Planets are believed to form in circumstellar disks around newly born stars at distances ranging from 0.1 to 10 AUs.
This location corresponds to milli-arcsecond scales at the distance of the closest star forming regions and to temperatures
ranging from a few hundred to a few thousand Kelvin. To conduct observations of close environments of such disks
at the milli-arcsecond scale, infrared interferometry is a suitable tool that can be employed to observe T Tauri, FU Ori
and Herbig Ae/Be stars. However, the data obtained so far consist of a small number of measurements which can only
constrain theoretical models. With the advent of recent multi-aperture interferometers, the interferometric data can be used
to reconstruct images independently of any parametric model, as is routinely done in the radio frequency range. On the
other hand, in the optical range, not enough measurements are available to univocally reconstruct an image and some a
priori must be introduced. In this contribution, we present systematic tests performed on the MiRA algorithm (an image
reconstruction algorithm developed for optical interferometry) in order to evaluate the feasibility of the technique. The
methodology allows deriving some practical rules for the user and has been applied to an YSO (HD 163296). I present the
results of the image reconstruction, providing the first images of a complex YSO.
The hydrogen emission line is a defining characteristic of young stellar objects probing the planet forming regions
of the disks. The limiting sensitivity of current interferometers has precluded it's detailed study. We'll review
our current understanding of hydrogen emission, recent results and project the science that can be achieved with
sensitive interferometers such as the PRIMA off-axis mode and GRAVITY.
We compare the quality of interferometric image reconstructions for two different sets of data: square of the
visibility plus closure phase (e.g. AMBER like case) and square of the visibility plus visibility phase (e.g.
PRIMA+AMBER or GRAVITY like cases). We used the Multi-aperture image Reconstruction Algorithm for
reconstructions of test cases under different signal-to-noise ratios and noisy data (squared visibilities and phases).
Our study takes into account noise models based on the statistics of visibility, phase and closure phase. We
incorporate the works developed by Tatulli and Chelly (2005) on the noise of the power-spectrum and closure
phase in the read-out and photon noise regimes,1 and by Colavita (1999) on the signal-to-noise ratio of the
visibility phase.2 The final images were then compared to the original one by means of positions and fluxes,
computing the astrometry and the photometry. For the astrometry, the precision was typically of tens of
microarcseconds, while for the photometry, it was typically of a few percent. Although both cases are suitable
for image restorations of real interferometric observations, the results indicate a better performance of phase
referencing (V2 + visibility phase) in a low signal-to-noise ratio scenario.
We present the work developed within the science team of the Very Large Telescope Interferometer Spectro-Imager (VSI) during the Phase A studies. VSI aims at delivering ~ 1 milliarcsecond resolution data cubes
in the near-infrared, with several spectral resolutions up to 12 000, by combining up to 8 VLTI telescopes. In
the design of an instrument, the science case plays a central role by supporting the instrument construction
decision, defining the top-level requirements and balancing design options. The overall science philosophy of
VSI was that of a general user instrument serving a broad community. The science team addressed themes
which included several areas of astrophysics and illustrated specific modes of operation of the instrument: a)
YSO disks and winds; b) Multiplicity of young stars; c) Exoplanets; d) Debris disks; e) Stellar surface imaging;
f) The environments of evolved stars; g) AGN tori; h) AGN's Broad Line Region; i) Supermassive black-holes;
and j) Microlensing. The main conclusions can be summarized as follows: a) The accessible targets and related
science are extremely sensitive to the instrument limiting magnitude; the instrument should be optimized for
sensitivity and have its own fringe tracker. b) Most of the science cases are readily achievable with on-axis fringe
tracking, off-axis fringe tracking enabling extra science. c) In most targets (YSOs, evolved stars and AGNs), the
interpretation and analysis of circumstellar/nuclear dust morphology requires direct access to the gas via spectral
resolved studies of emission lines, requiring at least a spectral resolution of 2 500. d) To routinely deliver images
at the required sensitivity, the number of telescopes in determinant, with 6 telescopes being favored. e) The
factorial increase in the number of closure phases and visibilities, gained in a single observation, makes massive
surveys of parameters and related science for the first time possible. f) High dynamic range imaging and very
high dynamic range differential closure phase are possible allowing the study of debris disks and characterization
of pegasides. g) Spectro-imaging in the near-infrared is highly complementary to ALMA, adaptive optics and
interferometric imaging in the thermal infrared.
The study of protoplanetary disks, where the planets are believed to form, will certainly allow the formation
of our Solar System to be understood. To conduct observations of these objects at the milli-arcsecond scale,
infrared interferometry provides the right performances for T Tauri, FU Ori or Herbig Ae/Be stars. However,
the only information obtained so far are scarce visibility measurements which are directly tested with models.
With the outcome of recent interferometers, one can foresee obtaining images reconstructed independently of the
models. In fact, several interferometers including IOTA and AMBER on the VLTI already provide the possibility
to recombine three telescopes at once and thus to obtain the data necessary to reconstruct images.
In this paper, we describe the use of MIRA, an image reconstruction algorithm developed for optical interferometry
data (squared visibilities and closure phases) by E. Thiébaut. We foresee also to use the spectral
information given by AMBER data to constrain even better the reconstructed images. We describe the use of
MIRA to reconstruct images of young stellar objects out of actual data, in particular the multiple system GW
Orionis (IOTA, 2004), and discuss the encountered difficulties.
In this paper, we study the feasibility of obtaining near-infrared spectra of bright extrasolar planets with the
2nd generation VLTI Spectro-Imager instrument (VSI), which has the required angular resolution to resolve
nearby hot Extrasolar Giant Planets (EGPs) from their host stars. Taking into account fundamental noises, we
simulate closure phase measurements of several extrasolar systems using four 8-m telescopes at the VLT and a
low spectral resolution (R = 100). Synthetic planetary spectra from T. Barman are used as an input. Standard
χ2-fitting methods are then used to reconstruct planetary spectra from the simulated data. These simulations
show that low-resolution spectra in the H and K bands can be retrieved with a good fidelity for half a dozen
targets in a reasonable observing time (about 10 hours, spread over a few nights). Such observations would
strongly constrain the planetary temperature and albedo, the energy redistribution mechanisms, as well as the
chemical composition of their atmospheres. Systematic errors, not included in our simulations, could be a serious
limitation to these performance estimations. The use of integrated optics is however expected to provide the
required instrumental stability (around 10-4 on the closure phase) to enable the first thorough characterisation
of extrasolar planetary emission spectra in the near-infrared.