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W. Krzeminski

Publications and source records attributed to W. Krzeminski.

At least 55 records · Page 3Linked to original sources
arXiv2000

Cluster AgeS Experiment: The Age and Distance of the Globular Cluster omega Centauri Determined from Observations of the Eclipsing Binary OGLEGC17

We use masses, radii, and luminosities of the detached eclipsing binary OGLEGC17 derived from photometric and spectroscopic observations to calculate the age and distance of the globular cluster omega Cen. Age versus turnoff mass and age versus luminosity relations from Girardi et al. (2000) yield two independent estimates of the age, 9.1<t<16.7 Gyr and 12.9<t<18.5 Gyr. The distance and distance modulus derived by use of the infrared versus surface brightness relation are d =5385+-300 pc and (m-M)_V=14.06+-0.11. Distances derived from our infrared surface brightness versus color relation and the Teff versus B-V color relation of Sekiguchi & Fukugita (2000) disagree by about 10 per cent. Major improvements in the accuracy in estimated age and distance can be made with better measurements of the masses of the components of OGLEGC17.

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arXiv2000

Clusters AgeS Experiment. RR Lyr Variables in the Globular Cluster NGC 6362

We present V and B-band CCD photometry of 35 RR Lyr stars in the southern hemisphere globular cluster NGC 6362. Fourier decomposition of the light curves was used to estimate the basic properties of these variables. From the analysis of the RRc stars we obtain a mean mass of M=0.531+/-0.014 $M_\odot$, luminosity log L=1.656+/-0.006, effective temperature Teff=7429+/-20, and helium abundance Y=0.292+/-0.002. The mean values of the absolute magnitude, metallicity (on Jurcsik's scale) and effective temperature for 14 RRab stars with "regular" light curves are: M_V=0.86+/-0.01, [Fe/H]=-0.93+/-0.04 and Teff=6555+/-25, respectively. From the B-V colors, periods and metallicities of the RRab stars we estimate the color excess for NGC 6362 to be equal to E(B-V)=0.08+/-0.01. Using this value we derive the colors of the blue and red edges of the instability strip in NGC 6362 to be $(B-V)^{BE}_0=0.17$ and $(B-V)^{RE}_0=0.43$. From the relations between the Fourier coefficients of RRab and RRc stars and their absolute magnitudes we estimate the apparent distance modulus to NGC 6362 to be (m-M)_V=14.46+/-0.10. From the mean value of log L of the RRc stars we obtain 14.59+/-0.03. The V-band light curves of three of the RRc stars exhibit changes in amplitude of over 0.1 mag on the time scale of around one week. Near the radial 1st overtone frequency we find one or two peaks, which strongly suggests that these variables belong to the newly identified group of non-radial pulsating RR Lyr stars.

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arXiv2000

RR Lyrae Variables in the Globular Cluster M5

We present V-band CCD photometry of 65 RR Lyr variables from the globular cluster M5. We have estimated the basic physical parameters for 16 RRc stars and 26 RRab stars using a Fourier decomposition of the light curves of the variables. The mean values of mass, luminosity, effective temperature and relative helium abundance for the RRc stars are measured to be ${\cal M}=0.54{\cal M}_\odot$, log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively. For the RRab variables the derived mean values of absolute magnitude, metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period is a function of metal abundance rather than Oosterhoff type. We find significant problems with the calibration of both the zero point and the scale of the luminosities measured with the Fourier technique. The apparent distance modulus derived from RRc stars is equal to $14.47\pm0.11$ and it is in good agreement with recent determinations. On the other hand distance modulus obtained from the sample of RRab stars (calibrated by the Baade-Wesselink observations of field RR Lyr variables) is significantly smaller and equal to $14.27\pm0.04$

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arXiv1999

A photometric survey for variable stars in the globular cluster M5

We present the results of a photometric survey for variable stars in the nearby globular cluster M5. A 14.8x22.8 arcmin^2 field centered on the cluster was monitored for a total of 37 hours with a CCD camera mounted on the 1.0-m Swope telescope. Five new variables were identified: four blue stragglers which are SX Phe pulsating variables, and an eclipsing binary with an orbital period of 0.466 d. The eclipsing binary lies near the main-sequence turnoff on the cluster color-magnitude diagram. We do not confirm the variability of any of the 10 eclipsing binaries identified in the cluster field by Reid (1996) and Yan & Reid (1996). The dwarf nova M5-V101 exhibited two outbursts with full amplitudes of about 2.7 mag during our observations. A V/B-V color-magnitude diagram of the surveyed field was obtained, and a possible extreme HB star located 2 mag below cutoff of the blue HB was identified.

