African Meridian B-Field Education and Research (AMBER) Array

The AMBER array contains four magnetometers and spans across the geomagnetic equator from L of 1 to an L of 1.4. In addition to filling the largest land-based gap in global magnetometer coverage, the AMBER array will address two fundamental areas of space physics: (1) the processes governing electrodynamics of the equatorial ionosphere as a function of latitude (or L-shell), local time, longitude, magnetic activity, and season, and (2) ULF pulsation strength and its connection with equatorial electrojet strength at low/mid-latitude regions. Satellite observations show unique equatorial ionospheric structures in the African sector, though these have not been confirmed by observation from the ground due to lack of ground-based instruments in the region. In order to have a complete global understanding of equatorial ionosphere motions, deployment of ground-based magnetometers in Africa is essential. One focus of IHY is the deployment of networks of small instruments, including the development of research infrastructure in developing nations through the United Nations Basic Space Science (UNBSS) Small Instrument Array. Therefore, AMBER magnetometer array in partnership with parallel US funded GPS receivers in Africa will allow us to understand the electrodynamics that governs equatorial ionosphere motions. While AMBER routinely observes the F region plasma drift mechanism (E × B drift), the GPS stations will monitor the structure of plasma at low/mid-latitudes in the African sectors. In addition to new scientific discoveries and advancing the space science research into Africa by establishing scientific collaborations between scientists in the developing and developed nations, the AMBER project also contributes to developing the basic science of heliophysics through cross-disciplinary studies of universal process. This includes the creation of sustainable research/training infrastructure within the developing nations (Africa).     

Introducing the near infrared VLTI instrument AMBER to its users

AMBER is the General User near infrared focal instrument of the Very Large Telescope Interferometer. It is a single mode, dispersed fringes, three telescope instrument. A limiting magnitude of the order of H=13 will allow tackling of a fair sample of extra galactic targets. A very high accuracy, in particular in color differential phase and closure phase modes gives good hope for very high dynamical range observations, possibly including hot extra solar planets. The relatively high maximum spectral resolution, up to 10000, will allow stellar activity observations. Between these extreme goals, AMBER has a wide range of applications including Young Stellar Objects, Evolved Stars, circumstellar material and many others. This revised version was published online in July 2006 with corrections to the Cover Date.     

Processing of AMBER data

This practice work session introduces the participants to the ESO tools available for the reduction of VLTI/AMBER data. They include Gasgano and Reflex. These tools are provided by ESO, and are based on amdlib, provided by the AMBER consortium.     

Unveiling the Launching Region of YSO Jets with AMBER

The interplay between accretion and ejection in the environment of young stellar objects (YSOs) is believed to be a crucial element in the star formation process. Since most of the properties of the models are set up in the first few AUs from the source (below the so-called Alfvèn surface), to validate and constrain the models observationally we need very high angular resolution. With HST (resolution ∼ 0.'1, i.e. about 14 AU in Taurus)we have been able to access the external border of the acceleration region, for jets in the Taurus-Auriga cloud. Here we see an onion-like kinematic structure in the first 200 AU of the flow, and indications for rotation around the symmetry axis for the resolved low/moderate velocity component. We have now planned observations with AMBER on the VLTI to investigate at 1 mas resolution (in J) the core of the central engine, down to 0.1 AU from the source. Here we describe a joint project by several Institutes in the AMBER consortium dedicated to the study of the morphology and detailed kinematics of a few selected targets. On one hand we will use the large UV coverage of the ATs to explore at medium spectral resolution the structure of the flow. On the other hand the large collection area of the UTs combined with the high resolution mode (R=10000) of AMBER will allow us to search for interesting kinematic features, among which signatures of rotation around the axis, that would constitute an important validation of the proposed models for the jet launching. This revised version was published online in July 2006 with corrections to the Cover Date.     

