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  • 1
    Electronic Resource
    Electronic Resource
    X-ray spectral diagnostics for coronal loop sizes and heating rates in the active K dwarf AB Doradus
    Amsterdam : Elsevier
    Advances in Space Research 10 (1990), S. 135-138 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(90)90132-J
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  • 2
    Electronic Resource
    Electronic Resource
    Magnetostatic equilibria for coronal loops on rotating stars (1991)
    Springer
    Solar physics 131 (1991), S. 269-289 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have investigated magnetostatic equilibria for coronal loops embedded in a potential magnetic field on a rotating star. We find that for any given star, there is a maximum value of the plasma pressure inside a single loop, above which no equilibrium exists. This maximum internal pressure depends on the ratio of the temperatures inside and outside the loop, and on the ratio of the plasma pressure to the magnetic pressure at the base of the external field. Thus, any loop of a large-scale field which is heated or cooled to a different temperature from its immediate surroundings, or which experiences a change in its internal pressure may eventually lose equilbrium. For some values of the base pressure and temperature ratio the relation between summit height and footpoint separation is double-valued. As the summit height of a loop is increased, its footpoint separation increases to a critical value, then decreases to zero at the maximum possible summit height. At the critical footpoint separation the slope of the loop height-footpoint separation relation becomes infinite, and no equilibrium solution exists for greater footpoint separations. We find also that the strength and scale of the field external to the flux tube is the most important factor in determining its maximum height. The effects of varying the stellar rotation rate - and, hence, the variation in pressure with height - are comparatively unimportant, even for very high rotation rates at which the point of balance between gravitational and centrifugal forces lies close to the stellar surface. In this case it is possible to find equilibrium loop solutions whose summits lie outside the centrifugal balance point. We have also investigated the effects of varying the stellar surface gravity. For stellar of fixed mass and rotation rate, the loop dimensions scale approximately linearly with the stellar radius.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151638
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  • 3
    Electronic Resource
    Electronic Resource
    Azimuthal structures in the wind and chromosphere of the herbig Ae star ab aur. preliminary results from the MUSICOS 1992 campaign (1994)
    Springer
    Solar physics 155 (1994), S. 185-193 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international project to facilitate and organize world-wide multi-site campaigns in high resolution spectroscopy, in view of obtaining a complete time coverage of various types of variable stellar phenomena. In the framework of this project a vast spectroscopic campaign was organized in December 1992, involving 8 sites well distributed in longitude around the Earth. The observations concerned three scientific programs, among which was the study of azimuthal structures in the wind and chromosphere of the pre-Main-Sequence Herbig Ae star AB Aur. The Hei 5876 Å line of AB Aur, which is formed in the expanding chromosphere of this star, in the innermost parts of its wind, was monitored at a resolution of 30000, nearly continuously for about 4 days. A spectacular variability of this line was discovered, the profile changing from pure emission to a composite profile including a deep absorption component in the course of a few hours. This variability can be the signature of azimuthal structures in the wind of AB Aur. We present the data collected during the campaign, and discuss possible interpretations of the spectacular variations of the Hei 5876 Å line.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00670738
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  • 4
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    WASP-80b has a dayside within the T-dwarf range (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-08
    Description: WASP-80b is a missing link in the study of exoatmospheres. It falls between the warm Neptunes and the hot Jupiters and is amenable for characterization, thanks to its host star's properties. We observed the planet through transit and during occultation with Warm Spitzer . Combining our mid-infrared transits with optical time series, we find that the planet presents a transmission spectrum indistinguishable from a horizontal line. In emission, WASP-80b is the intrinsically faintest planet whose dayside flux has been detected in both the 3.6 and 4.5 μm Spitzer channels. The depths of the occultations reveal that WASP-80b is as bright and as red as a T4 dwarf, but that its temperature is cooler. If planets go through the equivalent of an L–T transition, our results would imply that this happens at cooler temperatures than for brown dwarfs. Placing WASP-80b's dayside into a colour–magnitude diagram, it falls exactly at the junction between a blackbody model and the T-dwarf sequence; we cannot discern which of those two interpretations