bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of spatially and spectrally resolved solar data with numerical simulations
Authors: Cauzzi G. , Ramos A. Asensio , Reardon K. P. , Janssen K.
Published: May 2007

We present a detailed comparison between high resolution observations of a quiet solar region and simulated spectra in several 3-D snapshots of a realistic radiation-hydrodynamical simulation of the solar atmosphere. We find excellent agreement between the two data sets, thus providing strong support to the realism of the simulations, and confirming the high quality of the instrumentation. We propose that spatially resolved spectral data can be employed as a complementary tool to more classic analysis for investigating the hotly debated topic of solar abundances.
Magnetic fields in Herbig Ae stars
Authors: Yudin R. V. , Pogodin M. A. , Hubrig S. , Schöller M.
Published: May 2007

We present and discuss the results of a comprehensive analysis of magnetic field measurements for a group of seven young Herbig Ae/Be stars obtained using low-resolution spectropolarimetry with FORS1 at the VLT. The first definite evidence for the presence of surface magnetic fields has been found only recently in the Herbig Ae stars HD 139614, HD 31648 and HD 144432 (Hubrig et al. 2004; Hubrig et al. 2006) from the measurement of circular polarization in Ca II lines and hydrogen lines located in the spectral region from 3900 to 4900 Å. The unusual Herbig Ae star HD 190073 shows distinctive Zeeman features in the Ca II doublet which displays several components in both H and K lines. The most recent analysis carried out separately for different lines has shown that Zeeman features appear in metallic lines in all seven objects of our programme. The most prominent of them are seen in lines of the CaII doublet possibly of both photospheric and circumstellar origin. The circumstellar Balmer line components demonstrate the existence of a magnetic field in HD 144432 and HD 31648. The photospheric components of these lines with signs of magnetic field are present in HD 144668.
Testing convection in stellar models using detached eclipsing binaries
Authors: Southworth John , Bruntt Hans
Published: May 2007

The fundamental properties of detached eclipsing binary stars can be measured very accurately, which could make them important objects for constraining the treatment of convection in theoretical stellar models. However, only four or five pieces of information can be found for the average system, which is not enough. We discuss studies of more interesting and useful objects: eclipsing binaries in clusters and eclipsing binaries with pulsating components.
Reversed granulation in mid-photosphere of the Sun
Authors: Janssen Katja , Cauzzi Gianna
Published: May 2007

Reversed granulation is predicted by numerical simulations and has been observed in the wings of chromospheric lines. Here we present reversed granulation in high spatial, spectral and temporal resolution, extracted from the photospheric Fe I 7090.4 Å line core intensities, once taking into account the velocity effects. Correlations with the continuum are studied in terms of smoothing, time delay and temporal evolution. The latter appears to be superimposed with an oscillatory signal that suggests the presence of gravity waves in the mid-photosphere.
Effect of the microturbulence parameter on the Color-Magnitude Diagram
Authors: Montalbán J. , Nendwich J. , Heiter U. , Kupka F. , Paunzen E. , Smalley B.
Published: May 2007

Microturbulence is usually treated in model atmospheres as a free parameter (ξt) that allows to re-establish agreement among abundances derived from different lines. Even if this parameter is a consequence of treating a 3D problem as a 1D one, it seems clear that microturbulence is linked to the velocity field within the atmosphere, and therefore to convection in the external layers. The values of the parameter as determined from observations show a dependence both on effective temperature and on surface gravity. In this paper we study how the microturbulence parameter used in the atmosphere models affects the theoretical color-magnitude diagram (CMD). First, in the Main Sequence (MS) domain due to the dependence of the microturbulence parameter on Teff; and second, in the giant branch (Pre-main sequence and Red Giant Branch) where several photometric indexes show a large variation due to the increase of the microturbulence parameter as the stellar gravity decreases. We predict then a significant change in the CMD, as well as in the color-temperature calibrations, if variations of ξt such as those observationally determined are included in theoretical CMD computations.
Revealing the onset of free convection in terrestrial planet atmospheres
Authors: Els Sebastian G. , Vogiatzis Konstantinos
Published: May 2007

The site testing program for the Thirty-Meter-Telescope (TMT) is monitoring the seeing conditions above several mountains. Observations are presented which show that the ground layer seeing (0 m–500 m above the ground) is influenced by the onset of free convection. It is shown that under radiation dominated conditions, indicating free convection of the atmosphere above the site, the occurence of the very best seeing conditions is supressed. A quantitative threshold for these conditions is given, allowing to estimate the shortterm future stability of the ground layer seeing.
Modeling convection and zonal winds in giant planets
Authors: Evonuk Martha , Glatzmaier Gary A.
Published: May 2007

Three basic modeling approaches have been used to numerically simulate fluid turbulence and the banded zonal winds in the interiors and atmospheres of giant planets: shallow-water models, deep-shell Boussinesq models and deep-shell anelastic models. We review these models and discuss the approximations and assumptions upon which they are based. All three can produce banded zonal wind patterns at the surface. However, shallow-water models produce a retrograde (i.e., westward) zonal jet in the equatorial region, whereas strong prograde (i.e., eastward) equatorial jets exist on Jupiter and Saturn. Deep-shell Boussinesq models maintain prograde equatorial jets by the classic method of vortex stretching of convective columnar flows; however, they neglect the effects of the large density stratification in these giant planets. Deep-shell anelastic models account for density stratification and maintain prograde equatorial jets by generating vorticity as rising fluid expands and sinking fluid contracts, without the constraint of long thin convective columns.
Convection and mixing in stars: theory versus observations
Authors: Chiosi Cesare
Published: May 2007

