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Meusinger H.,Thuringer Landessternwarte Tautenburg | Balafkan N.,University of Leipzig
Astronomy and Astrophysics | Year: 2014

Aims. A tiny fraction of the quasar population shows remarkably weak emission lines. Several hypotheses have been developed, but the weak line quasar (WLQ) phenomenon still remains puzzling. The aim of this study was to create a sizeable sample of WLQs and WLQ-like objects and to evaluate various properties of this sample. Methods. We performed a search for WLQs in the spectroscopic data from the Sloan Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for nearly 105 quasar spectra. The final sample consists of 365 quasars in the redshift range z = 0.6 - 4.2 (z = 1.50 ± 0.45) and includes in particular a subsample of 46 WLQs with equivalent widths WMg ii< 11 Å and WC iv< 4.8 Å. We compared the luminosities, black hole masses, Eddington ratios, accretion rates, variability, spectral slopes, and radio properties of the WLQs with those of control samples of ordinary quasars. Particular attention was paid to selection effects. Results. The WLQs have, on average, significantly higher luminosities, Eddington ratios, and accretion rates. About half of the excess comes from a selection bias, but an intrinsic excess remains probably caused primarily by higher accretion rates. The spectral energy distribution shows a bluer continuum at rest-frame wavelengths 1500 Å. The variability in the optical and UV is relatively low, even taking the variability-luminosity anti-correlation into account. The percentage of radio detected quasars and of core-dominant radio sources is significantly higher than for the control sample, whereas the mean radio-loudness is lower. Conclusions. The properties of our WLQ sample can be consistently understood assuming that it consists of a mix of quasars at the beginning of a stage of increased accretion activity and of beamed radio-quiet quasars. The higher luminosities and Eddington ratios in combination with a bluer spectral energy distribution can be explained by hotter continua, i.e. higher accretion rates. If quasar activity consists of subphases with different accretion rates, a change towards a higher rate is probably accompanied by an only slow development of the broad line region. The composite WLQ spectrum can be reasonably matched by the ordinary quasar composite where the continuum has been replaced by that of a hotter disk. A similar effect can be achieved by an additional power-law component in relativistically boosted radio-quiet quasars, which may explain the high percentage of radio quasars. © 2014 ESO.


Hatzes A.P.,Thuringer Landessternwarte Tautenburg
Astronomy and Astrophysics | Year: 2014

Kepler-78b is a transiting Earth-mass planet in an 8.5 h orbit discovered by the Kepler Space Mission. We performed an analysis of the published radial velocity measurements for Kepler-78 in order to derive a refined measurement for the planet mass. Kepler-78 is an active star and radial velocity variations due to activity were removed using a floating chunk offset (FCO) method where an orbital solution was made to the data by allowing the velocity offsets of individual nights to vary. We show that if we had no a priori knowledge of the transit period the FCO method, used as a periodogram, would still have detected Kepler-78b in the radial velocity data. It can thus be effective at finding unknown short-period signals in the presence of significant activity noise.Using the FCO method while keeping the ephemeris and orbital phase fixed to the photometric values and using only data from nights where 6-10 measurements were taken results in a K-amplitude of 1.34 ± 0.25 m s-1, a planet mass of 1.31 ± 0.24 M, and a planet density of ρ = 4.5-2.0^{+2.2}$} g cm-3. Allowing the orbital phase to be a free parameter reproduces the transit phase to within the uncertainty. The corresponding density implies that Kepler-78b may have a structure that is deficient in iron and is thus more like the Moon. Although the various approaches that were used to filter out the activity of Kepler-78 produce consistent radial velocity amplitudes to within the errors, these are still too large to constrain the structure of this planet. The uncertainty in the mass for Kepler-78b is large enough to encompass models with structures ranging from Mercury-like (iron enriched) to Moon-like (iron deficient). A more accurate K-amplitude as well as a better determination of the planet radius are needed to distinguish between these models. © ESO, 2014.


