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A. Udalski

Publications and source records attributed to A. Udalski.

At least 523 records · Page 29Linked to original sources
arXiv1999

The Optical Gravitational Lensing Experiment. Variable Stars in Star Clusters of the Magellanic Clouds. I. Eclipsing Systems in the Clusters of the SMC

The list of 127 eclipsing stars in optical coincidence with star clusters from the SMC is presented. It was prepared using the catalogs of eclipsing systems and star clusters from the SMC based on observations collected during the OGLE-II microlensing project. Location of 12 eclipsing stars in the color-magnitude diagram of clusters allows to exclude their membership. Photometric data of 73 systems support their membership. The remaining 42 objects were found in loose, faint clusters and therefore no conclusive statement about their membership can be made. All presented data are available from the OGLE archive.

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arXiv1999

The Optical Gravitational Lensing Experiment. Monitoring of QSO 2237+0305

We present results from 2 years of monitoring of Huchra's lens (QSO 2237+0305) with the 1.3 m Warsaw telescope on Las Campanas, Chile. Photometry in the V band was done using a newly developed method for image subtraction. Reliable subtraction without Fourier division removes all complexities associated with the presence of a bright lensing galaxy. With positions of lensed images adopted from HST measurements it is relatively easy to fit the variable part of the flux in this system, as opposed to modeling of the underlying galaxy. For the first time we observed smooth light variation over a period of a few months, which can be naturally attributed to microlensing. We also describe automated software capable of real time analysis of the images of QSO 2237+0305. It is expected that starting from the next observing season in 1999 an alert system will be implemented for high amplification events (HAE) in this object. Time sampling and photometric accuracy achieved should be sufficient for early detection of caustic crossings.

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arXiv1999

The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. I. Double-Mode Cepheids in the Small Magellanic Cloud

We present a sample of 93 double-mode Cepheids detected in the 2.4 square degree area in the central part of the SMC. 23 stars from the sample pulsate in the fundamental mode and the first overtone while 70 objects are the first and second overtone pulsators. This is the largest sample of such type Cepheids detected in one environment so far. We analyze period ratio of double-mode Cepheids and Fourier parameters of decomposition of the light curves of these objects. We also present location of different type Cepheids from the SMC in the color-magnitude diagram and show their distribution of V-I color indices. We find one object which is probably a blend, either physical or optical, of two Cepheids pulsating in the fundamental mode.

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arXiv1999

The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. II. Single-Mode Second Overtone Cepheids in the Small Magellanic Cloud

We present a sample of 13 candidates for single-mode second overtone Cepheids discovered in the 2.4 square degree area of the SMC bar during the OGLE microlensing search. All candidates passed four photometric tests: analysis of Fourier parameters of the light curve, R_21 and ϕ_21, position in the period-luminosity diagram and location in the color-magnitude diagram. The stars form a very homogeneous group of objects with very similar light curves of low amplitude and almost sinusoidal shape. Their color indices B-V and V-I are also very similar and the objects determine the observational blue edge of the instability strip of the first overtone Cepheids. This is the first firm sample of the single-mode second overtone Cepheids detected so far.

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arXiv1998

The Optical Gravitational Lensing Experiment. Eclipsing Binary Stars in the Small Magellanic Cloud

We present the catalog of 1459 eclipsing binary stars detected in the central 2.4 square degree area of the Small Magellanic Cloud during the OGLE-II microlensing search. The sample includes objects brighter than I=20 mag with periods ranging from about 0.3 to 250 days. The average completeness of the catalog is about 80%. Statistics of the entire sample and well detached systems, suitable for distance determination, are also presented. The catalog, finding charts and BVI photometry data for all objects are available from the OGLE Internet archive.

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arXiv1998

The Optical Gravitational Lensing Experiment. Short Distance Scale to the LMC

We present {\it UBVI} photometry of the eclipsing binary HV2274 - the system which has been recently used for distance determination to the LMC by Guinan et al. (1998). We determine the interstellar reddening to the star, E(B-V)=0.149+/-0.015 mag, based on observed colors of the star. This value is in excellent agreement with the mean reddening towards HV2274 obtained from photometry of the red clump stars in the surrounding field. The reddening is almost twice as large as determined by Guinan et al. (1998). We discuss the consequences of reddening underestimate. Most likely HV2274 is located much closer with the distance modulus to the star and the LMC: m-M = 18.22+/-0.13 mag supporting the short distance scale to the LMC. Such a distance modulus is in excellent agreement with the recent distance determinations with RR Lyr and red clump stars.

