K. Vida

Leibniz Institute for Astrophysics Potsdam, Potsdam, Brandenburg, Germany

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Publications (77)147.56 Total impact

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    ABSTRACT: This paper describes an alternative approach for generating pointing models for telescopes equipped with serial kinematics, esp. equatorial or alt-az mounts. Our model construction does not exploit any assumption for the underlying physical constraints of the mount, however, one can assign various effects to the respective components of the equations. In order to recover the pointing model parameters, classical linear least squares fitting procedures can be applied. This parameterization also lacks any kind of parametric singularity. We demonstrate the efficiency of this type of model on real measurements with meter-class telescopes where the results provide a root mean square accuracy of 1.5-2 arcseconds.
    Experimental Astronomy 07/2015; DOI:10.1007/s10686-015-9472-z · 2.66 Impact Factor
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    ABSTRACT: We study the connection between the chromospheric and photospheric behaviour of the active late-type star FK Comae. We use spot temperature modelling, light curve inversion based on narrow- and wide-band photometric measurements, Halpha observations from 1997-2010, and Doppler maps from 2004-2010 to compare the behaviour of chromospheric and photospheric features. Investigating low-resolution Halpha spectra we find that the changes in the chromosphere seem to happen mainly on a time scale longer than a few hours, but shorter variations were also observed. According to the Halpha measurements prominences are often found in the chromosphere that reach to more than a stellar radius and are stable for weeks, and which seem to be often, but not every time connected with dark photospheric spots. The rotational modulation of the Halpha emission seems to typically be anticorrelated with the light curve, but we did not find convincing evidence of a clear connection in the long-term trends of the Halpha emission and the brightness of the star. In addition, FK Com seems to be in an unusually quiet state in 2009-2010 with very little chromospheric activity and low spot contrast, that might indicate the long-term decrease of activity.
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    ABSTRACT: We report the discovery of a new totally-eclipsing binary (RA=06:40:29.11; Dec=+38:56:52.2; J=2000.0; Rmax=17.2 mag) with an sdO primary and a strongly irradiated red dwarf companion. It has an orbital period of Porb=0.187284394(11) d and an optical eclipse depth in excess of 5 magnitudes. We obtained two low-resolution classification spectra with GTC/OSIRIS and ten medium-resolution spectra with WHT/ISIS to constrain the properties of the binary members. The spectra are dominated by H Balmer and He II absorption lines from the sdO star, and phase-dependent emission lines from the irradiated companion. A combined spectroscopic and light curve analysis implies a hot subdwarf temperature of Teff(spec) = 55 000 +/- 3000K, surface gravity of log g(phot) = 6.2 +/- 0.04 (cgs) and a He abundance of log(nHe/nH) = -2.24 +/- 0.40. The hot sdO star irradiates the red-dwarf companion, heating its substellar point to about 22 500K. Surface parameters for the companion are difficult to constrain from the currently available data: the most remarkable features are the strong H Balmer and C II-III lines in emission. Radial velocity estimates are consistent with the sdO+dM classification. The photometric data do not show any indication of sdO pulsations with amplitudes greater than 7mmag, and Halpha-filter images do not provide evidence of the presence of a planetary nebula associated with the sdO star.
