Difference imaging has proven to be a powerful technique for detecting and monitoring the variability of unresolved stellar sources in M 31. Using this technique in surveys of galaxies outside the Local Group could have many interesting applications. The goal of this paper is to test difference imaging photometry on Centaurus A, the nearest giant elliptical galaxy, at a distance of 4Mpc. We obtained deep photometric data with the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two months. Applying the difference imaging photometry package DIFIMPHOT, we produced high-quality difference images and detected variable sources. The sensitivity of the current observational setup was determined through artificial residual tests. In the resulting high-quality difference images, we detect 271 variable stars. We find a difference flux detection limit corresponding to m_R_~=24.5. Based on a simple model of the halo of Centaurus A, we estimate that a ground-based microlensing survey would detect in the order of 4 microlensing events per year due to lenses in the halo. Conclusions: Difference imaging photometry works very well at the distance of Centaurus A and promises to be a useful tool for detecting and studying variable stars in galaxies outside the local group. For microlensing surveys, a higher sensitivity is needed than achieved here, which would be possible with a large ground-based telescope or space observatory with wide-field imaging capabilities.
We present a catalog of photometrically variable stars discovered within two 21.3'x21.3' fields centered on the Cygnus OB2 association (Cyg OB2). There have hitherto been no deep optical variability studies of Cyg OB2, despite it being replete with early-type massive stars, likely due to the high and variable extinction (up to A_V_~20) that permeates much of the region. Here, we provide results of the first variability study with this combination of spatial coverage (~0.5deg) and photometric depth (R~21mag). We find 121 stars to be variable in both R and I bands, 115 of them newly discovered. Of the 121 variables, we identify 27 eclipsing binaries and eclipsing-binary candidates, 52 pulsating variables, and 20 potential Herbig Ae/Be stars. Confirming both the status and the cluster membership of the Herbig Ae/Be stars would address the uncertainty regarding the age and star formation history of Cyg OB2. We match our catalog to known variables and binaries in the region, Two Micron All Sky Survey near-IR data, and Chandra X-ray observations to find counterparts to new variables in other wavelengths.
We have identified 23 RR Lyrae stars and three possible anomalous Cepheids (ACs) among 84 candidate variables in the recently discovered Canes Venatici I (CVn I) dwarf spheroidal galaxy.
We present a new study of the variable star population in globular cluster 5 of the Fornax dSph, based on B and V time series photometry obtained with the MagIC camera of the 6.5m Magellan Clay telescope and complementary Hubble Space Telescope archive data. Light curves and accurate periodicities were obtained for 30 RR Lyrae stars and one SX Phoenicis variable. The RR Lyrae sample includes 15 fundamental-mode pulsators, 13 first-overtone pulsators, one candidate double-mode pulsator and one RR Lyrae star with uncertain type classification. The average and minimum periods of the ab-type RR Lyrae stars, <P_ab_>=0.590days, P_ab,min_=0.53297days and the position in the horizontal branch type-metallicity plane, indicate that the cluster has Oosterhoff-intermediate properties, basically confirming previous indications by Mackey & Gilmore, although with some differences both in the period and type classification of individual variables.
To obtain time-series photometry of the very crowded central regions of Galactic globular clusters. To obtain better angular resolution than has been previously achieved with conventional CCDs on ground-based telescopes. To complete, or improve, the census of the variable star population in those stellar systems. Images were taken using the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile. The telescope was equipped with an electron-multiplying CCD, and the short-exposure-time images obtained (ten images per second) were stacked using the shift-and-add technique to produce the normal-exposure-time images (minutes). Photometry was performed via difference image analysis. Automatic detection of variable stars in the field was attempted. The light curves of 12541 stars in the cores of ten globular clusters were statistically analysed to automatically extract the variable stars. We obtained light curves for 31 previously known variable stars (3 long-period irregular, 2 semi-regular, 20 RR Lyrae, 1 SX Phoenicis, 3 cataclysmic variables, 1 W Ursae Majoris-type and 1 unclassified) and we discovered 30 new variables (16 long-period irregular, 7 semi-regular, 4 RR Lyrae, 1 SX Phoenicis and 2 unclassified).
We have performed a photometric V,R,I CCD time-series analysis with a baseline of about eight years of the outer-halo globular cluster NGC 7492 with the aim of searching for new variables and using these (and the previously known variables) to determine the physical parameters of interest for the cluster (e.g. metallicity, absolute magnitude of the horizontal branch, distance, etc.). We used difference image analysis to extract precise light curves in the relatively crowded star field, especially towards the densely populated central region. Several different approaches were used for variability detection that recover the known variables and lead to new discoveries. We determined the physical parameters of the only RR0 star using a light-curve Fourier decomposition analysis. We found one new long-period variable and two SX Phe stars in the blue straggler region. We also present one candidate SX Phe star that requires follow-up observations.
We present the first time-series study of the ultra-faint dwarf galaxy Hercules. Using a variety of telescope/instrument facilities we secured about 50 V and 80 B epochs. These data allowed us to detect and characterize 10 pulsating variable stars in Hercules. Our final sample includes six fundamental-mode (ab-type) and three first-overtone (c-type) RR Lyrae stars, and one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, <P_ab_>=0.68days ({sigma}=0.03days), places Hercules in the Oosterhoff II group, as found for almost the totality of the ultra-faint dwarf galaxies investigated so far for variability. The RR Lyrae stars were used to obtain independent estimates of the metallicity, reddening, and distance to Hercules, for which we find [Fe/H]=-2.30+/-0.15dex, E(B-V)=0.09+/-0.02mag, and (m-M)_0_=20.6+/-0.1mag, in good agreement with the literature values. We have obtained a V, B-V color-magnitude diagram (CMD) of Hercules that reaches V~25mag and extends beyond the galaxy's half-light radius over a total area of 40'x36'. The CMD and the RR Lyrae stars indicate the presence of a population as old and metal-poor as (at least) the Galactic globular cluster M68.
Two nearby galaxies, IC 1613 and NGC 6822, were observed over four years to detect short period Cepheids and to obtain good light curves for Fourier decomposition. Since the program was carried out with a relatively small telescope, the Dutch 0.9-m at ESO-La Silla, the observations were performed without filter (white light), or Wh-band; the advantage of this technique is that the photon statistics correspond to those of V-band observations of similar exposure time obtained with a 2-m class telescope. In this paper we discuss some properties of Wh photometry and present the results of the observations of Field B in IC 1613 (3.8'x3.8'). A total of 65 images were obtained and the reduction was performed with DAOPHOT. The light curves of 4037 stars were measured, and the analysis using various techniques allowed the detection of 121 variable stars. We detected 51 population I Cepheids, while previously only 12 Cepheids were known in the same field. Most of these have short periods and some are first overtone mode pulsators. The other variable stars include one eclipsing binary, two suspected W Vir stars, 57 irregular and semi-regular red variables, 3 periodic red variables and 6 irregular blue variables.
The nearby galaxies IC 1613 and NGC 6822 were observed over four years to detect short period Cepheids and to obtain good light curves for Fourier decomposition. The program was carried out with a relatively small telescope, the Dutch 0.9m at ESO-La Silla, and the observations were performed without filter (white light), or Wh-band. In this paper we present the results of the observations of Fields C and D in IC 1613 (3.8'x3.8' each). The analysis of the light curves of 3297 stars in the two fields has allowed the detection of 92 variable stars. A total of 34 Population I Cepheids have been found, while only 7 of these objects were previously known in these fields. Most of the stars have short periods and some of them are first overtone mode pulsators. Among the other variable stars there are one eclipsing binary and five W Vir candidates. A nova event was observed in Field D.
