arXiv subjects

G. Pietrzynski

Publications and source records attributed to G. Pietrzynski.

At least 217 records · Page 12Linked to original sources

arXiv2002

Double-periodic blue variables in the Magellanic Clouds

We report the discovery, based on an inspection of the OGLE-II database, of a group of blue variables in the Magellanic Clouds showing simultaneously two kinds of photometric variability: a short-term cyclic variability with typical amplitude $ΔI \sim$ 0.05 mag and period $P_{1}$ between 4 and 16 days and a sinusoidal, long-term cyclic oscillation with much larger amplitude $ΔI \sim$ 0.2 mag with period $P_{2}$ in the range of 150-1000 days. We find that both periods seems to be coupled through the relationship $P_{2}$ = 35.2 $\pm$ 0.8 $P_{1}$. In general, the short term variability is reminiscent of those shown by Algol-type binaries. We propose that the long-term oscillation could arise in the precession of a elliptical disc fed by a Roche-lobe filling companion in a low mass ratio Algol system.

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arXiv2002

Microlensing of Relativistic Knots in the Quasar HE1104-1805

We present 3 years of photometry of the ``Double Hamburger'' lensed quasar, HE1104-1805, obtained on 102 separate nights using the OGLE 1.3-m telescope. Both the A and B images show variations, but with substantial differences in the lighcurves at all time delays. At the 310 day delay reported by Wisotzki and collaborators the difference lightcurve has an rms amplitude of 0.060 mag. The structure functions for the A and B images are quite different, with image A more than twice as variable as image B (a factor of 4 in structure function) on timescales of less than a month. Adopting microlensing as a working hypothesis for the uncorrelated variability, the short timescale argues for the relativistic motion of one or more components of the source. We argue that the small amplitude of the fluctuations is due to the finite size of the source with respect to the microlenses.

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arXiv2002

The Optical Gravitational Lensing Experiment. BVI Maps of Dense Stellar Regions. III. The Galactic Bulge

We present the VI photometric maps of the Galactic bulge. They contain VI photometry and astrometry of about 30 million stars from 49 fields of 0.225 square degree each in the Galactic center region. The data were collected during the second phase of the OGLE microlensing project. We discuss the accuracy of data and present color-magnitude diagrams of selected fields observed by OGLE in the Galactic bulge. The VI maps of the Galactic bulge are accessible electronically for the astronomical community from the OGLE Internet archive.

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arXiv2002

Light Curve Analysis of Be Star Candidates in the Small Magellanic Cloud

The recent discovery, based on a search of public OGLE data, of Be star candidates in the Small Magellanic Cloud showing spectacular photometric variations (Mennickent et al. 2002), has motivated further analysis of their light curves. Here we report the results of a statistical study of the light curves of Type-1, Type-2 and Type-3 stars. Type-3 stars show a bimodal period distribution, with a main, broad peak, between 20 and 130 days, and a narrower, secondary peak, between 140 and 210 days. We also find that, among Type-3 stars, the maximum amplitude of oscillation correlates with the system luminosity, in the sense that only low luminosity systems show large amplitude oscillations. In general, the amplitude of these oscillations tends to increase with the photometric period. A parametric study shows no correlation between the amplitude, duration and asymmetry of Type-1 outbursts, and amplitude of Type-2 jumps, with the available stellar photometric parameters.

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arXiv2002

The ARAUCARIA project: Deep near-infrared survey of nearby galaxies. I. The distance to the Large Magellanic Cloud from K-band photometry of red clump stars

We have obtained deep imaging in the near-infrared J and K bands for 2 nearby fields in the bar of the LMC with the ESO NTT telescope, under exquisite seeing conditions. The K, J-K color-magnitude diagrams constructed from these data are of outstanding photometric quality and reveal the presence of several hundreds of red clump stars. Using the calibration of Alves for the K-band absolute magnitude of Hipparcos-observed red clump stars in the solar neigbourhood we derive a distance modulus to our observed LMC fields of 18.487 mag. Applying a correction for the tilt of the LMC bar with respect to the line of sight according to the geometrical model of van der Marel et al., the corresponding LMC barycenter distance is 18.501 mag. If we adopt a K-band population correction of -0.03 mag, as done by Alves et al. 2002, to account for the difference in age and metallicity between the solar neighborhood and LMC red clump star populations, we obtain an LMC barycenter distance modulus of 18.471 mag from our data. This is in excellent agreement with the result of Alves et al., and of another very recent study of Sarajedini et al. (2002) obtained from K-band photometry. However, we emphasize that current model predictions about the uncertainties of population corrections seem to indicate that errors up to about 0.12 mag may be possible, probably in any photometric band. Therefore, work must continue to tighten the constraints on these corrections. We also determine the mean red clump star magnitude in our LMC fields in the J band, which could be a useful alternative to the K band should future work reveal that population effect corrections for red clump stars in the J band are smaller, or more reliably determined than those for the K band.

