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Balona L.A.,South African Astronomical Observatory | Breger M.,University of Texas at Austin | Catanzaro G.,National institute for astrophysics | Cunha M.S.,University of Porto | And 13 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We show that the star KIC4840675 observed by Kepler is composed of three stars with a rapidly rotating A-type star and two solar-type fainter companions. The A-type star is a δScuti variable with a dominant mode and many other modes of lower amplitude, including several low-frequency variations. The low-frequency variation with highest amplitude can be interpreted as rotational modulation with the light curve changing with time. However, the most interesting aspect of this star is a triplet of independent modes in the range 118-129d -1 (1.4-1.5mHz), which is far outside the range of typical δScuti frequencies. We discuss the possibility that these modes could be solar-like oscillations, oscillations of the roAp type or due to an unseen pulsating compact companion. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Olah K.,Konkoly Observatory MTA CsFK | Moor A.,Konkoly Observatory MTA CsFK | Kovari Z.,Konkoly Observatory MTA CsFK | Granzer T.,Leibniz Institute for Astrophysics Potsdam | And 3 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (IL? Hya, XX? Tri, and DM? UMa) known to exhibit V-band light variations between 0.m65-1.m05. Our aim is to find the origin of their large brightness variation. Methods. 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 - IC color-index calibration. Results. 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 it 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.m6 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. Conclusions. 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 mbol and Teff, and is supported by independent v sin i measurements. © ESO, 2014.

Strassmeier K.G.,Leibniz Institute for Astrophysics Potsdam | Weber M.,Leibniz Institute for Astrophysics Potsdam | Granzer T.,Leibniz Institute for Astrophysics Potsdam | Schanne L.,Observatory for Stellar Spectroscopy Volklingen | And 3 more authors.
Astronomische Nachrichten | Year: 2014

We present continuous and time-resolved R = 55 000 optical échelle spectroscopy of ε Aurigae from 2006-2013. Data were taken with the STELLA Echelle Spectrograph of the robotic STELLA facility at the Observatorio del Teide in Tenerife. Contemporaneous photometry with the Automatic Photoelectric Telescopes at Fairborn Observatory in Arizona is presented for the years 1996-2013. Spectroscopic observations started three years prior to the photometric eclipse and are still ongoing. A total of 474 high-resolution échelle spectra are analyzed and made available in this paper. We identify 368 absorption lines of which 161 lines show the characteristic sharp disk lines during eclipse. Another 207 spectral lines appeared nearly unaffected by the eclipse. From spectrum synthesis, we obtained the supergiant atmospheric parameters Teff = 7395 ± 70 K, log g ≈ 1, and [Fe/H] = +0.02 ± 0.2 with ξt = 9 km s-1, ζRT = 13 km s-1, and v sin i = 28 ± 3 km s-1. The residual average line broadening expressed in km s-1 varies with a period of 62.6 ± 0.7 d, in particular at egress and after the eclipse. Two-dimensional line-profile periodograms show several periods, the strongest with ≈110 d evident in optically thin lines as well as in the Balmer lines. Center-of-intensity weighted radial velocities of individual spectral lines also show the 110-d period but, again, additional shorter and longer periods are evident and are different in the Balmer lines. The two main spectroscopic Hα periods, ≈ 116 d from the line core and ≈ 150 d from the center-of-intensity radial velocities, appear at 102 d and 139 d in the photometry. The Hβ and Johnson V I photometry on the other hand shows two well-defined and phase-coherent periods of 77 d and 132 d. We conclude that Hα is contaminated by changes in the circumstellar environment while the Hβ and V I photometry stems predominantly from the non radial pulsations of the F0 supergiant. We isolate the disk-rotation profile from 61 absorption lines and found that low disk eccentricity generally relates to low disk rotational velocity (but not always) while high disk eccentricity always relates to high velocity. There is also the general trend that the disk-absorption in spectral lines with higher excitation potential comes from disk regions with higher eccentricity and thus also with higher rotational velocity. The dependency on transition probability is more complex and shows a bi-modal trend. The outskirts of the disk is distributed asymmetrically around the disk and appears to have been built up mostly in a tail along the orbit behind the secondary. Our data show that this tail continues to eclipse the F0 Iab primary star even two years after the end of the photometric eclipse. High-resolution spectra were also taken of the other, bona-fide, visual-binary components of ε Aur (ADS 3605BCDE). Only the C-component, a K3-4-giant, appears at the same distance than ε Aur but its radial velocity is in disagreement with a bound orbit. The other components are a nearby (≈ 7 pc) cool DA white dwarf, a G8 dwarf, and a B9 supergiant, and not related to ε Aur. The cool white dwarf shows strong DIB lines that suggest the existence of a debris disk around this star. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Olah K.,Konkoly Observatory MTA CsFK | Moor A.,Konkoly Observatory MTA CsFK | Strassmeier K.G.,Leibniz Institute for Astrophysics Potsdam | Borkovits T.,Konkoly Observatory MTA CsFK | And 3 more authors.
Astronomische Nachrichten | Year: 2013