We summarize the results of stellar evolution theory obtained over the past three decades and highlight the points of disagreement with the present-day observational information. Arguments are given to favour stellar models in which the classical treatment of convection and mixing is abandoned and more complex schemes are adopted.
The role of convection in AGB modeling
Authors: Ventura Paolo
Published: May 2007

The modeling of the Asymptotic Giant Branch phase is made highly uncertain by some still unsolved issues related to the input macro-physics used to calculate the stellar evolution, namely mass loss, nuclear cross sections, overshooting and convective modeling. We show that in the massive intermediate mass models, which achieve at the bottom of their convective envelope temperatures sufficiently high to favour an advanced nucleosynthesis, the treatment of convection plays a major role in determining the physical and chemical evolution of the stellar models during this evolutionary phase.
The DB gap of white dwarfs and semiconvection
Authors: Shibahashi Hiromoto
Published: May 2007

A remarkable and intriguing fact is that few DB white dwarfs, i.e., objects with a helium-rich atmosphere, are found in the effective temperature range between 45000-30000 K, and this exclusion zone is known as the DB gap. Since the temperatures of the blue and the red edges of the DB gap coincide with the effective temperatures where HeII/III and HeI/II convection zones show up respectively, convective mixing is suspected to be the cause of the appearance of DBs outside the DB gap. Chemical separation due to gravitational settling in the convectively stable atmosphere is then suspected to be the cause of the presence of the DB gap. The white dwarfs which turn back to DBs after the DB gap phase are expected to have a semiconvective layer, which is superadiabatic but convectively stable, when they are located near the red edge of the DB gap. Such white dwarfs are expected to be pulsationally unstable. Discovery of pulsation in them will open up a new class of pulsating white dwarfs to asteroseismic study.
The destruction of 3He by Rayleigh-Taylor instability on the first giant branch
Authors: Eggleton Peter P. , Dearborn David S. P. , Lattanzio John C.
Published: May 2007

Low-mass stars, ∼ 1–2 solar masses, near the Main Sequence are efficient at producing 3He, which they mix into the convective envelope on the giant branch and distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the observed cosmic abundance of 3He with the predictions of Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell. In this zone the burning of the 3He left behind by the retreating convective envelope is predominantly by the reaction 3He + 3He → 4He + 1H + 1H, a reaction which, untypically for stellar nuclear reactions, lowers the mean molecular weight, leading to a local minimum. This local minimum leads to Rayleigh-Taylor instability, and turbulent motion is generated which will continue ultimately up into the normal convective envelope. Consequently material from the envelope is dragged down sufficiently close to the burning shell that the He in it is progressively destroyed. Thus we are able to remove the threat that He production in low-mass stars poses to the Big Bang nucleosynthesis of 3He.

Some slow mixing mechanism has long been suspected, that connects the convective envelope of a red giant to the burning shell. It appears to be necessary to account for progressive changes in the C/C and N/C ratios on the First Giant Branch. We suggest that these phenomena are also due to the Rayleigh-Taylor-unstable character of the He-burning region.
Round table discussion on session D: stellar evolution, nucleosynthesis and convective mixing
Authors: Weiss Achim
Published: May 2007

Summary of the round table discussion following Session IV on “Stellar evolution, nucleosynthesis, and convective mixing”.
Si, O, Ne, and C shell burning
Authors: Meakin Casey , Arnett David
Published: May 2007

We simulate the reactive-hydrodynamic flow for a variety of convective shell burning epochs in supernova progenitor models. The neutrino-cooled stages of carbon, neon, oxygen, and silicon burning are simulated in two and three dimensions. Even in the absence of rotation significant symmetry breaking occurs (10% in rms variation in thermodynamic variables such as temperature and density). These distortions are caused by turbulent convection interacting with stably stratified boundaries. Strong interactions of multiple active shells is seen; it is mediated by waves generated by convection. Some implications for supernova progenitors are presented.
12C/13C ratios in giants of open clusters
Authors: Smiljanic Rodolfo , Gauderon R. , North P. , Barbuy B. , Charbonnel C. , Mowlavi N.
Published: May 2007

The carbon isotopic ratio,12C/13C, is a tracer of the mixing events during the evolution along the giant branch, due to the conversion of12C into13C (and14N) via the CN cycle. A decrease of this ratio from 90, the solar value, to 20–25, is expected due to the first dredge-up. However, ratios down to 3–4, the CN cycle equilibrium value, have been observed in giants of the field, of globular and of open clusters. Observations seem to indicate a non-standard mixing in the RGB, probably beginning in the luminosity bump, when the outward moving hydrogen burning shell crosses the molecular weight barrier left by the convective layer in its maximum extent. We are currently analyzing a sample of 24 giants in 8 open clusters for which we determined12C/13C from high resolution, high signal to noise spectra using spectrum synthesis. In this work we discuss the general characteristics of our results in comparison to previous analyses of giants in open clusters available in the literature.
Abundances of C, N and O as probes of mixing in low-mass helium-core burning stars
Authors: Tautvaišienė G. , Edvardsson B. , Puzeras E. , Ilyin I.
Published: May 2007

