arXiv subjects

G. Pietrzynski

Publications and source records attributed to G. Pietrzynski.

At least 91 records · Page 5Linked to original sources

arXiv2012

The Optical Gravitational Lensing Experiment. Optical Counterparts to the X-ray Sources in the Galactic Bulge

We present a sample of 209 variable objects - very likely optical counterparts to the X-ray sources detected in the direction of the Galactic center by the Galactic Bulge Survey (GBS) carried out with the Chandra satellite. The variable sources were found in the databases of the OGLE long term survey monitoring regularly the Galactic bulge since 1992. The counterpart candidates were searched based on the X-ray source position in the radius of 3."9. Optical light curves of the candidates comprise a full variety of variability types: spotted stars, pulsating red giants (potentially secondary stars of symbiotic variables), cataclysmic variables, eclipsing binary systems, irregular non-periodic objects including an AGN (GRS 1734-292). Additionally, we find that positions of 19 non-variable stars brighter than 16.5 mag in the OGLE databases are so well aligned with the X-ray positions (<0."75) that these objects are also likely optical counterparts to the GBS X-ray sources. We provide the OGLE astrometric and photometric information for all selected objects and their preliminary classifications. Photometry of the candidates is available from the OGLE Internet archive, http://ogle.astrouw.edu.pl

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arXiv2012

A possible binary system of a stellar remnant in the high magnification gravitational microlensing event OGLE-2007-BLG-514

We report the extremely high magnification (A > 1000) binary microlensing event OGLE-2007-BLG-514. We obtained good coverage around the double peak structure in the light curve via follow-up observations from different observatories. The binary lens model that includes the effects of parallax (known orbital motion of the Earth) and orbital motion of the lens yields a binary lens mass ratio of q = 0.321 +/- 0.007 and a projected separation of s = 0.072 +/- 0.001$ in units of the Einstein radius. The parallax parameters allow us to determine the lens distance D_L = 3.11 +/- 0.39 kpc and total mass M_L=1.40 +/- 0.18 M_sun; this leads to the primary and secondary components having masses of M_1 = 1.06 +/- 0.13 M_sun and M_2 = 0.34 +/- 0.04 M_sun, respectively. The parallax model indicates that the binary lens system is likely constructed by the main sequence stars. On the other hand, we used a Bayesian analysis to estimate probability distributions by the model that includes the effects of xallarap (possible orbital motion of the source around a companion) and parallax (q = 0.270 +/- 0.005, s = 0.083 +/- 0.001). The primary component of the binary lens is relatively massive with M_1 = 0.9_{-0.3}^{+4.6} M_sun and it is at a distance of D_L = 2.6_{-0.9}^{+3.8} kpc. Given the secure mass ratio measurement, the companion mass is therefore M_2 = 0.2_{-0.1}^{+1.2} M_sun. The xallarap model implies that the primary lens is likely a stellar remnant, such as a white dwarf, a neutron star or a black hole.

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arXiv2012

OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary

The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low-mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of $0.10\pm 0.01\ M_\odot$ and $0.09\pm 0.01\ M_\odot$. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population.

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arXiv2012

RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system

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 Lyr star OGLE-BLG-RRLYR-02792 was found to be a member in an eclipsing binary system4, as 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 M_sun and therefore cannot be a classical RR Lyrae star. Through models we find that its properties are best explained by the evolution of a close binary system that started with 1.4 M_sun and 0.8 M_sun stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 Gyr 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 H-R diagram occupied by RR Lyrae stars. We estimate that samples of RR Lyr stars may contain a 0.2 percent contamination with systems similar to this one, implying that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.

