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.     

Imaging stars and their environments with the VLTI

The opportunity to deliver high-angular resolution model-independent images is one of the most attractive prospects for optical/infrared interferometry. In this paper I use simple imaging simulations to identify some of the practical difficulties that may arise in achieving this goal with the VLTI. For the types of studies investigated here – stellar surface imaging and the mapping of emission line disks – it is likely that the key challenges will be to measure the visibility function on a sufficiently large range of baselines, and to secure accurate and reliable phase information. This revised version was published online in July 2006 with corrections to the Cover Date.     

MIDI – the 10 μm instrument on the VLTI

After more than five years of preparation, the mid-infrared interferometric instrument MIDI has been transported to Paranal where it will undergo testing and commissioning on theVery Large Telescope Interferometer VLTI from the end of 2002through large part of this year 2003. Thereafter it will be available as a user instrument to perform interferometric observations over the8 μm–13 μm wavelength range, with a spatial resolution of typically 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 3–4 mag or 1–2.5 Jy for self –fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument, laboratory tests, and expected performance, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

PRIMA for the VLTI – Science

The four main scientific objectives of PRIMA – the Phase-Referenced Imaging and Micro-arc second Astrometry facility for the VLTI – will be described:– extra-solar system characterization with astrometry, to detect planets and evaluate their mass, and imaging of the dust accretion disk,– galactic center study with astrometry(dynamics of the bulge stars) and imaging at 10μm (piercing the gas and dust clouds surrounding the galactic center),– observations of AGNs and other extra-galactic objects, too faint to be observed without PRIMA, for which partial imaging is needed to constrain their structuremodels,– micro-gravitational lensing event resolution (imaging and astrometry of their photo-center) in the Galactic Bulge and Magellanic Clouds, helping to determine directly the lens mass and distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

GENIE – The Darwin demonstrator

Darwin is a cornerstone mission of the Horizons 2000+ program of the European Space Agency. It has the express purpose of carrying out the first direct search for terrestrial exoplanets, and to achieve unprecedented spatial resolution in the infrared wavelength region. The detection and study of terrestrial exoplanets promises to usher in a new era in science and will affect a broad spectrum of disciplines. Further, the time line for implementation of such an instrument is now likely to be of the close order of 10 years, leading to possible answers to one of mankind's most fundamental questions in the second decade of the 21st century. It has been found that in order to realize am interferometer in space in the next 10–15 years, it is necessary to prepare the way through a number of intermediary steps – both on the ground and in space. In this context, we here describe GENIE – a ground based nulling interferometry experiment to be implemented at the VLTI in a partnership between ESA and ESO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interferometric Observation at Mid-Infrared Wave-lengths with MIDI

MIDI, the MID-Infrared Interferometricnterferometric Instrument for ESO's Very Large Telescope Interferometer (VLTI), will be the first instrument for combining mid-infrared light directly in order to obtain angular resolution up to 10 mas (assuming a 200 m baseline) in a wavelength range from 8 to 13 μm. Currently in the phase of commissioning at Paranal, the start of its scientific operation is expected for summer 2003. Direct interferometry at thermal infrared wavelengths demands special requirements on the instrument and also on the procedures of preparation of data reduction. Hereafter MIDI's different observing modes are described and an example for an interferometric observation is given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fluctuations of Solar Activity during the Declining Phase of the 11-Year Sunspot Cycle

The number of coronal mass ejections (CMEs) erupting from the Sun follows a trend similar to that of sunspot numbers during the rising and maximum phase of the solar cycle. In the declining phase, the CME number has large fluctuations, dissimilar to those of sunspot numbers. In several studies of solar – interplanetary and solar – terrestrial relationships, the sunspot numbers and the 2800-MHz flux (F10) are used as representative of solar activity. In the rising phase, this may be adequate, but in the declining phase, solar parameters such as CMEs may have a different behaviour. Cosmic-ray Forbush decreases may occur even when sunspot activity is low. Therefore, when studying the solar influence on the Earth, one has to consider that although geomagnetic conditions at solar maximum will be disturbed, conditions at solar minimum may not be necessarily quiet.     

Formation and Evolution of Intermediate Mass Black Hole X-Ray Binaries

The evolution of young (≲ 10 Myr) star clusters with a density exceeding about 10⁵ star pc⁻³ are strongly affected by physical stellar collisions during their early lifetime. In such environments the same star may participate in several tens to hundreds of collisions ultimately leading to the collapse of the star to a black hole of intermediate mass. At later time, the black hole may acquire a companion star by tidal capture or by dynamical – three-body – capture. When the captured star evolves it starts to fill its Roche-lobe and transfers mass to its accompanying black hole. This then leads to a bright phase of X-ray emission, which lasts for the remaining main-sequence lifetime of the donor. If the star captured by the intermediate mass black hole is relatively low mass ≲ 2 M⊙) the binary will also be visible as a bright source in gravitational waves. Based on empirical models we argue that, for as long as the donor remains on the main sequence, the source will be ultraluminous L_x >rsim 10⁴⁰ ergs^-1 for about a week every few month. When the donor star is more massive >15 M⊙, or evolved off the main sequence the bright time is longer, but the total accretion phase lasts much shorter.     

