Publications

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    ABSTRACT: The status of our work on binary classical cepheid systems in the Large Magellanic Cloud is presented. We report on results from our follow up of two eclipsing binary cepheids OGLE-LMC-CEP-0227 and OGLE-LMC-CEP-1812. Here we presented for the first time confirmation that a third cepheid OGLE-LMC-CEP-2532 is a true eclipsing binary cepheid with a period of 800 days. Two other very good candidates for eclipsing binaries detected during OGLE-IV survey are also discussed.
    07/2014;
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    ABSTRACT: We have obtained extensive high-quality spectroscopic observations of the OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which Soszynski et al. (2008) had identified as a candidate system for containing two classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing binary system the first ever known to consist of two classical Cepheid variables. After disentangling the four different radial velocity variations in the system we present the orbital solution and the individual pulsational radial velocity curves of the Cepheids. We show that both Cepheids are extremely likely to be first overtone pulsators and determine their respective dynamical masses, which turn out to be equal to within 1.5 %. Since the secondary eclipse is not observed in the orbital light curve we cannot derive the individual radii of the Cepheids, but the sum of their radii derived from the photometry is consistent with overtone pulsation for both variables. The existence of two equal-mass Cepheids in a binary system having different pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting challenge to stellar evolution and pulsation theories, and a more detailed study of this system using additional datasets should yield deeper insight about the physics of stellar evolution of Cepheid variables. Future analysis of the system using additional near-infrared photometry might also lead to a better understanding of the systematic uncertainties in current Baade-Wesselink techniques of distance determinations to Cepheid variables.
    The Astrophysical Journal 03/2014; 786(2). · 6.73 Impact Factor
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    ABSTRACT: We present a distance determination to the Small Magellanic Cloud (SMC) based on an analysis of four detached, long period, late type eclipsing binaries discovered by the OGLE Survey. The components of the binaries show negligible intrinsic variability. A consistent set of stellar parameters was derived with low statistical and systematic uncertainty. The absolute dimensions of the stars are calculated with a precision of better than 3%. The surface brightness - infrared color relation was used to derive the distance to each binary. The four systems clump around a distance modulus of (m - M)=18.99 with a dispersion of only 0.05 mag. Combining these results with the distance published by Graczyk et al. for the eclipsing binary OGLE SMC113.3 4007 we obtain a mean distance modulus to the SMC of 18.965 +/- 0.025 (stat.) +/- 0.048 (syst.) mag. This corresponds to a distance of 62.1 +/- 1.9 kpc, where the error includes both uncertainties. Taking into account other recent published determinations of the SMC distance we calculated the distance modulus difference between the SMC and the LMC equal to 0.458 +/- 0.068 mag. Finally we advocate mu_{SMC}=18.95 +/- 0.07 as a new "canonical" value of the distance modulus to this galaxy.
    The Astrophysical Journal 11/2013; 780(1). · 6.73 Impact Factor
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    ABSTRACT: A novel method of analysis of double-lined eclipsing binaries containing a radially pulsating star is presented. The combined pulsating-eclipsing light curve is built up from a purely eclipsing light curve grid created using an existing modeling tool. For every pulsation phase the instantaneous radius and surface brightness are taken into account, being calculated from the disentangled radial velocity curve of the pulsating star and from its out-of-eclipse pulsational light curve and the light ratio of the components, respectively. The best model is found using the Markov Chain Monte Carlo method. The method is applied to the eclipsing binary Cepheid OGLE-LMC-CEP-0227 (P_puls = 3.80 d, P_orb = 309 d). We analyze a set of new spectroscopic and photometric observations for this binary, simultaneously fitting OGLE V-band, I-band and Spitzer 3.6 {\mu}m photometry. We derive a set of fundamental parameters of the system significantly improving the precision comparing to the previous results obtained by our group. The Cepheid mass and radius are M_1 = 4.165 +/- 0.032 M_solar and R_1 = 34.92 +/- 0.34 R_solar, respectively. For the first time a direct, geometrical and distance-independent determination of the Cepheid projection factor is presented. The value p = 1.21 +/- 0.03(stat.) +/- 0.04(syst.) is consistent with theoretical expectations for a short period Cepheid and interferometric measurements for {\delta} Cep. We also find a very high value of the optical limb darkening coefficients for the Cepheid component, in strong disagreement with theoretical predictions for static atmospheres at a given surface temperature and gravity.