is the more likely. WASP-80b's flux density is as low as GJ 436b at 3.6 μm; the planet's dayside is also fainter, but bluer than HD 189733Ab's nightside (in the [3.6] and [4.5] Spitzer bands). Flux measurements on other planets with similar equilibrium temperatures are required to establish whether irradiated gas giants, such as brown dwarfs, transition between two spectral classes. An eventual detection of methane absorption in transmission would also help lift that degeneracy. We obtained a second series of high-resolution spectra during transit, using HARPS. We reanalyse the Rossiter–McLaughlin effect. The data now favour an aligned orbital solution and a stellar rotation nearly three times slower than stellar line broadening implies. A contribution to stellar line broadening, maybe macroturbulence, is likely to have been underestimated for cool stars, whose rotations have therefore been systematically overestimated.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    The Sun as a planet-host star: proxies from SDO images for HARPS radial-velocity variations (2016)
    Oxford University Press
    Details
    Publication Date: 2016-02-28
    Description: The Sun is the only star whose surface can be directly resolved at high resolution, and therefore constitutes an excellent test case to explore the physical origin of stellar radial-velocity (RV) variability. We present HARPS observations of sunlight scattered off the bright asteroid 4/Vesta, from which we deduced the Sun's activity-driven RV variations. In parallel, the Helioseismic and Magnetic Imager instrument on board the Solar Dynamics Observatory provided us with simultaneous high spatial resolution magnetograms, Dopplergrams and continuum images of the Sun in the Fe i 6173 Å line. We determine the RV modulation arising from the suppression of granular blueshift in magnetized regions and the flux imbalance induced by dark spots and bright faculae. The rms velocity amplitudes of these contributions are 2.40 and 0.41 m s –1 , respectively, which confirms that the inhibition of convection is the dominant source of activity-induced RV variations at play, in accordance with previous studies. We find the Doppler imbalances of spot and plage regions to be only weakly anticorrelated. Light curves can thus only give incomplete predictions of convective blueshift suppression. We must instead seek proxies that track the plage coverage on the visible stellar hemisphere directly. The chromospheric flux index $R^{\prime }_{HK}$ derived from the HARPS spectra performs poorly in this respect, possibly because of the differences in limb brightening/darkening in the chromosphere and photosphere. We also find that the activity-driven RV variations of the Sun are strongly correlated with its full-disc magnetic flux density, which may become a useful proxy for activity-related RV noise.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Three newly discovered sub-Jupiter-mass planets: WASP-69b and WASP-84b transit active K dwarfs and WASP-70Ab transits the evolved primary of a G4+K3 binary (2014)
    Oxford University Press
    Details
    Publication Date: 2014-10-10
    Description: We report the discovery of the transiting exoplanets WASP-69b, WASP-70Ab and WASP-84b, each of which orbits a bright star ( V  ~ 10). WASP-69b is a bloated Saturn-mass planet (0.26  M Jup , 1.06  R Jup ) in a 3.868-d period around an active, ~1-Gyr, mid-K dwarf. ROSAT detected X-rays 60±27 arcsec from WASP-69. If the star is the source then the planet could be undergoing mass-loss at a rate of ~10 12  g s –1 . This is one to two orders of magnitude higher than the evaporation rate estimated for HD 209458b and HD 189733b, both of which have exhibited anomalously large Lyman α absorption during transit. WASP-70Ab is a sub-Jupiter-mass planet (0.59  M Jup , 1.16  R Jup ) in a 3.713-d orbit around the primary of a spatially resolved, 9–10-Gyr, G4+K3 binary, with a separation of 3.3 arcsec (≥800 au). WASP-84b is a sub-Jupiter-mass planet (0.69  M Jup , 0.94  R Jup ) in an 8.523-d orbit around an active, ~1-Gyr, early-K dwarf. Of the transiting planets discovered from the ground to date, WASP-84b has the third-longest period. For the active stars WASP-69 and WASP-84, we pre-whitened the radial velocities using a low-order harmonic series. We found that this reduced the residual scatter more than did the oft-used method of pre-whitening with a fit between residual radial velocity and bisector span. The system parameters were essentially unaffected by pre-whitening.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The thermal emission of the exoplanet WASP-3b (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-05
    Description: We report the detection of thermal emission from the transiting hot Jupiter WASP-3b at 3.6, 4.5 and 8.0 μm using the Spitzer Space Telescope . We obtain planet-to-star flux ratios of $0.209^{+0.040}_{-0.028}$ , 0.282 ± 0.012 and $0.328^{+0.086}_{-0.055}\,$ per cent at these wavelengths, respectively, implying infrared brightness temperatures of $T_{3.6\,\mu {\rm m}}=2280^{+210}_{-150}\,{\rm K}$ , T 4.5 μm = 2400 ± 80 K and $T_{8.0\,\mu {\rm m}}=2210^{+390}_{-250}\,{\rm K}$ . We find that WASP-3b falls into an emerging class of highly irradiated planets whose measured temperatures suggest that the planets are dark and redistribute heat around the planet inefficiently. The latter is similarly concluded from 1D atmospheric model comparisons, which also favour the presence of an atmospheric temperature inversion. We compare the WASP-3 system to the proposed inversion–activity relation, finding that it hints at a more complex relation than a simple cut-off in activity implied by previous data. Using eclipse timings we also constrain e cos to be $-0.0006^{+0.0010}_{-0.0006}$ , suggesting that the eccentricity of WASP-3b can only be large for a narrow range of .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Planets and stellar activity: hide and seek in the CoRoT-7 system (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-02