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arXiv2012

The Optical Gravitational Lensing Experiment: Analysis of the bulge RR Lyrae population from the OGLE-III data

We have analyzed the data on 16,836 RR Lyrae (RR Lyr) variables observed toward the Galactic bulge during the third phase of the Optical Gravitational Lensing Experiment (OGLE-III), which took place in 2001-2009. Using these standard candles, we show that the ratio of total to selective extinction toward the bulge is given by R_I=A_I/E(V-I)=1.080+/-0.007 and is independent of color. We demonstrate that the bulge RR Lyr stars form a metal-uniform population, slightly elongated in its inner part. The photometrically derived metallicity distribution is sharply peaked at [Fe/H]=-1.02+/-0.18, with a dispersion of 0.25 dex. In the inner regions (|l|<3, |b|<4) the RR Lyr tend to follow the barred distribution of the bulge red clump giants. The distance to the Milky Way center inferred from the bulge RR Lyr is R_0=8.54+/-0.42 kpc. We report a break in the mean density distribution at a distance of ~0.5 kpc from the center indicating its likely flattening. Using the OGLE-III data, we assess that (4-7)x10^4 type ab RR Lyr variables should be detected toward the bulge area of the on going near-IR VISTA Variables in the Via Lactea (VVV) survey, where the uncertainty partially results from the unknown RR Lyr spatial density distribution within 0.2 kpc from the Galactic center.

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arXiv2012

The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55

The quantitative study of the physical properties and chemical abundances of large samples of massive blue stars at different metallicities is a powerful tool to understand the nature and evolution of these objects. Their analysis beyond the Milky Way is challenging, nonetheless it is doable and the best way to investigate their behavior in different environments. Fulfilling this task in an objective way requires the implementation of automatic analysis techniques that can perform the analyses systematically, minimizing at the same time any possible bias. As part of the ARAUCARIA project we carry out the first quantitative spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55 at 1.94 Mpc away. By applying the methodology developed in this work, we derive their stellar parameters, chemical abundances and provide a characterization of the present-day metallicity of their host galaxy. Based on the characteristics of the stellar atmosphere/line formation code FASTWIND, we designed and created a grid of models for the analysis of massive blue supergiant stars. Along with this new grid, we implemented a spectral analysis algorithm. Both tools were specially developed to perform fully consistent quantitative spectroscopic analyses of low spectral resolution of B-type supergiants in a fast and objective way. We present the main characteristics of our FASTWIND model grid and perform a number of tests to investigate the reliability of our methodology. The automatic tool is applied afterward to a sample of 12 B-type supergiant stars in NGC55, deriving the stellar parameters and abundances. The results indicate that our stars are part of a young population evolving towards a red supergiant phase. The derived chemical composition hints to an average metallicity similar to the one of the Large Magellanic Cloud, with no indication of a spatial trend across the galaxy.

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arXiv2012

OGLE-2008-BLG-510: first automated real-time detection of a weak microlensing anomaly - brown dwarf or stellar binary?

The microlensing event OGLE-2008-BLG-510 is characterised by an evident asymmetric shape of the peak, promptly detected by the ARTEMiS system in real time. The skewness of the light curve appears to be compatible both with binary-lens and binary-source models, including the possibility that the lens system consists of an M dwarf orbited by a brown dwarf. The detection of this microlensing anomaly and our analysis demonstrates that: 1) automated real-time detection of weak microlensing anomalies with immediate feedback is feasible, efficient, and sensitive, 2) rather common weak features intrinsically come with ambiguities that are not easily resolved from photometric light curves, 3) a modelling approach that finds all features of parameter space rather than just the `favourite model' is required, and 4) the data quality is most crucial, where systematics can be confused with real features, in particular small higher-order effects such as orbital motion signatures. It moreover becomes apparent that events with weak signatures are a silver mine for statistical studies, although not easy to exploit. Clues about the apparent paucity of both brown-dwarf companions and binary-source microlensing events might hide here.

