arXiv subjects

E. Plachy

Publications and source records attributed to E. Plachy.

At least 19 recordsLinked to original sources

arXiv2025

Exploring Low-Amplitude Variability in First Overtone Cepheids with TESS

Classical Cepheid stars that pulsate in the first overtone radial mode often exhibit additional periodicities at the millimagnitude level. Extensive studies of the OGLE data of the Magellanic Clouds have revealed distinct groups based on their period ratio with the first overtone mode. These groups are similar to those found in overtone RR Lyrae stars. Theoretical calculations suggest that some of the observed periodicities may be consistent with non-radial modes, while others remain unexplained. Currently, we only know of a handful of examples from the Galactic Cepheid sample that exhibit low-amplitude periodicities. The purpose of this study is to undertake a systematic search for low-amplitude variability in overtone Cepheids of the Milky Way in the photometric data of the full-frame images of the Transiting Exoplanet Survey Satellite, which were produced with the MIT Quick Look Pipeline. We applied standard Fourier analysis and classified the additional signals according to their period ratio to the overtone pulsation period. We found 127 stars in total to exhibit additional periodicities. In 17 stars, these can be identified as a second radial overtone. A further 83 stars were observed to display periodic signals with a ratio of $P_{\mathrm{x}}/P_{1\mathrm{O}}$ in the range 0.60$-$0.65. In 15 stars, the $P_{1\mathrm{O}}/P_{\mathrm{x}}$ is found to be near $\sim$0.68, of which six are also found to be in the previous group. Furthermore, we observed the presence of low-amplitude signals in 22 stars outside the aforementioned period ratios. It is possible that some of these may be direct detections of non-radial modes, with no harmonic frequency peak in the 0.60$-$0.65 period range. The TESS measurements revealed that the amplitudes and frequencies of these signals often vary within a TESS sector, a phenomenon that challenges theoretical models.

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arXiv2025

Connections between the cycle-to-cycle light curve and O-C variations of the non-Blazhko RR Lyrae stars

It has long been known that if the durations of the consecutive cycles of a pulsating star vary randomly, the O-C diagram could show quasi-periodic/irregular variations, even though the actual average period is constant. It is hypothesised that the period variation observed in many RR Lyrae stars, which are much faster and stronger than may be explained by an evolutionary origin, may in fact be caused by this cycle-to-cycle (C2C) variation effect. So far, quantitative studies have not really been performed, and space data have not been used at all. Our primary goal was to quantitatively analyse the O$-$C diagrams of RR Lyrae stars obtained from space photometry and explained by quasi-periodic or irregular periodic variations to see if they can be explained by random fluctuations in pulsation cycle length without assuming real period variations. We fitted statistical models to the O-C diagrams and tested their validity and fit. The necessary analysis of the light curves was performed using standard Fourier methods. We found that the vast majority of the O-C curves can be satisfactorily explained by assuming timing noise and the C2C variation without a real mean period variation. We have shown that the strength of the C2C variation is strongly dependent on the pulsation period and metallicity. These correlations suggests that turbulent convection may be behind the C2C variation. The additional frequencies of some RR Lyrae stars and their variation over time play only a marginal role in O-Cs. We have found new arguments that the phase jump phenomenon in RRc stars is in fact a continuous change, moreover, it could also be caused by the C2C variation.

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arXiv2024

MOCKA -- A PLATO mock asteroseismic catalogue: Simulations for gravity-mode oscillators