We present and analyze long-term optical photometric measurements of the three active stars V2253 Oph, IT Com and IS Vir. All three systems are single-lined spectroscopic binaries with an early K giant as primary component but in different stages of orbital-rotational synchronization. Our photometry is supplemented by 2MASS and WISE near-IR and mid-IR magnitudes and then used to obtain more accurate effective temperatures and extinctions. For V2253 Oph and IT Com, we found their spectral energy distributions consistent with pure photospheric emission. For IS Vir, we detect a marginal mid-IR excess which hints towards a dust disk. The orbital and rotational planes of IT Com appear tobe coplanar, contrary to previous findings in the literature. We apply a multiple frequency analysis technique to determine photometric periods, and possibly changes of periods, ranging from days to decades. New rotational periods of 21.55±0.03 d, 65.1±0.3 d, and 23.50±0.04 d were determined for V2253 Oph, IT Com, and IS Vir, respectively. Splitting of these periods led to tentative detections of differential surface rotations of δP/P ≈ 0.02 for V2253 Oph and 0.07 for IT Com. Using a time-frequency technique based on short-term Fourier transforms we present evidence of cyclic light variations of length ≈ 10 yr for V2253 Oph and 5-6 yr for IS Vir. A single flip-flop event has been observed for IT Com of duration 2-3 yr. Its exchange of the dominant active longitude had happened close to a time of periastron passage, suggesting some response of the magnetic activity from the orbital dynamics. The 21.55-d rotational modulation of V2253 Oph showed phase coherence also with the orbital period, which is 15 times longer than the rotational period, thus also indicating a tidal feedback with the stellar magnetic activity. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hareter M.,Konkoly Observatory MTA CSFK | Paparo M.,Konkoly Observatory MTA CSFK
Proceedings of the International Astronomical Union | Year: 2014

The star HD 51844 was observed in the CoRoT LRa02 as a target in the seismology field, which turned out to be an SB2 system. The 117-day light curve revealed δ Scuti pulsations in the range of 6 to 15 d-1 where four frequencies have amplitudes larger than 1.4 mmag, and a rich frequency spectrum with amplitudes lower than 0.6 mmag. Additionally, the light curve exhibits a 3-mmag brightening event recurring every 33.5 days with a duration of about 5 days. The radial velocities from spectroscopy confirmed that the star is an eccentric binary system with nearly identical masses and physical parameters. The brightening event in the light curve coincides with the maximum radial-velocity separation showing that the brightening is in fact caused by tidal distortion and/or reflected light. One component displays large line-profile variations, while the other does not show significant variation. The frequency analysis revealed a quintuplet structure of the four highest-amplitude frequencies, which is due to the orbital motion of the pulsating star. Copyright © International Astronomical Union 2014.

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