arXiv subjects

K. Oláh

Publications and source records attributed to K. Oláh.

At least 19 recordsLinked to original sources

arXiv2026

Sub-second cadence structure of optical flares on AD Leo

Stellar flares are sudden brightenings caused by magnetic reconnection and are frequently observed on late-type stars. High-cadence photometry of flares provides valuable insights into the mechanisms of these events, yet such observations remain scarce. We seek to explore the sub-second fine structure of stellar flares and assess the information content in high-speed photometry. New 0.3 s-cadence photometry from a six-year-long observing campaign of the active M-dwarf AD Leo is presented. We use time--frequency analysis to detect quasi-periodic pulsations in the decay phase of flares. We explore statistical measures of time series complexity of the detected flares to quantify the information gain achievable with high-cadence photometry. We detect 42 flares in 211 hours of observations. The flare frequency distribution is consistent with the previous literature. We find no quasi-periodic pulsations with periods below a few seconds, and identify two candidate signals with periods around 1 and 3 min. Using different measures of complexity on the binned flare light curves we confirm the advantages of high observing cadence. However, we also find a plateau up to a binning of ~4--5 s for a few complex flares, suggesting that an exposure time of a few seconds is usually enough to retain most of the information carried by a single-filter observation. New photometric observations of AD Leo revealed sub-structures of flare light curves on the timescale of a few seconds, but we found no features on timescales below that.

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arXiv2025

Magnetic activity on the young Sun: a case study of EK Draconis

Context. Young, solar analogue stars provide key insights into the early stages of stellar evolution, particularly in terms of magnetic activity and rotation. Their rapid rotation, high flaring rate, and enhanced surface activity make them ideal laboratories for testing stellar models or even the solar dynamo. Aims. Using long-term photometric data, we investigated the cyclic behaviour of EK Dra over the last century. We analyze its short-term activity based on 13 Transiting Exoplanet Survey Satellite (TESS) sectors. Applying Doppler imaging on high-resolution spectral data we investigate short and long-term spot evolution and surface differential rotation. Methods. We use Short-term Fourier-transform on a 120 years long archival photometric data in order to search for activity cycles. The short-term space photometry data is fitted with an analytic three-spot model, and we hand-select flares from it to analyze their phase and frequency distribution. Spectral synthesis is used to determine the astrophysical parameters of EK Dra. Using the iMap multi-line Doppler imaging code, we reconstruct 13 Doppler images. Differential rotation is derived by cross-correlating consecutive Doppler maps. Results. Long-term photometric data reveal a 10.7-12.1 year cycle that was persistently present for 120 years. In the more recent half of the light curve a 7.3-8.2 years-long signal is also visible. The distribution of the 142 flares in the TESS data shows no correlation with the rotational phase or with the spotted longitudes. The reconstructed Doppler images show a surface that varies from rotation to rotation, putting the lower limit of the spot lifetime between 10-15 days. Based on the cross-correlation of the Doppler maps, EK Dra has a solar-type differential rotation with a surface shear parameter of $α_{DR} = 0.030 \pm 0.008$.

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arXiv2025

Four decades of photometry of XX Trianguli, `the most spotted star' in the sky

Over the past 40 years the brightness variations of XX Tri, a single line RS CVn type binary system with a synchronized K-giant primary, has exceeded one magnitude in the V band. Although these changes are primarily caused by starspots, an additional activity-related mechanism may also be behind the long-term trend of overall brightness increase. By compiling the most complete photometric data set so far, we attempt to examine how the nature of seasonal-to-decadal changes can be linked to global magnetism. The long-term brightening of XX Tri was accompanied by a gradual increase in the effective temperature, which resulted in a blueing shift in the Herzsprung-Russell diagram. In the long term, a constant cycle of about 4 years is most strongly present in the entire data. Besides, we also found a modulation of about 11 years, and a slowly decreasing cycle of about 5.7-5.2 years. From the seasonal datasets we found that the most dominant rotation-related periods are scattered around the orbital period. From this we infer a solar-type surface differential rotation, although the surface shear is significantly smaller than that of the Sun. The 4-year cycle indicates flip-flop-like behavior: during this time, the 2-3 active longitudes usually present on the stellar surface are rearranged. The magnitude-range changes in the long term cannot be interpreted solely as changes in the number and size of spots; the unspotted brightness of XX Tri has also increased over the decades. This should alert users of photometric spot models to reconsider the basic concept of constant unspotted brightness in similar cases.