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arXiv2012

On the evolutionary and pulsation mass of Classical Cepheids: III. the case of the eclipsing binary Cepheid CEP0227 in the Large Magellanic Cloud

We present a new Bayesian approach to constrain the intrinsic parameters (stellar mass, age) of the eclipsing binary system CEP0227 in the LMC. We computed evolutionary models covering a broad range in chemical compositions and in stellar mass. Independent sets of models were constructed either by neglecting or by including a moderate convective core overshooting (beta=0.2) during central H-burning phases. Models were also constructed either by neglecting or by assuming a canonical (eta=0.4,0.8) or an enhanced (eta=4) mass loss rate. The solutions were computed in three different planes: luminosity-temperature, mass-radius and gravity-temperature. By using the Bayes Factor, we found that the most probable solutions were obtained in the gravity-temperature plane with a Gaussian mass prior distribution. The evolutionary models constructed by assuming a moderate convective core overshooting (beta=0.2) and a canonical mass loss rate (eta=0.4) give stellar masses for the primary Cepheid M=4.14^{+0.04}_{-0.05} M_sun and for the secondary M=4.15^{+0.04}_{-0.05} M_sun that agree at the 1% level with dynamical measurements. Moreover, we found ages for the two components and for the combined system t=151^{+4}_{-3} Myr that agree at the 5% level. The solutions based on evolutionary models that neglect the mass loss attain similar parameters, while those ones based on models that either account for an enhanced mass loss or neglect convective core overshooting have lower Bayes Factors and larger confidence intervals. The dependence on the mass loss rate might be the consequence of the crude approximation we use to mimic this phenomenon. By using the isochrone of the most probable solution and a Gaussian prior on the LMC distance, we found a distance modulus 18.53^{+0.02}_{-0.02} mag and a reddening value E(B-V)= 0.142^{+0.005}_{-0.010} mag that agree well with literature estimates.

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arXiv2012

One or more bound planets per Milky Way star from microlensing observations

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity$^{\bf 1,2}$ or transit$^{\bf 3}$ methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17--30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing$^{\bf 6\rm{\bf -}\bf 9}$, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing$^{\bf 10}$. These planets are at least as numerous as the stars in the Milky Way$^{\bf 10}$. Here we report a statistical analysis of microlensing data (gathered in 2002--07) that reveals the fraction of bound planets 0.5--10 AU (Sun--Earth distance) from their stars. We find that 17$_{\bf -9}^{\bf +6}$% of stars host Jupiter-mass planets (0.3--10 $\MJ$, where $\MJ {\bf = 318}$ $\Mearth$ and $\Mearth$ is Earth's mass). Cool Neptunes (10--30 $\Mearth$) and super-Earths (5--10 $\Mearth$) are even more common: their respective abundances per star are 52$_{\bf -29}^{\bf +22}$% and 62$_{\bf -37}^{\bf +35}$%. We conclude that stars are orbited by planets as a rule, rather than the exception.

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arXiv2011

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XIV. Classical and Type II Cepheids in the Galactic Bulge

The fourteenth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains Cepheid variables detected in the OGLE-II and OGLE-III fields toward the Galactic bulge. The catalog is divided into two main categories: 32 classical Cepheids (21 single-mode fundamental-mode F, four first-overtone 1O, two double-mode F/1O, three double-mode 1O/2O and two triple-mode 1O/2O/3O pulsators) and 335 type II Cepheids (156 BL Her, 128 W Vir and 51 RV Tau stars). Six of the type II Cepheids likely belong to the Sagittarius Dwarf Spheroidal Galaxy. The catalog data include the time-series photometry collected in the course of the OGLE survey, observational parameters of the stars, finding charts, and cross-identifications with the General Catalogue of Variable Stars. We discuss some statistical properties of the sample and compare it with the OGLE catalogs of Cepheids in the Large and Small Magellanic Clouds. Multi-mode classical Cepheids in the Galactic bulge show systematically smaller period ratios than their counterparts in the Magellanic Clouds. BL Her in the Galactic bulge stars seem to be brighter than the linear extension of the period-luminosity relation defined by the longer-period type II Cepheids. We also show individual stars of particular interest, like two BL Her stars with period doubling.

