arXiv subjects

A. Schwarzenberg-Czerny

Publications and source records attributed to A. Schwarzenberg-Czerny.

At least 37 records · Page 2Linked to original sources

arXiv2009

The beat Cepheids in the Magellanic Clouds: an analysis from the EROS-2 database

A number of microlensing dark-matter surveys have produced tens of millions of light curves of individual background stars. These data provide an unprecedented opportunity for systematic studies of whole classes of variable stars and their host galaxies. We aim to use the EROS-2 survey of the Magellanic Clouds to detect and study the population of beat Cepheids (BCs) in both Clouds. BCs pulsating simultaneously in the first overtone and fundamental modes (FO/F) or in the second and first overtone modes (SO/FO) are of particular interest. Using special software designed to search for periodic variables, we have scanned the EROS-2 data base for variables in the typical period range of Cepheids. Metallicities of FO/F objects were then calculated from linear nonadiabatic convective stellar models. We identify 74 FO/F BCs in the LMC and 41 in the SMC, and 173 and 129 SO/FO pulsators in the LMC and SMC, respectively; 185 of these stars are new discoveries. For nearly all the FO/F objects we determine minimum, mean, and maximum values of the metallicity. The EROS data have expanded the samples of known BCs in the LMC by 31%, in the SMC by 110%. The FO/F objects provide independent measures of metallicities in these galaxies. The mean value of metallicity is 0.0045 in the LMC and 0.0018 in the SMC.

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arXiv2008

Planets in Stellar Clusters Extensive Search. V. Search for planets and identification of 18 new variable stars in the old open cluster NGC 188

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 188, an old, rather populous cluster. We have monitored the cluster for more than 87 hours, spread over 45 nights. We have not detected any good transiting planet candidates. We have discovered 18 new variable stars in the cluster, bringing the total number of identified variables to 46, and present for them high precision light curves, spanning 15 months.

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arXiv2008

Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud

For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid HR 7308. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted. The light curve exhibits a previously unobserved and spectacular change in both mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum implies a radial velocity of 104 km s$^{-1}$ and a metallicity of -0.4$\pm$0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4$\pm$6.0 mas yr$^{-1}$ using EROS astrometry, which is compatible with data from the NOMAD catalogue. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion for this star, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.

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arXiv2008

Cluster AgeS Experiment (CASE): Deficiency of observed dwarf novae in globular clusters

We present the results of a search for dwarf novae (DNe) in globular clusters (GCs). It is based on the largest available homogeneous sample of observations, in terms of the time span, number of observations and number of clusters. It includes 16 Galactic GCs and yielded two new certain DNe: M55-CV1 and M22-CV2. All previously known systems located in our fields were recovered, too. We surveyed M4, M5, M10, M12, M22, M30, M55, NGC 288, NGC 362, NGC 2808, NGC 3201, NGC 4372, NGC 6362, NGC 6752, omega Cen (NGC 5139) and 47 Tuc (NGC 104). The discovery of two DNe, namely M55-CV1 and M22-CV2, was already reported by Kaluzny et al. (2005) and Pietrukowicz et al. (2005), respectively. In the remaining 14 GCs we found no certain new DNe. Our result raises the total number of known DNe in the Galactic globular clusters to 12 DNe, distributed among 7 clusters. Our survey recovered all three already known erupting cataclysmic variables (CVs) located in our fields, namely M5-V101, M22-CV1, and V4 in the foreground of M30. To assess the efficiency of the survey, we analyzed images with inserted artificial stars mimicking outbursts of the prototype dwarf novae SS Cyg and U Gem. Depending on the conditions, we recovered between 16-100 percent of these artificial stars. The efficiency seems to be predominantly affected by duty cycle/time sampling and much less by distance/magnitude. Except for saturated tiny collapsed cores of M30, NGC 362 and NGC 6752 (and also the dense core of NGC 2808) crowding effects in the V band were avoided by our image subtraction technique augmented with auxiliary unsaturated B-band images. Our results clearly demonstrate that in GCs common types of DNe are very rare indeed. However, great care must be taken before these conclusions can be extended to the CV population in GCs.

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arXiv2008

A large-scale survey for variable stars in M33

We have started a survey of M 33 in order to find variable stars and Cepheids in particular. We have obtained more than 30 epochs of g'r'i' data with the CFHT and the one-square-degree camera MegaCam. We present first results from this survey, including the search for variable objects and a basic characterization of the various groups of variable stars.

