@article{MTMT:34053151,
	title = {EI Eridani: A star under the influence. The effect of magnetic activity in the short and long term},
	url = {https://m2.mtmt.hu/api/publication/34053151},
	author = {Kriskovics, Levente and Kővári, Zsolt and Seli, Bálint Attila and Oláh, Katalin Ilona and Vida, Krisztián and Henry, G. W. and Granzer, T. and Görgei, A.},
	doi = {10.1051/0004-6361/202245767},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {674},
	unique-id = {34053151},
	issn = {0004-6361},
	abstract = {Context. Homogeneous photometric time series spanning decades provide a unique opportunity to study the long-term cyclic behavior of active spotted stars such as our target EI Eridani. In addition, with ultraprecise space photometry data, it is possible to investigate the accompanying flare activity in detail. However, the rotation period of ≈2 days for EI Eri makes it impossible to achieve time-resolved surface images from a single ground-based observing site. Therefore, for this purpose, spectroscopic data from a multi-site observing campaign are needed.},
	year = {2023},
	eissn = {1432-0746},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X; Seli, Bálint Attila/0000-0002-3658-2175; Görgei, A./0009-0002-1289-7946}
}

@article{MTMT:33538419,
	title = {Characteristics of flares on giant stars},
	url = {https://m2.mtmt.hu/api/publication/33538419},
	author = {Oláh, Katalin Ilona and Seli, Bálint Attila and Kővári, Zsolt and Kriskovics, Levente and Vida, Krisztián},
	doi = {10.1051/0004-6361/202243789},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {668},
	unique-id = {33538419},
	issn = {0004-6361},
	abstract = {Context. Although late-type dwarfs and giant stars are substantially different, their flares are thought to originate in similar physical processes and differ only by a scale factor in the energy levels. We study the validity of this approach.},
	year = {2022},
	eissn = {1432-0746},
	orcid-numbers = {Seli, Bálint Attila/0000-0002-3658-2175; Kővári, Zsolt/0000-0001-5160-307X}
}

@CONFERENCE{MTMT:33087071,
	title = {Stellar FIP effect from the empirical side},
	url = {https://m2.mtmt.hu/api/publication/33087071},
	author = {Seli, Bálint Attila and Van Driel Gesztelyi, Lídia and Baker, Deborah and Laming, J. Martin and Kővári, Zsolt and Oláh, Katalin Ilona and Kriskovics, Levente and Vida, Krisztián and Balázs, Lajos},
	booktitle = {44th COSPAR Scientific Assembly. Held 16-24 July},
	unique-id = {33087071},
	abstract = {The difference between the elemental abundances of the corona and the photosphere is an apparently common feature of stellar atmospheres. The abundance difference depends on the first ionization potential of the given element, so the phenomenon is known as the FIP effect. Here we explore the variation of the strength of the FIP effect for different types of stars, through the FIP bias parameter. Using a sample of 59 main sequence and evolved stars with known coronal abundances from the literature, we look for macroscopic, measurable parameters that affect the stellar FIP bias, and also re-evaluate the simple dependence on the effective temperature.},
	year = {2022},
	orcid-numbers = {Seli, Bálint Attila/0000-0002-3658-2175; Kővári, Zsolt/0000-0001-5160-307X}
}

@article{MTMT:32726535,
	title = {Extending the FIP bias sample to magnetically active stars},
	url = {https://m2.mtmt.hu/api/publication/32726535},
	author = {Seli, Bálint Attila and Oláh, Katalin Ilona and Kriskovics, Levente and Kővári, Zsolt and Vida, Krisztián and Balázs, Lajos and Laming, J. M. and Van Driel Gesztelyi, Lídia and Baker, D.},
	doi = {10.1051/0004-6361/202141493},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {659},
	unique-id = {32726535},
	issn = {0004-6361},
	abstract = {Context. The different elemental abundances of the photosphere and the corona are striking features of not only the Sun, but of other stars as well. This phenomenon is known as the first ionisation potential (FIP) effect, 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. Aims: We aim to extend the Teff−FIP bias relationship to a larger stellar sample and analyse the effect of other astrophysical parameters on the relation (e.g. surface gravity, age, activity indicators). Methods: 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 that may influence the stellar FIP bias. Results: Adding stars to the Teff−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 Teff 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. Conclusions: The new branch of the Teff−FIP bias diagram contains stars with higher activity level, in terms of X-ray flux and rotational velocity. The Rossby number also seems to be important, indicating possible dependence on the type of dynamo operating in these stars influencing their FIP bias. 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 they end 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.},
	year = {2022},
	eissn = {1432-0746},
	orcid-numbers = {Seli, Bálint Attila/0000-0002-3658-2175; Kővári, Zsolt/0000-0001-5160-307X}
}

