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Search Results: 1 - 10 of 309891 matches for " Andrew J. Donati "
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Effects of Indole-3-Acetic Acid on the Transcriptional Activities and Stress Tolerance of Bradyrhizobium japonicum
Andrew J. Donati, Hae-In Lee, Johan H. J. Leveau, Woo-Suk Chang
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0076559
Abstract: A genome-wide transcriptional profile of Bradyrhizobium japonicum, the nitrogen-fixing endosymbiont of the soybean plant, revealed differential expression of approximately 15% of the genome after a 1 mM treatment with the phytohormone indole-3-acetic acid (IAA). A total of 1,323 genes were differentially expressed (619 up-regulated and 704 down-regulated) at a two-fold cut off with q value ≤ 0.05. General stress response genes were induced, such as those involved in response to heat, cold, oxidative, osmotic, and desiccation stresses and in exopolysaccharide (EPS) biosynthesis. This suggests that IAA is effective in activating a generalized stress response in B. japonicum. The transcriptional data were corroborated by the finding that stress tolerance of B. japonicum in cell viability assays was enhanced when pre-treated with 1 mM IAA compared to controls. The IAA treatment also stimulated biofilm formation and EPS production by B. japonicum, especially acidic sugar components in the total EPS. The IAA pre-treatment did not influence the nodulation ability of B. japonicum. The data provide a comprehensive overview of the potential transcriptional responses of the symbiotic bacterium when exposed to the ubiquitous hormone of its plant host.
The Speed of Light in a Novel Gravitational Environment  [PDF]
Gianni Donati
Journal of Applied Mathematics and Physics (JAMP) , 2017, DOI: 10.4236/jamp.2017.510164
Abstract: In this paper, a review of the experiments, developments and ideas which have been presented on this topic together with its natural extension to the description of gravity, the ultimate dominating mystery of the universe, was taken out. A parallel mechanism was proposed between the emission of light and of neutrino by quantum leaps of electrons between the fixed energy levels of atomic orbits. The analysis of the neutron-proton mix of existing nuclides provides a rule for the calculation of the neutrino flux from matter and suggests both a medium for the transmission of light and a solution to the problem of gravitation.
Analytic and numerical models of the 3D multipolar magnetospheres of pre-main sequence stars
S. G. Gregory,J. -F. Donati
Physics , 2011, DOI: 10.1002/asna.201111621
Abstract: Traditionally models of accretion of gas on to T Tauri stars have assumed a dipole stellar magnetosphere, partly for simplicity, but also due to the lack of information about their true magnetic field topologies. Before and since the first magnetic maps of an accreting T Tauri star were published in 2007 a new generation of magnetospheric accretion models have been developed that incorporate multipole magnetic fields. Three-dimensional models of the large-scale stellar magnetosphere with an observed degree of complexity have been produced via numerical field extrapolation from observationally derived T Tauri magnetic maps. Likewise, analytic and magnetohydrodynamic models with multipolar stellar magnetic fields have been produced. In this conference review article we compare and contrast the numerical field extrapolation and analytic approaches, and argue that the large-scale magnetospheres of some (but not all) accreting T Tauri stars can be well described by tilted dipole plus tilted octupole field components. We further argue that the longitudinal field curve, whether derived from accretion related emission lines, or from photospheric absorption lines, provides poor constrains on the large-scale magnetic field topology and that detailed modeling of the rotationally modulated Stokes V signal is required to recover the true field complexity. We conclude by examining the advantages, disadvantages and limitations of both the field extrapolation and analytic approaches, and also those of magnetohydrodynamic models.
Doin' the twist: Secular changes in the surface differential rotation on AB Doradus
A. Collier Cameron,J. -F. Donati
Physics , 2001, DOI: 10.1046/j.1365-8711.2002.05147.x
Abstract: We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 days between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 days by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 days to 70 days initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have Delta Omega / Omega ~ 0.004, which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the mechanism of Applegate (1992).
