K. Vida

Konkoly Observatory, Budapeŝto, Budapest, Hungary

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Publications (66)111.74 Total impact

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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). · 6.73 Impact Factor
  • 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.
    04/2014;
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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.
    01/2014;
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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 10/2013; · 1.40 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.
    10/2013;
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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.
    09/2013;
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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.
    09/2013;
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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.
    09/2013;
  • K. Vida, K. 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.
    08/2013;
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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 on the fastest rotators in the order of 1-2 years, which is reported in this paper.
    06/2013;
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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.
    06/2013;
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    ABSTRACT: A total number of about 1400 photographic exposures were taken with the 16-cm astrograph of the Konkoly Observatory at Budapest between 1950 and 1954, on 130 plates. The plates were digitalized using an EPSON Perfection V750 flatbed transparency scanner. Photoelectric observations of RZ Lyr were collected with the 60-cm telescope of the Konkoly Observatory in 1958, 1959 with RCA 1P21, and in 1968, 1969 and 1972 with EMI 9052 B photomultiplier tubes. (4 data files).
    VizieR Online Data Catalog. 04/2013;
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    ABSTRACT: V405 And is a fast-rotating (P=0.465d) grazing eclipsing binary with two active components. Using multicolor photometric monitoring and radial velocity measurements, we find that the primary and the secondary components have masses of 0.49 and 0.21 solar masses, meaning that the primary probably possesses a radiative core and a convective envelope, while the secondary is fully convective. The radius of the low-mass component fits well the theoretical mass-radius relation. However, the radius of the primary is significantly larger than the predicted value, in fact, the discrepancy is far the highest from the four similar objects known.
    Central European Astrophysical Bulletin. 03/2013;
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    ABSTRACT: Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2 \mu m from the ground and between 3.6 \mu m and 160 \mu m from space. Methods. We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results. We find that the shapes of the light curves at different wavelengths are strictly self-similar and that the observed variability is related to a single physical process, most likely variable extinction. We suggest that the central source is episodically occulted by a dense dust cloud in the inner disk, and, based on the invariability of the far-infrared fluxes, we propose that it is a long-lived rather than a transient structure. In some respects, V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars. Conclusions. The example of V2492 Cyg demonstrates that the light variations of young eruptive stars are not exclusively related to changing accretion. The variability provided information on an azimuthally asymmetric structural element in the inner disk. Such an asymmetric density distribution in the terrestrial zone may also have consequences for the initial conditions of planet formation.
    Astronomy and Astrophysics 01/2013; · 5.08 Impact Factor
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    ABSTRACT: We present the analysis of the multicolour CCD observations in the 2010 season of the Blazhko RRc star TV Boo. TV Boo shows a complex Blazhko modulation dominated by two independent modulations with P_{fm1}=9.74 d and P_{fm2}=21.43 d long periods. Both modulation components appear in the frequency spectra as multiplet structures around the harmonics of the pulsation. The positive value of the asymmetry parameter (Q=+0.51) of the primary modulation suggests that it is similar in its character to the Blazhko effect of most of the modulated RRab stars. Interestingly, the secondary, lower-amplitude modulation exhibits a negative asymmetry parameter (Q=-0.22), which is an unusually low value when compared to the Blazhko-modulated RRab stars. Apart from the two modulation frequencies, the spectra also show an additional frequency f' and its linear combinations with the pulsation and the primary modulation f_{1}+f' and f_{1}-f'-f_{m1}. We conclude that the additional frequency most probably belongs to a non-radial mode.
    Astronomische Nachrichten 11/2012; · 1.40 Impact Factor