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arXiv2011

Quantitative Spectroscopy of Blue Supergiant Stars in the Disk of M81: Metallicity, Metallicity Gradient and Distance

The quantitative spectral analysis of low resolution Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosites, interstellar reddening and a new distance using the Flux-weighted Gravity--Luminosity Relationship (FGLR). Substantial reddening and extinction is found with E(B-V) ranging between 0.13 to 0.38 mag and an average value of 0.26 mag. The distance modulus obtained after individual reddening corrections is 27.7+/-0.1 mag. The result is discussed with regard to recently measured TRGB and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (~0.2 dex) in the inner disk (R<=5kpc) and slightly subsolar (~ -0.05 dex) in the outer disk (R>10 kpc) with a shallow metallicity gradient of 0.034 dex/kpc. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of HST color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5 Gyrs. This might be the result of gas infall from metal rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group a galaxy mass metallicity-relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of SDSS star forming galaxies.

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arXiv2011

First detection of period doubling in a BL Herculis type star. Observations and theoretical models

We report on the discovery of the first BL Herculis star displaying period-doubling behaviour as predicted by the hydrodynamic models of Buchler & Moskalik. The star, with P=2.4d, is located in the Galactic bulge and was discovered with OGLE-III photometry. We present new nonlinear convective models, which, together with recent evolutionary tracks, put constraints on the stellar parameters. In particular, we estimate the mass and metallicity of the object.

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arXiv2011

VVV DR1: The First Data Release of the Milky Way Bulge and Southern Plane from the Near-Infrared ESO Public Survey VISTA Variables in the Via Lactea

The ESO Public Survey VISTA Variables in the Via Lactea (VVV) started in 2010. VVV targets 562 sq. deg in the Galactic bulge and an adjacent plane region and is expected to run for ~5 years. In this paper we describe the progress of the survey observations in the first observing season, the observing strategy and quality of the data obtained. The observations are carried out on the 4-m VISTA telescope in the ZYJHKs filters. In addition to the multi-band imaging the variability monitoring campaign in the Ks filter has started. Data reduction is carried out using the pipeline at the Cambridge Astronomical Survey Unit. The photometric and astrometric calibration is performed via the numerous 2MASS sources observed in each pointing. The first data release contains the aperture photometry and astrometric catalogues for 348 individual pointings in the ZYJHKs filters taken in the 2010 observing season. The typical image quality is ~0.9-1.0". The stringent photometric and image quality requirements of the survey are satisfied in 100% of the JHKs images in the disk area and 90% of the JHKs images in the bulge area. The completeness in the Z and Y images is 84% in the disk, and 40% in the bulge. The first season catalogues contain 1.28x10^8 stellar sources in the bulge and 1.68x10^8 in the disk area detected in at least one of the photometric bands. The combined, multi-band catalogues contain more than 1.63x10^8 stellar sources. About 10% of these are double detections due to overlapping adjacent pointings. These overlapping multiple detections are used to characterise the quality of the data. The images in the JHKs bands extend typically ~4 mag deeper than 2MASS. The magnitude limit and photometric quality depend strongly on crowding in the inner Galactic regions. The astrometry for Ks=15-18 mag has rms ~35-175 mas.

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arXiv2011

The Magellanic Quasars Survey. II. Confirmation of 144 New Active Galactic Nuclei Behind the Southern Edge of the Large Magellanic Cloud

We quadruple the number of quasars known behind the Large Magellanic Cloud (LMC) from 55 (42 in the LMC fields of the third phase of the Optical Gravitational Lensing Experiment (OGLE)) to 200 by spectroscopically confirming 169 (144 new) quasars from a sample of 845 observed candidates in four ~3 deg^2 Anglo-Australian Telescope/AAOmega fields south of the LMC center. The candidates were selected based on their Spitzer mid-infrared colors, X-ray emission, and/or optical variability properties in the database of the OGLE microlensing survey. The contaminating sources can be divided into 115 young stellar objects (YSOs), 17 planetary nebulae (PNe), 39 Be and 24 blue stars, 68 red stars, and 12 objects classed as either YSO/PN or blue star/YSO. There are also 402 targets with either featureless spectra or too low signal-to-noise ratio for source classification. Our quasar sample is 50% (30%) complete at I = 18.6 mag (19.3 mag). The newly discovered active galactic nuclei (AGNs) provide many additional reference points for proper motion studies of the LMC, and the sample includes 10 bright AGNs (I < 18 mag) potentially suitable for absorption line studies. Their primary use, however, is for detailed studies of quasar variability, as they all have long-term, high cadence, continuously growing light curves from the microlensing surveys of the LMC. Completing the existing Magellanic Quasars Survey fields in the LMC and Small Magellanic Cloud should yield a sample of ~700 well-monitored AGNs, and expanding it to the larger regions covered by the OGLE-IV survey should yield a sample of ~3600 AGNs.