A large population of helium-core-burning clump stars of the Galactic field, revealed by the Hipparcos orbiting observatory, is an excellent target for the analysis of mixing processes in evolved low-mass metal-abundant stars. In this contribution we overview first result of 12C, 13C, N and O abundance determinations in our extensive project which we started with the aim at deriving the fundamental parameters and abundances of more than 20 chemical elements in a large sample of Galactic clump stars.
Isotopic abundances of Carbon and Oxygen in Oxygen-rich giant stars
Authors: Tsuji Takashi
Published: May 2007

16O/17O and 12C/13C ratios in 23 M giants are determined from high resolution IR spectra. The results are confronted with the current models on the convective mixing.
Inclusion of a time-dependent, non-local convection theory in a stellar evolution code
Authors: Weiss Achim , Flaskamp Martin
Published: May 2007

The non-local, time-dependent convection theory of Kuhfuß (1986) in both its one- and three-equation form has been implemented in the Garching stellar evolution code. We present details of the implementation and the difficulties encountered. Specific test cases have been calculated, among them a 5 M⊙ star and the Sun. These cases point out deficits of the theory. In particular, the assumption of an isotropic velocity field leads to too extensive overshooting and has to be modified at convective boundaries. Some encouraging aspects are indicated as well.
Pre-main Sequence models with convection described by 2D-RHD numerical simulations
Authors: Montalbán Josefina , D'Antona Francesca
Published: May 2007

The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory of convection (MLT) in which the αMLT = l/Hp parameter calibration is based on 2D–hydrodynamical models (Ludwig et al. 1999). These MLT–α2D stellar models and tracks are very similar to those computed with non–grey ATLAS9 atmospheric boundary conditions and Full Spectrum of Turbulence (FST) convection model both in the atmosphere and in the interior. As for the FST models, the comparison of the new tracks with the location on the HR diagram of pre–MS binaries is not completely satisfactory; and the pre–MS lithium depletion in the MLT–α2D tracks is still much larger than that expected from the observations of lithium in young open clusters. Thus, in spite of the fact that 2D RHD models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in pre–MS, should be smaller than in the MS.
Convective processes and hydromagnetic instabilities in core collapse supernova simulations
Authors: Obergaulinger M. , Hammer N. J. , Müller E.
Published: May 2007

Hydrodynamic and potentially also hydromagnetic instabilities play an important role in core-collapse supernovae. We report on simulations performed to investigate the effects of magnetic fields in simplified models. Furthermore, we present efforts to get rid of problems arising from the coordinate singularity at the symmetry axis and the associated boundary conditions.
Solar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars
Authors: Samadi R. , Belkacem K. , Goupil M.-J. , Kupka F. , Dupret M.-A.
Published: May 2007

Excitation of solar-like oscillations is attributed to turbulent convection and takes place at the upper-most part of the outer convective zones. Amplitudes of these oscillations depend on the efficiency of the excitation processes as well as on the properties of turbulent convection. We present past and recent improvements on the modeling of those processes. We show how the mode amplitudes and mode line-widths can bring information about the turbulence in the specific cases of the Sun and α Cen A.
Can we detect convection in the Sun?
Authors: Hanasoge Shravan M. , Duvall T. L. , Derosa M. L. , Miesch M. S.
Published: May 2007

We investigate the possibility of detecting deep convection in the Sun by computing travel-time shifts induced by convective flows interacting with propagating waves. The convection zone is modeled using a velocity profile taken from an Anelastic convection simulation. We present results obtained from a ray calculation of travel-time shifts. We compare these results with a full 3D calculation of the wave-flow interaction.
Testing Reynolds stress model in solar interior
Authors: Yang J. Y. , Li Y.
Published: May 2007

The Reynolds stress model (RSM) for turbulent convection motion is compared to the MLT in solar model. The free parameters involved in the RSM are also tested with the aid of helioseismology. It is found that, the structure of solar convection zone is differ from the MLT when using the RSM, especially for the Reynolds correlations and the temperature gradient. Both the local and non-local RSM can improve the calculated solar p-mode oscillation frequencies with the appropriate choice of the parameters' value.
A closure model for turbulent convection. Application to the excitation of p modes
Authors: Belkacem K. , Samadi R. , Goupil M.-J. , Kupka F. , Baudin F.
Published: May 2007