MIDI observations of 1459 Magnya: First attempt of interferometric observations of asteroids with the VLTI

The Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) can be used to obtain direct determination of the sizes and the albedos of asteroids. We present results of the first attempt to carry out interferometric observations of asteroids with the Mid Infrared Interferometric Instrument (MIDI) at the VLTI. Our target was 1459 Magnya. This is the only V-type asteroid known to exist in the outer main-belt, and its IRAS-albedo turns out to be rather low for an object of this taxonomic class. Interferometric fringes were not detected, very likely due to the fact that the flux emitted by the asteroid was lower than expected and below the MIDI threshold for fringe detection. However, by fitting the Standard Thermal Model to the N-band infrared flux measured by MIDI in photometric mode and to the visible absolute magnitude, obtained from quasi-simultaneous B- and V-band photometric observations, we have derived a geometric visible albedo of 0.37±0.06 and an effective diameter of 17±1 km. This new estimate of the albedo differs from that previously obtained by IRAS and is more consistent with the V-type taxonomic classification of 1459 Magnya.     

Galactic Diffuse Polarized Emission

Diffuse polarized emission by synchrotron is a key tool to investigate magnetic fields in the Milky Way, particularly the ordered component of the large scale structure. Key observables are the synchrotron emission itself and the RM is by Faraday rotation. In this paper the main properties of the radio polarized diffuse emission and its use to investigate magnetic fields will be reviewed along with our current understanding of the galactic magnetic field and the data sets available. We will then focus on the future perspective discussing RM-synthesis – the new powerful instrument devised to unlock the information encoded in such an emission – and the surveys currently in progress like S-PASS and GMIMS.     

Long-Term Variations in the Growth and Decay Rates of Sunspot Groups

Using the combined Greenwich (1874 – 1976) and Solar Optical Observatories Network (1977 – 2009) data on sunspot groups, we study the long-term variations in the mean daily rates of growth and decay of sunspot groups. We find that the minimum and the maximum values of the annually averaged daily mean growth rates are ≈ 52% day⁻¹ and ≈ 183% day⁻¹, respectively, whereas the corresponding values of the annually averaged daily mean decay rates are ≈ 21% day⁻¹ and ≈ 44% day⁻¹, respectively. The average value (over the period 1874 – 2009) of the growth rate is about 70% more than that of the decay rate. The growth and the decay rates vary by about 35% and 13%, respectively, on a 60-year time scale. From the beginning of Cycle 23 the growth rate is substantially decreased and near the end (2007 – 2008) the growth rate is lowest in the past about 100 years. In the extended part of the declining phase of this cycle, the decay rate steeply increased and it is largest in the beginning of the current Cycle 24. These unusual properties of the growth and the decay rates during Cycle 23 may be related to cause of the very long declining phase of this cycle with the unusually deep and prolonged current minimum. A ≈ 11-year periodicity in the growth and the decay rates is found to be highly latitude and time dependent and seems to exist mainly in the 0° – 10° latitude interval of the southern hemisphere. The strength of the known approximate 33 – 44-year modulation in the solar activity seems to be related to the north-south asymmetry in the growth rate. Decreasing and increasing trends in the growth and the decay rates indicate that the next 2 – 3 solar cycles will be relatively weak.     

Interferometric calibrations and astronomical calibrators

This chapter gives some indications on the selection of suitable astronomical calibrators for interferometric observations. After an introduction to interferometric calibrations, we will focus on the criteria, steps and tools developed by ESO to prepare VLTI observations and to select calibrators. Although the tools described here are focused on VLTI, the problem of astronomical calibrators is the same for the other interferometers and can be approached in the same way.     

A Preliminary Estimate of the Size of the Coming Solar Cycle 24, based on Ohl’s Precursor Method

For many purposes (e.g., satellite drag, operation of power grids on Earth, and satellite communication systems), predictions of the strength of a solar cycle are needed. Predictions are made by using different methods, depending upon the characteristics of sunspot cycles. However, the method most successful seems to be the precursor method by Ohl and his group, in which the geomagnetic activity in the declining phase of a sunspot cycle is found to be well correlated with the sunspot maximum of the next cycle. In the present communication, the method is illustrated by plotting the 12-month running means aa(min ) of the geomagnetic disturbance index aa near sunspot minimum versus the 12-month running means of the sunspot number Rz near sunspot maximum [aa(min ) versus Rz(max )], using data for sunspot cycles 9 – 18 to predict the Rz(max ) of cycle 19, using data for cycles 9 – 19 to predict Rz(max ) of cycle 20, and so on, and finally using data for cycles 9 – 23 to predict Rz(max ) of cycle 24, which is expected to occur in 2011 – 2012. The correlations were good (∼+0.90) and our preliminary predicted Rz(max ) for cycle 24 is 142±24, though this can be regarded as an upper limit, since there are indications that solar minimum may occur as late as March 2008. (Some workers have reported that the aa values before 1957 would have an error of 3 nT; if true, the revised estimate would be 124±26.) This result of the precursor method is compared with several other predictions of cycle 24, which are in a very wide range (50 – 200), so that whatever may be the final observed value, some method or other will be discredited, as happened in the case of cycle 23.     

