O. von der Lühe

AMBER, the near-infrared spectro-interferometric three-telescope VLTI instrument

Context: Optical long-baseline interferometry is moving a crucial step forward with the advent of general-user scientific instruments that equip large aperture and hectometric baseline facilities, such as the Very Large Telescope Interferometer (VLTI). Aims: AMBER is one of the VLTI instruments that combines up to three beams with low, moderate and high spectral resolutions in order to provide milli-arcsecond spatial resolution for compact astrophysical sources in the near-infrared wavelength domain. Its main specifications are based on three key programs on young stellar objects, active galactic nuclei central regions, masses, and spectra of hot extra-solar planets. Methods: These key science goals led to scientific specifications, which were used to propose and then validate the instrument concept. AMBER uses single-mode fibers to filter the entrance signal and to reach highly accurate, multiaxial three-beam combination, yielding three baselines and a closure phase, three spectral dispersive elements, and specific self-calibration procedures. Results: The AMBER measurements yield spectrally dispersed calibrated visibilities, color-differential complex visibilities, and a closure phase allows astronomers to contemplate rudimentary imaging and highly accurate visibility and phase differential measurements. AMBER was installed in 2004 at the Paranal Observatory. We describe here the present implementation of the instrument in the configuration with which the astronomical community can access it. Conclusions: .After two years of commissioning tests and preliminary observations, AMBER has produced its first refereed publications, allowing assessment of its scientific potential.

Basic physical parameters of a selected sample of evolved stars

We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the reference abundance, we determine the [Fe/H] for the whole sample. These metallicities, together with the Teff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B−V)0 and log g) using theoretical isochrones and a Bayesian estimation method. The (B−V)0 values so estimated turn out to be in excellent agreement (to within ∼0.05 mag) with the observed (B−V), confirming the reliability of the Teff−(B−V)0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1 <θ< 10 mas interval (or, alternatively, finding mean differences of just 6%). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ∼4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius.

Interferometric data reduction with AMBER/VLTI. Principle, estimators, and illustration

Aims. In this paper, we present an innovative data reduction method for single-mode interferometry. It has been specifically developed for the AMBER instrument, the three-beam combiner of the Very Large Telescope Interferometer, but it can be derived for any single-mode interferometer. Methods. The algorithm is based on a direct modelling of the fringes in the detector plane. As such, it requires a preliminary calibration of the instrument in order to obtain the calibration matrix that builds the linear relationship between the interferogram and the interferometric observable, which is the complex visibility. Once the calibration procedure has been performed, the signal processing appears to be a classical least-square determination of a linear inverse problem. From the estimated complex visibility, we derive the squared visibility, the closure phase, and the spectral differential phase. Results. The data reduction procedures have been gathered into the so-called amdlib software, now available for the community, and are presented in this paper. Furthermore, each step in this original algorithm is illustrated and discussed from various on-sky observations conducted with the VLTI, with a focus on the control of the data quality and the effective execution of the data reduction procedures. We point out the present limited performances of the instrument due to VLTI instrumental vibrations which are difficult to calibrate.

Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI

We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wave- length of 10 µm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 µm to 13.5 µm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 µm are found to be from 1 AU to 10 AU. The 10 µm sizes of our sample stars correlate with the slope of the 10-25 µm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different ge- ometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irra- diated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way.

MIDI - The 10 mu m 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.

Speckle interferometry with adaptive optics corrected solar data

Context. Adaptive optics systems are used on several advanced solar telescopes to enhance the spatial resolution of the recorded data. In all cases, the correction remains only partial, requiring post-facto image reconstruction techniques such as speckle interferometry to achieve consistent, near-diffraction limited resolution. Aims. This study investigates the reconstruction properties of the Kiepenheuer-Institut Speckle Interferometry Package (KISIP) code, with focus on its phase reconstruction capabilities and photometric accuracy. In addition, we analyze its suitability for real-time reconstruction. Methods. We evaluate the KISIP program with respect to its scalability and the convergence of the implemented algorithms with dependence on several parameters, such as atmospheric conditions. To test the photometric accuracy of the final reconstruction, comparisons are made between simultaneous observations of the Sun using the ground-based Dunn Solar Telescope and the space-based Hinode/SOT telescope. Results. The analysis shows that near real-time image reconstruction with high photometric accuracy of ground-based solar observations is possible, even for observations in which an adaptive optics system was utilized to obtain the speckle data.