    The Astrophysical Journal 05/2015; 808(2). DOI:10.1088/0004-637X/808/2/179 · 6.28 Impact Factor
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    ABSTRACT: The overtone and multi-mode RR Lyrae stars in the globular cluster M3 are studied using a 200-d long, $B,V$ and $I_{\mathrm C}$ time-series photometry obtained in 2012. 70\% of the 52 overtone variables observed show some kind of multi-periodicity (additional frequency at ${f_{0.61}}={f_{\mathrm {1O}}}/0.61$ frequency ratio, Blazhko effect, double/multi-mode pulsation, period doubling). A signal at 0.587 frequency ratio to the fundamental-mode frequency is detected in the double-mode star, V13, which may be identified as the second radial overtone mode. If this mode-identification is correct, than V13 is the first RR Lyrae star showing triple-mode pulsation of the first three radial modes. Either the Blazhko effect or the ${f_{0.61}}$ frequency (or both of these phenomena) appear in 7 double-mode stars. The $P_{\mathrm{1O}}/P_{\mathrm{F}}$ period ratio of RRd stars showing the Blazhko effect are anomalous. A displacement of the main frequency component at the fundamental-mode with the value of modulation frequency (or its half) is detected in three Blazhko RRd stars parallel with the appearance of the overtone-mode pulsation. The ${f_{0.61}}$ frequency appears in RRc stars that lie at the blue side of the double-mode region and in RRd stars, raising the suspicion that its occurrence may be connected to double-mode pulsation. The changes of the Blazhko and double-mode properties of the stars are also reviewed using the recent and archive photometric data.
    The Astrophysical Journal Supplement Series 04/2015; 219(2). DOI:10.1088/0067-0049/219/2/25 · 14.14 Impact Factor
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    ABSTRACT: We introduce the Fly's Eye Camera System, an all-sky monitoring device intended to perform time domain astronomy. This camera system design will provide complementary data sets for other synoptic sky surveys such as LSST or Pan-STARRS. The effective field of view is obtained by 19 cameras arranged in a spherical mosaic form. These individual cameras of the device stand on a hexapod mount that is fully capable of achieving sidereal tracking for the subsequent exposures. This platform has many advantages. First of all it requires only one type of moving component and does not include unique parts. Hence this design not only eliminates problems implied by unique elements, but the redundancy of the hexapod allows smooth operations even if one or two of the legs are stuck. In addition, it can calibrate itself by observed stars independently from both the geographical location (including northen and southern hemisphere) and the polar alignment of the full mount. All mechanical elements and electronics are designed within the confines of our institute Konkoly Observatory. Currently, our instrument is in testing phase with an operating hexapod and reduced number of cameras.
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    ABSTRACT: Context. Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifestations of the stellar dynamo working underneath. Therefore, such observations are important for stellar dynamo theory and useful constraints for solar dynamo studies as well. Aims. The active K1-giant component of the long-period RS CVn-type binary system sigma Gem and its global surface flow pattern is revisited. Methods. We refine the differential rotation law from recovering the spot migration pattern. We apply a detailed cross-correlation technique to a unique set of 34 time-series Doppler images recovered using data from 1996/97. By increasing the number of the available cross-correlation function maps from the formerly used 4 to 17 we expect a more robust determination of the differential surface rotation law. In addition, we present a new time-series Doppler imaging study of sigma Gem using our advanced surface reconstruction code iMap for a dataset collected in 2006/07. Results. Results from the reprocessed cross-correlation study confirm that the star performs antisolar-type differential rotation with a surface shear alpha of -0.04 +/- 0.01, i.e., almost a factor of two stronger compared to the previously claimed value. We also confirm the evidence of a global poleward spot migration with an average velocity of 0.21 +/- 0.03 km/s, in accordance with theoretical predictions. From the new observations we obtain three subsequent Doppler images. The time evolution of these images confirms the antisolar-type differential rotation of the same amount.
    Astronomy and Astrophysics 11/2014; 573. DOI:10.1051/0004-6361/201424138 · 4.48 Impact Factor
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    ABSTRACT: We study three representative, overactive spotted K giants (IL Hya, XX Tri, and DM UMa) known to exhibit V-band light variations between 0.65-1.05 mags. Our aim is to find the origin of their large brightness variation. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42 yr for IL Hya, 28 yr for XX Tri, and 34 yr for DM UMa. For one target, IL Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V-I_C color-index calibration. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in IL Hya and up to 54% in XX Tri over the course of our observations. We emphasize that for IL Hya this is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX Tri it is even about one third. The long-term, 0.6 mag V-band variation of DM UMa is more difficult to explain because little or no B-V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL Hya, a radius change of about 9% is suggested from m_bol and T_eff, and is supported by independent vsin(i) measurements.