With ESA's PLATO space mission set for launch in December 2026, a new photometric legacy and a future of new scientific discoveries await. In this work we investigate PLATO's potential for observing pulsating stars across the Hertzsprung-Russell diagram as part of the PLATO Complimentary Science program (PLATO-CS). Specifically, a PLATO mock asteroseismic catalogue (MOCKA) of intermediate to massive stars is presented as a benchmark to highlight the asteroseismic yield of PLATO-CS in a quantitative way. MOCKA includes simulations of $\beta$~Cephei, slowly pulsating B (SPB), $\delta$~Scuti, $\gamma$~Doradus, RR Lyrae, Cepheid, hot subdwarf, and white dwarf stars. In particular, main-sequence gravity (g) mode pulsators are of interest as some of these stars form an important foundation for the scientific calibration of PLATO. MOCKA is based on a magnitude limited ($G\lesssim17$) \textit{Gaia} catalogue and is a product of realistic end-to-end \texttt{PlatoSim} simulations of stars for the first PLATO pointing field in the Southern hemisphere, which will be observed for a minimally 2-yr duration. We show that an abundant spectrum of frequencies is achievable across a wide range of magnitudes and co-pointing PLATO cameras. Within the magnitude limited regimes simulated ($G \lesssim 14$ for $\gamma$~Doradus stars and $G \lesssim 16$ for SPB stars) the dominant g-mode frequency is recovered in more than $95\%$ of the cases. MOCKA help us to understand the limits of the PLATO mission as well as highlight the opportunities to push astrophysics beyond current stellar models. All data products of this paper are made available to the community for further exploration. The key data products of MOCKA are the magnitude limited \textit{Gaia} catalogue of the first PLATO pointing field, together with fully reduced light curves from multi-camera observations for each pulsation class.

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arXiv2024

On the incidence rate of RR Lyrae stars with non-radial modes

Over the recent years, additional low-amplitude non-radial modes were detected in many of the first-overtone RR Lyrae stars. These non-radial modes form a characteristic period ratio with the dominant first-overtone mode of around 0.61. The incidence rate of this phenomenon changes from population to population. It is also strongly dependent on the quality of the analyzed data. Current models explaining these additional signals involve non-radial modes of degrees 8 and 9. Using synthetic horizontal branch populations, we investigate the incidence rate of first-overtone RR Lyrae stars with non-radial modes depending on the population properties, i.e., ages and metallicities. We compare our results with the observed results for globular clusters and the numerous collection of field first-overtone RR Lyrae stars to test the predictions of the models. We used synthetic horizontal branches combined with pulsation models to predict how the incidence rate would depend on the age and metallicity of the population. To test whether the results based on synthetic horizontal branches are realistic, we compared them to incidence rates observed by TESS in first-overtone field RR Lyrae stars, using photometric metallicity values from a newly established calibration for TESS. The analysis of synthetic horizontal branches showed that the incidence rate decreases with decreasing metallicity. We inferred photometric metallicity for RR Lyrae stars observed by TESS and showed that the theoretical predictions are in agreement with the observations. Using the same method, we also conclude that the metallicity distribution of RR Lyrae stars showing an additional mode with a period-ratio around $0.68$ appears to be different from that of both all first-overtone stars and those showing additional non-radial modes.

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arXiv2023

Recent advances in the photometric investigation of classical pulsators with the Kepler/K2 and TESS missions

Nearly continuous, densely sampled, space-based photometry allows us to recover the finest details in the light variations of stars. The number of such light curves have been increasing rapidly in the last few years thanks to the extended mission of the Kepler space telescope and the launch of the TESS mission. This new era brings us new perspectives in RR Lyrae and Cepheid studies, where low amplitude phenomena can be studied in a wide range of individual stars and on a statistical basis. In this proceedings I review the recent investigations of the Kepler, K2 and TESS fields, as well as the challenges in accurately reducing high-quality photometry.

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arXiv2023

Frequency analysis of the first-overtone RR Lyrae stars based on the Extended Aperture Photometry from the K2 data