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arXiv2025

Starspots on eclipsing giant stars I.: The sample and eclipse mapping examples

Spotted stars in eclipsing binary systems allow us to gather significant information about the stellar surface inhomogeneities that is otherwise impossible from only photometric data. Starspots can be scanned using the eclipse (or transit) mapping technique, which takes advantage of the passage of a companion star (or planet) in front of a spotted giant star in a binary system. Based on the characteristics of their ultra-precise space photometric light curves, we compile a list of eclipsing binaries whose primary component is a spotted subgiant or giant star, with the aim of applying the eclipse mapping technique to them. Eclipsing binaries with giant primaries were selected from Transiting Exoplanet Survey Satellite (TESS) light curves by visual inspection. Spots showing up as bumps during eclipses are modeled with an eclipse mapping technique specialized for two stars, and the number of spots are found with the help of Bayes factors. The full light curves themselves were analyzed with time series spot modeling, and the results of the two approaches were compared. We present a catalog of 29 eclipsing close binaries with active giant components and analyze TIC 235934420, TIC 271892852 and TIC 326257590 from the Continuous Viewing Zones (CVZ) of TESS. Remarkable agreement is found between the starspot temperatures, sizes, and longitudes from the eclipse mapping results and the corresponding full light curve solutions. Spots are always present at the substellar points of the tidally locked binaries. Data from the TESS CVZ allow us to follow the changes of spot patterns on yearly timescales.

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arXiv2024

Magnetic activity under tidal influences in the 2+2~hierarchical quadruple system V815 Herculis

Tidal forces in close binaries and multiple systems that contain magnetically active component are supposed to influence the operation of magnetic dynamo. Through synchronization the tidal effect of a close companion helps maintain fast rotation, thus supporting an efficient dynamo. At the same time, it can also suppress the differential rotation of the convection zone, or even force the formation of active longitudes at certain phases fixed to the orbit. V815 Her is a four-star system consisting of two close binaries orbiting each other, one of which contains an active G-type main-sequence star. Therefore, the system offers an excellent opportunity to investigate the influence of gravitational effects on solar-type magnetic activity using different methods.

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arXiv2024

Searching for stellar CMEs in the Praesepe and Pleiades clusters

On the Sun, the energetic, erupting phenomena of flares and coronal mass ejections (CMEs) often occur together. While space-based photometry has revealed frequent white-light flares for vast numbers of stars, only a handful of coronal mass ejections have been detected. Space-based photometry reveals the timing and detailed structure of flares. To detect CME signatures, however, optical spectroscopy is essential, as the ejected plasma can be detected by Doppler-shifted emission bumps in the Balmer-regions. We present a dedicated ground-based multi-object spectroscopic observations of the young, nearby Praesepe (600 Myr) and Pleiades (135 Myr) clusters to detect CMEs and flares parallel with the observations of Praesepe by the TESS satellite. During the 10 days of overlapping observations, we did not find any obvious signs of CMEs or flares in the H$α$ region.

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arXiv2023

EI Eridani: a star under the influence -- The effect of magnetic activity in the short and long term

We use our photometric time series of more than forty years to analyze the long-term behaviour of EI Eri. Flare activity is investigated using space-borne photometric data obtained with TESS. The MUSICOS campaign aimed to achieve high-resolution spectroscopic observations from many sites around the globe, so that uninterrupted phase coverage of EI Eri became available. We use these data to reconstruct successive surface temperature maps of the star in order to study the changes of starspots on a very short timescale. We use long-term, seasonal period analysis of our photometric time series to study changes in the rotational period. Short-term Fourier-transform is also applied to look for activity cycle-like changes. We also study the phase and frequency distribution of hand-selected flares. We apply our multi-line Doppler imaging code to reconstruct four consecutive Doppler images. These images are also used to measure surface differential rotation by our cross-correlation technique. In addition, we carry out tests to demonstrate how Doppler imaging is affected by the fact that the data came from several different instruments with different spectral resolutions. Seasonal period analysis of the light curve reveals a smooth, significant change in period, possibly indicating the evolution of active latitudes. Temperature curves from $B-V$ and $V-I$ show slight differences, indicating the activity of EI Eri is spot dominated. Short-term Fourier transform reveals smoothly changing cycles between 4.5--5.5 and 8.9--11.6 years. The time-resolved spotted surface of EI Eri from Doppler imaging enabled us to follow the evolution of the different surface features. Cross-correlating the consecutive Doppler maps reveal surface shear of $α=0.036\pm0.007$. Our tests validate our approach and show that the surface temperature distribution is adequately reconstructed by our method.