    Astronomy and Astrophysics 09/2014; 572. DOI:10.1051/0004-6361/201424695 · 4.48 Impact Factor
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    ABSTRACT: We investigate the surface spot activity of the rapidly rotating, lithium-rich active single K-giant DI Psc to measure the surface differential rotation and understand the mechanisms behind the Li-enrichment. Doppler imaging was applied to recover the surface temperature distribution of DI Psc in two subsequent rotational cycles using the individual mapping lines Ca I 6439, Fe I 6430, Fe I 6421 and Li I 6708. Surface differential rotation was derived by cross-correlation of the subsequent maps. Difference maps are produced to study the uniformity of Li-enrichment on the surface. These maps are compared with the rotational modulation of the Li I 6708 line equivalent width. Doppler images obtained for the Ca and Fe mapping lines agree well and reveal strong polar spottedness, as well as cool features at lower latitudes. Cross-correlating the consecutive maps yields antisolar differential rotation with shear coefficient -0.083 +- 0.021. The difference of the average and the Li maps indicates that the lithium abundance is non-activity related. There is also a significant rotational modulation of the Li equivalent width.
    Astronomy and Astrophysics 08/2014; DOI:10.1051/0004-6361/201424263 · 4.48 Impact Factor
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    ABSTRACT: The Fly's Eye Project is a high resolution, high coverage time-domain survey in multiple optical passbands: our goal is to cover the entire visible sky above the 30° horizontal altitude with a cadence of ~3 min. Imaging is going to be performed by 19 wide-field cameras mounted on a hexapod platform resembling a fly’s eye. Using a hexapod developed and built by our team allows us to create a highly fault-tolerant instrument that uses the sky as a reference to define its own tracking motion. The virtual axis of the platform is automatically aligned with the Earth’s rotational axis; therefore the same mechanics can be used independently from the geographical location of the device. Its enclosure makes it capable of autonomous observing and withstanding harsh environmental conditions. We briefly introduce the electrical, mechanical and optical design concepts of the instrument and summarize our early results, focusing on sidereal tracking. Due to the hexapod design and hence the construction is independent from the actual location, it is considerably easier to build, install and operate a network of such devices around the world.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
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    ABSTRACT: We have searched the Kepler light curves of ~3900 M-star targets for evidence of periodicities that indicate, by means of the effects of starspots, rapid stellar rotation. Several analysis techniques, including Fourier transforms, inspection of folded light curves, 'sonograms', and phase tracking of individual modulation cycles, were applied in order to distinguish the periodicities due to rapid rotation from those due to stellar pulsations, eclipsing binaries, or transiting planets. We find 178 Kepler M-star targets with rotation periods, P_rot, of < 2 days, and 110 with P_rot < 1 day. Some 30 of the 178 systems exhibit two or more independent short periods within the same Kepler photometric aperture, while several have three or more short periods. Adaptive optics imaging and modeling of the Kepler pixel response function for a subset of our sample support the conclusion that the targets with multiple periods are highly likely to be relatively young physical binary, triple, and even quadruple M star systems. We explore in detail the one object with four incommensurate periods all less than 1.2 days, and show that two of the periods arise from one of a close pair of stars, while the other two arise from the second star, which itself is probably a visual binary. If most of these M-star systems with multiple periods turn out to be bound M stars, this could prove a valuable way of discovering young hierarchical M-star systems; the same approach may also be applicable to G and K stars. The ~5% occurrence rate of rapid rotation among the ~3900 M star targets is consistent with spin evolution models that include an initial contraction phase followed by magnetic braking, wherein a typical M star can spend several hundred Myr before spinning down to periods longer than 2 days.
