arXiv subjects

Johanna Jurcsik

Publications and source records attributed to Johanna Jurcsik.

17 recordsLinked to original sources

arXiv2025

RR Lyrae stars with variable mean magnitudes

Context. A number of RR Lyrae stars show variable mean magnitudes in the OGLE survey light curves of the Galactic bulge. Hitherto this phenomenon was not studied, as it was generally assumed to be related to problems with the photometry. Aims. We investigate whether the mean magnitude variability of RR Lyrae variables is due to genuine astrophysical phenomena. Methods. We make use of the extended, and in many cases overlapping, light curves from multiple microlensing surveys, to study RR Lyrae stars with apparent mean-magnitude variations. A modified Fourier-series based fitting method is introduced to analyze the light curves showing mean-magnitude variations. Data from infrared surveys are also used to construct spectral energy distributions (SEDs). Results. 72 stars are presented where the mean-magnitude variations are most probably of genuine astrophysical origin, and not the result of problems with the photometry. The ratio of variation between the V and I bands is compatible with variable extinction by dust in most cases, but no infrared excess is detected in the SEDs. The occurrence rate of the phenomenon, after correcting for selection effects, is $\sim0.9\%$ among RR Lyrae variables in the OGLE bulge fields.

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arXiv2023

Photometric metallicities of fundamental-mode RR Lyr stars from Gaia $G$-band photometry of globular-cluster variables

Photometric metallicity formulae of fundamental-mode RR Lyr (RRab) stars are presented using globular-cluster data exclusively. The aim is to check whether this selection may help increasing the overall accuracy of the fits and eliminating the systematic bias of the photometric results, namely that they tend to overestimate [Fe/H] of the most metal-poor variables. The $G$-band time-series data available in the Gaia DR3 archive and a new compilation of the published spectroscopic globular cluster [Fe/H] values on a uniform solar reference metallicity scale are utilized. We have derived a new ${\mathrm{[Fe/H]}}_{\mathrm{phot}}- P,φ_{31}$ formula, and have diagnosed that no significant increase in the accuracy of the fit can be achieved using non-linear or multi-parameter formulae. The best result is obtained when different formulae are applied for variables with Oosterhoff-type I and II properties. However, even this solution cannot eliminate the systematic bias of the results completely. This separation of the variables has also led to the conclusion that the photometric estimates of the [Fe/H] are less reliable for the Oo-type II variables than for the Oo-type I sample. Published ${\mathrm{[Fe/H]}}_{\mathrm{phot}}$ values and the results of the available photometric formulae in the Gaia $G$-band are compared with the present results. It is found that each of the solutions yields very similar results, with similar accuracy and systematic biases. Major differences are detected only in the zero-points of the [Fe/H] scales, and these offsets are larger than differences in the accepted solar reference values would explain.

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arXiv2023

Toward more accurate RR Lyrae metallicities

By using a large sample of published spectroscopic iron abundances, we point out the importance of gravity correction in deriving more accurate metal abundances for RR Lyrae stars. For the 197 stars with multiple spectra we find overall [Fe/H] standard deviations of 0.167 (as published), 0.145 (shifted by data source zero points) and 0.121 (both zero point-shifted and gravity-corrected). These improvements are significant at the ~2 sigma level at each correction step, leading to a clearly significant improvement after both corrections applied. The higher quality of the gravity-corrected metallicities is strongly supported also by the tighter correlation with the metallicities predicted from the period and Fourier phase phi_31. This work underlines the need for using some external estimates of the temporal gravity in the chemical abundance analysis rather than relying on a full-fetched spectrum fit that leads to large correlated errors in the estimated parameters.