Estimating Fried’s parameter from a time series of an arbitrary resolved object imaged through atmospheric turbulence

A method to obtain an estimate of Fried’s seeing parameter r0 from time series of an arbitrarily shaped, resolved structure that exhibits degradation resulting from atmospheric turbulence is presented. The basic idea is to evaluate the ratio of the observed squared modulus of the average Fourier transform and the observed average power spectrum. The theory of the method is developed, and the influence of noise on the ratio is discussed. The method has been applied to five consecutive time series of observations of solar granulation under different seeing conditions. The power spectra, which are reconstructed with appropriate theoretical modulation transfer functions, converge.

Constraining the wind launching region in Herbig Ae stars: AMBER/VLTI spectroscopy of HD 104237

This work has been partly supported by the MIUR COFIN grant 2003/027003-001 and 025227/2004 to the INAFOsservatorio Astrofisico di Arcetri. This project has benefited from funding from the French Centre National de la Recherche Scientifique (CNRS) through the Institut National des Sciences de l’Univers (INSU) and its Programmes Nationaux (ASHRA, PNPS). The authors from the French laboratories would like to thank the successive directors of the INSU/CNRS directors. C. Gil work was supported in part by the Fundac¸˜ao para a Ciˆencia e a Tecnologia through project POCTI/CTE-AST/55691/2004 from POCTI,with funds from the European program FEDER.

A substellar companion around the intermediate-mass giant star HD 11977

We report the discovery of a substellar companion to the intermediate-mass star HD 11977 (G5 III). Radial velocities of this star have been monitored for five years with FEROS at the 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in of K 1 = Chile. Based on the collected data we calculated an orbital solution with a period of P = 711 days, a semi-amplitude of K 1 = 105 ms -1 , and an eccentricity of e = 0.4. The period of the radial-velocity variation is longer than that of the estimated stellar rotation, rendering it unlikely that rotational modulation is the source of the variation in the radial velocity. This hypothesis is supported by the absence of a correlation between stellar activity indicators and radial-velocity variation. By determining a primary stellar mass of M * = 1.91 M ○. , the best-fit minimum mass of the companion and semi-major axis of the orbit are m 2 sin i = 6.54 M Jup and a 2 = 1.93 AU, respectively. An upper limit for the mass of the companion of m 2 ? 65.5 M Jup has been calculated from HIPPARCOS astrometric measurements. Although the possibility of a brown-dwarf companion cannot be excluded, HD 11977 B is one of the few planet candidates detected around an intermediate-mass star. The progenitor main-sequence star of HD 11977 is probably an A-type star. This discovery gives an indirect evidence for planetary companions around early type main-sequence stars.

Precise radial velocity measurements of G and K giants Multiple systems and variability trend along the Red Giant Branch

We present the results of our radial velocity (RV) measurements of G and K giants, concentrating on the presence of multiple systems in our sample. Eighty-three giants have been observed for 2.5 years with the fiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. Seventy-seven stars (93%) of the targets have been analyzed for RV variability using simultaneous Th-Ar calibration and a cross-correlation technique. We estimate the long-term precision of our measurement as better than 25 m s −1 . Projected rotational velocities have been measured for most stars of the sample. Within our time-base only 21 stars (or 27%) show variability below 2σ, while the others show RV variability with amplitudes up to several km s −1 . The large amplitude (several km s −1 ) and shape (high eccentricity) of the RV variations for 11 of the program stars are consistent with stellar companions, and possibly brown dwarf companions for two of the program stars. In those systems for which a full orbit could be derived, the companions have minimum masses from ∼0.6 Mdown to 0.1 M� .T o these multiple systems we add the two candidates of giant planets already discovered in the sample. This analysis shows that multiple systems contribute substantially to the long-term RV variability of giant stars, with about 20% of the sample being composed of multiple systems despite screening our sample for known binary stars. After removing binaries, the range of RV variability in the whole sample clearly decreases, but the remaining stars retain a statistical trend of RV variability with luminosity: luminous cool giants with B − V ≥ 1.2 show RV variations with σRV > 60 m s −1 , while giants with B − V < 1.2 including those in the clump region exhibit less variability or they are constant within our accuracy. The same trend is observed with respect to absolute visual magnitudes: brighter stars show a larger degree of variability and, when plotted in the RV variability vs. magnitude diagram a trend of increasing RV scatter with luminosity is seen. The amplitude of RV variability does not increase dramatically, as predicted, for instance, by simple scaling laws. At least two luminous and cooler stars of the sample show a correlation between RV and chromospheric activity and bisector asymmetry, indicating that in these two objects RV variability is likely induced by the presence of (chromospheric) surface structures.