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.     

Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase,Gnevyshev Gap,and Length of Minimum

According to research results from solar-dynamo models, the northern and southern hemispheres may evolve separately throughout the solar cycle. The observed phase lag between the northern and southern hemispheres provides information regarding how strongly the hemispheres are coupled. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 – 23, we determine how out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric phase differences range from 0 – 11, 0 – 14, and 2 – 19 months for the rising, maximum, and declining periods, respectively. The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar cycle. An analysis of the sunspot cycle double peak, or Gnevyshev gap, is conducted to determine if the double-peak is caused by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which the hemispheres couple, we measured the magnetic flux crossing the Equator by examining Kitt Peak Vacuum Telescope and SOLIS magnetograms for Solar Cycles 21 – 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the Equator, length of solar minima, and phase lag between the hemispheres.     

Determination of the Rotation Periods of Solar Active Longitudes

There are two types of active longitudes (ALs) in terms of the distribution of sunspot areas: long-lived and intra-cyclic ALs. The rotation period of the long-lived ALs has been determined by a new method in this paper. The method is based on the property of ALs to be maintained over several cycles of solar activity. The daily values of sunspot areas for 1878 – 2005 are analyzed. It is shown that the AL positions remain almost constant over a period of about ten cycles, from cycle 13 to cycle 22. The rotation period was found to be 27.965 days during this period. The dispersion in AL positions is about 26° from cycle to cycle, which is half of the dispersion observed in the Carrington system. The ALs in the growth phase of the activity cycle are more stable and pronounced. The excess in solar activity in the ALs over adjacent longitudinal intervals is about 12 – 14%. It is shown that only one long-lived AL can be observed at one time on the Sun, as a rule.     

Origin of the Submillimeter Radio Emission During the Time-Extended Phase of a Solar Flare

Solar flares observed in the 200 – 400 GHz radio domain may exhibit a slowly varying and time-extended component which follows a short (few minutes) impulsive phase and can last for a few tens of minutes to more than one hour. The few examples discussed in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present a detailed analysis of the time-extended phase of the 27 October 2003 (M6.7) flare, combining 1 – 345 GHz total-flux radio measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, radiated by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7 – 16 MK and 1 – 3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 10⁴ K. These results, which may also apply to other millimeter–submillimeter radio events, are not consistent with the expectations from standard semiempirical models of the chromosphere and transition region during flares, which predict observable radio emission from the chromosphere at all frequencies where the corona is transparent.     

The path towards high-contrast imaging with the VLTI: the Hi-5 project

The development of high-contrast capabilities has long been recognized as one of the top priorities for the VLTI. As of today, the VLTI routinely achieves contrasts of a few 10^??3 in the near-infrared with PIONIER (H band) and GRAVITY (K band). Nulling interferometers in the northern hemisphere and non-redundant aperture masking experiments have, however, demonstrated that contrasts of at least a few 10^??4 are within reach using specific beam combination and data acquisition techniques. In this paper, we explore the possibility to reach similar or higher contrasts on the VLTI. After reviewing the state-of-the-art in high-contrast infrared interferometry, we discuss key features that made the success of other high-contrast interferometric instruments (e.g., integrated optics, nulling, closure phase, and statistical data reduction) and address possible avenues to improve the contrast of the VLTI by at least one order of magnitude. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 \(\upmu \)m), a sweet spot to image and characterize young extra-solar planetary systems. Finally, we address the science cases of a high-contrast VLTI imaging instrument and focus particularly on exoplanet science (young exoplanets, planet formation, and exozodiacal disks), stellar physics (fundamental parameters and multiplicity), and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed. This project is called Hi-5 for High-contrast Interferometry up to 5 μm.     

New Plateau de Bure observations of M 1–92; unveiling the core

M 1–92 is a very well studied bipolar pPN that can be considered an archetype of this type of sources; it shows a clear axial symmetry, along with the kinematics characteristic of this class of envelopes around post-AGB stars. We performed sub-arcsecond resolution observations of the J=2–1 rotational line of ¹³CO in M 1–92 with the new extended configurations of the IRAM Plateau de Bure interferometer, for studying the morphology and velocity field of the molecular gas better in the nebula, particularly in its central parts. We found that the equatorial structure dividing the two lobes is a thin flat disk, which expands radially with a velocity proportional to the distance to the central stellar system. The kinetic age of this equatorial flow is very similar to that measured in the two lobes, suggesting that the whole structure was formed as a result of a single event some 1200 yr ago, after which the nebula reached an expansion velocity field with axial symmetry. The small widths and velocity dispersion in the gas forming the lobe walls confirm that the acceleration responsible for the nebular shape could not last more than 100–120 yr. In view of the similarity to η Car, we speculate on the possibility that the whole nebula was formed as a result of a magneto-rotational explosion in a common-envelope system. The study of the possible importance of this mechanism in the context of global PNe and pPNe reshaping should be one on the fields in which future ALMA observations will make a crucial contribution. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).