Ammler-Von Eiff M.,Thuringer Landessternwarte Tautenburg | Ammler-Von Eiff M.,University of Gottingen | Ammler-Von Eiff M.,Max Planck Institute for Solar System Research | Reiners A.,University of Gottingen
Astronomy and Astrophysics | Year: 2012

Context. The Sun displays differential rotation that is intimately connected to the solar dynamo and hence related to solar activity, the solar cycle, and the solar wind. Considering the detectability and habitability of planets around other stars it is important to understand the role of differential rotation in other stars. Aims. We present projected rotational velocities and new measurements of the rotational profile of some 180 nearby stars with spectral types A-F. The results are consolidated by a homogeneous compilation of basic stellar data from photometry and the identification of multiple stellar systems. New and previous measurements of rotation by line profile analysis are compiled and made available. Methods. The overall broadening profile is derived analysing spectral line shape from hundreds of spectral lines by the method of least-squares deconvolution, reducing spectral noise to a minimum. The effect of differential rotation on the broadening profile is best measured in inverse wavelength space by the first two zeros of its Fourier transform. Results. Projected rotational velocity vsini is measured for more than 110 of the sample stars. Rigid and differential rotation can be distinguished in 56 cases where vsini > 12 km s-1. We detect differential rotation rates of δΩ/Ω = 5% and more. Ten stars with significant differential rotation rates are identified. The line shapes of 46 stars are consistent with rigid rotation, even though differential rotation at very low rates might still be possible in these cases. The strongest amount of relative differential rotation (54%) detected by line profile analysis is found among F stars. Conclusions. As of now, 33 differential rotators detected by line profile analysis have been confirmed. The frequency of differential rotators decreases towards high effective temperature and rapid rotation. There is evidence for two populations of differential rotators, one of rapidly rotating A stars at the granulation boundary with strong horizontal shear and one of mid-to late-F type stars with moderate rates of rotation and less shear. The gap in between can only partly be explained by an upper bound found for the horizontal shear of F stars. Apparently, the physical conditions change at early-F spectral types. The range of horizontal shear observed for mid-type F stars is reproduced by theoretical calculations while there seems to be a discrepancy in period dependence for late-F stars. © ESO 2012.


Hatzes A.P.,Thuringer Landessternwarte Tautenburg
Nature | Year: 2014

Characterization studies now have a dominant role in the field of exoplanets. Such studies include the measurement of an exoplanet's bulk density, its brightness temperature and the chemical composition of its atmosphere. The use of space telescopes has played a key part in the characterization of transiting exoplanets. These facilities offer astronomers data of exquisite precision and temporal sampling as well as access to wavelength regions of the electromagnetic spectrum that are inaccessible from the ground. Space missions such as the Hubble Space Telescope, Microvariability and Oscillations of Stars (MOST), Spitzer Space Telescope, Convection, Rotation and Planetary Transits (CoRoT), and Kepler have rapidly advanced our knowledge of the physical properties of exoplanets and have blazed a trail for a series of future space missions that will help us to understand the observed diversity of exoplanets. © 2014 Macmillan Publishers Limited. All rights reserved.


Van Weeren R.J.,Leiden University | Bruggen M.,Jacobs University Bremen | Rottgering H.J.A.,Leiden University | Hoeft M.,Thuringer Landessternwarte Tautenburg
Monthly Notices of the Royal Astronomical Society | Year: 2011

Galaxy clusters grow by mergers with other clusters and galaxy groups. These mergers create shock waves within the intracluster medium (ICM) that can accelerate particles to extreme energies. In the presence of magnetic fields, relativistic electrons form large regions emitting synchrotron radiation, the so-called radio relics. Behind the shock front, synchrotron and inverse Compton losses cause the radio spectral index to steepen away from the shock front. In a binary cluster merger, two shock waves are generated which move diametrically outwards along the merger axis. Two radio relics can then form on both sides of the cluster centre. An example of such a cluster is CIZA J2242.8+5301, where very clear spectral steepening in the downstream region is observed. The main relic has a total extent of 1700kpc, while its width is only 55kpc. Together with the high observed polarization fraction, this implies that the relic is seen very close to edge-on which makes it easier to constrain the merger geometry. Here we present hydrodynamical simulations of idealized binary cluster mergers with the aim of constraining the merger scenario for this cluster. From our simulations, we find that CIZA J2242.8+5301 is probably undergoing a merger in the plane of the sky (less than 10° from edge-on) with a mass ratio (M1:M2) of about 2:1, and an impact parameter ≲400kpc. We find that the core passage of the clusters happened about 1 Gyr ago. We conclude that double relics can set constraints on the mass ratios, impact parameters, time-scales and viewing geometry of binary cluster mergers, which is particularly useful when detailed X-ray observations are not available. In addition, the presence of large radio relics can be used to constrain the degree of clumping in the outskirts of the ICM, which is important to constrain the baryon fraction, density and entropy profiles, around the virial radius and beyond. We find that the amplitude of density fluctuations, with sizes of ≲200kpc, in the relic in CIZA J2242.8+5301 is not larger than 30 per cent. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Hartmann M.,Thuringer Landessternwarte Tautenburg | Guenther E.W.,Thuringer Landessternwarte Tautenburg | Hatzes A.P.,Thuringer Landessternwarte Tautenburg
Astrophysical Journal | Year: 2010