Additional low-amplitude signals are observed in many RR Lyrae stars, beside the pulsations in radial modes. The most common ones are short-period signals forming a period ratio of around 0.60--0.65 with the first overtone, or long-period signals forming a period ratio of around 0.68. The RR Lyrae stars may also exhibit quasi-periodic modulation of the light curves, known as the Blazhko effect. We used the extensive sample of the first-overtone RR Lyrae stars observed by the Kepler telescope during the K2 mission to search for and characterize these low-amplitude additional signals. K2 data provides space-based photometry for a statistically significant sample. Hence this data is excellent to study in detail pulsation properties of RR Lyrae stars. We used K2 space-based photometry for RR Lyrae candidates from Campaigns 0-19. We selected RR Lyrae stars pulsating in the first overtone and performed a frequency analysis for each star to characterize their frequency contents. We classified 452 stars as first-overtone RR Lyrae. From that sample, we selected 281 RR$_{0.61}$ stars, 67 RR$_{0.68}$ stars, and 68 Blazhko stars. We found particularly interesting stars which show all of the above phenomena simultaneously. We detected signals in RR$_{0.61}$ stars that form period ratios lower than observed for the majority of stars. These signals likely form a new sequence in the Petersen diagram, around a period ratio of 0.60. In 32 stars we detected additional signals that form a period ratio close to that expected in RRd stars, but the classification of these stars as RRd is uncertain. We also report a discovery of additional signals in eight stars that form a new group in the Petersen diagram around the period ratio of 0.465-0.490. The nature of this periodicity remains unknown.

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arXiv2023

Time series analysis of bright TESS RRc stars: Additional modes, phase variations and more

Using two years of data from the TESS space telescope, we have investigated the time series of 633 overtone pulsating field RR Lyrae (RRc) stars. The majority of stars (82.8 per cent) contain additional frequencies beyond the main pulsation. In addition to the frequencies previously explained by the $\ell = 8$ and $\ell = 9$ non-radial modes, we have identified a group of stars where the additional frequencies may belong to the $\ell = 10$ non-radial modes. We found that stars with no additional frequencies are more common among stars with shorter periods, while stars with longer periods almost always show additional frequencies. The incidence rate and this period distribution both agree well with the predictions of recent theoretical models. The amplitude and phase of additional frequencies are varying in time. The frequencies of different non-radial modes appearing in a given star seem to vary on different timescales. We have determined a 10.4 per cent incidence rate for the Blazhko effect. For several stars we have detected continuous annual-scale phase change without significant amplitude variation. This type of variation offers a plausible explanation for the `phase jump' phenomenon reported in many RRc stars. The main pulsation frequency could show quasi-periodic phase and amplitude fluctuations. This fluctuation is clearly related to additional frequencies present in the star: stars with two non-radial modes show the strongest fluctuations, while stars with no such modes show no fluctuations at all. The summation of the phase fluctuation over time may explain the O-C variations that have long been known for many non-Blazhko RRc stars.

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arXiv2022

Gaia DR3: Specific processing and validation of all-sky RR Lyrae and Cepheid stars -- The Cepheid sample

Context. Cepheids are pulsating stars that play a crucial role in several astrophysical contexts. Among the different types, the Classical Cepheids are fundamental tools for the calibration of the extragalactic distance ladder. They are also powerful stellar population tracers in the context of Galactic studies. The Gaia Third Data Release (DR3) publishes improved data on Cepheids collected during the initial 34 months of operations. Aims. We present the Gaia DR3 catalogue of Cepheids of all types, obtained through the analysis carried out with the Specific Object Study (SOS) Cep&RRL pipeline. Methods. We discuss the procedures adopted to clean the Cepheid sample from spurious objects, to validate the results, and to re-classify sources with a wrong outcome from the SOS Cep&RRL pipeline. Results. The Gaia DR3 includes multi-band time-series photometry and characterisation by the SOS Cep&RRL pipeline for a sample of 15,006 Cepheids of all types. The sample includes 4,663, 4,616, 321 and 185 pulsators, distributed in the LMC, SMC, M31 and M33, respectively, as well as 5 221 objects in the remaining All Sky sub-region which includes stars in the MW field/clusters and in a number of small satellites of our Galaxy. Among this sample, 327 objects were known as variable stars in the literature but with a different classification, while, to the best of our knowledge, 474 stars have not been reported before to be variable stars and therefore they likely are new Cepheids discovered by Gaia.

