Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

The first and last items are a definitive need for the study of extra-solar planets while the second is more
of interest for stellar formation (and AGN) studies in general since the complex topographic nature of nearby
environment of young stars cannot rely on model fitting only and rather requires real imaging capabilities.
In addition, if low to medium spectral resolutions are needed for most of the objectives, in stellar formation
studies, a spectro-imaging approach, combining both high angular resolution within not a too small field and a
spectroscopic analysis of each resolution pixel, would be ideal.
It must be noted that all instrumental approaches useful for extra-planetary studies are not retained : for
instance, astrometric capabilities, that are expected to be quite productive in some years from now but have
been chosen as their priority by other institutes.
The guiding orders of magnitude Let’s recall the basic numbers that guided the high-level definition of
the current instrumentation for these priorities to be followed.
• Spectral range: in the range 1-2.5 µm (J, H and K bands), emission is at 1000 K to 3000 K; going further
allows to probe cooler material and going shortward allows to probe the important region around Hα.
• Spatial resolution: in nearby (100 pc) star formation regions, 1 mas typically corresponds to 0.10 AU.
Investigating planet formation requires at least one milliarcsec resolution. At the wavelengths quoted
above, this implies interferometric baselines of about one hundred meters. In AGN, the outer size of dust
tori are expected to be around 3 pc. Since 10 mas @ 15 Mpc (closest AGN) corresponds to 0.5 pc, the
same resolution power is necessary.
• Sensitivity: YSO are fainter than K=5 and AGN than K=7-9. The required instrumentation is directly
constrained by this sensitivity threshold
• Accuracy: in AGN and YSO, the requested dynamic range is of the order of 1:100 to 1:1000. For the
exoplanets, the brightest Pegasids are almost 10 4 times fainter than their host stars (at 2 µm). Therefore
a measurement accuracy between 1% and 0.1% is compulsory and better is required for some specific
objectives.
10.2.2 Focusing on three complementary axii
The exposed priorities lead to the following possible directions of instrumental development, constrained by
the high-level requirements above mentioned, organized here in the three complementary families previously
analyzed :
• Improving Very High Contrast imaging capabilities Associated to a typically low spectral resolution,
VHC imaging points toward optimized AO-based imagers like the VLT Planet Finder (2 nd generation
VLT instrumentation) or nulling-type interferometers like, in the long term, DARWIN or TPF-I whose
precursors could be the PEGASE project proposed to CNES in the context of the free-flyers CfT or the
GENIE-on-ice ALLADIN project for Antarctica. The related developments are : Adaptive optics with
large number of actuators, system studies or sub-systems (e.g. fast detectors) for VHC contrast imaging
optimization, integrated optics at thermal wavelengths optimized for excellent characteristics (of spatial
filtering, low level of leakage for instance).
• Improving imaging capabilities of complex objects Associated to low-to-medium spectral resolution,
better imaging capabilities of interferometers point toward an increase of the number of baselines ;
while present-day optical interferometers (IOTA with IONIC, VLTI with AMBER, CHARA with MIRC)
are just beginning using closure phase with three telescopes, there is a good prospect for increasing the
u,v-plane coverage as the PdB IRAM interferometer did in the past when going from the initial 3 antennae
to the current 6 antennae, i.e. a factor x15 in the phase information level. The related developments are :
continuing integrated optics development toward more complex and performant components (recombinators
with 4, 6 or 8 beams ; improved throughput ; other functionalities like optimized fringe tracking
components) ; AO system development dedicated to large interferometers auxiliary telescopes.
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