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arXiv1998

RR Lyrae Variables in the Globular Cluster M55. The First Evidence for Non Radial Pulsations in RR Lyr Stars

We present the results of a photometric study of RR Lyrae variables in the field of the globular cluster M55. We have discovered nine new RR Lyrae stars, increasing the number of known variables in this cluster to 15 objects. Five of the newly discovered variables belong to Bailey type RRc and two to type RRab. Two background RRab stars are probable members of the Sagittarius dwarf galaxy. Fourier decomposition of the light curves was used to derive basic properties of the present sample of RR Lyrae variables. From an analysis of the RRc variables we obtain a mean mass of $M=0.53\pm0.03 M_\odot$, luminosity $\log L=1.75\pm0.01$, effective temperature $T_{eff}=7193\pm27$ K, and helium abundance $Y=0.27\pm0.01$. Based on the $B-V$ colors, periods and metallicities of the RRab stars we estimate the value of the color excess for M55 to be equal to $E(B-V)=0.11\pm0.03$. Using this value we derive the colors of the blue and red edges of the instability strip in M55. The blue edge lies at $(B-V)_0=0.20$ mag and the red edge lies at $(B-V)_0=0.38$ mag. We estimate the values of the visual apparent and dereddened distance moduli to be $13.65\pm0.11$ and $13.31\pm0.11$, respectively. The light curves of three of the RRc variables exhibit changes in amplitude of over 0.1 mag on the time scale of less than a week, rather short for the Blazhko effect, but with no evidence for another radial pulsational frequency. However we do detect other periodicities which are clearly visible in the light curve after removing variations with the first overtone radial frequency. This is strong evidence for the presence of non-radial pulsations, a behavior common for $δ$ Scuti stars but not yet observed among RR Lyr variables.

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arXiv1998

BVI CCD Photometry of the Globular Cluster 47 Tuc

We present $BVI$ photometry for about 16000 stars from a 220 arcmin$^{2}$ field centered 8 arcmin east of the center of 47 Tuc. We have identified eight likely blue stragglers located in the outer parts of the cluster. Four of these objects are easy targets for spectroscopic studies with ground-based telescopes. Six candidates for blue horizontal branch stars were identified. However, it is possible that all or most of them belong in fact to the SMC halo. One faint blue star being candidate for a cataclysmic variable was found close to the cluster center. The average $I$-band magnitude for stars forming the red giant branch clump is determined at $I_{0}=13.09\pm 0.005 $mag. This in turn implies distance modulus of the cluster $(m-M)_{0,47Tuc}=13.32\pm 0.03\pm 0.036 $mag (statistical plus systematic error), if we adopt $M_{I,m}=-0.23 \pm 0.03 $mag for the average absolute luminosity of {\em Hipparcos}-calibrated clump giants, following Paczyński & Stanek and Stanek & Garnavich. This distance modulus of 47 Tuc is lower by $0.2-0.25 $mag than its recent estimates based on Hipparcos parallaxes for subdwarfs. We discuss possible reasons for this discrepancy. The photometric data are available through the anonymous ftp service.

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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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arXiv1996

The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters - II. Fields 5139D-F in Omega Centauri

Three fields located close to the center of the globular cluster Omega Cen were surveyed in a search for variable stars. We present V-band light curves for 32 periodic variables. This sample includes 10 SX Phe stars, 19 eclipsing binaries, and three likely spotted variables (FK Com or RS CVn type stars). Only 4 of these variables were previously known (including two objects reported in Paper I). All SX Phe stars and 8 eclipsing binaries from our sample belong to blue stragglers. Five binaries occupy positions on the upper main-sequence of the cluster and another four are located at the base of the subgiant branch. Of particular interest is detection of a detached binary system with a period P=4.64 day. Further study of this star can provide direct information about properties of turnoff stars in Omega Cen. A detached or semi-detached system located slightly above the horizontal branch of Omega Cen was also discovered. All SX Phe stars and most of eclipsing binaries discovered in our survey are likely cluster members. We present V vs. V-I color-magnitude diagrams for the monitored fields. A populous group of likely hot subdwarfs as well as numerous candidates for blue stragglers can be noted in these diagrams. Our data indicate that Omega Cen possesses the largest number of blue stragglers among all galactic globulars.