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arXiv2021

Extending the FIP bias sample to magnetically active stars. Challenging the FIP bias paradigm?

The different elemental abundances of the photosphere and the corona are striking features of not only the Sun, but other stars as well. This phenomenon is known as the FIP effect (FIP stands for first ionization potential), and its strength can be characterized by the FIP bias, the logarithmic abundance difference between low- and high-FIP elements in the corona, compared to the photosphere. The FIP bias was shown to depend on the surface temperature of the star. We compiled FIP bias and other parameters for 59 stars for which coronal composition is available, now including evolved stars. Using principal component analysis and linear discriminant analysis, we searched for correlations with other astrophysical parameters within the sample which may influence the stellar FIP bias. Adding stars to the $T_{\rm eff}-$FIP bias diagram unveiled new features in its structure. In addition to the previously known relationship, there appears to be a second branch, a parallel sequence about 0.5 dex above it. While the $T_{\rm eff}$ remains the main determinant of the FIP bias, other parameters such as stellar activity indicators also have influence. We find three clusters in the FIP bias determinant parameter space. One distinct group is formed by the evolved stars. Two groups contain main sequence stars in continuation separated roughly by the sign change of the FIP-bias value. The new branch of the $T_{\rm eff}-$FIP bias diagram contains stars with higher activity level, in terms of X-ray flux and rotational velocity. The two main sequence clusters run from the earliest spectral types of A-F with shallow convection zones through G-K-early M stars with gradually deeper convection zones, and end up with the fully convective M dwarf stars, depicting the change of the dynamo type with the internal differences of the main sequence stars in connection with the FIP-bias values.

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arXiv2021

A confined dynamo: magnetic activity of the K-dwarf component in the pre-cataclysmic binary system V471 Tauri

We scrutinize the red dwarf component in the eclipsing binary system V471 Tau in order to unravel relations between different activity layers from the stellar surface through the chromosphere up to the corona. We aim at studying how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. We use space photometry, high resolution spectroscopy and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. From K2 photomery we find that 5-10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimate a weak differential rotation. From the flare activity we derive a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high resolution spectroscopy we reconstruct four Doppler images for different epochs which reveal an active longitude, that is, a permanent dominant spot facing the white dwarf. From short term changes in the consecutive Doppler images we derive a weak solar-type surface differential rotation with 0.0026 shear coefficient, similar to that provided by photometry. The long-term evolution of X-ray luminosity reveals a possible activity cycle length of 12.7 ys, traces of which were discovered also in the H$α$ spectra. We conclude that the magnetic activity of the red dwarf component in V471 Tau is strongly influenced by the close white dwarf companion. We confirm the presence of a permanent dominant spot (active longitude) on the red dwarf facing the white dwarf. The weak differential rotation of the red dwarf is very likely the result of tidal confinement by the companion. We find that the periodic appearance of the inter-binary H$α$ emission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.

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arXiv2021

KIC 2852961 -- a superflaring red monster in the Kepler field

Superflares on giant stars have up to 100,000 times more energy than the high energy solar flares. However, it is disputed, whether scaling up a solar-type dynamo could explain such a magnitude difference. We investigate the flaring activity of KIC 2852961, a late-type spotted giant. We seek for flares in the Kepler Q0-Q17 datasets by an automated technique together with visual inspection. Flare occurence rate and flare energies are analyzed and compared to flare statistics of different targets with similar flare activity at different energy levels. We find that the flare energy distribution of KIC 2852961 does not seem to be consistent with that of superflares on solar-type stars. Also, we believe that in case of KIC 2852961 spot activity should have an important role in producing such superflares.

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arXiv2021

Towards the true number of flaring giant stars in the Kepler field. Are there flaring specialities associated with the giant nature?

