Stellar differential rotation from direct star-spot tracking     

A.Collier Cameron  J.-F.Donati  M.Semel 《Monthly notices of the Royal Astronomical Society》2002,330(3):699-706

On the Sun, the rotation periods of individual sunspots not only trace the latitude-dependence of the surface rotation rate, but also provide clues as to the amount of subsurface fluid shear. In this paper we present the first measurements of stellar differential rotation made by tracking the rotation of individual star-spots with sizes comparable to the largest sunspots. To achieve this we re-analyse four sequences of densely sampled, high signal-to-noise ratio echelle spectra of AB Doradus spanning several stellar rotations in 1996 December. Using spectral subtraction, least-squares deconvolution and matched-filter analysis, we demonstrate that it is possible to measure directly the velocity amplitudes and rotation periods of large numbers of individual star-spots at low to intermediate latitude. We derive values for the equatorial rotation rate and the magnitude of the surface differential rotation, both of which are in excellent agreement with those obtained by Donati & Collier Cameron from cross-correlation of Doppler images derived a year earlier in 1995 December, and with a re-analysis of the 1996 data by the χ ² landscape method. The differences between the rotation rates of individual spots and the fitted differential rotation law are substantially greater than the observational errors. The smaller spots show a greater scatter about the mean relation than the larger ones, which suggests that buffeting by turbulent supergranular flows could be responsible.  相似文献   

The global magnetic topology of AB Doradus   总被引：1，自引：0，他引：1  

M. Jardine  A. Collier Cameron  J.-F. Donati 《Monthly notices of the Royal Astronomical Society》2002,333(2):339-346

We have used Zeeman–Doppler maps of the surface field of the young, rapid rotator AB Dor  ( P _rot=0.514 d)  to extrapolate the coronal field, assuming it to be potential. We find that the topology of the large-scale field is very similar in all three years for which we have images. The corona divides cleanly into regions of open and closed field. The open field originates in two mid-latitude regions of opposite polarity separated by about 180° of longitude. The closed field region forms a torus extending almost over each pole, with an axis that runs through these two longitudes. We have investigated the effect on the global topology of different forms of flux in the unobservable hemisphere and in the dark polar spot where the Zeeman signal is suppressed. The flux distribution in the unobservable hemisphere affects only the low-latitude topology, whereas the imposition of a unidirectional polar field forces the polar cap to be open. This contradicts observations that suggest that the closed field corona extends to high latitudes and leads us to propose that the polar cap may be composed of multipolar regions.  相似文献   

A search for starlight reflected from HD 75289b     

Christopher Leigh  rew Collier Cameron  Stephane Udry  Jean-François Donati  Keith Horne  David James  Alan Penny 《Monthly notices of the Royal Astronomical Society》2003,346(2):L16-L20

We have used a Doppler tomographic analysis to conduct a deep search for the starlight reflected from the planetary companion to HD 75289. In four nights on VLT(UT2)/UVES in 2003 January, we obtained 684 high-resolution echelle spectra with a total integration time of 26 h. We establish an upper limit on the geometric albedo of the planet   p < 0.12  (to the 99.9 per cent significance level) at the most probable orbital inclination   i ≃ 60°  , assuming a grey albedo, a Venus-like phase function and a planetary radius   R _p= 1.6 R_Jup  . We are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.  相似文献   

Dynamo processes and activity cycles of the active stars AB Doradus, LQ Hydrae and HR 1099     

J.-F. Donati  A. Collier Cameron  M. Semel  G. A. J. Hussain  P. Petit  B. D. Carter  S. C. Marsden  M. Mengel  A. López Ariste  S. V. Jeffers  D. E. Rees 《Monthly notices of the Royal Astronomical Society》2003,345(4):1145-1186

  相似文献   

Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099     

P. Petit  J.-F. Donati  G. A. Wade  J. D. Landstreet  S. Bagnulo  T. Lüftinger  T. A. A. Sigut  S. L. S. Shorlin  S. Strasser  M. Aurière  J. M. Oliveira 《Monthly notices of the Royal Astronomical Society》2004,348(4):1175-1190

  相似文献   

How well do integrated assessment models represent non-CO2 radiative forcing?     

