arXiv subjects

G. Michalska

Publications and source records attributed to G. Michalska.

At least 19 recordsLinked to original sources

arXiv2023

A CCD Search for Variable Stars in the Open Cluster NGC 6611

We present the results of the $UBVI_C$ variability survey in the young open cluster NGC 6611 based on observations obtained during 34 nights spanning one year. In total, we found 95 variable stars. Most of these stars are classified as periodic and irregular pre-main sequence (PMS) stars. The analysis of the $JHK_S$ 2MASS photometry and four-colour IRAC photometry revealed 165 Class II young stellar sources, 20 of which are irregular variables and one is an eclipsing binary. These classifications, complemented by $JHK$ UKIDSS photometry and $riHα$ VPHAS photometry, were used to identify 24 candidates for classical T Tauri stars and 30 weak-lined T Tauri stars. In addition to the PMS variables, we discovered eight $δ$ Scuti candidates. None of these were previously known. Furthermore, we detected 17 eclipsing binaries where two were previously known. Based on the proper motions provided by the Gaia EDR3 catalogue, we calculated the cluster membership probabilities for 91 variable stars. For 61 variables, a probability higher than 80% was determined, which makes them cluster members. Only 25 variables with a probability less than 20% were regarded to be non-members.

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arXiv2021

BRITE observations of $ν$ Centauri and $γ$ Lupi, the first non-eclipsing members of the new class of nascent binaries

Results of an analysis of the BRITE-Constellation and SMEI photometry and radial-velocity observations, archival and new, of two single-lined spectroscopic binary systems $ν$ Centauri and $γ$ Lupi are reported. In the case of $γ$ Lup AB, a visual binary, an examination of the light-time effect shows that component A is the spectroscopic binary. Both $ν$ Cen and $γ$ Lup exhibit light variations with the orbital period. The variations are caused by the reflection effect, i.e. heating of the secondary's hemisphere by the early-B main sequence (MS) primary component's light. The modelling of the light curves augmented with the fundamental parameters of the primary components obtained from the literature photometric data and Hipparcos parallaxes, shows that the secondary components are pre-MS stars, in the process of contracting onto the MS. $ν$ Cen and $γ$ Lup A are thus found to be non-eclipsing counterparts of the B2 IV eclipsing binary (and a $β$ Cephei variable) 16 (EN) Lac, the B5 IV eclipsing binary (and an SPB variable) $μ$ Eri, and the recently discovered LMC nascent eclipsing binaries.

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arXiv2020

Ensemble asteroseismology of pulsating B-type stars in NGC 6910

Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsating stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $β$ Cep stars, including three that are hybrid $β$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $β$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_\odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $χ$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars.

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arXiv2018

Variability survey in NGC 6910, the open cluster rich in $β$ Cephei-type stars

NGC 6910 is the northern hemisphere open cluster known to be rich in $β$ Cephei-type stars. Using four-season photometry obtained in Białków (Poland) and Xinglong (China) observatories, we performed variability survey of NGC 6910. As the result, we found over 100 variable stars in the field of the cluster, including many stars showing variability due to pulsations and binarity. Thanks to the spectroscopic observations, we also detected changes in the profiles of spectral lines of $β$ Cep stars, caused by pulsations.

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arXiv2017

Pulsations in close binaries from the BRITE point of view

Using BRITE photometric data for several close binary systems we address the problem of damping pulsations in close binary systems due to proximity effects. Because of small number statistics, no firm conclusion is given, but we find pulsations in three relatively close binaries. The pulsations in these binaries have, however, very low amplitudes.

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arXiv2017

UVSat: a concept of an ultraviolet/optical photometric satellite

Time-series photometry from space in the ultraviolet can be presently done with only a few platforms, none of which is able to provide wide-field long-term high-cadence photometry. We present a concept of UVSat, a twin space telescope which will be capable to perform this kind of photometry, filling an observational niche. The satellite will host two telescopes, one for observations in the ultraviolet, the other for observations in the optical band. We also briefly show what science can be done with UVSat.

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arXiv2017

Spectroscopic survey of Kepler stars. II. FIES/NOT observations of A- and F-type stars

We have analysed high-resolution spectra of 28 A and 22 F stars in the Kepler field, observed with the FIES spectrograph at the Nordic Optical Telescope. We provide spectral types, atmospheric parameters and chemical abundances for 50 stars. Balmer, Fe I, and Fe II lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The stars analysed include chemically peculiar stars of the Am and Lambda Boo types, as well as stars with approximately solar chemical abundances. The wide distribution of projected rotational velocity, Vsini, is typical for A and F stars. The microturbulence velocities obtained are typical for stars in the observed temperature and surface gravity ranges. Moreover, we affirm the results of Niemczura et al., that Am stars do not have systematically higher microturbulent velocities than normal stars of the same temperature.