Oscillations of stellar p modes excited by turbulent convection are investigated. In the uppermost part of the solar convection zone, radiative cooling is responsible for the formation of turbulent plumes, hence the medium is characterized with downdrafts and updrafts. The motivation is to take the asymmetry of up- and downflows created by turbulent plumes into account through an adapted closure model. We built a generalized two-scale mass-flux model (GTFM) that considers both the skew introduced by the presence of two flows and the effect of turbulence onto each flow. In order to apply the GTFM to the solar case, we introduce the plume dynamics as modelled by Rieutord & Zahn (1995) and construct a closure model with plumes (CMP). The CMP enables to express third- and fourth-order velocity correlation products in terms of the second-order ones. When comparing with 3D numerical simulation results, the CMP improves the agreement for the fourth order moments in comparison with the quasi-normal approximation (QNA) or the classical mass-flux model (MFM). This excitation model reproduces the maximum of the power supplied to solar p modes, when compared with GOLF observations.
Monitoring of the cool ZZ Ceti star PG 2303+243
Authors: Pakštienė E. , Solheim J.-E. , Handler G. , Steininger B. , Rodler F. , Paparo M. , Bognar Z. , Patterson R. , Reed M.
Published: May 2007

Because PG 2303+243 had not been observed since 1992, we have arranged the mini-campaign of six observatories on PG 2303+243 in 2004 Sep. 5-21 for the more detailed frequency analysis of variations. The amplitudes of the most frequencies change even during the one night and some frequencies can disappear or appear in one year. Probably it is resonance effect. We have estimated 69 frequencies of pulsations and their amplitudes according the observations from 2004 and 36 frequencies and its amplitudes from 2005.
Effect of convection modeling on the non-adiabatic seismic observables of δ Scuti stars
Authors: Montalbán Josefina , Dupret Marc-Antoine
Published: May 2007

Pulsation frequencies in δ Scuti stars do not fall in the asymptotic domain, therefore, multi-band photometric methods are currently used to identify the pulsation modes. Theoretical photometric amplitude ratios and phase differences between photometric bands depend, however, on the treatment of convection in surface stellar layers. In this poster we present the results of applying the non-adiabatic analysis by Dupret et al. (2003) to δ Scuti stellar models computed by using the FST (Canuto et al. 1996, CGM) treatment of convection in the interior and in the atmosphere. We determine the amplitude ratios and phase difference for several bands in the Strömgren photometric system, and we compare the results obtained with the FST treatment and with the classical Mixing-length theory. We show that the differences in the external thermal structure are clearly reflected in the photometric phase differences.
Rotation of the solar convection zone from helioseismology
Authors: Christensen-Dalsgaard Jørgen
Published: May 2007

Helioseismology has provided very detailed inferences about rotation of the solar interior. Within the convection zone the rotation rate roughly shares the latitudinal variation seen in the surface differential rotation. The transition to the nearly uniformly rotating radiative interior takes place in a narrow tachocline, which is likely important to the operation of the solar magnetic cycle. The convection-zone rotation displays zonal flows, regions of slightly more rapid and slow rotation, extending over much of the depth of the convection zone and converging towards the equator as the solar cycle progresses. In addition, there is some evidence for a quasi-periodic variation in rotation, with a period of around 1.3 yr, at the equator near the bottom of the convection zone.
Theoretical estimates of the convective turnover time for low-mass, rotating pre-main sequence stars
Authors: Mendes L. T. S. , Landin N. R. , Vaz L. P. R.
Published: May 2007

The Rossby number Ro is an important quantity related to the well-known magnetic activity-rotation correlation for main sequence, solar-type stars. For such stars, Ro can be obtained by a semi-empirical relationship between the convective turnover time τc and the B-V colour index, but an equivalent activity-rotation correlation seems not to exist for pre-main sequence stars. In this work we report theoretical estimates of τc for low-mass, rotating pre-main sequence stars under either the Full Spectrum of Turbulence (FST) or the classical Mixing Length Theory (MLT) convection models. The results for the MLT models show that the lower the convection efficiency the higher τc, while the FST models give τc lower than those for the MLT. The presence of a parametric magnetic field lowers the convection efficiency, resulting in smaller τc values.
Near-surface shear layer dynamics
Authors: Brandenburg Axel
Published: May 2007

The outer surface layers of the sun show a clear deceleration at low latitudes. This is generally thought to be the result of a strong dominance of vertical turbulent motions associated with strong downdrafts. This strong negative radial shear should not only contribute to amplifying the toroidal field locally and to expelling magnetic helicity, but it may also be responsible for producing a strong prograde pattern speed in the supergranulation layer. Using simulations of rotating stratified convection in cartesian boxes located at low latitudes around the equator it is shown that in the surface layers patterns move in the prograde direction on top of a retrograde mean background flow. These patterns may also be associated with magnetic tracers and even sunspot proper motions that are known to be prograde relative to the much slower surface plasma.
Dynamos of giant planets
Authors: Busse F. H. , Simitev R.
Published: May 2007

Possibilities and difficulties of applying the theory of magnetic field generation by convection flows in rotating spherical fluid shells to the Giant Planets are outlined. Recent progress in the understanding of the distribution of electrical conductivity in the Giant Planets suggests that the dynamo process occurs predominantly in regions of semiconductivity. In contrast to the geodynamo the magnetic field generation in the Giant Planets is thus characterized by strong radial conductivity variations. The importance of the constraint on the Ohmic dissipation provided by the planetary luminosity is emphasized. Planetary dynamos are likely to be of an oscillatory type, although these oscillations may not be evident from the exterior of the planets.
Photospheric magnetoconvection
Authors: Cameron Robert , Vögler Alexander , Schüssler Manfred
Published: May 2007