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arXiv2022

RRab variables with identical light-curve shapes at different pulsation periods

In this paper, we report on the detection of RRab stars with quasi-identical-shape light curves but period differences as large as $0.05-0.21$ d using the Galactic bulge data of the OGLE-IV survey. We have examined stars with shorter periods than the Oosterhoff~I ridge of the bulge. These stars generally have smaller amplitudes and larger Fourier phase-differences than the typical bulge RRab stars have at the same period. Many of these "anomalous" stars have good-quality light curves without any sign of the Blazhko modulation. Examining their Fourier parameters revealed that several of these stars show very similar light-curve to the typical bulge RR Lyrae. We found hundreds of quasi-identical-shape light-curve pairs with different periods between the "anomalous"- and the "normal"-position RRab stars based on the OGLE $I$-band data. The OGLE $V$-band, and the archive VVV and MACHO surveys $K_s$-, $b$- and $r$-band data of these stars were also checked for light-curve-shape similarity. Finally, 149 pairs with identical-shape light curves in each available photometric band were identified. Calculating the physical properties of the variables using empirical formulae, on average, $-0.5$~dex, $-0.13$~mag, 0.67, and 165~K differences between the [Fe/H], $M_V$, $R/R_\odot$, and $T_{\mathrm{eff}}$ values of the members of the pairs are derived, being the short-period stars less metal-poor, fainter, smaller and hotter than the long-period variables. To explain the existence of variables with different physical properties and pulsation periods but with identical-shape light curves is a challenging task for modelling.

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arXiv2021

Studies of RR Lyrae Variables in Binary Systems. I.: Evidence of a Trimodal Companion Mass Distribution

We present 87 candidates for RR Lyrae variable stars in binary systems, based on our new search using the light-travel time effect (LTTE) and observed - calculated ($O-C$) diagrams in the Galactic bulge time-series photometry of the Optical Gravitational Lensing Experiment. Out of these, 61 are new candidates, while 26 have been announced previously. Furthermore, 12 stars considered as binary candidates in earlier works are discarded from the list, either as they were found to have $O-C$ diagrams incompatible with the LTTE or because their long-term periodicity is definitely caused by the Blazhko effect. This sample of RR Lyrae binary candidates allows us to draw the first firm conclusions about the population of such objects: no candidate has an orbital period below 1000 days, while their occurrence rate steadily increases with increasing period, and peaks between 3000 and 4000 days; however, the decrease in the number of stars toward even longer periods is probably the result of observational biases. The eccentricities show a very significant concentration between 0.25 and 0.3, with a quarter of candidates found in this single bin, overlaid on an otherwise flat distribution between 0.05 and 0.6. Only six stars have higher inferred eccentricities above 0.6. Lastly, the distribution of the mass functions is highly peculiar, exhibiting strong trimodality. We interpret these modes as the presence of three distinct groups of companions, with typical inferred masses of $\sim0.6$, $\sim0.2$, and $\sim0.067\,\mathrm{M}_\odot$, which can be associated with populations of white dwarf and main sequence, red dwarf, and brown dwarf companions, respectively.

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arXiv2021

On the photometric metallicity of Oosterhoff-type I and II RRab stars

We are revising the consistency of photometric metallicity formulae widely used for fundamental-mode RR Lyrae (RRab) variables, based on their V- and I-band light curves, published by Jurcsik and Kovács (1996) and Smolec (2005), respectively. 293 RRab variables belonging to 10 globular clusters, all simultaneously containing stars of both Oosterhoff types, are selected for this purpose. We find that on average, the V-band formula results in higher estimated metallicities by about 0.05 dex than the I-band formula. Moreover, we detect a dependency on the Oosterhoff class of the variables for both formulae, as well. Using the V-band formula, Oo I stars are 0.05-0.10 dex more metal rich than Oo II stars of the same cluster. Although with less significance, but the I-band results indicate a reversed trend. Therefore, we surmise that the average difference we have found between the V- and I-band formulae is the consequence of the total sample being dominated by Oo I variables.

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arXiv2019

Blazhko type fundamental-mode RR Lyrae stars in the globular cluster, M3

Blazhko stars from the extended $BVI_{\mathrm C}$ time series of RRab stars in the globular cluster, M3, are analysed. This is the largest sample of Blazhko stars with full details of their Blazhko properties in a homogeneous stellar system. Blazhko periods and light-curve solutions are determined/estimated for 83 fundamental-mode variables. The lack of phase modulation in Oosterhoff-type~II stars and the diminishing relative strength of the modulation in long-period Oosterhoff-type~I stars are the regular tendencies found between the pulsation and modulation properties of Blazko stars. Similarly to previous results, no modulation of the longest-period stars is detected. The onset of the modulation in a previously regular RRab star, and the similar distribution of modulated and non-modulated stars imply that the modulation is a temporal property of RRL stars, which may occur at any time in any RRab star except the coolest ones. Comparing the modulation periods in M3 and in other samples of Blazhko stars, the mean log$(P_{\mathrm {mod}})$ value is found to depend on the metallicity of the system. The separation of the temperature- and radius-change induced variations supports our previous finding that the photometric radius variation does not show any modulation. The pulsation-averaged mean brightness and temperature of Blazhko stars are found to be larger in the large-amplitude phase of the modulation than in the small-amplitude phase. The larger the amplitude of the modulation, the larger changes of the mean parameters are detected.