Second generation instrumentation for the VLTI: The french VLTI connection

We describe the current French ideas for the instrumentation of the second generation of the VLTI. Instruments concepts addressed include: integrated optics beam combiner, extension of MIDI to a four beam facility, extension of AMBER to the visible and a densified pupil direct imaging beam combiner. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of the atmospheric refraction in a single-mode instrument – application to AMBER/VLTI

This paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibres. We show the analytical approach which allowed us to assess the need to correct the refraction in J and H bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near-infrared focal beam combiner of the Very Large Telescope Interferometric mode, but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos to take into account the atmospheric turbulence effect after correction by the European Southern Observatory system Multi-Application Curvature Adaptive Optics. The optomechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J and H bands. First observations in J band of a bright star, α Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single-mode instrument, and validated the operating law.     

Probing the magnetosphere in young stars with AMBER/VLTI

The exact geometry of the interface region between the accretion disk and the stellar surface in young stars has crucial implications for both the origin of mass-loss and the regulation of stellar angular momentum. We discuss proposed AMBER/VLTI observations of the Paβ line emitting region that will put the first direct constraints on the magnetospheric accretion flow scenario in active young stellar systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extragalactic Astronomy with the VLTI: a new window on the Universe

Interferometry in the optical and near infrared has so far played a marginal role in Extragalactic Astronomy. Active Galactic Nuclei are the brightest and most compact extragalactic sources, nonetheless only a very limited number could be studied with speckle interferometry and none with long baseline interferometry. The VLTI will allow the study of moderately faint extragalactic objects with very high spatial resolution thus opening a new window on the universe. With this paper we focus on three scientific cases to show how AMBER and MIDI can be used to tackle open issues in extragalactic astronomy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interferometry: The tool to study giant, supergiant and Mira stars

Late-type giant stars have been traditional targets for infrared interferometers. They are bright and big and are therefore easy targets to resolve and to detect. Considerable progress has been and is being made on the spatial structure of these objects thanks to existing interferometers. Beyond the classical measurement of their diameters, pulsations have been directly detected, spatial intensity distributions are more and more understood and more important, consistent scenarios for both spectroscopic and interferometric measurements are on the verge to be validated. All this has been possible with prototype instruments having a small number of baselines and very limited spectral capabilities. AMBER and MIDI will surely open a new era with high spectral resolution, high efficiency and imaging capabilities. This is not an exhaustive review of all the work done in the field but rather a presentation of the context. A recent review of Mira star observations with interferometers was recently written (Scholz, 2003).A brief science case is first introduced in this paper. Achievements with high angular resolution single-telescope techniques are presented. The contributions of optical and infrared interferometers are then explained. Eventually, some hints about the possible progress with VLTI are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long Range Science Perspectives for the VLTI

VLTI interferometry will allow imaging of galactic and extragalactic sources with milliarcsecond angular resolution. For moderately bright sources the spectral resolution will be of the order of 10000. These capabilities will allow detailed studies of solar system objects, stars, proto-planetary systems and the detection of hot extra-solar planets. The observations of galactic nuclei will allow unprecedented measurements of physical parameters in these systems. VLTI will be a prime instrument to study the immediate environment of the massive black hole at the center of the Milky Way. With the exception of a few `self-referencing' sources the observations of extragalactic nuclei will benefit from an extended capability for simultaneous measurements of nearby reference sources for fringe tracking. With beam combination instruments like AMBER, MIDI, PRIMA, and GENIE the VLTI will reach full maturity at a time when other interferometric instruments at different wavelengths will be fully operational. Most important are ALMA (in the mm- and sub-mm-domain), LOFAR and SKA (in the radio meter to centimeter domain) and of course VLB-networks in the radio, and other – at that time –well developed interferometers in the optical. A major scientific potential of future scientific VLTI programs will lie in an efficient combination of these high angular resolution capabilities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fast Calculation of Integration of Bremsstrahlung Radiation from Electrons with Power-law Distribution