    Monthly Notices of the Royal Astronomical Society 08/2013; 436(2). · 5.52 Impact Factor
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    ABSTRACT: Motivated by an amazing range of reported distances to the nearby Local Group spiral galaxy M33, we have obtained deep near-infrared photometry for 26 long-period Cepheids in this galaxy with the ESO VLT. From the data we constructed period-luminosity relations in the J and K bands which together with previous optical VI photometry for the Cepheids by Macri et al. were used to determine the true distance modulus of M33, and the mean reddening affecting the Cepheid sample with the multiwavelength fit method developed in the Araucaria Project. We find a true distance modulus of 24.62 for M33, with a total uncertainty of +- 0.07 mag which is dominated by the uncertainty on the photometric zero points in our photometry. The reddening is determined as E(B-V)=0.19 +- 0.02, in agreement with the value used by the HST Key Project of Freedman et al. but in some discrepancy with other recent determinations based on blue supergiant spectroscopy and an O-type eclipsing binary which yielded lower reddening values. Our derived M33 distance modulus is extremely insensitive to the adopted reddening law. We show that the possible effects of metallicity and crowding on our present distance determination are both at the 1-2% level and therefore minor contributors to the total uncertainty of our distance result for M33.
    The Astrophysical Journal 05/2013; 773(1). · 6.73 Impact Factor
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    ABSTRACT: We performed a new and accurate fit of light and radial velocity curves of the Large Magellanic Cloud (LMC) Cepheid—OGLE-LMC-CEP-0227—belonging to a detached double-lined eclipsing binary system. We computed several sets of nonlinear, convective models covering a broad range in stellar mass, effective temperature, and chemical composition. The comparison between theory and observations indicates that current theoretical framework accounts for luminosity—V and I band—and radial velocity variations over the entire pulsation cycle. Predicted pulsation mass—M = 4.14 ± 0.06 M ☉—and mean effective temperature—Te = 6100 ± 50 K—do agree with observed estimates with an accuracy better than 1σ. The same outcome applies, on average, to the luminosity amplitudes and to the mean radius. We find that the best-fit solution requires a chemical composition that is more metal-poor than typical LMC Cepheids (Z = 0.004 versus 0.008) and slightly helium enhanced (Y = 0.27 versus 0.25), but the sensitivity to He abundance is quite limited. Finally, the best-fit model reddening—E(V – I) = 0.171 ± 0.015 mag—and the true distance modulus corrected for the barycenter of the LMC—μ0, LMC = 18.50 ± 0.02 ± 0.10 (syst) mag—agree quite well with similar estimates in the recent literature.
    The Astrophysical Journal Letters 04/2013; 768(1):L6. · 6.35 Impact Factor
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    ABSTRACT: In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.
    Nature 03/2013; 495(7439):76-9. · 38.60 Impact Factor
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    ABSTRACT: A preliminary distance etimate to SMC 108.1.14904, a long-period eclipsing binary in the Small Magellanic Cloud, is presented. The binary system contains two bright, non-active G-type giants. Its orbital period is 185 days and the orbit is circular. Using surface brightness calibration, we obtain a distance modulus to the system of (m-M)= 19.02 +/- 0.04 (statistical) +/- 0.05 (systematic) mag, where the systematic error is dominated by uncertainties in the surface brightness calibration. This is a second eclipsing binary in the SMC analysed by our team.