In order to investigate the dependence of planet formation on stellar mass, we have been monitoring a sample of F-type main-sequence stars with the 2.0m Alfred-Jensch telescope of the Thüringer Landessternwarte Tautenburg. This survey is based on high-precision radial velocity (RV) measurements using the coudé échelle spectrograph and an iodine absorption cell. We present RV measurements of the F7 V star HD 8673 that show a long-term variability of 1634 days with a semi-amplitude K = 288 m s-1 that can be explained most reasonably by an orbiting sub-stellar companion with a minimum mass of 14.2 M Jup in a high-eccentricity (e = 0.723) Keplerian orbit. © 2010. The American Astronomical Society. All rights reserved..


Hatzes A.P.,Thuringer Landessternwarte Tautenburg
Astronomy and Astrophysics | Year: 2016

Context. Radial velocity measurements initially showed evidence that the M dwarf GL 581 might host up to six planetary companions. Two of these, GL 581g and GL 581d had orbital distances in the so-called "habitable zone" of the star. The existence of both of these planets have been called into question. Additional radial velocity measurements for GL 581g could not confirm its presence. A study of Hα in GL 581 showed that changes in this activity indicator correlated with radial velocity variations attributed to GL 581d. Thus two planets that were important for studies of habitable planets may be artifacts of stellar activity. Aims. Previous investigations analyzing the same activity data have reached different conclusions regarding the existence of GL 581d. We therefore investigated the Hα variations for GL 581 to assess the nature of the radial velocity variations attributed to the possible planet GL 581d. Methods. We performed a Fourier analysis of the published Hα measurements for GL 581. Fourier components were selectively found and removed in a so-called pre-whitening process thus isolating any variations at the orbital frequency of GL 581d. Results. The frequency analysis yields five significant frequencies, one of which is associated with the 66.7 d orbital period of the presumed planet Gl 581d. The Hα variations at this period show sine-like variations that are 180° out-of-phase with the radial velocity variations of GL 581d. This is seen in the full data set that spans almost 7 years, as well as a subset of the data near the end of the time series that had good temporal sampling over 230 days. Furthermore, no significant temporal variations are found in the ratio of the amplitudes of the Hα index and radial velocity variations. This provides additional evidence that the radial velocity signal attributed to GL 581d is in fact due to stellar activity. Conclusions. The analysis confirms the anti-correlation of the radial velocity of GL 581d with the Hα equivalent width and provides additional strong evidence that the signal of GL 581d is intrinsic to the star. © ESO, 2016.


Tkachenko A.,Catholic University of Leuven | Lehmann H.,Thuringer Landessternwarte Tautenburg | Smalley B.,Keele University | Uytterhoeven K.,Institute of Astrophysics of Canarias | Uytterhoeven K.,University of La Laguna
Monthly Notices of the Royal Astronomical Society | Year: 2013

The Kepler satellite missionwas designed to search for transiting exoplanets and delivers single band-pass light curves of a huge number of stars observed in the Cygnus-Lyra region. At the same time, it opens a new window for asteroseismology. In order to accomplish one of the required preconditions for the asteroseismic modelling of the stars, namely knowledge of their precise fundamental parameters, ground-based spectroscopic and/or photometric follow-up observations are needed. We aim to derive fundamental parameters and individual abundances for a sample of 18 γ Dor/δ Sct and 8 slowly pulsating B (SPB)/β Cep candidate stars in the Kepler satellite field of view. We use the spectral synthesis method to model newly obtained, high-resolution spectra of 26 stars in order to derive their fundamental parameters like Teff, log g, v sin i, ξ, [M/H] and individual abundances with high accuracy. The stars are then placed into the log(Teff)-log(g) diagram and the obtained spectroscopic classification is compared to the existing photometric one. For most A- and F-type stars, the derived Teff values agree within the measurement errors with the values given in the Kepler Input Catalog (KIC). For hot stars, the KIC temperatures appear to be systematically underestimated, in agreement with previous findings. We also find that the temperatures derived from our spectra agree reasonably well with those derived from the spectral energy distribution fitting. According to their position in the log(Teff)-log(g) diagram, two stars are expected γ Dor stars, four stars are expected δ Sct stars and four stars are possibly δ Sct stars at the blue edge of the instability strip. Two stars are confirmed SPB variables, and one star falls into the SPB instability region but its parameters might be biased by binarity. Two of the four stars that fall into the δ Sct instability region show γ Dor-type oscillation in their light curves implying that γ Dor-like oscillations are much more common among the δ Sct stars than what is theoretically expected. Moreover, one of the stars located at the hot border of the δ Sct instability strip is classified as δ Sct-γ Dor hybrid pulsator from its light-curve analysis. Given that these findings are fully consistent with recent investigations, we conclude that a revision of the γ Dor and δ Sct instability strips is essential.© 2013 The Authors Published by Oxford University Press on behalf of The Royal Astronomical Society.


