim, Montgomery and Winget [1] to perform a seismological analysis. We find acceptable model fits with masses between 0.60 and 0.70 M⊙. The hydrogen layer mass of the acceptable models are almost always between 10- and 10-6 M*. In addition to our seismological results, we also show our analysis of individual light curve segments. Considering the non-sinusoidal shape of the light curve and the Fourier spectra of segments showing large amplitude variations, the importance of non-linear effects in the pulsation is clearly seen. [ABSTRACT FROM AUTHOR]

We have undertaken a project to discover new type II Cepheids in two large-area sky surveys. However, most of the candidates from the surveys differ from type II Cepheids in their amplitude and period distributions and they form a separate, cooler sequence relative to the Cepheids in a temperature—gravity plot. We refer to these objects as “Cepheid-like” stars. Many exhibit unstable pulsation which we illustrate with representative light curves. [ABSTRACT FROM AUTHOR]

We present a new analysis of the light curve for the secondary star in the PSR B1957+20 system. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100% coverage in the R-band. We also have a number of new Ks band data points which we use to constrain the IR magnitude of the system. We model this with the Eclipsing Light Curve code. From our best fit model we are able to constrain the system inclination to 63.9 ± 2.4° for pulsar masses ranging from 1.35 – 1.9 M⊙. The pulsar mass is unconstrained. We also find that the secondary is not filling its Roche lobe which has important consequences for evolutionary models of this system. The temperature of the un-irradiated side of the companion is in agreement with previous estimates. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The period-V-amplitude plane for fundamental mode pulsator RR Lyrae stars in the MACHO Bulge fields is examined. Only stars with “normal” light curves are used in the analysis. Although the RR Lyraes in Bulge globular clusters divide into two distinct groups based on the average periods [12], no gap between Oosterhoff I and II stars in the Bulge field star sample is seen. The main Δ log P distribution of the Bulge RR0 Lyrae field star population is shifted by about 0.02 days with regard to the Milky Way Oosterhoff I population. Stars with Δ log P>0.06 days are seen in the sample, a rare property of RR Lyrae in Milky Way globular clusters. A comparative study of the Bulge field stars with similar metallicities but different Oosterhoff types is carried out, and it is shown that Bulge field RR0 Lyrae variables with OoII-like Δ log P values are about 0.2 mag brighter than RR0 Lyrae variables with OoI-like Δ log P value. Hence, an RR Lyrae MVRR-[Fe/H] relation may be inaccurate when considering populations with different Δ log P. [ABSTRACT FROM AUTHOR]

We present results of the search for variable stars in a sample of Local Group isolated dwarf galaxies, namely LGS3, IC1613, Cetus, and Tucana, based on very deep HST/ACS data. About 900 variables and candidates have been found in total. Among the confirmed variables, we identified about 700 RR Lyrae stars, 60 Cepheids, and 50 eclipsing binaries from their light-curve morphology and location in the color-magnitude diagrams. We find that the properties of these variables reflect details of the star formation history of the galaxies. In particular, our in depth study of the RR Lyrae population of the dSph Tucana reveals interesting details about the early evolution of this galaxy, and the origin of its stellar population gradients. We find that the fainter RR Lyrae stars, also having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample, suggesting that a metallicity gradient must have appeared very early on in the history of this galaxy. We also compare the location of the Cepheids found in the galaxies of our sample in the period-luminosity diagram with the Magellanic Clouds Cepheids of the OGLE collaboration, and find that the Cepheids in the gas-rich dwarf galaxies are classical Cepheids, while Cetus and Tucana harbor only anomalous Cepheids. [ABSTRACT FROM AUTHOR]

HD 49330 (B0.5IVe) has been observed with CoRoT during the LRA1 run and presented an outburst of moderate amplitude (0.03 mag). The analysis of its light curve revealed many frequencies in the Fourier spectrum typical of p and g pulsation modes with amplitude variations along the run closely correlated to the outburst. The simultaneous spectroscopic observations allowed us to identify some p and g modes. Using these results, we performed a preliminary seismic modeling of the Be star HD 49330 taking into account its rapid rotation. [ABSTRACT FROM AUTHOR]