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arXiv1998

Optical Gravitational Lensing Experiment. Population Effects on the Mean Brightness of the Red Clump Stars

We present an empirical test indicating that the mean I-band magnitude of the red clump stars, used as the standard candle in the recent distance determinations to the Magellanic Clouds and other objects, is age independent for intermediate age (2-10 Gyr) stars. Fifteen star clusters of age ~1.5-12 Gyr from the LMC and SMC (ESO121SC03, SL663, NGC2155, NGC2121, SL388, SL862, NGC121, L1, KRON3, NGC416, L113, NGC339, L11, NGC419, NGC411) were observed and their color-magnitude diagrams are presented. The mean I-band brightness of the red clump in these clusters is constant and its mean extinction-free magnitude is: I_0=17.88+/-0.05 mag and I_0=18.31+/-0.07 mag at the mean metallicity of -0.8 dex and -1.2 dex for the LMC and SMC clusters, respectively. For older objects (> 10 Gyr) the brightness of the red clump, which converts into the red part of the horizontal branch, fades by about 0.3-0.4 mag, setting an important limitation on the red clump stars method of distance determination. The red clump distance moduli to the Magellanic Clouds from the new independent data set are: m-M=18.18+/-0.06 mag and m-M=18.65+/-0.08 mag for the LMC and SMC, respectively, in very good agreement with previous determinations. Weak dependence of the mean I-band brightness of the red clump on metallicity and its independence of age for intermediate age population (2-10 Gyr) of stars as well as the most precise calibration as compared to other standard candle candidates makes the red clump stars method one of the most accurate steps in the distance scale ladder.

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arXiv1998

The Optical Gravitational Lensing Experiment. Photometry of the MACHO-SMC-98-1 Binary Microlensing Event

We present photometry of the unique binary microlensing event MACHO-SMC-98-1 collected by the OGLE group. Particularly interesting observation was made close to the first caustic crossing which was not covered by observations of other groups. It allows to test proposed models of which Model~1 proposed by PLANET group seems to be in the best agreement with the OGLE observations.

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arXiv1998

The Optical Gravitational Lensing Experiment. BVI Maps of Dense Stellar Regions. I. The Small Magellanic Cloud

We present three color, BVI maps of the Small Magellanic Cloud. The maps contain precise photometric and astrometric data for about 2.2 million stars from the central regions of the SMC bar covering ~2.4 square degrees on the sky. Mean brightness of stars is derived from observations collected in the course of the OGLE-II microlensing search from about 130, 30 and 15 measurements in the I, V and B-bands, respectively. Accuracy of the zero points of photometry is about 0.01 mag, and astrometry 0.15 arcsec (with possible systematic error up to 0.7 arcsec). Color-magnitude diagrams of observed fields are also presented. The maps of the SMC are the first from the series of similar maps covering other OGLE fields: LMC, Galactic bulge and Galactic disk. The data are very well suited for many projects, particularly for the SMC which has been neglected photometrically for years. Because of potentially great impact on many astrophysical fields we decided to make the SMC data available to the astronomical community from the OGLE Internet archive.

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arXiv1998

The Optical Gravitational Lensing Experiment. The Catalog of Clusters in the Small Magellanic Cloud

We present the catalog of clusters found in the area of ~2.4 square degree in the central regions of the Small Magellanic Cloud. The catalog contains data for 238 clusters, 72 of them are new objects. For each cluster equatorial coordinates, radii, approximate number of members, cross-identification, finding chart and color magnitude diagrams: V-(B-V) and V-(V-I) are provided. Photometric data for all clusters presented in the catalog are available from the OGLE Internet archive.

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arXiv1998

Optical Gravitational Lensing Experiment. The Distance Scale: Galactic Bulge - LMC - SMC

We analyze the mean luminosity of three samples of field RRab Lyr stars observed in the course of the OGLE microlensing experiment: 73 stars from the Galactic bulge and 110 and 128 stars from selected fields in the LMC and SMC, respectively. The fields are the same as in the recent distance determination to the Magellanic Clouds with the red clump stars method by Udalski et al (1998). We determine the relative distance scale d_{GB}:d_{LMC}:d_{SMC} equal to: (0.194+/-0.010):1.00:(1.30+/-0.08). We calibrate our RR Lyr distance scale with the recent calibration of Gould and Popowski (1998) based on statistical parallaxes. We obtain the following distance moduli to the Galactic bulge, LMC and SMC: m-M=14.53+/-0.15, m-M=18.09+/-0.16 and m-M=18.66+/-0.16 mag. We use the RR Lyr mean V-band luminosity at the Galactic bulge metallicity as the reference brightness and analyze the mean, I-band luminosity of the red clump stars in objects with different ages and metallicities. We add to our analysis the metal poor Carina dwarf galaxy which contains old RR Lyr stars and intermediate age red clump population. We find a weak dependence of the mean red clump brightness on metallicity and we calibrate its zero point with the nearby local red clump stars measured by Hipparcos: M^{RC}_I=(0.09+/-0.03) *[Fe/H]^{RC}-0.23+/-0.03. Our revised red clump distance moduli to the Galactic bulge, LMC and SMC are m-M=14.53+/-0.06, m-M=18.13+/-0.07 and m-M=18.63+/-0.07 mag, respectively. The distance modulus to the Carina galaxy is m-M=19.84+/- 0.07 mag. Excellent agreement of RR Lyr and red clump distances which have independent absolute calibrations confirms the short distance scale to the LMC.