Mathijs J. H. M. Harmsen  Detlef P van Vuuren  Maarten van den Berg  Andries F Hof  Chris Hope  Volker Krey  Jean-Francois Lamarque  Adriana Marcucci  Drew T Shindell  Michiel Schaeffer 《Climatic change》2015,133(4):565-582

  相似文献   

Magnetic cycles of HR 1099 and LQ Hydrae     

J.-F. Donati 《Monthly notices of the Royal Astronomical Society》1999,302(3):457-481

  相似文献   

In situ correlations between PAH and biotransformation activities in mussels and fish     

Jean-Francois Narbonne  Philippe Garrigues  Anne Mathieu  Christophe Raoux  Daniel Ribera  Jean-Louis Monod  Aldo Viarengo  Jean-Pierre Salaün  Marc Lafaurie 《Marine environmental research》1989,28(1-4)

  相似文献   

Bubble growth in slightly supersaturated albite melt at constant pressure     

Don R. Baker  Phyllis Lang  Jean-Francois Bergevin  Liping Bai 《Geochimica et cosmochimica acta》2006,70(7):1821-1838

Bubble growth experiments were performed in a piston-cylinder by hydrating albite melt with ∼11 wt.% H₂O at 550 MPa followed by rapid decompression at 1 MPa s⁻¹ to pressures of 450 or 400 MPa. At these conditions the melt was supersaturated with ∼0.5 or ∼1.5 wt.% H₂O, respectively, which caused rapid exsolution and bubble growth. Results at 1200 °C demonstrate that portions of the initial cumulative bubble-area distributions may be characterized by a power law with an exponent near 1, but they rapidly evolve to exponential distributions and approach a unimodal distribution after 32 h of growth. This evolution occurs by the growth of larger bubbles at the expense of smaller ones. The growth rate of the average bubble radius in these experiments is described by a power law whose exponent is 0.35, close to the theoretical exponent of 1/3 for phase growth in which coalescence is dominated by Ostwald ripening of the bubbles. Over the range of pressures and water contents investigated at 1200 °C, the bubble-size distributions and growth rate are not significantly affected by changes in the amount of exsolved water or by splitting the decompression path into two steps. Similar decompression experiments at 800 °C are dominated by smaller bubbles than in the 1200 °C experiments and also demonstrate exponential cumulative size distributions, but consistently contain a small fraction of larger bubbles. The growth rate of these bubble radii cannot be fit with a power law, but a logarithmic dependence of the bubble radii on time is possible, suggesting a difference in the growth mechanisms at low and high temperatures. This difference is attributed to the orders of magnitude changes in melt viscosity and water diffusion in the melt as the temperature varies from 800 to 1200 °C. At 1200 °C the transport properties of albite melt resemble those of natural basaltic melts whereas at 800 °C the properties are similar to those of andesitic to dacitic melts. The decompression rate used in this study exceeds natural rates by one to two orders of magnitude. Thus, these results indicate that natural mafic-to-intermediate magmas supersaturated with only a small excess of water should easily nucleate bubbles during ascent and that bubble growth in mafic magmas will proceed much more rapidly than in andesitic to dacitic magmas. Intermediate composition magmas also may be capable of forming bimodal bubble-size distributions even in the case when only one nucleation event occurred. The rapid evolution of the bubble-size distribution from a power law to an exponential may be useful in constraining the time duration between bubble nucleation and the quenching of natural samples.  相似文献   

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation     

Nita Sahai  Young J. Lee  Huifang Xu  Jean-Francois Gaillard 《Geochimica et cosmochimica acta》2007,71(13):3193-3210

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca₅(PO₄)₃OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO_2(g) and O_2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe^III[(OH)₃(PO₄)(As^VO₄)]·nH₂O or Fe^III[(OH)₃( PO₄)(As^VO₄)(As^IIIO₃)_minor]·nH₂O, where As^IIIO₃ is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe^III(OH)₃ and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe^III[(OH)₃, PO₄]·nH₂O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO₄·2H₂O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.  相似文献