Magnetic fields, convective motions and radiative energy transport all strongly influence each other in the solar photosphere. Attempts to understand, at a quantitative level, the diverse phenomona seen in the photosphere have relied on increasingly powerful computers since the 1980's. This approach has been remarkably successful and some recent examples, particularly cases where all three ingredients are important, will be discussed.
Generation of coupled global and local magnetic fields by a cellular MHD dynamo
Authors: Getling A. V. , Simitev R. D. , Busse F. H.
Published: May 2007

The convection-driven MHD dynamo in a rotating spherical shell is simulated numerically. Convection cells are regarded as a connecting link between the global and local electromagnetic processes. Local (in many cases, bipolar) magnetic structures are regularly produced by convection cells. Dynamo regimes in “thick” and “thin” shells are discussed. In the first case, the “general” magnetic field maintained by the dynamo has a sign-alternating dipolar component, which varies cyclically, although not periodically. The local structures, as they disintegrate, change into background fields, which drift toward the poles. From time to time, reversals of the magnetic fields in the polar regions occur, as “new” background fields expel the “old” fields. In the second case, the system settles down to a nearly stationary regime without polarity reversals.
Round table discussion of session G: MHD convection and dynamos
Authors: Toomre Juri
Published: May 2007

Vigorous discussion ensued about conditions under which both small-scale and global-scale dynamo action would be realized within real stars where the flow fields are expected to be highly turbulent and the magnetic Prandtl numbers small. Our nearest star reminds us that intricate boundary-layer phenomena may have to also be considered, such as the presence of a tachocline of rotational shear at the base of the solar convection zone revealed by helioseismology, which suggests that an interface dynamo may be at work to produce the observed 22-year cycles of large-scale magnetic activity.
Is solar convection responsible for the local amplification and structuring of magnetic fields? (Observational test of the hypothesis)
Authors: Getling A. V. , Bao X. M.
Published: May 2007

Full-vector magnetograms and Dopplergrams of selected areas of the solar photosphere are used to compute the vertical component of the right-hand side of the vector induction equation. We attempt to find a criterion for the action of the convective mechanism of amplification and structuring of magnetic fields using the distributions of this quantity and of the vertical magnetic-field component.
Quasi-regular structures in the solar photosphere (trenching in the brightness relief): Algorithmic treatment
Authors: Getling A. V. , Buchnev A. A.
Published: May 2007

The results of applying two algorithmic techniques to time-averaged images of the solar granulation are presented. Some quasi-regular structures, which are hidden in the intense noise due to the presence of light blotches in the images, can be detected in this way.
Spectral properties of magnetohydrodynamic convection with mean horizontal temperature gradient
Authors: Skandera D. , Müller W.-Ch.
Published: May 2007

Spectral properties of convective magnetohydrodynamic (MHD) turbulence in two and three dimensions are studied by means of direct numerical simulations (Skandera D. & Müller W.-C. 2006). The investigated system is set up with a mean horizontal temperature gradient in order to avoid a development of elevator instabilities in a fully periodic box. All simulations are performed without mean magnetic field. The applied resolution is 5123 and 20482. The MHD equation are solved by a numerical code (Müller & Biskamp 2000) that uses a standard pseudospectral scheme. For removing of aliasing errors a spherical truncation method is employed. Obtained results are compared with predictions of various existing phenomenological theories for magnetohydrodynamic and convective turbulence (Müller & Biskamp 2000). While the three-dimensional system is found to operate in a Kolmogorov-like regime where buoyant forces have a negligible impact on the turbulence dynamics (relatively low Rayleigh number achieved in the simulation; Ra ∼106), the two-dimensional system exhibits interesting irregular quasi-oscillations between a buoyancy dominated Bolgiano-Obukhov-like regime of turbulence and a standard Iroshnikov-Kraichnan-like regime of turbulence (Müller & Biskamp 2000). The most important parameter determining the turbulent regime of 2D magnetoconvection, apart from a high Rayleigh number, seems to be the mutual alignment of velocity and magnetic fields. The non-linear dynamics and the interplay between individual fields are examined with different transfer functions that confirm basic assumptions about directions of energy transfer in spectral space. Kinetic, magnetic and temperature energy are transported by a turbulent cascade from large to smaller scales. The local/nonlocal character of the transport is tested for several individual terms in the governing equations. Moreover, other statistical quantities, e.g. probability density functions, are computed as well. A passive character of the temperature field in the investigated three-dimensional magnetoconvection is supported by computations of intermittency using extended self-similarity. The intermittency of the Elsasser field z+ is in agreement with results from numerical simulations of isotropic MHD turbulence (Müller & Biskamp 2000). The intermittency of the temperature field is found to approximately agree with results of passive scalar measurements in hydrodynamic turbulence (Ruiz-Chavarria, Baudet & Ciliberto 1996).
Concluding remarks
Authors: Zahn Jean-Paul
Published: May 2007