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arXiv1998

Optical Gravitational Lensing Experiment. Distance to the Magellanic Clouds with the Red Clump Stars: Are the Magellanic Clouds 15% Closer than Generally Accepted?

We present a new distance determination to the Large and Small Magellanic Clouds using the newly developed red clump stars method (Paczynski and Stanek 1998). This new, single-step, Hipparcos calibrated method seems to be one of the most precise techniques of distance determination with very small statistical error due to large number of red clump stars usually available. The distances were determined independently along four lines-of-sight located at opposite sides of each Magellanic Cloud. The results for each line-of-sight are very consistent. For the SMC we obtain the distance modulus: m-M=18.56+/-0.03+/-0.06 mag (statistical and systematic errors, respectively) and for the LMC: m-M=18.08+/-0.03+/-0.12} mag where systematic errors are mostly due to uncertainty in reddening estimates. Both distances will be refined and systematic errors reduced when accurate reddening maps for our fields are available. Distance moduli to both Magellanic Clouds are ~0.4 mag smaller than generally accepted values. The modulus to the LMC is in good agreement with the recent determinations from RR Lyrae type stars and upper limit resulting from the SN1987A echo. We suspect that the distance to the LMC and SMC is shorter by about 15% than previously assumed: 42 kpc and 52 kpc, respectively. We also present our color-magnitude diagrams around the red clump for the LMC and SMC. We identify vertical red clump, first noted by Zaritsky and Lin (1997), in the color-magnitude diagram of both Magellanic Clouds and we interpret it as an evolutionary feature rather than unknown stellar population between the LMC and our Galaxy.

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arXiv1997

Optical Gravitational Lensing Experiment. Photometry of the MACHO-SMC-1 Microlensing Candidate

We present photometric observations of the MACHO-SMC-1 microlensing candidate collected by the OGLE-2 project. We show light curves of both components of the 1.6 arcsec blend: microlensed star and its optical companion. We find the contribution of the optical companion to the total flux to be 24% and confirm presence of the small amplitude periodic oscillations in the light curve of the lensed star with the period of 5.096 days and amplitude 0.05 mag. The lensed star is probably an ellipsoidal binary system.

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arXiv1997

The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters - IV. Fields 104A-E in 47 Tuc

Five fields located close to the center of the globular cluster NGC 104=47 Tuc were surveyed in a search for variable stars. We present V-band light curves for 42 variables. This sample includes 13 RR Lyr stars -- 12 of them belong to the Small Magellanic Cloud (SMC) and 1 is a background object from the galactic halo. Twelve eclipsing binaries were identified -- 9 contact systems and 3 detached/semi-detached systems. Seven eclipsing binaries are located in the blue straggler region on the cluster color-magnitude diagram (CMD) and four binaries can be considered main-sequence systems. One binary is probably a member of the SMC. Eight contact binaries are likely members of the cluster and one is most probably a foreground star. We show that for the surveyed region of 47 Tuc, the relative frequency of contact binaries is very low as compared with other recently surveyed globular clusters. The sample of identified variables also includes 15 red variables with periods ranging from about 2 days to several weeks. A large fraction of these 15 variables probably belong to the SMC but a few stars are likely to be red giants in 47 Tuc. VI photometry for about 50 000 stars from the cluster fields was obtained as a by product of our survey.

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arXiv1997

The Optical Gravitational Lensing Experiment. The Catalog of Periodic Variable Stars in the Galactic Bulge. V. Periodic Variables in Fields: MM5-A, MM5-B, MM7-A and MM7-B

We present the fifth part of the OGLE Catalog of Periodic Variable Stars in the Galactic bulge. 574 variable stars: 44 pulsating, 369 eclipsing and 161 miscellaneous type were detected in four fields located symmetrically in galactic lattitude around the Galactic center: MM5-A, MM5-B, MM7-A and MM7-B. The Catalog and individual observations are available in digital form from the OGLE Internet archive.

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arXiv1996

Modelling the Galactic Bar Using Red Clump Giants

The color-magnitude diagrams of $\sim 7 \times 10^5$ stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic $l$ fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by $20-30°$, with axis ratios corresponding to $x_0:y_0:z_0=3.5:1.5:1$. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.

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arXiv1996

The Optical Gravitational Lensing Experiment. The General Catalog of Stars in the Galactic Bulge. I. The Stars in the Central Baade's Window OGLE Field BWC

This paper presents the first part of the Optical Gravitational Lensing Experiment (OGLE) General Catalog of Stars in the Galactic bulge. The Catalog is based on observations collected during the OGLE microlensing search. This part contains 33196 stars brighter than I=18mag identified in the Baade's Window BWC field. Stars from remaining 20 OGLE fields will be presented in similar form in the next parts of the Catalog. The Catalog is available to the astronomical community over the Internet network.

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