@article{MTMT:32106991,
	title = {A confined dynamo: Magnetic activity of the K-dwarf component in the pre-cataclysmic binary system V471 Tauri},
	url = {https://m2.mtmt.hu/api/publication/32106991},
	author = {Kővári, Zsolt and Kriskovics, Levente and Oláh, Katalin Ilona and Odert, P. and Leitzinger, M. and Seli, Bálint Attila and Vida, Krisztián and Borkovits, Tamás and Carroll, T.},
	doi = {10.1051/0004-6361/202140707},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {650},
	unique-id = {32106991},
	issn = {0004-6361},
	abstract = {Context. Late-type stars in close binary systems can exhibit strong magnetic activity owing to rapid rotation supported by tidal locking. On the other hand, tidal coupling may suppress the differential rotation which is a key ingredient of the magnetic dynamo. Aims: We studied the red dwarf component in the eclipsing binary system V471 Tau in order to unravel the relations between the different activity layers, from the stellar surface through the chromosphere up to the corona. Our aim is to study how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. Methods: We used space photometry, high-resolution spectroscopy, and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. We applied a light curve synthesis program to extract the eclipsing binary model and to analyze the residual light variations. Photometric periods were obtained using a Fourier-based period search code. We searched for flares by applying an automated flare detection code. Spectral synthesis was used to derive or specify some of the astrophysical parameters. Doppler imaging was used to reconstruct surface temperature maps, which were cross-correlated to derive surface differential rotation. We applied different conversion techniques to make it possible to compare the X-ray emissions obtained from different space instruments. Results: From the K2 photometry we found that 5-10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimated a weak differential rotation. From the flare activity we derived a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high-resolution spectroscopy we reconstructed 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 derived a weak solar-type surface differential rotation with αDR = 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 yr, traces of which were also discovered in the Hα spectra. Conclusions: 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 Hα emission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.},
	keywords = {Astrophysics - Solar and Stellar Astrophysics; starspots; binaries: eclipsing; Stars: late-type; Stars: activity; stars: flare; stars: individual: V471 Tau},
	year = {2021},
	eissn = {1432-0746},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X; Seli, Bálint Attila/0000-0002-3658-2175; Borkovits, Tamás/0000-0002-8806-496X}
}

@CONFERENCE{MTMT:32079892,
	title = {KIC 2852961 – a superflaring red monster in the Kepler field},
	url = {https://m2.mtmt.hu/api/publication/32079892},
	author = {Kővári, Zsolt and Oláh, Katalin Ilona and Günther, M. N. and Vida, Krisztián and Kriskovics, Levente and Seli, Bálint Attila},
	booktitle = {The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS20.5)},
	doi = {10.5281/zenodo.4555348},
	unique-id = {32079892},
	abstract = {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.},
	keywords = {Astrophysics - Solar and Stellar Astrophysics; LATE-TYPE STARS; MAGNETIC ACTIVITY; FLARES; stars: individual: KIC 2852961; superflares},
	year = {2021},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X; Seli, Bálint Attila/0000-0002-3658-2175}
}

@CONFERENCE{MTMT:32079888,
	title = {Upper limits on the CME frequency of solar-like stars},
	url = {https://m2.mtmt.hu/api/publication/32079888},
	author = {Leitzinger, Martin and Odert, Petra and Greimel, Robert and Vida, Krisztián and Kriskovics, Levente and Guenther, Eike W. and Korhonen, Heidi and Koller, Florian and Kővári, Zsolt and Hanslmeier, Arnold and Lammer, Helmut},
	booktitle = {The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS20.5)},
	doi = {10.5281/zenodo.4562940},
	unique-id = {32079888},
	abstract = {In the last years efforts have been made to determine parameters of stellar coronal mass ejections (CMEs), on the one hand via acquiring dedicated observing time at telescopes and on the other hand via searching data archives. Here we present a search for CMEs on solar-like stars using optical spectroscopic data from the Polarbase and ESO HARPS Phase 3 archives. For detecting stellar CMEs we use the signature of filaments/prominences being ejected from a star, which is Doppler-shifted emission/absorption occurring on the blue side of Balmer lines, as filaments/prominences are very pronounced in Balmer lines. Using more than 3700 hours of on-source time of 425 stars we aim for a statistical determination of CME parameters, such as projected velocity, occurrence frequency, and mass. The target stars are nearby objects and consist of F-K main-sequence stars of various ages. We find no signature of CME activity and a very low level of flaring activity (10 out of 425 stars). Comparing this to results from the Kepler mission, the fraction of flaring stars is more or less consistent. Comparing extrapolated Hα flare rates to the sparse detection of flares reveals that we could have detected more flares. We therefore determined the full-disk Hα signal of one of the strongest solar flares in the last solar cycles. This showed that we would have needed data with higher S/N to detect such a flare in our data. Finally, we compared the observed upper limits of CME rates of our target stars to modelled CME rates. The modelled CME rates are mostly below the observationally determined upper limits, indicating that most on-source times per star were too short to detect stellar CMEs with this method. The sparse detection of flares and the non-detection of CMEs may be explained by biases naturally introduced by using archival data, as well as a a low level of activity of the target stars. We conclude with a short report on ongoing and future activities of the search for stellar CMEs.},
	keywords = {Cool stars on the main sequence},
	year = {2021},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X}
}