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arXiv2002

The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign - Supplement

The photometric data collected during 2001 season OGLE-III planetary/low luminosity object transit campaign were reanalyzed with the new transit search technique -- the BLS method by Kovacs, Zucker and Mazeh. In addition to all presented in our original paper transits, additional 13 objects with transiting low-luminosity companions were discovered. We present here a supplement to our original catalog -- the photometric data, light curves and finding charts of all 13 new objects. The model fits to the transit light curves indicate that a few new objects may be Jupiter-sized (R<1.6 R_Jup). OGLE-TR-56 is a particularly interesting case. Its transit has only 13 mmag depth, short duration and a period of 1.21190 days. Model fit indicates that the companion may be Saturn-sized if the passage were central. Spectroscopic follow-up observations are encouraged for final classification of the transiting objects as planets, brown dwarfs or late M-type dwarf stars. We also provide the most recent ephemerides of other most promising planetary transits: OGLE-TR-10 and OGLE-TR-40 based on observations collected in June 2002. All photometric data are available to the astronomical community from the OGLE Internet archive.

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arXiv2002

The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign

We present results of an extensive photometric search for planetary and low-luminosity object transits in the Galactic disk stars commencing the third phase of the Optical Gravitational Lensing Experiment -- OGLE-III. Photometric observations of three fields in the direction of the Galactic center (800 epochs per field) were collected on 32 nights during time interval of 45 days. Out of the total of 5 million stars monitored, about 52~000 Galactic disk stars with photometry better than 1.5% were analyzed for flat-bottomed eclipses with the depth smaller than 0.08 mag. Altogether 46 stars with transiting low-luminosity objects were detected. For 42 of them multiple transits were observed, a total of 185, allowing orbital period determination. Transits in two objects: OGLE-TR-40 and OGLE-TR-10, with the radii ratio of about 0.14 and estimate of the radius of the companion 1.0-1.5R_Jup, resemble the well known planetary transit in HD 209458. The sample was selected by the presence of apparent transits only, with no knowledge on any other properties. Hence, it is very well suited for general study of low-luminosity objects. The transiting objects may be Jupiters, brown dwarfs, or M dwarfs. Future determination of the amplitude of radial velocity changes will establish their masses, and will confirm or refute the reality of the so called ``brown dwarf desert''. The low-mass stellar companions will provide new data needed for the poorly known mass-radius relation for the lower main sequence. All photometric data are available to the astronomical community from the OGLE Internet archive.

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arXiv2002

Optical Gravitational Lensing Experiment. OGLE-1999-BUL-19: The First Multi-Peak Parallax Event

We describe a highly unusual microlensing event, OGLE-1999-BUL-19, which exhibits multiple peaks in its light curve. The Einstein radius crossing time for this event is approximately one year, which is unusually long. We show that the motion of the Earth induces these multiple peaks in the light curve, since the relative transverse velocity of the lens projected into the observer plane is very small (v = 12.5 km/s). This is the lowest velocity so far published and we believe that this is the first multiple-peak parallax event ever observed. We also believe that this event may be exhibiting slight binary-source signatures in addition to these parallax-induced multiple peaks. With spectroscopic observations it is possible to test this `parallax plus binary-source' hypothesis and (if this hypothesis turns out to be correct) to simultaneously fit both models and obtain a measurement of the lens mass. Furthermore, spectroscopic observations could also supply information regarding the lens properties, possibly providing another avenue for determining the lens mass. We found that most of the I-band blending is probably caused by light from the lens or a binary companion to the source. However, in the V-band, there appears to be a second blended source 0.35" away from the lensed source. HST observations will be very useful for understanding the nature of the blends. We also suggest that a radial velocity survey of all parallax events will be very useful for further constraining the lensing kinematics and understanding the origins of these events and the excess of long events toward the bulge.