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arXiv2022

Gaia Data Release 3. Summary of the variability processing and analysis

Context. Gaia has been in operations since 2014. The third Gaia data release expands from the early data release (EDR3) in 2020 by providing 34 months of multi-epoch observations that allowed us to probe, characterise and classify systematically celestial variable phenomena. Aims. We present a summary of the variability processing and analysis of the photometric and spectroscopic time series of 1.8 billion sources done for Gaia DR3. Methods. We used statistical and Machine Learning methods to characterise and classify the variable sources. Training sets were built from a global revision of major published variable star catalogues. For a subset of classes, specific detailed studies were conducted to confirm their class membership and to derive parameters that are adapted to the peculiarity of the considered class. Results. In total, 10.5 million objects are identified as variable in Gaia DR3 and have associated time series in G, GBP, and GRP and, in some cases, radial velocity time series. The DR3 variable sources subdivide into 9.5 million variable stars and 1 million Active Galactic Nuclei/Quasars. In addition, supervised classification identified 2.5 million galaxies thanks to spurious variability induced by the extent of these objects. The variability analysis output in the DR3 archive amounts to 17 tables containing a total of 365 parameters. We publish 35 types and sub-types of variable objects. For 11 variable types, additional specific object parameters are published. An overview of the estimated completeness and contamination of most variability classes is provided. Conclusions. Thanks to Gaia we present the largest whole-sky variability analysis based on coherent photometric, astrometric, and spectroscopic data. Later Gaia data releases will more than double the span of time series and the number of observations, thus allowing for an even richer catalogue in the future.

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arXiv2021

HD344787: a true Polaris analogue?

Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale, but they are also important objects per se, allowing us to put constraints on the physics of intermediate-mass stars and the pulsation theories. We have investigated the peculiar DCEP HD 344787, which is known to exhibit the fastest positive period change among DCEPs along with a quenching amplitude of the light variation. We have used high-resolution spectra obtained with HARPS-N@TNG for HD 344787 and the more famous Polaris DCEP, to infer their detailed chemical abundances. Results from the analysis of new time-series photometry of HD 344787 obtained by the TESS satellite are also reported. The double mode nature of HD344787 pulsation is confirmed by analysis of the TESS light curve, although with rather tiny amplitudes of a few tens of millimag. This is an indication that HD344787 is on the verge of quenching the pulsation. Analysis of the HARPS-N@TNG spectra reveals an almost solar abundance and no depletion of carbon and oxygen. Hence, the star appears to have not gone through the first dredge-up. Similar results are obtained for Polaris. Polaris and HD344787 are confirmed to be both most likely at their first crossing of the instability strip (IS). The two stars are likely at the opposite borders of the IS for first overtone DCEPs with metal abundance Z=0.008. A comparison with other DCEPs which are also thought to be at their first crossing allows us to speculate that the differences we see in the Hertzsprung-Russell diagram might be due to differences in the properties of the DCEP progenitors during the main sequence phase.

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arXiv2020

TESS observations of Cepheid stars: first light results

We present the first analysis of Cepheid stars observed by the TESS space mission in Sectors 1 to 5. Our sample consists of 25 pulsators: ten fundamental mode, three overtone and two double-mode classical Cepheids, plus three Type II and seven anomalous Cepheids. The targets were chosen from fields with different stellar densities, both from the Galactic field and from the Magellanic System. Three targets have 2-minute cadence light curves available by the TESS Science Processing Operations Center: for the rest, we prepared custom light curves from the full-frame images with our own differential photometric FITSH pipeline. Our main goal was to explore the potential and the limitations of TESS concerning the various subtypes of Cepheids. We detected many low amplitude features: weak modulation, period jitter, and timing variations due to light-time effect. We also report signs of non-radial modes and the first discovery of such a mode in an anomalous Cepheid, the overtone star XZ Cet, which we then confirmed with ground-based multicolor photometric measurements. We prepared a custom photometric solution to minimize saturation effects in the bright fundamental-mode classical Cepheid, $β$ Dor with the lightkurve software, and we revealed strong evidence of cycle-to-cycle variations in the star. In several cases, however, fluctuations in the pulsation could not be distinguished from instrumental effects, such as contamination from nearby sources which also varies between sectors. Finally, we discuss how precise light curve shapes will be crucial not only for classification purposes but also to determine physical properties of these stars.