Meusinger H.,Thuringer Landessternwarte Tautenburg | Weiss V.,Thuringer Landessternwarte Tautenburg
Astronomy and Astrophysics | Year: 2013

Aims. Although the variability in the ultraviolet and optical domain is one of the major characteristics of quasars, the dominant underlying mechanisms are still poorly understood. There is a broad consensus on the relationship between the strength of the variability and such quantities as time-lag, wavelength, luminosity, and redshift. However, evidence on a dependence on the fundamental parameters of the accretion process is still inconclusive. This paper is focused on the correlation between the ultraviolet quasar long-term variability and the accretion rate. Methods. We compiled a catalogue of about 4000 quasars including individual estimators for the variability strength derived from the multi-epoch photometry in the SDSS Stripe 82, virial black hole masses M derived from the Mg ii line, and mass accretion rates ဠfrom the Davis-Laor scaling relation. Several statistical tests were applied to evaluate the correlations of the variability with luminosity, mass, Eddington ratio, and accretion rate. Results. We confirm the existence of significant anti-correlations between the variability estimator V and the accretion rate á€, the Eddington ratio ε, and the bolometric luminosity L bol, respectively. The Eddington ratio is tightly correlated with á€. A weak, statistically not significant positive trend is indicated for the dependence of V on M. As a side product, we find a strong correlation of the radiative efficiency η with M in our sample. We show via numerical simulations that this trend is most likely produced by selection effects in combination with the mass errors and the use of the scaling relation for á€. The anti-correlations of V with á€, ε, and Lbol cannot be explained in such a way. The strongest anti-correlation is found between V and á€. However, it is difficult to decide which of the quantities L,ε, and ဠis intrinsically correlated with V and which of the observed correlations of V are produced by the L - ε - ဠrelation. A V - ဠanti-correlation is qualitatively expected for the strongly inhomogeneous accretion disks. We argue that the observed amplitudes of the variability at far-UV wavelengths, the stochastic nature of variability, and the variability time-scales are not adequately explained by the simple multi-temperature black-body model of a standard disk and suggest to check whether the strongly inhomogeneous disk model is capable of reproducing these observations better. © ESO, 2013.


Lehmann H.,Thuringer Landessternwarte Tautenburg | Southworth J.,Keele University | Tkachenko A.,Catholic University of Leuven | Pavlovski K.,University of Zagreb
Astronomy and Astrophysics | Year: 2013

Context. KIC 10661783 is an eclipsing binary that shows δ Scuti-like oscillations. More than 60 pulsation frequencies have been detected in its light curve as observed by the Kepler satellite. Aims. We want to determine the fundamental stellar and system parameters of the eclipsing binary as a precondition for asteroseismic modelling of the pulsating component and to establish whether the star is a semi-detached Algol-type system. Methods. We measured the radial velocities of both components from new high-resolution spectra using TODCOR and compute the orbit using PHOEBE. We used the KOREL program to decompose the observed spectra into its components, and analysed the decomposed spectra to determine the atmospheric parameters. For this, we developed a new computer program for the normalisation of the KOREL output spectra. Fundamental stellar parameters are determined by combining the spectroscopic results with those from the analysis of the Kepler light curve. Results. We obtain Teff, log g, vsini, and the absolute masses and radii of the components, together with their flux ratio and separation. Whereas the secondary star rotates synchronously with the orbital motion, the primary star rotates subsynchronously by a factor of 0.75. The newly determined mass ratio of 0.0911 is higher than previously thought and means a detached configuration is required to fit the light curve. Conclusions. With its low orbital period and very low mass ratio, the system shows characteristics of the R CMa-type stars but differs from this group by being detached. Its current state is assumed to be that of a detached post-Algol binary system with a pulsating primary component. © 2013 ESO.

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