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arXiv2002

On Be Star Candidates and Possible Blue Pre-Main Sequence Objects in the Small Magellanic Cloud

Recently the OGLE experiment has provided accurate light curves and colours for about 2 millions stars in the Small Magellanic Cloud. We have examined this database for its content of Be stars, applying some selection criteria, and we have found a sample of $\sim$ 1000 candidates. Some of these stars show beautiful light curves with amazing variations never observed in any Galactic variable. We find outbursts in 13% of the sample (Type-1 stars), high and low states in 15%, periodic variations in 7%, and the usual variations seen in Galactic Be stars in 65% of the cases. The Galactic counterparts of Type-1 objects could be the outbursting Be stars found by Hubert & Floquet (1998) after the analysis of Hipparcos photometry. We discuss the possibility that Type-1 stars could correspond to Be stars with accreting white dwarf companions or alternatively, blue pre-main sequence stars surrounded by thermally unstable accretion disks. We provide coordinates and basic photometric information for these stars and some examples of light curves.

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arXiv2002

The Optical Gravitational Lensing Experiment. Catalog of high proper motion stars towards the Magellanic Clouds

We present a Catalog of high proper motion (HPM) stars detected in the foreground of central parts of the Magellanic Clouds. The Catalog contains 2161 objects in the 4.5 square degree area towards the LMC, and 892 HPM stars in the 2.4 square degree area towards the SMC. The Catalog is based on observations collected during four years of the OGLE-II microlensing survey. The Difference Image Analysis (DIA) of the images provided candidate HPM stars with proper motion as small as 4 mas/yr. These appeared as pseudo-variables, and were all measured astrometrically on all CCD images, providing typically about 400 data points per star. The reference frame was defined by the majority of background stars, most of them members of the Magellanic Clouds. The reflex motion due to solar velocity with respect to the local standards of rest is clearly seen. The largest proper motion in our sample is 363 mas/yr. Parallaxes were measured with errors smaller than 20% for several stars.

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arXiv2002

Variable Stars in the Field of Young Open Cluster NGC 581

We present results of the search for variable stars in the field of young open cluster NGC 581. Based on 19 nights of observations, 6 new variable stars were discovered. Two of them turned out to be eclipsing binary systems. Another two detected variable stars are most probably of gamma Dor type. During our observations one of the known Be stars located in our field of view showed irregular variations of brightness, typical for this class of stars. The sixth variable star is a pulsating red giant.

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arXiv2002

Photometric standard stars in the BVI system in a wide field centered on the spiral galaxy NGC 300

Based on 13 nights of observations of four fields in NGC 300, we have set up an extensive sequence of stars with accurate BVI photometry covering a relatively large (25 x 25 arcmin) region centered on this galaxy. This sequence of standard stars is very useful for calibrating the photometry of variable stars and other objects in NGC 300 and other galaxies obtained from wide field mosaic images. Our standard star list contains B, V and I measurements for 390 stars. The accuracy of the zero points in the V filter and B-V color is better than 0.02 mag, and about 0.03 mag for the V-I color. We found very good agreement between our measurements and those previously obtained by Walker for 26 stars near NGC 300.

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arXiv2002

Difference Image Analysis of the OGLE-II Bulge Data. III. Catalog of 200,000 Candidate Variable Stars

We present the first edition of a catalog of variable stars from OGLE-II Galactic Bulge data covering 3 years: 1997-1999. Typically 200-300 I band data points are available in 49 fields between -11 and 11 degrees in galactic longitude, totaling roughly 11 square degrees in sky coverage. Photometry was obtained using the Difference Image Analysis (DIA) software and tied to the OGLE data base with the DoPhot package. The present version of the catalog comprises 221,801 light curves. In this preliminary work the level of contamination by spurious detections is still about 10%. Parts of the catalog have only crude calibration, insufficient for distance determinations. The next, fully calibrated, edition will include the data collected in year 2000. The data is accessible via FTP. Due to the data volume, we also distribute DAT tapes upon request.

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arXiv2002

The Optical Gravitational Lensing Experiment. Difference Image Analysis of LMC and SMC data. The Catalog

We present the first edition of a catalog of variable stars found in the Magellanic Clouds using OGLE-II data obtained during four years: 1997-2000. The catalog covers about 7 square degrees of the sky - 21 fields in the Large Magellanic Cloud and 11 fields in the Small Magellanic Cloud. All variables were found with the Difference Image Analysis (DIA) software. The catalog is divided into two sections. The DC section contains FITS reference images (obtained by co-adding 20 best frames for each field) and profile photometry (DoPHOT) of all variable stars on those images. The AC section contains flux variations and magnitudes of detected variable stars obtained with DIA as well as with DoPHOT. The results are available in electronic form via FTP and through WWW interface from the OGLE internet archive. Typically, there are about 400 I-band data points and about 30 V and B-band data points for over 68,000 variables. A detailed analysis and classification of variable stars will be presented elsewhere. The FTP catalog contains approximately 2Gbytes of data.