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arXiv2019

Gaia Data Release 2: All-sky classification of high-amplitude pulsating stars

More than half a million of the 1.69 billion sources in Gaia Data Release 2 (DR2) are published with photometric time series that exhibit light variations during the 22 months of observation. An all-sky classification of common high-amplitude pulsators (Cepheids, long-period variables, Delta Scuti / SX Phoenicis, and RR Lyrae stars) is provided for stars with brightness variations greater than 0.1 mag in G band. A semi-supervised classification approach was employed, firstly training multi-stage random forest classifiers with sources of known types in the literature, followed by a preliminary classification of the Gaia data and a second training phase that included a selection of the first classification results to improve the representation of some classes, before the improved classifiers were applied to the Gaia data. Dedicated validation classifiers were used to reduce the level of contamination in the published results. A relevant fraction of objects were not yet sufficiently sampled for reliable Fourier series decomposition, consequently classifiers were based on features derived from statistics of photometric time series in the G, BP, and RP bands, as well as from some astrometric parameters. The published classification results include 195,780 RR Lyrae stars, 150,757 long-period variables, 8550 Cepheids, and 8882 Delta Scuti / SX Phoenicis stars. All of these results represent candidates whose completeness and contamination are described as a function of variability type and classification reliability. Results are expressed in terms of class labels and classification scores, which are available in the vari_classifier_result table of the Gaia archive.

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arXiv2018

Wandering near the red edge: photometric observations of three cool ZZ Ceti stars

We summarize our findings on three cool ZZ Ceti type pulsating white dwarfs. We determined eight independent modes in HS 0733+4119, of which seven are new findings. For GD 154, we detected two new eigenmodes, and the recurrence of the pulsational behaviour first observed in 1977. We discuss that GD 154 does not only vary its pulsations between a multiperiodic and a quasi-monoperiodic phase, but there are also differences between the relative amplitudes of the near-subharmonics observed in the latter phase. In the complex pulsator, Ross 808, we compared the pre- and post Whole Earth Telescope campaign measurements, and determined two new frequencies besides the ones observed during the campaign. Studying these stars can contribute to better understanding of pulsations close to the empirical ZZ Ceti red edge. All three targets are in that regime of the ZZ Ceti instability strip where short-term amplitude variations or even outbursts are likely to occur, which are not well-understood theoretically.

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arXiv2018

The s process in AGB stars as constrained by a large sample of Barium stars

Context. Barium (Ba) stars are dwarf and giant stars enriched in elements heavier than iron produced by the slow neutron-capture process (s process). They belong to binary systems where the primary star evolved through the asymptotic giant branch (AGB) phase,during which it produced the s-process elements and transferred them onto the secondary, now observed as a Ba star. Aims. We compare the largest homogeneous set of Ba giant star observations of the s-process elements Y, Zr, La, Ce, and Nd with AGB nucleosynthesis models to reach a better understanding of the s process in AGB stars. Methods. By considering the light-s (ls: Y and Zr) heavy-s (hs: La, Ce, and Nd) and elements individually, we computed for the first time quantitative error bars for the different hs-element/ls-element abundance ratios, and for each of the sample stars. We compared these ratios to low-mass AGB nucleosynthesis models. We excluded La from our analysis because the strong La lines in some of the sample stars cause an overestimation and unreliable abundance determination, as compared to the other observed hs-type elements. Results. All the computed hs-type to ls-type element ratios show a clear trend of increasing with decreasing metallicity with a small spread (less than a factor of 3). This trend is predicted by low-mass AGB models where 13C is the main neutron source. The comparison with rotating AGB models indicates the need for the presence of an angular momentum transport mechanism that should not transport chemical species, but significantly reduce the rotational speed of the core in the advanced stellar evolutionary stages. This is an independent confirmation of asteroseismology observations of the slow down of core rotation in giant stars, and of rotational velocities of white dwarfs lower than predicted by models without an extra angular momentum transport mechanism.