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arXiv2008

Curious Variables Experiment (CURVE). Three Periodicities of BF Ara

We report CCD photometry of the dwarf nova BF Ara throughout fifteen consecutive nights in quiescence. Light curve in this interval is dominated by a large amplitude around 0.8 mag modulation consisting two periods. Higher amplitude signal is characterized by period of 0.082159(4) days, which was increasing at the rate of dotP/Psh = 3.8(3)* 10^{-5}. Weaker and stable signal has period of 0.084176(21) days. Knowing the superhump period of BF Ara determined by Kato et al. (2003) and equal to 0.08797(1) days, the first modulation is interpreted as quiescent negative superhump arising from retrograde precesion of titled accretion disk and the latter one as an orbital period of the binary. The respective period excess and defect are epsilon_+ = 4.51% +/- 0.03% and epsilon_- = -2.44% +/- 0.02%. Thus BF Ara is yet another in-the-gap nova with mass ratio q of around 0.21.

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arXiv2006

Detection of Beat Cepheids in M33 and Their Use as a Probe of the M33 Metallicity Distribution

Our analysis of the Deep CFHT M33 variability survey database has uncovered 5 Beat Cepheids (BCs) that are pulsating in the fundamental and first overtone modes. With {\it only} the help of stellar pulsation theory and of mass--luminosity (M-L) relations, derived from evolutionary tracks, we can accurately determine the metallicities Z of these stars. The [O/H] metallicity gradient of -0.16 dex/kpc that is inferred from the M33 galacto-centric distances of these Cepheids and from their 'pulsation' metallicities is in excellent agreement with the standard spectroscopic metallicity gradients that are determined from H II regions, early B supergiant stars and planetary nebulae. Beat Cepheids can thus provide an additional, independent probe of galactic metallicity distributions.

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arXiv2005

Efficient analysis in planet transit surveys

With the growing number of projects dedicated to the search for extrasolar planets via transits, there is a need to develop fast, automatic, robust methods with a statistical background in order to efficiently do the analysis. We propose a modified analysis of variance (AoV) test particularly suitable for the detection of planetary transits in stellar light curves. We show how savings of labor by a factor of over 10 could be achieved by the careful organization of computations. Basing on solid analytical statistical formulation, we discuss performance of our and other methods for different signal-to-noise and number of observations.

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arXiv2005

Cluster AgeS Experiment (CASE): Dwarf Novae and a Probable Microlensing Event in the Globular Cluster M22

We report the identification of a new cataclysmic variable (denoted as CV2) and a probable microlensing event in the field of the globular cluster M22. Two outbursts were observed for CV2. During one of them superhumps with P_sh=0.08875 d were present in the light curve. CV2 has an X-ray counterpart detected by XMM-Newton. A very likely microlensing event at a radius of 2.3 arcmin from the cluster center was detected. It had an amplitude of Delta_V=0.75 mag and a characteristic time of 15.9 days. Based on model considerations we show that the most likely configuration has the source in the Galactic bulge with the lens in the cluster. Two outbursts were observed for the already known dwarf nova CV1.

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arXiv2005

Cluster AgeS Experiment (CASE): SX Phe stars from the globular cluster Omega Centauri

We present an analysis and interpretation of oscillation spectra for all 69 SX Phoenicis stars discovered in the field of the cluster. For most of the stars we have reliable absolute magnitudes and colors. Except of one, or perhaps two, objects, the stars are cluster members. Their pulsational behaviour is very diversified. Multiperiodic variability with at least part of the excited modes being nonradial is most common but there are also many cases of high amplitude, presumably radial mode, pulsators. In a number of such cases we have evidence for two radial modes being excited. Parameters of radial mode pulsators are in most cases consistent with standard evolutionary models for stars in the mass range 0.9-1.15 Mo. However, in four cases we have evidence that the masses are significantly lower than expected. Three objects show frequency triplets that may be interpreted in terms of rotational frequency splitting of l=1 modes. Implied equatorial velocities of rotation are from 10 to over 100 km/s. Nearly all measured frequencies fall in the ranges predicted for unstable modes. Two cases of low frequency variability are interpreted as being caused by tidal distortion induced by close companions.

