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M. Szymanski

Publications and source records attributed to M. Szymanski.

72 records · Page 4Linked to original sources
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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arXiv1994

The Optical Gravitational Lensing Experiment. Deep Photometry of the Sagittarius Dwarf Spheroidal Galaxy

We present deep CCD photometry of a field in the newly discovered dwarf spheroidal galaxy in Sagittarius (hereafter Sgr), and of a nearby control field. These data (totaling ~3.5 hrs exposure time in V and I) were used to produce color-magnitude (CM) diagrams reaching I ~ 22.3 in both fields. After statistically removing the field-stars from the CM diagram we find that Sgr is dominated by a moderately old (age ~ 10 Gyr) population, significantly younger than a typical globular-cluster population. There is some evidence for a weak intermediate-age component or alternatively a population of blue stragglers. These results confirm that Sgr is a bona fide dwarf spheroidal (dSph) galaxy, the ninth found orbiting the Milky Way. We have discovered nine short-period variables in the Sgr field, well in excess of the number found in the control field. Seven of these stars have similar apparent magnitudes and are almost certainly RR~Lyr stars in Sgr. We have used the minimum-light colors of the RRab variables to determine the reddening of the Sgr field to be E\vi\ = 0.22. For an assumed RR~Lyr luminosity of M_V = +0.6 we conclude that the distance of Sgr is 25.2 +/- 2.8 kpc. The properties of the Sgr giant branch and upper main sequence are consistent with a mean metallicity of [Fe/H] ~ -1.1 +/- 0.3.

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arXiv1994

Are the Ogle Microlenses in the Galactic Bar?

The analysis of the first two years of OGLE data revealed 9 microlensing events of the galactic bulge stars, with the characteristic time scales in the range $ 8.6 < t_0 < 62 $ days, where $ t_0 = R_E / V $. The optical depth to microlensing is 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. We argue that the lenses are likely to be ordinary stars in the galactic bar, which has its long axis elongated towards us. A relation between $ t_0 $ and the lens masses remains unknown until a quantitative model of bar microlensing becomes available. At this time we have no evidence that the OGLE events are related to dark matter. The geometry of lens distribution can be determined observationally when the microlensing rate is measured over a larger range of galactic longitudes, like $ -10^o < l < +10^o $, and the relative proper motions of the galactic bulge (bar) stars are measured with the HST.

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arXiv1994

The Optical Gravitational Lensing Experiment. the Early Warning System

The discoveries of 17 microlensing event candidates have been reported over the last year by three teams conducting unprecedented mass photometric searches in the direction of the Galactic bulge and the Magellanic Clouds. These include 10 events found by the OGLE collaboration, 5 by the MACHO team and 2 by the EROS team. All searches have the main goal to detect dark matter in our Galaxy. The detection of 17 event candidates proves that the microlensing is a powerful tool in the search for dark matter, and it may be used for reliable mass determination when the geometry of the event is known. Here we present the first microlensing event, OGLE~\#11, discovered in real time, using the newly implemented "Early Warning System". We describe our system which makes it possible to monitor and study in great details any very rare phenomena, not only lensing events, with a broad array of instruments almost immediately after they have changed their brightness.

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arXiv1994

The Optical Gravitational Lensing Experiment. OGLE #7: Binary Microlens or a New Unusual Variable?

We present the light curve of an unusual variable object, OGLE #7, detected during the OGLE search for microlensing events. After one season of being in a low, normal state, the star brightened by more than 2~mag with a characteristic double-maximum shape, and returned to normal brightness after 60 days. We consider possible explanations of the photometric behavior of OGLE #7. The binary microlens model seems to be the most likely explanation -- it reproduces well the observed light curve and explains the observed colors of OGLE #7. The characteristic time scale of the OGLE #7 event, $t_E$, is equal to 80 days, the longest observed to date. The binary microlens model predicts that the spectrum of the star should be composite, with roughly 50% of its light in the $I$-band coming from a non-lensed source.

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arXiv1994

The Optical Depth to Gravitational Microlensing in the Direction of the Galactic Bulge

We present the analysis of the first two years of the OGLE search for gravitational lenses towards the Galactic bulge. We detected 9 microlensing events in an algorithmic search of $ \sim 10^8 $ measurements of $ \sim 10^6 $ stars. The characteristic time scales are in the range $ 8.6 < t_0 < 62 $ days, where $ t_0 = R_E / V $. The distribution of amplitudes is consistent with theoretical expectation. The stars seem to be drawn at random from the overall distribution of the observed bulge stars. We find that the optical depth to microlensing is larger than $ ( 3.3 \pm 1.2 ) \times 10^{-6}$, in excess of current theoretical estimates.

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arXiv1994

Color-Magnitude Diagram Distribution of the Bulge Red Clump Stars - Evidence for the Galactic Bar

The color-magnitude diagrams of $\sim 5\times 10^5$ stars obtained for 13 fields towards the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump stars. We find that the distributions of the extinction-adjusted apparent magnitudes of the red clump stars in fields lying at $l=\pm5°$ in galactic longitude differ by $0.37\pm0.025\;mag$. Assuming that the intrinsic luminosity distribution of the red clump stars is the same on both sides of the Galactic center, this implies that the distances to the red clump stars in the two fields differ by a factor of $1.185\pm0.015$. A plausible explanation of the observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or bar, which is inclined to the line of sight by no more than $45°$, with the part of the bar at the positive galactic longitude being closer to us. This agrees rather well with other studies indicating the presence of the bar in the center of the Galaxy. Color-magnitude diagram data are accessible over the computer network with anonymous {\tt ftp}.

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arXiv1994

The Distribution of Galactic Disk Stars in Baade's Window

The color-magnitude diagram of $ \sim 3 \times 10^5 $ stars obtained for Baade's Window towards the Galactic bulge with the OGLE project reveals a surprisingly narrow main sequence due to galactic disk stars at a distance of $d \sim 2 $ kpc, i.e. at the location of the Sagittarius spiral arm. A more careful analysis indicates there is an excess in the number of disk stars by a factor $\sim 2$ between us and $d \sim 2.5$ kpc and a rapid drop by a factor $\sim 10$ beyond that distance. It is unlikely that the observed structure is an artifact of the interstellar extinction, but careful determination of the extinction is needed before the structure is firmly established. The narrow main sequence extends down to stars as faint as $M_{_V} \approx 7$, i.e. it is composed of old stars. This is not expected in a conventional disk model, or in a conventional model of a spiral structure. However, a strong concentration of old stars towards spiral arms has been noticed in some other galaxies, like M51 (Rix \& Rieke 1993), with the near infrared surface photometry. We have also found that the relative distribution of red clump and red giant stars in the Galactic bulge implies that there is a relatively young stellar population present there.

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