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arXiv2001

The Optical Gravitational Lensing Experiment. Cepheids in the Galaxy IC1613: No Dependence of the Period--Luminosity Relation on Metallicity

We present results of the search for Cepheids in the galaxy IC1613 carried out as a sub-project of the OGLE-II microlensing survey. 138 Cepheids were found in the 14.2x14.2 arcmin region in the center of the galaxy. We present light curves, VI photometry and basic data for all these objects, as well as color-magnitude diagram of the observed field. The Period--Luminosity (PL) diagrams for IC1613 fundamental mode Cepheids for VI and interstellar extinction insensitive index W_I are constructed. Comparison of PL relations in metal poor galaxy IC1613 ([Fe/H]~-1.0 dex) with relations in metal richer Magellanic Clouds allows us to study dependence of Cepheid PL relations on metallicity in the wide range of metallicities covered by these three galaxies. The slopes of PL relations in IC1613 are identical as in the Magellanic Clouds. The comparison of brightness of Cepheids with the magnitudes of the tip of the red giant branch stars and RR Lyr stars in all three objects provides information on the stability of zero points of PL relations in the I and V-band, respectively. We find that the zero points of PL relations are constant to better than $\pm0.03$ mag over the entire range of covered metallicities. Thus, the most important conclusion of the paper is that the Cepheid PL relations do not depend on metallicity. Additionally we determine the mean distance to IC1613, based on the common distance scale resulting from four major stellar distance indicators: Cepheids, RR Lyr, TRGB and red clump stars. The distance modulus to IC1613 is equal to (m-M)_IC1613=24.20 mag with the standard deviation of +/-0.02 mag from six measurements and systematic uncertainty of calibrations of +/-0.07 mag.

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arXiv2001

Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?

We describe the discovery of the longest microlensing event ever observed, OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 641 days. The high quality data obtained with difference image analysis shows a small but significant parallax signature. This parallax effect allows one to determine the Einstein radius projected onto the observer plane as rE^hat ~ 29.2AU. The transverse velocity projected onto the observer plane is about 79km/s. We argue that the lens is likely to be have a mass of at least a few solar masses, i.e., it could be a stellar black hole. The black hole hypothesis can be tested using the astrometric microlensing signature with the soon-to-be installed Advanced Camera for Surveys on board the Hubble Space Telescope. Deep X-ray and radio images may also be useful for revealing the nature of the object.

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arXiv2001

Difference Image Analysis of the OGLE-II bulge data. II. Microlensing events

We present a sample of microlensing events discovered in the Difference Image Analysis (DIA) of the OGLE-II images collected during 3 observing seasons, 1997--1999. 4424 light curves pass our criteria on the presence of a brightening episode on top of a constant baseline. Among those, 512 candidate microlensing events were selected visually. We designed an automated procedure, which unambiguously selects up to 237 best events. Including 8 candidate events recovered by other means, a total of 520 light curves are presented in this work. In addition to microlensing events, the larger sample contains certain types of transients, but is also strongly contaminated by artifacts. All 4424 light curves in the weakly filtered group are available electronically, with the intent of showing the gray zone between microlensing events and variable stars, as well as artifacts, to some extent inevitable in massive data reductions. We welcome suggestions for improving the selection process before the full analysis of complete 4 seasons of the OGLE-II bulge data. Selection criteria for binary events can also be investigated with our extended sample.

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arXiv2001

A Catalog of OB Associations in the spiral galaxy NGC 300

We present results of a search for OB associations in NGC 300. Using an automatic and objective method (PLC technique) 117 objects were found. Statistical tests indicate that our sample is contaminated by less than 10 % of spurious detections due to random concentrations of blue stars. Spatial distributions of detected associations and H II regions are strongly correlated. The size distribution reveals a significant peak at about 60 $μ$rad which corresponds to 125 parsecs if a distance modulus of 26.66 mag is assumed. Besides the objects with sizes corresponding to typical associations we also found several much larger objects. A second level application of our detection method revealed that most of these are composed of smaller subgroups, with sizes of about 100 pc.

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