    Proceedings of the International Astronomical Union 02/2013; 8(S289):222-225.
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    ABSTRACT: We present a precise and accurate measurement of the distance to the Large Magellanic Cloud based on late-type eclipsing-binary systems. Our results provide curently the most accurate zero point for the extragalactic distance scale.
    Proceedings of the International Astronomical Union 02/2013; 8(S289):169-172.
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    ABSTRACT: We present non-linear hydrodynamic pulsation models for OGLE-BLG-RRLYR-02792 - a 0.26M_sun pulsator, component of the eclipsing binary system, analysed recently by Pietrzynski et al. The star's light and radial velocity curves mimic that of classical RR Lyrae stars, except for the bump in the middle of the ascending branch of the radial velocity curve. We show that the bump is caused by the 2:1 resonance between the fundamental mode and the second overtone - the same mechanism that causes the Hertzsprung bump progression in classical Cepheids. The models allow to constrain the parameters of the star, in particular to estimate its absolute luminosity (approx 33L_sun) and effective temperature (approx 6970K, close to the blue edge of the instability strip). We conduct a model survey for the new class of low mass pulsators similar to OGLE-BLG-RRLYR-02792 - products of evolution in the binary systems. We compute a grid of models with masses corresponding to half (and less) of the typical mass of RR Lyrae variable, 0.20M_sun<=M<=0.30M_sun, and discuss the properties of the resulting light and radial velocity curves. Resonant bump progression is clear and may be used to distinguish such stars from classical RR Lyrae stars. We present the Fourier decomposition parameters for the modelled light and radial velocity curves. The expected values of the phi_31 Fourier phase for the light curves differ significantly from that observed in RR Lyrae stars, which is another discriminant of the new class.
    Monthly Notices of the Royal Astronomical Society 10/2012; 428(4). · 5.52 Impact Factor
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    ABSTRACT: This is a continuation of our study of open clusters based on the 2--Micron All Sky Survey photometry. Here we present the results of the mass function analysis for 599 known open clusters in the Milky Way. The main goal of this project is a study of the dynamical state of open clusters, the mass segregation effect and an estimate of the total mass and the number of cluster members. We noticed that the cluster size (core and overall radii) decreases along dynamical evolution of clusters. The cluster cores evolve faster than the halo regions and contain proportionally less low-mass stars from the beginning of the cluster dynamical evolution. We also noticed, that the star density decreases for the larger clusters. Finally, we found an empirical relation describing the exponential decrease of the mass function slope with the dynamical evolution of clusters.
    Acta Astronomica -Warsaw and Cracow- 09/2012; · 2.68 Impact Factor
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    ABSTRACT: RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies. There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system, because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses M[symbol see text] and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with M[symbol see text] and 0.8M[symbol see text]stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung-Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.
    Nature 04/2012; 484(7392):75-7. · 38.60 Impact Factor
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    Bogumil Pilecki, Piotr Konorski, Marek Gorski
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    ABSTRACT: The RV analysis tool integrates widely used methods of radial velocity determination (CCF, TODCOR, BF) in an easy to use graphical environment. No advanced knowledge of these methods is required to use it. The obtained velocities may be immediately analyzed with the same tool as it comprises flexible fitting of orbital parameters, which includes the third body influence and pulsational velocities of the components. These features together help to establish the most accurate combination of templates, spectrum range, and method. Scripting functionality is to be implemented in the future.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):301-302.
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    Information Bulletin on Variable Stars. 04/2012;
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    ABSTRACT: We have analyzed the long period, double-lined eclipsing binary system OGLE SMC113.3 4007 (SC10 137844) in the SMC. The binary lies in the north-eastern part of the galaxy and consists of two evolved, well detached, non-active G8 giants. The orbit is eccentric with e = 0.311 and the orbital period is 371.6 days. Using extensive high-resolution spectroscopic and multi-color photometric data we have determined a true distance modulus of the system of m-M=18.83 +/- 0.02 (statistical) +/- 0.05 (systematic) mag using a surface brightness - color relation for giant stars. This method is very insensitive to metallicity and reddening corrections and depends only very little on stellar atmosphere model assumptions. Additionally, we derived very accurate, at the level of 1%-2%, physical parameters of both giant stars, particularly their masses and radii, making our results important for comparison with stellar evolution models. Our analysis underlines the high potential of late-type, double-lined detached binary systems for accurate distance determinations to nearby galaxies.