We aim to give a reliable estimate of the number of flaring giant stars in the Kepler field. By analyzing the flaring activity of these stars we explore their flare statistics and the released flare energies. The role of oscillation in suppressing magnetic activity is also investigated. On a sample of flaring giant stars we search for flaring specialities which may be associated with the giant nature. We search for flares using the full K1 data on a sample of 706 stars compiled from two lists of flaring giants found in the literature. In the end, we confirm only 61 stars as flaring giants. Among these 61 flaring giants we found only six which also show oscillations; we suggest that a large fraction of the 61 flaring giants are members of spectroscopic binaries which are proven already for 11. Tests are carried out to correct the detection bias at low flare energies for a subsample of 19 further studied, frequently flaring stars. For these 19 stars flare energy distributions and flare frequency diagrams (FFDs) are constructed. The number of detected flares on giant stars correlate only weakly with the rotational periods. The 61 confirmed flaring giant stars make up only about 0.3% of the entire giant star population in the Kepler database, in contrast with previous estimates of about an order higher percentage. No strong correlation was found between the stellar properties and the flaring characteristics. The wide scale of the flaring specialities are hardly related to the giant nature, if at all. This, together with the finding that the observed flare durations correlate with flare energies, regardless of the flare energy level and stellar luminosity class, suggest common background physics in flaring stars, or in other words, a general scaling effect behind.

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arXiv2020

Superflares on the late-type giant KIC 2852961 -- Scaling effect behind flaring at different energy levels

The most powerful superflares reaching 10$^{39}$erg bolometric energy are from giant stars. The mechanism behind flaring is supposed to be the magnetic reconnection, which is closely related to magnetic activity including starspots. However, it is poorly understood, how the underlying magnetic dynamo works and how the flare activity is related to the stellar properties which eventually control the dynamo action. We analyse the flaring activity of KIC 2852961, a late-type giant star, in order to understand how the flare statistics are related to that of other stars with flares and superflares and what the role of the observed stellar properties in generating flares is. We search for flares in the full Kepler dataset of the star by an automated technique together with visual inspection. We set a final list of 59 verified flares during the observing term. We calculate flare energies for the sample and perform a statistical analysis. The stellar properties of KIC 2852961 are revised and a more consistent set of parameters are proposed. The cumulative flare energy distribution can be characterized by a broken power-law, i.e. on the log-log representation the distribution function is fitted by two linear functions with different slopes, depending on the energy range fitted. We find that the total flare energy integrated over a few rotation periods correlates with the average amplitude of the rotational modulation due to starspots. Flares and superflares seem to be the result of the same physical mechanism at different energetic levels, also implying that late-type stars in the main sequence and flaring giant stars have the same underlying physical process for emitting flares. There might be a scaling effect behind generating flares and superflares in the sense that the higher the magnetic activity the higher the overall magnetic energy released by flares and/or superflares.

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arXiv2019

Magnetic activity of the young solar analogue V1358 Ori

Young, fast rotating single stars can show dramatically different magnetic signatures and levels of magnetic activity as compared with the Sun. While losing angular momentum due to magnetic breaking and mass loss through stellar winds, the stars gradually spin down resulting in decreasing levels of activity. Studying magnetic activity on such solar analogues plays a key role in understanding the evolution of solar-like stars and allows a glimpse into the past of the Sun as well. In order to widen our knowledge of the magnetic evolution of the Sun and solar-like stars, magnetic activity of the young solar analogue V1358 Ori is investigated. Fourier analysis of long-term photometric data is used to derive rotational period and activity cycle length, while spectral synthesis is applied on high resolution spectroscopic data in order to derive precise astrophysical parameters. Doppler imaging is performed to recover surface temperature maps for two subsequent intervals. Cross-correlation of the consecutive Doppler maps is used to derive surface differential rotation. The rotational modulation of the chromospheric activity indicators is also investigated. An activity cycle of $\approx 1600$ days is detected for V1358 Ori. Doppler imaging revealed a surface temperature distribution dominated by a large polar cap with a few weaker features around the equator. This spot configuration is similar to other maps of young solar analogues from the literature, and supports recent model predictions. We detected solar-like surface differential rotation with a surface shear parameter of $α= 0.016 \pm 0.010$ which fits pretty well to our recently proposed empirical relation between rotation and differential rotation. The chromospheric activity indicators showed a rotational modulation.