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arXiv2019

Revisiting the Kepler non-Blazhko RR Lyrae sample: Cycle-to-cyle variations and additional modes

We analyzed the long and short cadence light curves of the Kepler non-Blazhko RRab stars. We prepared the Fourier spectra, the Fourier amplitude and phase variation functions, time-frequency representation, the O-C diagrams and their Fourier contents. Our main findings are: (i) All stars which are brighter a certain magnitude limit show significant cycle-to-cycle light curve variations. (ii) We found permanently excited additional modes for at least one third of the sample and some other stars show temporarily excited additional modes. (iii) The presence of the Blazhko effect was carefully checked and identified one new Blazhko candidate but for at least 16 stars the effect can be excluded. This fact has important consequences. Either the cycle-to-cycle variation phenomenon is independent from the Blazhko effect and the Blazhko incidence ratio is still much lower (51%-55%) than the extremely large (>90%) ratio published recently. The connection between the extra modes and the cycle-to-cycle variations is marginal.

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arXiv2018

New Galactic multi-mode Cepheids from the ASAS-SN Survey

A systematic search for multi-mode Cepheids using the database of the ASAS-SN survey has led to the detection of thirteen new double-mode and two triple-mode Cepheids in the Galactic disk. These discoveries have increased the number of Galactic disk multi-mode Cepheids by 33%. One of the new triple-mode variables pulsates simultaneously in the fundamental and in the first and the second radial overtone modes and the other in the first three radial overtone modes. Overtone triple-mode Cepheids were identified only in the Galactic bulge and in the Large and Small Magellanic Clouds previously.

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arXiv2018

A Data-driven Study of RR Lyrae Near-IR Light Curves: Principal Component Analysis, Robust Fits, and Metallicity Estimates

RR~Lyrae variables are widely used tracers of Galactic halo structure and kinematics, but they can also serve to constrain the distribution of the old stellar population in the Galactic bulge. With the aim of improving their near-infrared photometric characterization, we investigate their near-infrared light curves, as well as the empirical relationships between their light curve and metallicities using machine learning methods. We introduce a new, robust method for the estimation of the light-curve shapes, and hence the average magnitudes of RR~Lyrae variables in the $K_\mathrm{S}$ band, by utilizing the first few principal components (PCs) as basis vectors, obtained from the PC analysis of a training set of light curves. Furthermore, we use the amplitudes of these PCs to predict the light-curve shape of each star in the $J$-band, allowing us to precisely determine their average magnitudes (hence colors), even in cases where only one $J$ measurement is available. Finally, we demonstrate that the $K_\mathrm{S}$-band light-curve parameters of RR~Lyrae variables, together with the period, allow the estimation of the metallicity of individual stars with an accuracy of $\sim 0.2 - 0.25$ dex, providing valuable chemical information about old stellar populations bearing RR~Lyrae variables. The methods presented here can be straightforwardly adopted for other classes of variable stars, bands, or for the estimation of other physical quantities.

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arXiv2009

New Results of the Konkoly Blazhko Group

During the recent years the Konkoly Blazhko Group (PIs Johanna Jurcsik and Béla Szeidl, co-workers Ádám Sódor, Zsombor Hurta and several undergraduate, graduate students) published new important results of Blazhko variables in 15 reviewed Journal articles. These results utilize multicolor CCD observations obtained with an automatic 60 cm telescope, and also previously unpublished Konkoly archive photometric data. Our light curves are the most extended multicolor data-sets ever obtained for a Blazhko variable, the observations cover each phase of the pulsation and the modulation as well. We have detected many previously unknown features of the light curve modulation, and based on the different band's observations we also revealed the underlying variations of the mean physical parameters during the Blazhko cycle. In my contribution the main achievements of the Konkoly Blazhko Group are summarised.