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arXiv2011

The Optical Gravitational Lensing Experiment. High Proper Motion Stars in the OGLE-III Data for Magellanic Clouds Fields

We present the results of a search for High Proper Motion (HPM) stars, i.e. the ones with μ> 100 mas/yr, in the direction to the Magellanic Clouds. This sky area was not examined in detail as the high stellar density hampers efforts in performing high-quality astrometry. Altogether 549 HPM stars were found with median uncertainties of proper motions per coordinate equal to 0.5 mas/yr. The fastest HPM star has the proper motion of 722.19 +/- 0.74 mas/yr. For the majority of objects (70%) parallaxes were also measured. The highest value found is π= 91.3 +/-1.6 mas. The parallaxes were used to estimate absolute magnitudes which enriched with color information show that 21 of HPM stars are white dwarfs. Other 23 candidate white dwarfs were selected of HPM stars with no measurable parallaxes using color-magnitude diagram. The search for common proper motion binaries revealed 27 such pairs in the catalog. The completeness of the catalog is estimated to be > 80% and it is slightly higher than for previous catalogs in the direction to the Magellanic Clouds.

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arXiv2011

The Baade-Wesselink p-factor applicable to LMC Cepheids

Context. Recent observations of LMC Cepheids bring new constraints on the slope of the period-projection factor relation (hereafter Pp relation) that is currently used in the Baade-Wesselink (hereafter BW) method of distance determination. The discrepancy between observations and theoretical analysis is particularly significant for short period Cepheids Aims. We investigate three physical effects that might possibly explain this discrepancy: (1) the spectroscopic S/N that is systematically lower for LMC Cepheids (around 10) compared to Galactic ones (up to 300), (2) the impact of the metallicity on the dynamical structure of LMC Cepheids, and (3) the combination of infrared photometry/interferometry with optical spectroscopy. Methods. To study the S/N we use a very simple toy model of Cepheids. The impact of metallicity on the projection factor is based on the hydrodynamical model of delta Cep already described in previous studies. This model is also used to derive the position of the optical versus infrared photospheric layers. Results. We find no significant effect of S/N, metallicity, and optical-versus-infrared observations on the Pp relation. Conclusions. The Pp relation of Cepheids in the LMC does not differ from the Galactic relation. This allows its universal application to determine distances to extragalactic Cepheids via BW analysis.

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arXiv2011

Discovery of period doubling in BL Herculis stars of the OGLE survey. Observations and theoretical models

We report the discovery of a period doubling behaviour in a 2.4d BL Herculis-type variable of the Galactic bulge. Another bulge BL Her-type star (P=2.25d) is a strong period doubling candidate. Both objects have been identified with the OGLE-III photometry. Possibility of period doubling in this type of pulsators has been predicted almost twenty years ago by Buchler & Moskalik. Our finding is the first observational confirmation of their theoretical results. Discovery of the first BL Herculis star showing the period doubling effect motivates a new theoretical investigation with the state-of-the-art convective pulsation codes. We present the results of initial model survey, specifically aimed at studying the observed period-doubled BL Her variable. All of our non-linear models have P=2.4d. The computations confirm that the period doubling effect is caused by the 3:2 resonance between the fundamental mode and the first overtone, as indicated by earlier radiative models of Buchler & Moskalik. Comparison of the computed and the observed light curves allows to constrain the parameters of the star, in particular its metallicity, which appears to be high, Z is approximately 0.01. The recent evolutionary tracks put also constraint on the mass of the star, which is close to M=0.50 solar masses.

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arXiv2011

The Araucaria Project. Accurate determination of the dynamical mass of the classical Cepheid in the eclipsing system OGLE-LMC-CEP-1812

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 Pietrzynski et al. (2010). 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.

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