    The Astrophysical Journal 05/2014; 788(2). DOI:10.1088/0004-637X/788/2/114 · 6.28 Impact Factor
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    K. Vida · K. Oláh · R. Szabó
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    ABSTRACT: We analyse light curves covering four years of 39 fast-rotating ($P_\mathrm{rot}< 1d$) late-type active stars from the Kepler database. Using time-frequency analysis (Short-Term Fourier-Transform), we find hints for activity cycles of 300-900 days at 9 targets from the changing typical latitude of the starspots, which, with the differential rotation of the stellar surface change the observed rotation period over the activity cycle. We also give a lowest estimation for the shear parameter of the differential rotation, which is ~0.001 for the cycling targets. These results populate the less studied, short period end of the rotation-cycle length relation.
    Monthly Notices of the Royal Astronomical Society 04/2014; 441(3). DOI:10.1093/mnras/stu760 · 5.23 Impact Factor
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    ABSTRACT: The driving objective of the Fly's Eye Project is a high resolution, high coverage time-domain survey in multiple optical passbands: our goal is to cover the entire visible sky above the 30 deg horizontal altitude with a cadence of 3 min. Imaging is intended to perform with 19 wide-field cameras mounted on a hexapod platform. The essence of the hexapod allows us to build an instrument that does not require any kind of precise alignment and, in addition, the similar mechanics can be involved independently from the geographical location of the device. Here we summarize our early results with a single camera, focusing on the sidereal tracking as it is performed with the hexapod built by our group.
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    ABSTRACT: In this paper we briefly summarize the design concepts of the Fly's Eye Camera System, a proposed high resolution all-sky monitoring device which intends to perform high cadence time domain astronomy in multiple optical passbands while still accomplish a high \'etendue. Fundings have already been accepted by the Hungarian Academy of Sciences in order to design and build a Fly's Eye device unit. Beyond the technical details and the actual scientific goals, this paper also discusses the possibilities and yields of a network operation involving ~10 sites distributed geographically in a nearly homogeneous manner. Currently, we expect to finalize the mount assembly -- that performs the sidereal tracking during the exposures -- until the end of 2012 and to have a working prototype with a reduced number of individual cameras sometimes in the spring or summer of 2013.
    11/2013; 334(9). DOI:10.1002/asna.201211962
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    ABSTRACT: We introduce an iterative spectral disentangling technique combined with Doppler imaging in order to recover surface temperature maps for both components of double-lined active binary systems. Our method provides an opportunity to separate spectra of the active components while minimizing the unwanted disturbances on the given line profile from the other component. The efficiency of the method is demonstrated on real data of the double-lined RS CVn-type binary V824 Ara. The resulting Doppler images reveal cool spots on the polar regions as well as low-latitude features on both of the stars. Moreover, both components have hot spots, that are facing each other. This may indicate interconnection between the stellar magnetic fields.
    Astronomische Nachrichten 11/2013; 334(9). DOI:10.1002/asna.201211974 · 1.12 Impact Factor
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    K. Vida · L. Kriskovics · K. Oláh
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    ABSTRACT: Long-term photometric measurements in a sample of ultrashort-period (P ≈ 0.5 days or less) single and binary stars of different interior structures are analysed. A loose correlation exists between the rotational rate and cycle lengths of active stars, regardless of their evolutionary state and the corresponding physical parameters. The shortest cycles are expected for the fastest rotators of the order of 1–2 years, which is reported in this paper. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 11/2013; 334(9). DOI:10.1002/asna.201211973 · 1.12 Impact Factor
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    ABSTRACT: We present a new Doppler imaging study for the Li-rich single K-giant DI Psc. Surface temperature maps are reconstructed for two subsequent rotation cycles. From the time evolution of the spot distribution antisolar-type differential rotation pattern is revealed. We show marks of non-uniform Li-abundance as well. The possible connection between the current evolutionary phase of the star and its magnetic activity is briefly discussed.