Weak and Strong Field Dynamos: from the Earth to the stars
J. Morin,E. Dormy,M. Schrinner,J. -F. Donati
Physics , 2011, DOI: 10.1111/j.1745-3933.2011.01159.x
Abstract: Observations of magnetism in very low mass stars recently made important progress, revealing characteristics that are now to be understood in the framework of dynamo theory. In parallel, there is growing evidence that dynamo processes in these stars share many similarities with planetary dynamos. We investigate the extent to which the weak \emph{vs} strong field bistability predicted for the geodynamo can apply to recent observations of two groups of very low mass fully-convective stars sharing similar stellar parameters but generating radically different types of magnetic fields. Our analysis is based on previously published spectropolarimetric and spectroscopic data. We argue that these can be interpreted in the framework of weak and strong field dynamos.
The chromospheric emission of solar-type stars in the young open clusters IC 2391 and IC 2602
S. C. Marsden,B. D. Carter,J. -F. Donati
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15319.x
Abstract: In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High resolution spectroscopic data were obtained for over 50 F, G, and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid-rotators (vsini > 20 km/s) with five of the stars being classified as ultra-rapid rotators (vsini > 100 km/s). The emission levels in the Calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against Rossby number (NR) the star's chromospheric emission levels show a plateau in the emission for Log(NR) < -1.1 indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars show little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators.
Temporal fluctuations in the differential rotation of cool active stars
J. -F. Donati,A. Collier Cameron,P. Petit
Physics , 2003, DOI: 10.1046/j.1365-2966.2003.07101.x
Abstract: This paper reports positive detections of surface differential rotation on two rapidly rotating cool stars at several epochs, by using stellar surface features (both cool spots and magnetic regions) as tracers of the large scale latitudinal shear that distorts the convective envelope in this type of stars. We also report definite evidence that this differential rotation is different when estimated from cool spots or magnetic regions, and that it undergoes temporal fluctuations of potentially large amplitude on a time scale of a few years. We consider these results as further evidence that the dynamo processes operating in these stars are distributed throughout the convective zone rather than being confined at its base as in the Sun. By comparing our observations with two very simple models of the differential rotation within the convective zone, we obtain evidence that the internal rotation velocity field of the stars we investigated is not like that of the Sun, and may resemble that we expect for rapid rotators. We speculate that the changes in differential rotation result from the dynamo processes (and from the underlying magnetic cycle) that periodically converts magnetic energy into kinetic energy and vice versa. We emphasise that the technique outlined in this paper corresponds to the first practical method for investigating the large scale rotation velocity field within convective zones of cool active stars, and offers several advantages over asteroseismology for this particular purpose and this specific stellar class.
Linear spectropolarimetry of Ap stars: a new degree of constraint on magnetic structure
G. A. Wade,J. -F. Donati,G. Mathys,N. Piskunov
Physics , 1998,
Abstract: We present preliminary results from a programme aimed at acquiring linear spectropolarimetry of magnetic A and B stars. Linear polarization in the spectral lines of these objects is due to the Zeeman effect, and should provide detailed new information regarding the structure of their strong magnetic fields. To illustrate the impact of these new data, we compare observed circular and linear polarization line profiles of 53 Cam with the profiles predicted by the magnetic model by Landstreet (1988). Linear polarization in the spectral lines of all stars studied is extremely weak; in most cases, below the threshold of detectability even for very high SNRs. In order to overcome this problem, we employ the Least-Squares Deconvolution (LSD) multi-line analysis technique in order to extract low-noise mean line profiles and polarization signatures from our echelle spectra. Tests show that these mean signatures can be modelled as real spectral lines, and have the potential to lead to high-resolution maps of the magnetic and chemical abundance surface distributions.
Stellar differential rotation from direct starspot tracking
A. Collier Cameron,J. -F. Donati,M. Semel
Physics , 2001, DOI: 10.1046/j.1365-8711.2002.05120.x
Abstract: 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 starspots with sizes comparable to the largest sunspots. To achieve this we re-analyse four sequences of densely-sampled, high signal-to-noise 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 starspots 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 (1997) from cross-correlation of Doppler images derived a year earlier in 1995 December, and with a re-analysis of the 1996 data by the method of Donati et al (2000). 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.
The global magnetic topology of AB Doradus
M. Jardine,A. Collier Cameron,J. F. Donati
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05394.x
Abstract: We have used Zeeman-Doppler maps of the surface field of the young, rapid rotator AB Dor (P = 0.514 days) 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 degrees 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.
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