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arXiv2018

Chaotic dynamics in the pulsation of DF Cygni, as observed by Kepler

Pulsations of RV Tauri-type variable stars can be governed by chaotic dynamics. However, observational evidence for this happening is usually hard to come by. Here we use the continuous, 4-year-long observations of the Kepler space telescope to search for the signs of chaos in the RVb-type pulsating supergiant, DF Cygni. We use the Global Flow Reconstruction method to estimate the quantitative properties of the dynamics driving the pulsations of the star. The secondary, long-term light variation, i.e., the RVb phenomenon was removed in the analysis with the Empirical Mode Decomposition method. Our analysis revealed that the pulsation of DF Cyg could be described as a chaotic signal with a Lyapunov dimension of ~2.8. DF Cyg is only the third RV Tau star, and the first of the RVb subtype, where the nonlinear analysis indicates that low-dimensional chaos may explain the peculiarities of the pulsation.

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arXiv2018

Diversity of dynamical phenomena in type II Cepheids of the OGLE collection

We have analysed the photometry of 924 type II Cepheids from the Galactic bulge observed by the Optical Gravitational Lensing Experiment. Our goal was to detect and analyse various dynamical phenomena, including, e.g. multi-mode pulsation, period-doubled pulsation, period-k pulsation, or quasi-periodic modulation of pulsation. First examples of double-mode BL Her-type stars, pulsating simultaneously in the fundamental and first overtone modes are reported. Two double-mode pulsators are not isolated cases however, but form an extension of recently identified class of long-period RRd stars. Period doubling effect was detected in three BL Her-type stars: two were identified previously and one is a new discovery. Through identification of numerous period-doubled W Vir stars we show that the appearance of the period doubling effect, and hence the transition towards RV Tau class, is a smooth process that starts at pulsation periods slightly above 15d. Interchange of the deep and shallow pulsation cycles is identified in tens of RV Tau variables. One RV Tau star is a strong candidate for period-4 pulsation. Quasi-periodic modulation of pulsation in all sub-classes of type II Cepheids is reported for the first time. Modulation was detected in 16 BL Her, 9 W Vir and in 7 RV Tau stars. In the last case, modulation is the most likely scenario, but two other scenarios, excitation of non-radial modes, or period-4 pulsation, can also account for the observed pulsation pattern. Irregular changes of pulsation amplitude and period on various time-scales are common in all subgroups of type II Cepheids.

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arXiv2018

K2 Photometry of RR Lyrae Stars

Thousands of RR Lyrae stars have been observed by the \textit{Kepler} space telescope so far. We developed a photometric pipeline tailored to the light variations of these stars, called the Extended Aperture Photometry (EAP). We present the comparison of our photometric solutions for Campaigns 0 through 6 with the other pipelines available, e.g., SAP/PDCSAP, K2P2, EVEREST, and others. We focus on the problems caused by instrumental effects and the detectability of the low-amplitude additional modes.

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arXiv2018

Gaia Data Release 2: Summary of the variability processing & analysis results

The Gaia Data Release 2 (DR2): we summarise the processing and results of the identification of variable source candidates of RR Lyrae stars, Cepheids, long period variables (LPVs), rotation modulation (BY Dra-type) stars, delta Scuti & SX Phoenicis stars, and short-timescale variables. In this release we aim to provide useful but not necessarily complete samples of candidates. The processed Gaia data consist of the G, BP, and RP photometry during the first 22 months of operations as well as positions and parallaxes. Various methods from classical statistics, data mining and time series analysis were applied and tailored to the specific properties of Gaia data, as well as various visualisation tools. The DR2 variability release contains: 228'904 RR Lyrae stars, 11'438 Cepheids, 151'761 LPVs, 147'535 stars with rotation modulation, 8'882 delta Scuti & SX Phoenicis stars, and 3'018 short-timescale variables. These results are distributed over a classification and various Specific Object Studies (SOS) tables in the Gaia archive, along with the three-band time series and associated statistics for the underlying 550'737 unique sources. We estimate that about half of them are newly identified variables. The variability type completeness varies strongly as function of sky position due to the non-uniform sky coverage and intermediate calibration level of this data. The probabilistic and automated nature of this work implies certain completeness and contamination rates which are quantified so that users can anticipate their effects. This means that even well-known variable sources can be missed or misidentified in the published data. The DR2 variability release only represents a small subset of the processed data. Future releases will include more variable sources and data products; however, DR2 shows the (already) very high quality of the data and great promise for variability studies.

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