@article{MTMT:31922644,
	title = {Toward the true number of flaring giant stars in the Kepler field: Are their flaring specialities associated with their being giant stars?},
	url = {https://m2.mtmt.hu/api/publication/31922644},
	author = {Oláh, Katalin Ilona and Kővári, Zsolt and Günther, M. N. and Vida, Krisztián and Gaulme, P. and Seli, Bálint Attila and Pál, András},
	doi = {10.1051/0004-6361/202039674},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {647},
	unique-id = {31922644},
	issn = {0004-6361},
	year = {2021},
	eissn = {1432-0746},
	orcid-numbers = {Oláh, Katalin Ilona/0000-0001-5160-307X; Kővári, Zsolt/0000-0001-5160-307X; Seli, Bálint Attila/0000-0002-3658-2175}
}

@article{MTMT:31992819,
	title = {Napflerek, csillagflerek, szuperflerek},
	url = {https://m2.mtmt.hu/api/publication/31992819},
	author = {Kővári, Zsolt},
	journal-iso = {METEOR CSILLAGÁSZATI ÉVKÖNYV},
	journal = {METEOR CSILLAGÁSZATI ÉVKÖNYV},
	volume = {2021},
	unique-id = {31992819},
	issn = {0866-2851},
	year = {2020},
	pages = {254-264},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X}
}

@article{MTMT:31602441,
	title = {Superflares on the late-type giant KIC 2852961. Scaling effect behind flaring at different energy levels},
	url = {https://m2.mtmt.hu/api/publication/31602441},
	author = {Kővári, Zsolt and Oláh, Katalin Ilona and Günther, M. N. and Vida, Krisztián and Kriskovics, Levente and Seli, Bálint Attila and Bakos, G. Á. and Hartman, J. D. and Csubry, Z. and Bhatti, W.},
	doi = {10.1051/0004-6361/202038397},
	journal-iso = {ASTRON ASTROPHYS},
	journal = {ASTRONOMY & ASTROPHYSICS},
	volume = {641},
	unique-id = {31602441},
	issn = {0004-6361},
	abstract = {Context. The most powerful superflares reaching 1039 erg bolometric energy are from giant stars. The mechanism behind flaring is thought 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 that eventually control the dynamo action. Aims: We analyze the flaring activity of KIC 2852961, a late-type giant star, in order to understand how its flare statistics are related to those of other stars with flares and superflares, and to understand the role of the observed stellar properties in generating flares. Methods: We searched for flares in the full Kepler dataset of KIC 2852961 using an automated technique together with visual inspection. We cross-matched the flare-like events detected by the two different approaches and set a final list of 59 verified flares during the observing term. We calculated flare energies for the sample and performed a statistical analysis. Results: 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; that is to say, 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. Conclusions: Flares and superflares seem to be the result of the same physical mechanism at different energy 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 the generation of flares and superflares in the sense that the higher the magnetic activity, the higher the overall magnetic energy released by flares and/or superflares.},
	keywords = {Astrophysics - Solar and Stellar Astrophysics; Stars: late-type; Stars: activity; stars: flare; stars: individual: KIC 2852961; stars: individual: TIC 137220334; stars: individual: 2MASS J19261136+3803107},
	year = {2020},
	eissn = {1432-0746},
	orcid-numbers = {Kővári, Zsolt/0000-0001-5160-307X; Seli, Bálint Attila/0000-0002-3658-2175}
}