Since the SSW (Solar Soft Ware) has been developed to the version 2, in comparison with the version 1 the performance of the integration calculation of the Bremsstrahlung radiation caused by high-energy electrons is improved. On the basis of the version 2 of SSW, we have made further improvement on the integration calculation. The comparison between a few schemes of Bremsstrahlung integration shows that our final scheme is faster than the version 2 for 2∼5 times. Compared with the version 1 and version 2 of SSW, the integration accuracy has been also improved a lot, even under the default control accuracy there will be no spikes appearing in photon spectra. In addition, the time consumption of integration is no longer sensitive to the upper limit of the integration. The improvement of the integration performance enables the accurate Bremsstrahlung cross-section be used to calculate the Bremsstrahlung integration. Compared with the result of approximate Bremsstrahlung cross-section, the result of accurate Bremsstrahlung cross-section gives a little smaller photon flux (≤4%), and the integration time increases about 30%.     

Testing the Models for Jet Generation with Hubble Space Telescope Observations

Many magneto-hydrodynamic (MHD) models have been developed to describe the acceleration and collimation of stellar jets, in the framework of an infall/outflow process. Thanks to high angular resolution instrumentation, such as the one on-board the Hubble Space Telescope (HST), we are finally able to test observationally the proposed ideas. We present the results obtained by us from the first 0”.1 resolution spectra of the initial portion (within 100–200 AU from the source) of the outflows from visible T Tauri stars, taken with the Space Telescope Imaging Spectrograph (STIS). We obtain the jet morphology, kinematics and excitation in different velocity intervals, and we derive the jet mass and momentum fluxes. These results confirm the predictions of magneto-centrifugal models for the jet launch. Recently we have also found indications for rotation in the peripheral regions of several flows. The derived rotational motions appear to be in agreement with the expected extraction of angular momentum from the star/disk system caused by the jet, which in turn allows the star to accrete up to its final mass. Improvements to resolution are expected from observations with STIS in the ultraviolet, and with the forthcoming AMBER spectrometer to be mounted at the VLTI.     

Virtual Versus Real Jets: New Clues from the Hubble Space Telescope

During past years high angular (<1″) resolution imaging has provided useful information about the propagation of “real” jets. Recently, in addition, the spectrograph Hubble's Space Telescope Imaging spectrograph (STIS) on board the Hubble Space Telescope (HST) has finally allowed us to test the magneto-centrifugal paradigm for the jet launching. I present results from HST/STIS spectra at 0.″1 resolution of small-scale jets from T Tauri stars in their initial 140 AU (1″). The jet morphology, kinematics and excitation in different velocity intervals are derived, from which we calculate mass and momentum fluxes. Even more interestingly, we find indications for rotation around the symmetry axis in the peripheral regions of the flow. The investigated component of the wind appears to originate in the disk at a distance of 0.5-2 AU from the star, and it extracts at least 60% of the inner disk angular momentum. These results confirm for the first time the validity of the magneto-centrifugal approach for the jet launching, and constitute a benchmark to test models and simulations. In the near future, near-infrared (NIR) interferometry with AMBER/VLTI and with the LBTI will permit to observe the jet engine down to 0.1 AU from the source, where the acceleration of the jet takes place.     

Numerical integration of dynamical systems with Lie series

The integration of the equations of motion in gravitational dynamical systems—either in our Solar System or for extra-solar planetary systems—being non integrable in the global case, is usually performed by means of numerical integration. Among the different numerical techniques available for solving ordinary differential equations, the numerical integration using Lie series has shown some advantages. In its original form (Hanslmeier and Dvorak, Astron Astrophys 132, 203 1984), it was limited to the N-body problem where only gravitational interactions are taken into account. We present in this paper a generalisation of the method by deriving an expression of the Lie terms when other major forces are considered. As a matter of fact, previous studies have been done but only for objects moving under gravitational attraction. If other perturbations are added, the Lie integrator has to be re-built. In the present work we consider two cases involving position and position-velocity dependent perturbations: relativistic acceleration in the framework of General Relativity and a simplified force for the Yarkovsky effect. A general iteration procedure is applied to derive the Lie series to any order and precision. We then give an application to the integration of the equation of motions for typical Near-Earth objects and planet Mercury.     