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    ABSTRACT: The nearby, bright, almost completely unreddened Type Ia supernova 2011fe in M101 provides a unique opportunity to test both the precision and the accuracy of the extragalactic distances derived from SNe Ia light curve fitters. We apply the current, public versions of the independent light curve fitting codes MLCS2k2 and SALT2 to compute the distance modulus of SN 2011fe from high-precision, multi-color (BVRI) light curves. The results from the two fitting codes confirm that 2011fe is a "normal" (not peculiar) and only slightly reddened SN Ia. New unreddened distance moduli are derived as 29.21 +/- 0.07 mag (D ~ 6.95 +/- 0.23$ Mpc, MLCS2k2), and 29.05 +/- 0.07 mag (6.46 +/- 0.21 Mpc, SALT2). Despite the very good fitting quality achieved with both light curve fitters, the resulting distance moduli are inconsistent by 2 sigma. Both are marginally consistent (at ~1 sigma) with the HST Key Project distance modulus for M101. The SALT2 distance is in good agreement with the recently revised Cepheid- and TRGB-distance to M101. Averaging all SN- and Cepheid-based estimates, the absolute distance to M101 is ~6.6 +/- 0.5 Mpc.
    Astronomy and Astrophysics 07/2012; · 5.08 Impact Factor
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    ABSTRACT: We present our successful Chandra program designed to identify, with sub-arcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was discovered by Swift and Fermi. Thanks to our rapid target of opportunity request, Chandra clearly detected the X-ray afterglow, though no optical afterglow was found in deep optical observations. The host galaxy was clearly detected in the optical and near-infrared band, with the best photometric redshift of z=1.31_{-0.23}^{+0.46} (90% confidence), making it one of the highest known short GRB redshifts. Furthermore, we see an offset of 1.0 +- 0.2 arcseconds, which corresponds to 8.4 +- 1.7 kpc, between the host and the afterglow position. We discuss the importance of using Chandra for obtaining sub-arcsecond X-ray localizations of short GRB afterglows to study GRB environments.
    The Astrophysical Journal 05/2012; 766(1). · 6.73 Impact Factor
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    ABSTRACT: V405 And is an ultrafast-rotating (P_rot ~ 0.46 days) eclipsing binary. The system consists of a primary star with radiative core and convective envelope, and a fully convective secondary. Theories have shown, that stellar structure can depend on magnetic activity, i.e., magnetically active M-dwarfs should have larger radii. Earlier light curve modelling of V405 And indeed showed this behaviour: we found that the radius of the primary is significantly larger than the theoretically predicted value for inactive main sequence stars (the discrepancy is the largest of all known objects), while the secondary fits well to the mass-radius relation. By modelling our recently obtained light curves, which show significant changes of the spotted surface of the primary, we can find further proof for this phenomenon.
    Proceedings of the International Astronomical Union 05/2012;
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    ABSTRACT: We use more than three decades-long photometry to study the activity patterns on the two fast-rotating subgiant components in EI Eri (G5IV) and V711 Tau (K1IV). From yearly mean rotational periods from the light curves, we find that EI Eri, with well-measured solar-type differential rotation, always has spots from the equator to high latitudes. The measured differential rotation of V711 Tau is controversial, and in any case is very small. The spots on the K1IV star in V711 Tau seem to be tidally locked. The physical parameters of the two systems are similar, with one remarkable difference: EI Eri has a low mass M4-5 dwarf companion, whereas V711 Tau has a G5V star in the system, thus their mass centers are in very different positions. This may modify the whole internal structure of the active stars, causing marked differences in their surface features.
    IAU Symposium; 04/2012
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    ABSTRACT: The differentially rotating convective envelope is an indispensable element of the magnetic dynamo working in RS CVn-systems. Tidal coupling is responsible for maintaining fast rotation, and also the observed high level of magnetic activity. In this work, we compare the physical properties of two well known RS CVn-type binaries, that is the long-period system σ Gem and the ellipsoidal variable ζ And. For the comparison, we use the results obtained from processing time-series Doppler images. We also aim at understanding how differential rotation could be affected by tidal forces in such close binaries.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):197-198.

Publication Stats

239 Citations
111.74 Total Impact Points

Institutions

  • 2007–2013
    • Konkoly Observatory
      Budapeŝto, Budapest, Hungary
    • Charles University in Prague
      • Matematicko-fyzikální fakulta
      Praha, Hlavni mesto Praha, Czech Republic
  • 2012
    • University of Copenhagen
      • Dark Cosmology Centre (DARK)
      København, Capital Region, Denmark
  • 2007–2012
    • Hungarian Academy of Sciences
      Budapeŝto, Budapest, Hungary
  • 2010
    • Pontifical Catholic University of Chile
      • Instituto de Astrofísica
      Santiago, Region Metropolitana de Santiago, Chile
  • 2009–2010
    • University of Geneva
      Genève, Geneva, Switzerland
    • The Budapest Observatory
      Budapeŝto, Budapest, Hungary
  • 2006–2009
    • Eötvös Loránd University
      • Department of Astronomy
      Budapest, Budapest fovaros, Hungary