    The Astrophysical Journal 03/2012; 750(2). · 6.73 Impact Factor
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    ABSTRACT: An overview of our ongoing photometric and spectroscopic monitoring of the \varepsilon Aurigae eclipse in 2009-2011 is presented. The obtained until now observational data is described and discussed. The changes in the spectrum, which can be attributed to the eclipsing disk, are illustrated with the variations in the profiles and the equivalent widths of selected spectral features. The mean radial velocities measured in our spectra are compared with a recent spectroscopic orbital solution. The duration of eclipses is also discussed briefly.
    Bulgarian Astronomical Journal. 01/2012;
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    ABSTRACT: We have analyzed the double-lined eclipsing binary system OGLE-LMC-CEP-1812 in the LMC and demonstrate that it contains a classical fundamental mode Cepheid pulsating with a period of 1.31 days. The secondary star is a stable giant. We derive the dynamical masses for both stars with an accuracy of 1.5%, making the Cepheid in this system the second classical Cepheid with a very accurate dynamical mass determination, following the OGLE-LMC-CEP-0227 system studied by Pietrzyński et al. The measured dynamical mass agrees very well with that predicted by pulsation models. We also derive the radii of both components and accurate orbital parameters for the binary system. This new, very accurate dynamical mass for a classical Cepheid will greatly contribute to the solution of the Cepheid mass discrepancy problem, and to our understanding of the structure and evolution of classical Cepheids.
    The Astrophysical Journal Letters 11/2011; 742(2):L20. · 6.35 Impact Factor
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    ABSTRACT: The main goal of our project is to obtain a complete picture of individual open clusters from homogeneous data and then search for correlations between their astrophysical parameters. The near-infrared JHKS photometric data from the 2-Micron All Sky Survey were used to determine new coordinates of the centres, angular sizes and radial density profiles for 849 open clusters in the Milky Way. Additionally, age, reddening, distance, and linear sizes were also derived for 754 of them. For these open clusters our results are in satisfactory agreement with the literature data. The analysed sample contains open clusters with ages in the range from 7 Myr to 10 Gyr. The majority of these clusters are located up to 3 kpc from the Sun, less than 0.4 kpc from the Galactic Plane and 6 - 12 kpc from the Galactic Centre. The majority of clusters have core radii of about 1.5 pc and the limiting radii of the order of 10 pc. We notice that in the near-infrared, open clusters seem to be greater than in optical bands. We notice that a paucity of clusters is observed at Galactic longitudes range from 140{\deg} to 200{\deg} which probably reflects the real spatial distribution of open clusters in the Galaxy. The lack of clusters was also found in earlier studies.
    Acta Astronomica -Warsaw and Cracow- 07/2011; 61. · 2.68 Impact Factor
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    ABSTRACT: This paper has been withdrawn by the author because has been send to other journal.
    12/2010;
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    ABSTRACT: We report on optical observations of Nova Eri 2009 obtained at the Nicolaus Copernicus University Observatory (Torun, Poland) and the Olsztyn Planetarium and Astronomical Observatory. Using a 60 cm Cassegrain telescope (Torun) and a 25 cm Schmidt-Cassegrain telescope (Olsztyn) we estimated the V brightness of the Nova to ~8.30+/-0.04 mag and ~8.59+/-0.01 mag on Nov. 27.00 UT and Nov. 27.91 UT.
    The Astronomer's Telegram. 01/2009;

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