    Proceedings of the International Astronomical Union 10/2013; 9(S302). DOI:10.1017/S1743921314002567
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    ABSTRACT: We present a time-series Doppler imaging study of the K-subgiant component in the RS CVn-type binary system IL Hya (P orb=12.905 d). From re-processing the unique long-term spectroscopic dataset of 70 days taken in 1996/97, we perform a thorough cross-correlation analysis to derive surface differential rotation. As a result we get solar-type differential rotation with a shear value alpha of 0.05, in agreement with preliminary suggestions from previous attempts. A possible surface pattern of meridional circulation is also detected.
    Proceedings of the International Astronomical Union 09/2013; 9(S302). DOI:10.1017/S1743921314002543
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    ABSTRACT: As a response to the Kepler white paper call, we propose to keep Kepler pointing to its current field of view and continue observing thousands of large amplitude variables (Cepheid, RR Lyrae and delta Scuti stars among others) with high cadence in the Kep-Cont Mission. The degraded pointing stability will still allow observation of these stars with reasonable (better than millimag) precision. The Kep-Cont mission will allow studying the nonradial modes in Blazhko-modulated and first overtone RR Lyrae stars and will give a better view on the period jitter of the only Kepler Cepheid in the field. With continued continuous observation of the Kepler RR Lyrae sample we may get closer to the origin of the century-old Blazhko problem. Longer time-span may also uncover new dynamical effects like apsidal motion in eclipsing binaries. A continued mission will have the advantage of providing unprecedented, many-years-long homogeneous and continuous photometric data of the same targets. We investigate the pragmatic details of such a mission and find a number of advantages, especially the minimal need of reprogramming of the flight software. Another undeniable advantage of the current field of view is the completed, ongoing and planned ground-based follow-up observations and allocated telescope times focusing on the current field. We emphasize that while we propose this continuation as an independent mission, we can easily share slots with e.g. planetary mission with a strong belief that both (or more) communities can still benefit from Kepler's current capabilities.
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    ABSTRACT: Cross-correlation of consecutive Doppler images is one of the most common techniques used to detect surface differential rotation (hereafter DR) on spotted stars. The disadvantage of a single cross-correlation is, however, that the expected DR pattern can be overwhelmed by sudden changes in the apparent spot configuration. Another way to reconstruct the image shear using Doppler imaging is to include a predefined latitude-dependent rotation law in the inversion code (`sheared image method'). However, special but not unusual spot distributions, such like a large polar cap or an equatorial belt (e.g., small random spots evenly distributed along the equator), can distort the rotation profile similarly as the DR does, consequently, yielding incorrect measure of the DR from the sheared image method. To avoid these problems, the technique of measuring DR from averaged cross-correlations using time-series Doppler images (`ACCORD') is introduced and the reliability of this tool is demonstrated on artificial data.
    Proceedings of the International Astronomical Union 09/2013; 9(S302). DOI:10.1017/S1743921314002063
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    Krisztián Vida · Katalin Oláh
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    ABSTRACT: Using data of fast-rotating active dwarf stars in the Kepler database, we perform time-frequency analysis of the light curves in order to search for signs of activity cycles. We use the phenomenon that the active region latitudes vary with the cycle (like the solar butterfly diagram), which causes the observed rotation period to change as a consequence of differential rotation. We find cycles in 8 cases of the 39 promising targets with periods between of 300-900 days.
    Proceedings of the International Astronomical Union 08/2013; 9(S302). DOI:10.1017/S1743921314002142

Publication Stats

390 Citations
147.56 Total Impact Points

Institutions

  • 2014
    • Leibniz Institute for Astrophysics Potsdam
      Potsdam, Brandenburg, Germany
  • 2009–2014
    • Hungarian Academy of Sciences
      Budapeŝto, Budapest, Hungary
  • 2007–2014
    • Konkoly Observatory
      Budapeŝto, Budapest, Hungary
  • 2006–2009
    • Eötvös Loránd University
      • Department of Astronomy
      Budapest, Budapest fovaros, Hungary
  • 2004
    • University of Szeged
      • Department of Experimental Physics
      Algyő, Csongrád, Hungary