Thermal convection occurs in most objects that populate our Universe, whenever radiation is insufficient to transport the heat because the medium is too opaque. In astrophysical objects convection involves a wide range of spatial and temporal scales - experts call this turbulence - which makes it rather difficult to model. For this reason convection remains one of the major uncertainties when modeling stars and planets, and this is partly true also for accretion disks. However, substantial progress has been achieved during the past years, both in the numerical simulation of convective regions and in the observation of convective flows by various new techniques.
Antigen presentation by MHC class II molecules
Authors: Weenink Sarah M , Gautam Anand M
Published: May 2007

The tremendous explosion in the field of MHC research in the last 5 years has significantly advanced our understanding of antigen processing pathways, particularly with regard to details of MHC class II‐mediated antigen presentation. MHC class II molecules at the surface of antigen presenting cells present antigenie peptides to CD4+ T helper cells. However for effective cell surface antigen presentation, a number of highly synchronized events must first take place intracellularly. The monomorphic protein, invariant chain (Ii), is a crucial participant in MHC class II antigen presentation. Acting as a molecular chaperone, this molecule escorts the newly synthesized class II heterodimers from the endoplasmic reticulum into the endosomal system. During this manoeuvre, the interaction of Ii with class II serves to prevent premature association of antigenic peptide. Once the complex reaches the acidic environment of the endosomes, Ii is proteolytically degraded and dissociates, leaving the class II binding site available for binding antigenic peptide derived from exogenous proteins. The final Ii fragment to be displaced. CLIP (class II‐associated invariant chain peptides), must be physically removed from the class II binding groove with assistance from another MHC‐encoded molecule. DM. The interaction of DM with class II also aids in the subsequent rapid loading of high‐affinity antigen‐derived peptides into the MHC class II groove. The stable peptide‐loaded complexes are now ready to exit the endocytic compartments to present their peptide antigen to specific T helper cells at the cell surface.
Mechanism of antigen‐driven somatic hypermutation of rearranged immunoglobulin V(D)J genes in the mouse
Authors: Steele Edward J , Rotheneluh Harald S , Blanden Robert V
Published: May 2007

Available data relevant to the mechanism of somatic hypermutation have been critically evaluated in the context of alternative models: (i) error‐generating reverse transcription (RT) followed by homologous recombination: and (ii) error‐prone DNA replication/repair. A set of basic principles concerning somatic hypermutation has also been formulated and a revised and expanded ‘RT‐Mutatorsome” concept (analogous to telomerase) is presented which is consistent with these principles and all data on the distribution of somatic mutations in normal and Ig transgenic mice carrying particular V(D)J and flanking region constructs. It is predicted that in the mouse VH and Vk loci, the J–C intronic Enhancer‐Nuclear Matrix Attachment Region (Ei/MAR) contains a unique sequence motif or secondary structure which ensures that only V(D)J sequences mutate whilst other regions of the genome are not mutated.
Cellular immunity: The final paradigm?
Authors: Attfield Derrick C
Published: May 2007

Cytotoxic T lymphocyte activation and most probably tolerance induction, is dependent on antigen presentation by a specialized group of cells. APC, including macrophages, dendritic cells and B cells. Since T lymphocytes are, at least, the majority of the time MHC molecule class‐specific, CD8+ T cells require antigen presented by MHC class I molecules. MHC class I molecules are, however, restricted to presenting endogenously produced antigenic peptides. Most threats to the organism are of exogenous origin and do not uniformly affect all or even most of the cells of an organism. This precludes the likelihood that any number of APC would be involved in every threatening situation, which raises the important question of how T lymphocytes are indeed activated, especially CD8+ T cells (MHC class I restricted).
Molecular mimicry: Can epitope mimicry induce autoimmune disease?
Authors: Davies Janet M
Published: May 2007

Mimicry of host antigens by infectious agents may induce cross‐reactive autoimmune responses to epitopes within host proteins which, in susceptible individuals, may tip the balance of immunological response versus tolerance toward response and subsequently lead to autoimmune disease. Epitope mimicry may indeed be involved in the pathogenesis of several diseases such as post‐viral myocarditis or Chagas disease, but for many other diseases in which it has been implicated, such as insulin‐dependent diabetes mellitis or rheumatoid arthritis, convincing evidence is still lacking. Even if an epitope mimic can support a cross‐reactive T or B cell response in vitro, its ability to induce an autoimmune disease in vivo will depend upon the appropriate presentation of the mimicked host antigen in the target tissue and, in the case of T cell mimics, the ability of the mimicking epitope to induce a proliferative rather than anergizing response upon engagement of the MHC‐peptide complex with the T cell receptor. B cell presentation of mimicking foreign antigen to T cells is a possible mechanism for instigating an autoimmune response to self antigens that in turn can lead to autoimmune disease under particular conditions of antigen presentation, secondary signalling and effector cell repertoire. In this review evidence in support of epitope mimicry is examined in the light of the necessary immunological considerations of the theory.
Quantitative reverse transcriptase‐PCR amplification of cytokine mRNA in liver biopsy specimens using a non‐competitive method
Authors: Bishop GA , Rokahr KL , Lowes M , McGuinness PH , Napoli J , Decruz DJ , Wong W‐Y , McCaughan GW
Published: May 2007