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arXiv1996

The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters -I. Fields 5139A-C in Omega Centauri

Three fields covering the central part of the globular cluster Omega Cen were surveyed in a search for variable stars. We present V-band light curves for 22 periodic variables: 9 SX~Phe stars, 7 contact binaries, 5 detached or semi-detached binaries, and one spotted variable (FK Com or RS CVn type star). Only 2 of these variables were previously known. All SX Phe stars and all contact binaries from our sample belong to blue stragglers. Observed properties of these stars are consistent with their cluster membership. Of particular interest is detection of two well detached binaries with periods P=1.50 day and P=2.47 day. Further study of these two binaries can provide direct information about properties of turnoff stars in Omega Cen. An uncomplete light curve of a Mira variable known as V2 was obtained. We present V vs. V-I color-magnitude diagrams for the monitored part of the cluster.

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arXiv1995

Modelling the Galactic Bar Using Red Clump Stars

The color-magnitude diagrams of $\sim 1 \times 10^6$ stars obtained for 19 fields towards the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump stars. We found that the distributions of the extinction-adjusted apparent magnitudes of red clump stars in fields lying at $l=\pm5°$ in galactic longitude differ by $\sim 0.4\; mag$. A plausible explanation of this observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or a bar, which is inclined to the line of sight by no more than $45°$. The part of the bar at the positive galactic longitude is closer to us. Work is now under way to model the Galactic bar by fitting the observed luminosity functions in the red clump region for various fields. Preliminary results indicate that the angle of the inclination of the bar to the line of sight can be as small as $\sim20°$. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.

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arXiv1994

The Optical Gravitational Lensing Experiment. Follow-Up Observations of the Macho Microlensing Event in the Galactic Bulge

We present follow-up observations of a gravitational microlensing candidate found by the MACHO collaboration in the Galactic bulge. The photometric data cover a period near the maximum of the event and may be used to construct complete light curve. The position of the lensed star in the color magnitude diagram suggests that it is a Galactic bulge star located on the subgiant branch.

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arXiv1994

The Optical Gravitational Lensing Experiment. Variable stars in the Sculptor dwarf speroidal galaxy

The central area of the Sculptor dwarf galaxy was surveyed for variable stars as a side-program of the OGLE project. Light curves in the V band were obtained for 226 RR Lyr stars and for 3 anomalous cepheids. One previously unknown anomalous cepheid was identified. We discovered also two variables located at the tip of the red giant branch of Sculptor. Out of 226 RR Lyr variables 135 were classified as RRab, 88 as RRc and 2 as RRd. Distribution of periods for RRab stars shows a sharp cut-off at P=0.475 days. This implies that the bulk of Sculptor RR Lyr stars has metallicity [Fe/H]\leq -1.7 on the Zinn-West scale. The average V magnitudes of RRab variables are correlated with their periods. This effect is most probably caused by the spread of metallicities exhibited by variables. Based on the average V magnitudes of RR Lyr stars the apparent distance modulus of Sculptor was determined at (m-M)_{V}=19.71. The color-magnitude diagram of Sculptor reaching V\approx 21.4 and I\approx 20.6 is presented. The observed width of the upper part of the red giant branch indicates range of metallicities -2.2\leq [Fe/H]\leq -1.6 for the Sculptor giants. The observed distribution of stars along the horizontal branch and average value of metallicity imply that age of Sculptor is similar to ages of the relatively young globular clusters from the outer galactic halo. The data about RR Lyr variables in four nearby dwarf galaxies are summarized and discussed briefly.

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arXiv1994

Results from the Optical Gravitational Lensing Experiment (OGLE)

The analysis of the first three years of the OGLE data revealed 12 microlensing events of the Galactic bulge stars, with the characteristic time scales in the range $ 8.6 < t_0 < 80 $ days, where $ t_0 = R_E / V $. A complete sample of nine events gave the optical depth to gravitational microlensing larger than $(3.3 \pm 1.2) \times 10^{-6}$, in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. The lenses are likely to be ordinary stars in the Galactic bar, which has its long axis elongated towards us. At this time we have no evidence that the OGLE events are related to dark matter. The OGLE color magnitude diagrams reveal the presence of the Galactic bar and a low density inner disk region $ \sim 4 $ kpc in radius. A catalogue of a few thousand variable stars is in preparation.

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arXiv1994

Evidence for the Galactic Bar from the Two Color Photometry of the Bulge Red Clump Stars

The Optical Gravitational Lensing Experiment (OGLE, Udalski et al.~1994a; Paczyński et al.~1994b -- these proceedings; and references therein) is an extensive photometric search for the rare cases of gravitational microlensing of Galactic bulge stars by foreground objects. It provides a huge data base (Szymański \& Udalski 1993), from which color-magnitude diagrams have been compiled (Udalski et al.~1993, 1994b). Here we discuss the use a of well-defined population of bulge red clump stars to investigate the presence of the bar in our Galaxy. The results of our earlier studies are described by Stanek et al.~(1994).

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