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arXiv2015

The 2003-4 multisite photometric campaign for the Beta Cephei and eclipsing star 16 (EN) Lacertae with an Appendix on 2 Andromedae, the variable comparison star

A multisite photometric campaign for the Beta Cephei and eclipsing variable 16 Lacertae is reported. 749 h of high-quality differential photoelectric Stromgren, Johnson and Geneva time-series photometry were obtained with ten telescopes during 185 nights. After removing the pulsation contribution, an attempt was made to solve the resulting eclipse light curve by means of the computer program EBOP. Although a unique solution was not obtained, the range of solutions could be constrained by comparing computed positions of the secondary component in the Hertzsprung-Russell diagram with evolutionary tracks. For three high-amplitude pulsation modes, the uvy and the Geneva UBG amplitude ratios are derived and compared with the theoretical ones for spherical-harmonic degrees l <= 4. The highest degree, l = 4, is shown to be incompatible with the observations. One mode is found to be radial, one is l = 1, while in the remaining case l = 2 or 3. The present multisite observations are combined with the archival photometry in order to investigate the long-term variation of the amplitudes and phases of the three high-amplitude pulsation modes. The radial mode shows a non-sinusoidal variation on a time-scale of 73 yr. The l = 1 mode is a triplet with unequal frequency spacing, giving rise to two beat-periods, 720.7 d and 29.1 yr. The amplitude and phase of the l = 2 or 3 mode vary on time-scales of 380.5 d and 43 yr. The light variation of 2 And, one of the comparison stars, is discussed in the Appendix.

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arXiv2013

Fundamental stellar and accretion disc parameters of the eclipsing binary DQ Velorum

To add to the growing collection of well-studied double periodic variables (DPVs) we have carried out the first spectroscopic and photometric analysis of the eclipsing binary DQ Velorum to obtain its main physical stellar and orbital parameters. Combining spectroscopic and photometric observations that cover several orbital cycles allows us to estimate the stellar properties of the binary components and the orbital parameters. We also searched for circumstellar material around the more massive star. We separated DQ Velorum composite spectra and measured radial velocities with an iterative method for double spectroscopic binaries. We obtained the radial velocity curves and calculated the spectroscopic mass ratio. We compared our single-lined spectra with a grid of synthetic spectra and estimated the temperature of the stars. We modeled the V-band light curve with a fitting method based on the simplex algorithm, which includes an accretion disc. To constrain the main stellar parameters we fixed the mass ratio and donor temperature to the values obtained by our spectroscopic analysis. We obtain donor and gainer masses, the radii, and temperatures for the stellar components. We find that DQ Vel is a semi-detached system consisting of a B3V gainer and an A1III donor star plus an extended accretion disc around the gainer. The disc is filling 89% of the gainer Roche lobe and it has a concave shape that is thicker at its edge than at its centre. We find a significant sub-orbital frequency of 0.19 d^{-1} in the residuals of the V-band light curve, which we interpret as a pulsation of an slowly pulsating B-type (SPB) of a gainer star. We also estimate the distance to the binary (3.1 kpc) using the absolute radii, apparent magnitudes, and effective temperatures of the components found in our study.

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arXiv2012

The massive eclipsing system ALS 1135 and variable stars in the field of the distant OB association Bochum 7

Using photometric and spectroscopic observations of the double-lined early-type eclipsing binary system ALS 1135, a member of the distant OB association Bochum 7, we derived the new physical and orbital parameters of its components. The masses of both components were derived with an accuracy better than 1 per cent, their radii, with an accuracy better than 3 per cent. Since the primary's mass is equal to about 25 sun masses, its radius was subsequently used to derive the age of the system which is equal to 4.3+/-0.5 Myr. The result shows that this method represents a viable alternative to isochrone fitting. A photometric search of the field of ALS 1135 resulted in the discovery of 17 variable stars, including seven pulsating ones. One of them is an SPB star belonging to Vel OB1, the other six are delta Scuti stars. Of the six delta Scuti stars three might belong to Vel OB1, the other two are likely members of Bochum 7. Given the age of Bochum 7, these two stars are probably pre-main sequence pulsators. In addition, we provide UBVI_C photometry for about 600 stars in the observed field.

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arXiv2012

International observational campaigns of the last two eclipses in EE Cephei: 2003 and 2008/9

Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depth and durations of the eclipses. Multicolour photometric data and spectroscopic observations at both low and high resolution were collected. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure. We considered the possibility of disk precession. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a "bump" that is very clear in all the UBVRI light curves. Variations in the spectral line profiles seem to be recurrent during each cycle. NaI lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the H_alpha emission. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the "bump" observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11-12 P_orb) and predicted depth of about 2 mag the forthcoming eclipse in 2014.