Reverse transcriptase‐PCR (RT‐PCR) amplification of mRNA is often the only technique able to detect expression of cytokine mRNA In small samples. The aim of this work was to investigate the utility of a non‐competitive RT‐PCR which used external standards to quantitate TNF‐α mRNA in liver biopsy specimens from liver transplant patients. It involved removal of aliquots from the PCR reaction at successive cycles, followed by dot‐blotting of the samples onto nylon membrane and hybridization with a radioactively‐labelled internal probe. Phosphorimage analysis of the labelled membranes allowed quantitation of the relative amount of PCR product at successive cycles. Plots of log(counts) versus cycle number showed straight lines in the exponential phase of amplification. The slopes of these lines showed the efficiency of amplification, which ranged from 76 to 87% for liver biopsy samples. Estimation of liver biopsy levels of TNF‐α in two separate PCR amplifications showed low inter‐assay variability (r2 = 0.98), Comparison of two separate cDNA syntheses also showed good correlation (r2 = 0.81, P < 0.0001), although not as good as for the PCR alone. This shows that variation in efficiency of cDNA synthesis is likely to contribute as much or more to variability of the analysis as variations in PCR amplification.
Tandem peptide epitopes facilitate CD4‐dependent activation of T cell clones
Authors: Hayball John D , Fidler Sarah J , Palliser Deborah , Rees Ann DM , Lamb Jonathan R , Lake Richard A
Published: May 2007

Peptides that consist of two tandemly repeated epitopes joined by a flexible linker have an increased affinity for class II molecules and are more potent at inducing prolireration of T cell clones than monomeric epitopes. The increase in potency of peptides with two epitopes for individual T cell clones is proportional to the relative CD4 dependence of the clones. We show that epitope dimers activate T cell clones that respond sub‐optimally to monomeric epitopes presented by APC from HIV‐infected donors. We hypothesize that HIV+ APC normally fail to stimulate the clones because virally encoded gpl20 sequesters CD4 from the activation complex, but epitope dimers overcome this effect because they are better able to recruit CD4.

The αβ heterodimer of human class II (HLA‐DR1) is further ordered as a dimer of heterodimers (superdimer) at least in its crystal form. Since class II molecules have an open‐ended antigen binding groove, the superdimer is theoretically permissive of stable binding of two peptide epitopes linked in tandem. Our data support a role for the MHC class II dimer of heterodimers in amplifying the proliforative response of T cells to antigen by dint of the superdimers having a higher affinity for CD4 than the nominal class II αβ heterodimers.
TCRβ‐independent development of CD4+ CD8+ thymocytes observed in a strain of scid mice
Authors: Shibata S , Asano T , Ogura A , Hashimoto N , Hayakawa J , Naiki M , Doi K
Published: May 2007

Mice homozygous for severe combined immunodeficiency (scid) mutation usually halt their thymocyte development at the CD4−CD8− double negative (DN) stage due to their inability of TCR gene rearrangement. In this study, we report that SCID‐bg mice, which were originally generated by mating CB‐17‐scid mice with KSN‐bg mice. spontaneously develop dominant CD4+CD8+ double positive (DP) thymocytes. Their thymi were mainly composed of DN, CD4−CD8+ and DP cells, and the majority of them did not present CD3. Similarly, they lacked TCRβ expression both on cell surface and in cytoplasm, which suggests that the thymocyte development to the DP stage observed m SCID‐bg mice was independent of CD3 and TCRβ expression. In spite of significant DP thymocytes in SCID‐bg mice, the histology of their thymi was not so different from those of CB‐17‐scid mice. Analysis of bone marrow cells in SCID‐bg mice showed that the development of B lineage cells was not altered when compared with CB‐17‐scid mice. These findings point out the fact that thymocytes in SCID‐bg mice have a peculiar characteristic compared to CB‐17‐scid mice, and provide evidence of TCRβ‐independent development of thymocytes to the DP stage.
Inhibition of IL‐2 production by nitric oxide: A novel self‐regulatory mechanism for Th1 cell proliferation
Authors: Taylor‐Robinson Andrew W
Published: May 2007

Cloned Th1 cells, but not Th2 cells, specific for malaria antigen, produce nitric oxide (NO) when activated with specific antigen or Con A. Furthermore. NO inhibits proliferation of, and production of IL‐2 and IFN‐γ by, Th1 but not Th2 cells, Here, it is demonstrated that the inhibition of Th1 cell proliferation by the NO donor S‐nitroso‐N‐acetyl penicillamine (SNAP) can be reversed by the addition of rIL‐2 but not of rIFN‐γ, suggesting that the inhibition of Th1 cells by NO may be presenting the production of IL‐2. Dose‐response studies showed that Th1 cells produce optimal levels of IL‐2 and a proliferative response, and no detectable NO, when stimulated with relatively low concentrations of antigen or mitogen in vitro. As the antigen/mitogen increased, however, high levels of NO were produced, accompanied by a concomitant reduction in IL‐2 secretion and T cell proliferation. The proliferation of, and IL‐2‐IFN‐γ production by, naive CD4+ T cells from normal spleens activated with Con A in vitro can be similarly inhibited by SNAP. These results suggest that NO may serve as a self‐regulator molecule preventing the over‐expansion of Th1 cells. Unrestricted Th1 cell activity has been implicated in a range of immunopathologies and autoimmune diseases. The proposed mechanism for down‐regulation of Th1 cell function may also account for the suppression of lymphocyte proliferation observed in malaria infections.
Vaccines for control of fertility and hormone‐dependent cancers
Authors: Talwar GP
Published: May 2007