Symplectic integrators for long-term integrations in celestial mechanics

We discuss the use of symplectic integration algorithms in long-term integrations in the field of celestial mechanics. The methods' advantages and disadvantages (with respect to more common integration methods) are discussed. The numerical performance of the algorithms is evaluated using the 2-body and circular restricted 3-body problems. Symplectic integration methods have the advantages of linear phase error growth in the 2-body problem (unlike most other methods), good conservation of the integrals of the motion, good performance for moderately eccentric orbits, and ease of use. Its disadvantages include a relatively large number of force evaluations and an inability to continuously vary the step size.     

Partial derivatives used in trajectory estimation

The Peano-Baker method is applied to the integration of the variational equations to produce the partial derivatives used in satellite navigation. In this method the analytic form of the state transition partial derivatives can be factored so that numerical integration is applied only to the departures from a simplified analytical model.The advantage of using the Peano-Baker approach rather than direct integration of the variational equations is that with the Peano-Baker method numerical integration can be performed adequately with low order formulae and relatively large step sizes. Numerical results are indicated.     

Long-term behavior of the motion of Pluto over 5.5 billion years

The motion of Pluto is said to be chaotic in the sense that the maximum Lyapunov exponent is positive: the Lyapunov time (the inverse of the Lyapunov exponent) is about 20 million years. So far the longest integration up to now, over 845 million years (42 Lyapunov times), does not show any indication of a gross instability in the motion of Pluto. We carried out the numerical integration of Pluto over the age of the solar system (5.5 billion years ≈ 280 Lyapunov times). This integration also did not give any indication of chaotic evolution of Pluto. The divergences of Keplerian elements of a nearby trajectory at first grow linearly with the time and then start to increase exponentially. The exponential divergences stop at about 420 million years. The divergences in the semi-major axis and the mean anomaly ( equivalently the longitude and the distance) saturate. The divergences of the other four elements, the eccentricity, the inclination, the argument of perihelion, and the longitude of node still grow slowly after the stop of the exponential increase and finally saturate.     

Laboratory verification of the Active Particle-induced X-ray Spectrometer(APXS) on the Chang'e-3 mission

In the Chang'e-3 mission, the Active Particle-induced X-ray Spectrometer(APXS) on the Yutu rover is used to analyze the chemical composition of lunar soil and rock samples. APXS data are only valid are only if the sensor head gets close to the target and integration time lasts long enough. Therefore, working distance and integration time are the dominant factors that affect APXS results. This study confirms the ability of APXS to detect elements and investigates the effects of distance and time on the measurements. We make use of a backup APXS instrument to determine the chemical composition of both powder and bulk samples under the conditions of different working distances and integration times. The results indicate that APXS can detect seven major elements, including Mg, Al, Si, K, Ca, Ti and Fe under the condition that the working distance is less than 30 mm and having an integration time of 30 min. The statistical deviation is smaller than 15%. This demonstrates the instrument's ability to detect major elements in the sample. Our measurements also indicate the increase of integration time could reduce the measurement error of peak area, which is useful for detecting the elements Mg, Al and Si. However, an increase in working distance can result in larger errors in measurement, which significantly affects the detection of the element Mg.     

Coronal Holes and Icosahedral Symmetry

The study of the expansion of the solar wind out of a system of coronal holes is continued. To this end, we consider the numerical integration of partial differential equations for problems with icosahedral symmetry, in general. First, employing Weyl theory, orbifold coordinates are introduced. Second, the icosahedral coordinates are discussed in detail. Third, following an analysis of the properties of these coordinates and the derivation of a few expressions useful for grid construction, various alternatives for the distribution of lattice points required for numerical integration are considered. A comparison of these numerical grids motivates the choice of a specific grid optimized for the numerical integration carried out in the accompanying paper by Kalish et al.(2002). This revised version was published online in July 2006 with corrections to the Cover Date.