    Description: Since the discovery of the transiting super-Earth CoRoT-7b, several investigations have yielded different results for the number and masses of planets present in the system, mainly owing to the star's high level of activity. We re-observed CoRoT-7 in 2012 January with both HARPS and CoRoT, so that we now have the benefit of simultaneous radial-velocity and photometric data. This allows us to use the off-transit variations in the star's light curve to estimate the radial-velocity variations induced by the suppression of convective blueshift and the flux blocked by starspots. To account for activity-related effects in the radial velocities which do not have a photometric signature, we also include an additional activity term in the radial-velocity model, which we treat as a Gaussian process with the same covariance properties (and hence the same frequency structure) as the light curve. Our model was incorporated into a Monte Carlo Markov Chain in order to make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We measure the masses of planets b and c to be 4.73 ± 0.95 and 13.56 ± 1.08 M , respectively. The density of CoRoT-7b is (6.61 ± 1.72)( R p /1.58 R ) –3  g cm –3 , which is compatible with a rocky composition. We search for evidence of an additional planet d, identified by previous authors with a period close to 9 d. We are not able to confirm the existence of a planet with this orbital period, which is close to the second harmonic of the stellar rotation at ~7.9 d. Using Bayesian model selection, we find that a model with two planets plus activity-induced variations is most favoured.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star (2016)
    Oxford University Press
    Details
    Publication Date: 2016-04-09
    Description: We present the discovery by the WASP-South survey of WASP-121 b, a new remarkable short-period transiting hot Jupiter. The planet has a mass of $1.183_{-0.062}^{+0.064}$ M Jup , a radius of 1.865 ± 0.044 R Jup , and transits every $1.274\,9255_{-0.000\,0025}^{+0.000\,0020}$ days an active F6-type main-sequence star ( V = 10.4, $1.353_{-0.079}^{+0.080}$ M , 1.458 ± 0.030 R , T eff = 6460 ± 140 K). A notable property of WASP-121 b is that its orbital semimajor axis is only ~1.15 times larger than its Roche limit, which suggests that the planet is close to tidal disruption. Furthermore, its large size and extreme irradiation (~7.1 10 9  erg s –1 cm –2 ) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAnsiting Planets and PlanetesImals Small Telescope, we indeed detect its emission in the z ' -band at better than ~4, the measured occultation depth being 603 ± 130 ppm. Finally, from a measurement of the Rossiter–McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of $257{^{\circ}_{.}} 8_{-5{^{\circ}_{.}} 5}^{+5{^{\circ}_{.}} 3}$ . This result may suggest a significant misalignment between the spin axis of the host star and the orbital plane of the planet. If confirmed, this high misalignment would favour a migration of the planet involving strong dynamical events with a third body.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Warm Spitzer occultation photometry of WASP-26b at 3.6 and 4.5 {mu}m (2013)
    Oxford University Press
    Details
    Publication Date: 2013-05-21
    Description: We present new warm Spitzer occultation photometry of WASP-26 at 3.6 and 4.5 μm along with new transit photometry taken in the g, r and i bands. We report the first detection of the occultation of WASP-26b, with occultation depths at 3.6 and 4.5 μm of 0.001 26 ± 0.000 13 and 0.001 49 ± 0.000 16 corresponding to brightness temperatures of 1825 ± 80 and 1725 ± 89 K, respectively. We find that the eccentricity of the orbit is consistent with a circular orbit at the 1 level ( $e=0.0028 ^{+ 0.0097}_{- 0.0022}$ , 3 upper limit e 〈 0.04). According to the activity–inversion relation of Knutson et al., WASP-26b is predicted to host a thermal inversion. The brightness temperatures deduced from the eclipse depths are consistent with an isothermal atmosphere, although the planet may host a weak thermal inversion given the uncertainties on these values. The data are equally well fitted by atmospheric models with or without a thermal inversion. We find that variation in activity of solar-like stars does not change enough over the time-scales of months or years to change the interpretation of the Knutson et al. activity–inversion relation, provided that the measured activity level is averaged over several nights. Further data are required to fully constrain the thermal structure of the atmosphere because the planet lies very close to the boundary between atmospheres with and without a thermal inversion.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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