We present preliminary results from the ground-based ESO/FLAMES observational program set up for the spectroscopic study of variable stars observed in the CoRoT exoplanet fields. In the Initial Run (IRa01) and the two first long anticenter runs LRa01 and LRc01 about 9000 variable stars have been identified by the CVC (Corot Variable Classifier). The multi-objects FLAMES medium resolution (R∼7000) spectroscopic observation of CoRoT variable stars from the IRa01 and LRa01 fields allows us to classify the variable stars accurately. In particular, we identified the Be stars and will determine their fundamental parameters by synthetic spectral fitting and by taking into account NLTE and rapid rotation effects. Confronting these parameters to the CoRoT light curves of these objects will allow us to test for correlations between stellar parameters and pulsation properties in a statistical way, in particular to study the instability strips. Moreover, knowing the fundamental parameters of the stars will allow us to perform seismic modeling. [ABSTRACT FROM AUTHOR]

Our main goal is to establish a firm empirical calibration of the surface brightness vs. (V-K) color relation for early type stars based on high quality spectroscopic and infrared observations of early-type detached eclipsing systems in the Large Magellanic Cloud (LMC). Our calibration of this relation will allow distance determinations accurate to about 2.5% to a single object located well beyond the Magellanic Clouds. This will let us calibrate other distance indicators, including period-luminosity relations for Cepheids. The first step of the project is to determine precise parameters for a sample of B-type systems in the LMC. We have already made a preliminary analysis of light and radial velocity curves for selected objects and measured their masses and radii. Here we present the results for one such system. [ABSTRACT FROM AUTHOR]

The high precision data obtained by the Kepler satellite allows us to detect hybrid-type pulsator candidates more accurately than the data obtained by ground-based observations. In this study, we present preliminary results on the new analysis of the Kepler light curve and high-resolution spectroscopic observations of the pulsating Am star KIC 9204718. Our tentative analysis shows that the star has hybrid-type pulsational characteristics. [ABSTRACT FROM AUTHOR]

Long time surveys yield many light curves among their byproducts, useful to identify candidate Be stars that can be confirmed by spectroscopic follow-ups. We present in this work different techniques we have used to find Be star candidates in the Galaxy, the Magellanic Clouds and the OGLE-IV Gaia South Ecliptic Pole Field. [ABSTRACT FROM AUTHOR]

We report the discovery of 24 new "dippers" - young stars with disks that show 1-3 day long abrupt drops in their light curves. The search was based on the K2 Campaign 2 observations of the Upper Sco and ρ Oph star forming regions. [ABSTRACT FROM AUTHOR]

It is well known that the residuals of the multifrequency analysis of d Scuti stars are correlated, giving rise to challenging features such as a plateau (HD 50844, HD 50870, HD 49434, . . .) of non-resolved frequencies with amplitude higher than the expected noise level. Several hypotheses have been advanced to explain these features: effective convection, rotation, non-linear interactions, etc. We have recently demonstrated that in some cases the underlying function describing the light variations of δ Scuti stars has not the property of being analytic. The strong consequence of this result is that the Fourier expansion on which the harmonic analysis is based could be not justified. In order to know the extension of this phenomenon among δ Scuti stars, we have used photometric data from CoRoT seismofield and a set of Kepler stars. The results show that this inconsistency in the application of harmonic analysis is almost ubiquitous to the d Scuti pulsating stars. [ABSTRACT FROM AUTHOR]

The possibility of a direct mass measurement of RR Lyrae variable stars in binary systems has remained elusive for many years, due to the apparent paucity of such systems. Motivated by our recent success of finding a population of high confidence candidates, we have been continuing the search for RR Lyrae variables in binary systems towards the Galactic bulge. We describe the preliminary results of our project in these proceedings. [ABSTRACT FROM AUTHOR]

Our thinking about eclipsing binary stars has undergone a tremendous change in the last decade. Eclipsing binary stars are one of nature's best laboratories for determining the fundamental physical properties of stars and thus for testing the predictions of theoretical models. Some of the largest ongoing variable star surveys include the Catalina Real-time Transient Survey (CRTS) and the VISTA Variables in the Vía Láctea survey (VVV). They both contain a large amount of photometric data and plenty of information about eclipsing binaries that wait to be extracted and exploited. Here we briefly describe our efforts in this direction. [ABSTRACT FROM AUTHOR]

A variability search has been performed in the Galactic center, using the nearinfrared images from the Vista Variables in the Vía Láctea (VVV) Survey. Light curves contain 89 epochs in the KS band. A total of 353 variable stars were found, of which only 47 are already present in the literature. [ABSTRACT FROM AUTHOR]