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arXiv2005

Cluster AgeS Experiment (CASE): Detection of a dwarf nova in the globular cluster M55

We report the detection of a dwarf nova (DN) in the core region of the globular cluster M55. Six outbursts were observed during 8 observing seasons spanning the period 1997-2004. The variable has an X-ray counterpart detected on images taken with the ROSAT telescope. Although we cannot offer proof of cluster membership, one can see that both the position on the H-R diagram and the X-ray flux are consistent with a bright DN at the cluster distance. According to our outburst statistics, no more than one similar DN could remain undetected in our field of view, centred at the cluster core.

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arXiv2004

Cluster AgeS Experiment. Catalog of Variable Stars in the Globular Cluster Omega Centauri

We present results of a photometric survey for variable stars in the field of the globular cluster Omega Centauri. The observed region was centered roughly on the cluster core and covered 644 arcmin^2. The cluster was monitored on 59 nights in 1999 and 2000. A total of 117 new variables were identified. Among them there are 16 RR Lyr-type stars, 35 SX Phe variables and 26 eclipsing binaries. A comprehensive catalog including all variable stars so far reported from the cluster field is presented. We list basic photometric properties and provide finding charts for a total of 392 objects. For 313 of them new BV light curves were obtained. The presented sample includes several interesting variables, such as SX Phe stars with extremely short periods of pulsation and several candidates for pulsating K giants. Optical counterparts to 9 X-ray sources detected by XMM and Chandra telescopes were identified: all of them are likely to be foreground variables not related to the cluster.

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arXiv2003

Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster Omega Centauri as standard candles

New photometry of RRab and RRc stars in Omega Cen is used to calibrate their absolute magnitudes M_V as a function of: a) metallicity; and b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the CASE project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables we obtained a relation of M_V=(0.26\pm 0.08)[Fe/H]+(0.91\pm 0.13). A dereddened distance modulus to the LMC based on that formula is $μ_{0}=18.56\pm 0.14$ mag. The second calibration of M_V, which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of $μ_{0}=18.51\pm 0.07$ mag.

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arXiv2003

Curious Variables Experiment (CURVE). Superhump Period Change Pattern in KS UMa and Other Dwarf Novae

We report extensive photometry of the dwarf nova KS UMa throughout its 2003 superoutburst till quiescence. During the superoutburst the star displayed clear superhumps with a mean period of Psh = 0.070092(23) days. In the middle stage of superoutburst the period was increasing with a rate of $\dot P/P = (21\pm12)\times 10^{-5}$ and later was decreasing with a rate of $\dot P/P = -(21\pm8)\times 10^{-5}$. At the end of superoutburst and during first dozen days of quiescence the star was showing late superhumps with a mean period of Plate = 0.06926(2) days. This phenomenon was observed even 30 days after beginning of the superoutburst. In quiescence the star shows quasi-periodic modulations with amplitude reaching 0.5 mag. The most common structure observed during this stage was sinusoidal wave characterized by a period of about 0.1 days. Comparing KS UMa to other SU UMa stars we conclude that this group of dwarf novae shows decreasing superhump periods at the beginning and the end of superoutburst but increasing period in the middle phase.

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arXiv2003

Variability of accretion flow in the core of the Seyfert galaxy NGC 4151

We analyze observations of the Seyfert galaxy NGC 4151 covering 90 years in the optical band and 27 years in the 2-10 keV X-ray band. We compute the Normalized Power Spectrum Density (NPSD), the Structure Function (SF) and the Autocorrelation Function (ACF) for these data. The results show that the optical and X-ray variability properties are significantly different. X-ray variations are predominantly in the timescale range of 5 - 1000 days. The optical variations have also a short timescale component which may be related to X-ray variability but the dominant effect is the long timescale variability, with timescales longer than $\sim$ 10 years. We compare our results with observations of NGC 5548 and Cyg X-1. We conclude that the long timescale variability may be caused by radiation pressure instability in the accretion disk, although the observed timescale in NGC 4151 is by a factor of few longer than expected. X-ray variability of this source is very similar to what is observed in Cyg X-1 but scaled with the mass of the black hole, which suggests that the radiation pressure instability does not affect considerably the X-ray production.

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arXiv2001

Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55

We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distribution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be (m-M)_V=13.86 +- 0.25 mag.

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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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