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arXiv2005

On the distribution of the modulation frequencies of RR Lyrae stars

For the first time connection between the pulsation and modulation properties of RR Lyrae stars has been detected. Based on the available data it is found that the possible range of the modulation frequencies, i.e, the possible maximum value of the modulation frequency depends on the pulsation frequency. Short period variables (P < 0.4 d) can have modulation period as short as some days, while longer period variables (P > 0.6 d) always exhibit modulation with P_mod > 20 d. We interpret this tendency with the equality of the modulation period with the surface rotation period, because similar distribution of the rotational periods is expected if an upper limit of the total angular momentum of stars leaving the RGB exists. The distribution of the projected rotational velocities of red and blue horizontal branch stars at different temperatures shows a similar behaviour as v_rot derived for RR Lyrae stars from their modulation periods. This common behaviour gives reason to identify the modulation period with the rotational period of the modulated RR Lyrae stars.

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arXiv2003

The metallicity range of variables in M3

The recently published spectroscopic metallicities of RR Lyrae stars in M3 (Sandstrom, K., Pilachowski, C. A., and Saha, A. 2001, AJ 122, 3212) though show a relatively wide range of the [Fe/H] values, the conclusion that no metallicity spread is real has been drawn, as no dependence on either minimum temperature or period was detected. Comparing these spectroscopic metallicities with [Fe/H] calculated from the Fourier parameters of the light curves of the variables a correlation between the [Fe/H] values appears. As a consequence of the independence of the spectroscopic and photometric metallicities, this correlation points to the reality of a metallicity spread. The absolute magnitudes of these stars follow a similar trend along both the spectroscopic and photometric metallicities as the general $\mathrm{M_V - [Fe/H]}$ relation predicts, which strengthens that the detected metallicity range is real.

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arXiv2000

Period changes in omega Centauri RR Lyrae stars

A century of observations of omega Centauri allows us to accurately follow the period changes of a large sample of variable stars. Although the period changes are composed of monotone and irregular changes, period increases dominate in most of the RRab stars. The typical period increase rates are of the same order of magnitude as model calculations predict for the evolved, redward phase of horizontal branch evolution. For the 44 well observed RRab stars which show monotonic period changes, a mean rate of 0.15 d/Myr has been found. The period changes of the first overtone RRc type stars show a much more complex, irregular behavior.

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arXiv2000

Identification and astrometry of variables in M3

We present identification and astrometry of all previously known or suspected variables along with the discovery of six new variables in the globular cluster M3. The number of the catalogized variables increased to 274 by including all the confirmed, previously known variables and the new discoveries. The precise and homogeneous astrometry, as well as the clarification of misapprehensions in the preceding identifications are done by using overlapping fields from a wide-field Schmidt-camera, a 1m RCC telescope, and HST archive observations from the center of the cluster. The astrometric positions can serve as a direct input to any photometry which needs the accurate centers of the variables.

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arXiv1998

Spatial metallicity asymmetry in $ω$ Centauri

The compilation of the metallicity values of $ω$~Centauri stars led to detection of spatial asymmetry in the metallicity distribution of the bright giants. It was found that the most and least metal-poor objects are separated along galactic latitude. Statistical tests show that the chance occurrence of the phenomenon is very low. The subgiant and the RR~Lyrae samples do not show, however, similar segregation of the different metallicity objects. The physical parameters of the variables calculated by using empirical formulae indicate that the bulk of the RRab stars comprises a very homogeneous group regarding both their metallicity and mass values. These stars are most probably already in the evolved stage of their horizontal branch evolution.

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arXiv1996

On the Light Curve - Luminosity Relation of RR Lyrae Stars

We use a large sample of RRab stars in globular clusters and in the Sculptor dwarf galaxy to decipher the relation between the Fourier decomposition and the luminosity. For fixing the zero point of the relation, we use the latest Baade-Wesselink (BW) results. It is shown that the most plausible representation of the absolute brightness (in V color) consists of a linear expression of the period and of the Fourier parameters A_1 and Phi_31 (computed also from the V light curve). We derive an average M_V = Alpha [Fe/H] + Beta relation from all the available Fourier data. Our results exclude any values of Alpha larger than 0.19, in agreement with most of the BW and evolutionary studies. We give age and reddened distance modulus estimations for the clusters entering in our analysis.

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