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arXiv2011

The evolution stage and massive disc of the interacting binary V393 Scorpii

V393 Scorpii is a bright Galactic Double Periodic Variable showing a long photometric cycle of approx. 253 days. We present new VIJK photometric time series for V393 Scorpii along with the analysis of ASAS V-band photometry. We disentangled all light curves into the orbital and long cycle components. The ASAS V-band orbital light curve was modeled with two stellar components plus a circumprimary optically thick disc assuming a semidetached configuration. We present the results of this calculation, giving physical parameters for the stars and the disc, along with general system dimensions. Our results are in close agreement with those previously found by Mennickent et al. (2010) from IR spectroscopy and the modeling of the spectral energy distribution. The stability of the orbital light curve suggests that the stellar + disc configuration remains stable during the long cycle. Therefore, the long cycle should be produced by an additional variable and not-eclipsed emitting structure. We discuss the evolutionary stage of the system finding the best match with one of the evolutionary models of van Rensbergen et al. (2008). According to these models, the system is found to be after an episode of fast mass exchange that transferred 4 M_sun from the donor to the gainer in a period of 400.000 years. We argue that a significant fraction of this mass has not been accreted by the gainer but remains in an optically thick massive (about 2 M_sun) disc-like surrounding pseudo-photosphere whose luminosity is not driven by viscosity but probably by reprocessed stellar radiation. Finally, we provide the result of our search for Galactic Double Periodic Variables and briefly discuss the outliers beta Lyr and RX Cas.

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arXiv2010

Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910

As a result of the variability survey in Chi Persei and NGC6910, the number of Beta Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational properties, in particular the frequency spectra, of Beta Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B type stars in Chi Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of Beta Cep-type stars and maybe also for other B-type pulsators.

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arXiv2010

Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars

CONTEXT: Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. AIMS: To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. METHODS: An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC884. Fifteen different instruments collected almost 77500 CCD images in 1286 hours. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 micromag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field.

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arXiv2009

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VII. NGC 1502

We present results of variability search in the field of the young open cluster NGC 1502. Eight variable stars were discovered. Of six other stars in the observed field that were suspected for variability, we confirm variability of two, including one beta Cep star, NGC 1502-26. The remaining four suspects were found to be constant in our photometry. In addition, UBVI photometry of the well-known massive eclipsing binary SZ Cam was obtained. The new variable stars include: two eclipsing binaries of which one is a relatively bright detached system with an EA-type light curve, an alpha2 CVn-type variable, an SPB candidate, a field RR Lyrae star and three other variables showing variability of unknown origin. The variability of two of them is probably related to their emission in Halpha, which has been measured by means of the alpha index obtained for 57 stars brighter than V=16 mag in the central part of the observed field. Four other non-variable stars with emission in Halpha were also found. Additionally, we provide VI photometry for stars down to V=17 mag and UB photometry for about 50 brightest stars in the observed field. We also show that the 10-Myr isochrone fits very well the observed color-magnitude diagram if a distance of 1 kpc and mean reddening, E(V-I)=0.9 mag, are adopted.

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arXiv2009

Light Curves of Galactic Eclipsing Double Periodic Variables

We present results of the investigation of the nature of double periodic variables (DPVs). We have selected a sample of Galactic eclipsing DPVs for a multiwavelength photometric study aimed to reveal their nature. The short orbital periodicity and the cyclic variability are decoupled and separately investigated. Shapes of orbital light curves are consistent with semi-detached binaries. The amplitude of the long cycle is always larger in redder bandpasses.

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arXiv2008

The eclipsing LMC star OGLE05155332--6925581: a clue for Double Periodic Variables

We investigate the nature of OGLE05155332-6925581, one of the brightest members of the enigmatic group of Double Periodic Variables (DPVs) recently found in the Magellanic Clouds. The modeling of archival orbital light curves (LCs), along with the analysis of the radial velocities suggest that this object is a semi--detached binary with the less massive star transferring matter to the more massive and less evolved star, in an Algol--like configuration. We find evidence for additional orbital variability and H$α$ emission, likely caused by an accretion disc around the primary star. As in the case of $βLyr$ the circumprimary disc seems to be more luminous than the primary, but we do not detect orbital period changes. We find that the LC follows a loop in the color--magnitude diagram during the long cycle; the system is redder when brighter and the rising phase is bluer than during decline. Infrared excess is also present. The source of the long--term periodicity is not eclipsed, indicating its circumbinary origin. Strong asymmetries, discrete absorption components (DACs) and a $γ$ shift are new and essential observational properties in the infrared H I lines. The DACs strength and RV follow a saw--teeth pattern during the orbital cycle. We suggest that the system experiences supercycles of mass outflow feeding a circumbinary disc. Mass exchange and mass loss could produce comparable but opposite effects in the orbital period on a long time scale, resulting in a quasi--constancy of this parameter.

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arXiv2007

Pulsating components of eclipsing binaries in the ASAS-3 catalog

As a result of the search among about 11 000 stars from the public ASAS-3 database, we report detection of pulsating components in eleven eclipsing binaries. In particular, we have found three classical Algols, MX Pav, IZ Tel, and VY Mic, with $δ$ Sct-type primary components. In six other eclipsing binaries, the short-period variability can also be interpreted in terms of $δ$ Sct-type pulsations, but in these systems both components are probably main-sequence stars. In HD 99612, the pulsation mode shows significant amplitude decrease during the time interval covered by observations. In addition, we find variability in one of the components of the eclipsing and double-lined spectroscopic O-type binary ALS 1135 which we interpret as a $β$ Cep-type pulsation. Finally, we find Y Cir to be a good candidate for an SPB star in an eclipsing binary system.

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