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arXiv2019

Complex Rotational Modulation of Rapidly Rotating M-Stars Observed with TESS

We have searched for short periodicities in the light curves of stars with $T_{\rm eff}$ cooler than 4000 K made from 2-minute cadence data obtained in TESS sectors 1 and 2. Herein we report the discovery of 10 rapidly rotating M-dwarfs with highly structured rotational modulation patterns among 10 M dwarfs found to have rotation periods less than 1 day. Star-spot models cannot explain the highly structured periodic variations which typically exhibit between 10 and 40 Fourier harmonics. A similar set of objects was previously reported following K2 observations of the Upper Scorpius association (Stauffer et al. 2017). We examine the possibility that the unusual structured light-curves could stem from absorption by charged dust particles that are trapped in or near the stellar magnetosphere. We also briefly explore the possibilities that the sharp structured features in the lightcurves are produced by extinction by coronal gas, by beaming of the radiation emitted from the stellar surface, or by occultations of spots by a dusty ring that surrounds the star. The latter is perhaps the most promising of these scenarios. Most of the structured rotators display flaring activity, and we investigate changes in the modulation pattern following the largest flares. As part of this study, we also report the discovery of 371 rapidly rotating M-dwarfs with rotational periods below 4 hr, of which the shortest period is 1.63 hr.

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arXiv2019

Surface magnetic activity of the fast-rotating G5-giant IN Comae, central star of the faint planetary nebula LoTr 5

On the AGB, low to intermediate mass stars blow away their outer envelopes, forming planetary nebulae. Interaction between the planetary nebula and its central progenitor is poorly understood and even more complex when the central object is a binary star with a magnetically active component, like it is the case for IN Com. We aim to quantify the surface activity of the cool binary component of IN Com and aim to explain its origin. We need a better understanding of how central binary stars in planetary nebulae evolve and how this evolution could develop such magnetically active stars like IN Com. We present 13 Doppler images covering six months in 2017 and use them to measure the surface differential rotation. Hitherto unpublished photometric data from between 1989-2017 are presented. We apply Fourier-transformation to both photometry and spectra. Very high resolution spectra are used to update IN Com's astrophysical parameters by means of spectral synthesis. Doppler images show cool and warm spots coexisting with an average surface temperature contrast of -1000K and +300K with respect to the effective temperature. Approximately 8% of the surface is covered with cool spots and 3% with warm spots. A cool polar spot is seen in all images. We found anti-solar surface differential rotation with a shear coefficient of -0.026+/-0.005 and an equatorial rotation period of 5.973+/-0.008 d. We reconfirm the 5.9-day rotation period of the cool star from photometry, radial velocities, and H alpha line-profile variations. A long-term V-brightness variation with a likely period of 7.2yr is found. It appears in phase with the orbital radial velocity of the binary system in the sense brightest at highest velocity and faintest at lowest velocity. We redetermine [Ba/Fe], [Y/Fe] and [Sr/Fe] ratios and confirm the overabundance of these s-process elements in the atmosphere of IN Com.

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arXiv2018

Eclipsing Spotted Giant Star With K2 and Historical Photometry

Context. Stars can maintain their observable magnetic activity from the PMS to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence since on the giant branch the stars spend only about 10% of their main sequence lifetime. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters of an active giant arises when it is a member of an eclipsing binary system. Aims. We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars. Method. We have combined the K2 data with archival HATNet and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRI photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs. Results. We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the HR diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses. Conclusions. We further evaluate the proposition that active giants have masses that are found to be generally higher by traditional methods than are indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars.

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arXiv2018

Cycle period, differential rotation and meridional flow for early M dwarf stars

Recent observations suggest the existence of two characteristic cycle times for early-type M stars dependent on the rotation period. They are of order one year for the fast rotators ($P_{\rm rot}<1$ day) and of order 4 years for the slower rotators. Additionally, the equator-to-pole differences of the rotation rates with $δΩ$ up to 0.03 rad d$^{-1}$ are known from Kepler data for the fast-rotating stars. These values are well-reproduced by the theory of large-scale flows in rotating convection zones on the basis of the $Λ$ effect. The resulting amplitudes $u^{\rm m}$ of the bottom value of the meridional circulation allows the calculation of the travel time from pole to equator at the base of the convection zone of early-type M stars. These travel times strongly increase with rotation period and they always exceed the observed cycle periods. Therefore, the operation of an advection-dominated dynamo in early M dwarfs, where the travel time must always be shorter than the cycle period, is not confirmed by our model nor the data.

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arXiv2017

Rotation--differential rotation relationships for late-type single and binary stars from Doppler imaging

From our sample of spotted late-type stars showing surface differential rotation we find that the relationship between the rotation period and the surface shear coefficient $α=ΔΩ/Ω_{\rm eq}$ is significantly different for single stars compared to members in close binaries. Single stars follow a general trend that $α$ increases with the rotation period. However, differential rotation of stars in close binary systems shows much weaker dependence on the rotation, if any, suggesting that in such systems tidal forces operate as a controlling mechanism of differential rotation.

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