This article traces and reviews the development of vaccines against two reproductive hormones, luteinizing hormone release hormone (LHRH) and human chorionic gonadotrophin (hCG). The heterospecies dimer (HSD)‐dehydrogenase hCG vaccine is the first to have completed phase II clinical trials providing evidence for the prevention of pregnancy in humans by immunization with this vaccine. The vaccine acts without blocking ovulation or disturbance of menstrual bleeding. Its effect is fully reversible and fertility is regained with decline of the antibodies. The antibody response in humans is directed against immunodominant epitopes located in the core part of βhCG, distinct from the C‐terminal region, However, problems arising from carrier‐conjugated vaccines against ‘self’ hormones such as carrier‐induced immunosuppression on repeated immunization are evoked and solutions discussed. The LHRH vaccine, a model semi‐synthetic vaccine, can be used in both sexes and has applications in regulation of fertility of animals. It has therapeutic applications as a cost‐effective approach in clinical conditions where LHRH agonists and antagonists have therapeutic action. The vaccine has undergone phase 1 and H clinical trials in advanced‐stage carcinoma of prostate patients with encouraging results.
The role of dendritic cells in T cell activation
Authors: Ni K , O'Neill HC
Published: May 2007

Dendritic cells (DC) are distinguishable from other antigen‐presenting cells by their potent antigen‐presenting capacity. They are not only efficient at presenting peptide antigen but can also process and present soluble protein antigens to antigen‐specific T cells and cloned T cell lines. They are very strong stimulators of both allogeneic and syngeneic mixed lymphocyte reactions and have a unique capacity to stimulate naive T cells. The potent functional capacity of DC is related to a high‐level expression of major histocompatibility complex class I/II molecules and constitutive expression of costimulatory molecules, such as CD80/CD86 as well as heat stable antigen. CD40 and the leucocyte function antigen (LFA) family of adhesion molecules. Recent studies have shown that DC are also involved in regulation of the immune response via induction of both central and peripheral tolerance.
Elution and partial characterization of immunoglobulins bound to ovine placenta
Authors: Omwandho Charles A , Hall Amanda L , Falconer John , Roberts Timothy K
Published: May 2007

Immunoglobulins were eluted from ovine placentae and characterized by immunoprecipitation, electrophoresis, western blotting and ELISA. IgG was shown to comprise the bulk of placental‐bound immunoglobulins while smaller amounts of IgM and only trace amounts of IgA were demonstrated. Results suggest that ovine placental IgG eluted by surgical cannulation of the uterine blood vessels in situ is similar to that eluted from postpartum placentae in vitro, implying that there may be some transfer of antibodies across the maternal side of the placental barrier to the trophoblast. These antibodies are rich in IgG, and IgG2, have a relative molecular weight of 158 kDa, and bind to an 80 kDa peptide prepared from pre‐acidified ovine placental cotyledons.

We propose that the binding of placental IgG to the 80 kDa antigen may prevent immunological rejection of the foetus by competitively excluding cytotoxic cells of maternal origin such as NK cells. Also, given that a similar antigen (80 kDa) has been reported in humans and equines and shown to be saturated with IgG in term placentae, we propose that this antigen may be conserved in several mammalian species for reproductive purposes. Consequently, we suggest that the ovine placental IgG and the 80 kDa antigen may be suitable as models for the study of maternal‐foetal interactions in mammalian pregnancies.
Characterization of ovine umbilical vein endothelial cells and their expression of cell adhesion molecules: Comparative study with human endothelial cells
Authors: Grooby Warwick L , Krishnan Ravi , Russ Graeme R
Published: May 2007

This paper reports the isolation and characterization of sheep umbilical vein endothelial cells (ShUVEC) and examines the kinetics of the expression of intercellular cell adhesion molecule‐1 (ICAM‐1) and vascular cell adhesion molecule‐l (VCAM‐I). In culture, the cells demonstrate the classical endothelial cell phenotype, including the expression of von Willebrand Factor (vWF), the metabolism of acetylated‐low density lipoprotein (Ac‐LDL) and the typical cobblestone monolayer appearance. Northern analysis of the expression of ICAM‐1 and VCAM‐I mRNA by ShUVEC stimulated with either LPS or recombinant ovine (ro)‐TNE‐α or IL‐lβ demonstrated peak RNA expression for both molecules at between 2 and 6 h after stimulation. Maximum VCAM‐1 protein expression occurred between 6 and 12 h after stimulation with ro‐TNF‐α, followed by a slight decrease to a plateau extending beyond 48 h. The levels and kinetics of expression of additional cell surface proteins on ShUVEC, including P‐sclectin (CD62P). β1 integrin (CD29) and ovine MHC classes I and II, were found to be almost identical to that observed on human endothelial cells (HUVEC) cultured under similar conditions. Based on the above results, we conclude that the ShUVEC represent the ovine equivalent of HUVEC, both phenotypically and functionally.