APrSMIDI Aperture Synthesis in the MIDInfrared 10 mm

APrèS-MIDI Aperture Synthesis in the MID-Infrared (10 mm) with the VLTI B. Lopez, Ph. Mathias, P. Antonelli, N. Berruyer, Y. Bresson, O. Chesneau, M. Dugué, A. Dutrey, A. Glazenborg, U. Graser, Th. Henning, S. Lagarde, Ch. Leinert, G. Perrin, A. Roussel, E. Thiebaut Acknowlegments : V. Coudé du Foresto, F. Delplancke, J. Gay, A. Glindemann, K. Meisenheimer, D. Mourard, R. Petrov, J. Steinacker, J. -C. Valtier

MIDI APre. S-MIDI (4 VLTI beams -> 2 beams) Science Cases : Active Galaxic Nuclei Disc of Young Stellar Objects Envelopes of evolved stars …

Le code de transfert radiatif, MC-TRANSF, exemple d’application : structure disque de la nébuleuse post-AGB ‘The Egg Nebula’. Model parameters Density law

Expected results by increasing difficulty : • Apparent sizes of discs • Inclinations • Via models (assuming a geometry) : inner radius, optical depth, nature and typical size of dust, temperature, … • Repartition of the dust material, structures like gaps

MIDI instrument APre. S-MIDI Optical interface (4 beams -> 2 beams) ? l 1 4 photometric beams or l 2 ?

Details of the main optics Top view Pyramidal Miroir à facettemirror Segmented mirrors Miroirs segmentés D 100 mm F 700 mm Décalage 1 mm Collimator Collimateur D 50 mm F 855 mm D 10 mm Tilt +/- 0. 82° Plane Plan demirror renvoi D 100 mm 4 renvois plans 4 plane mirrors D 50 mm

The concept proposed : a pupil recombination (with tilt angles) Si (e-i(u+ui)(k+dki)P(u))

Considerations about the Signal to Noise Ratio Beams MIDI 2 APre. S-MIDI 4 Baselines 1 Background from 2 tel. Beam splitter 1 6 4 0 per baseline : Closure phases 3 SNR APre. S-MIDI = 1/3 SNR MIDI 0 globally : SNR APre. S-MIDI = 6*(3 ? )*1/3 SNR MIDI Additional background of about 15 % due to 5 -8 mirrors added to the 30 -35 mirrors of the VLTI (a lost of reflectivity is also to considere). Background limited noise regime in the 40 pixel diameter pupil using the full N band in 0. 1 second Exposure time to increase when using filters external fringe tracking needed.

Expected performances of APre. S-MIDI Sensitivity (as for MIDI): N=4 with UTs, N=7 -8 with UTs and the use of an external fringe tracking Angular resolution: 10 mas at 10 microns Recombination mode: pupil plane with tilt angle, up to 4 telescope beams allowed Spectral mode: using narrow filters (R=30) with 4 beams (dispersed mode with 3 beams ? ) Field of view : corresponding to Airy disc sizes. Expected imaging performance: one image reconstructed by aperture synthesis from about 10 hours of effective observation using 4 telescopes.

Simulated image at 10 mm (Tuthill et al. 2000, Ap. J 543, 284)

Aperture synthesis at 10 mm with the VLTI, expected efficiency Simulated image at 10 mm 16 mas pixel size 3 telescopes reconstruced image 4 telescopes reconstruced image with MEM 4 telescopes, 6 nights 397 spectral densities, 264 phase closures, SNR=20

Status of the project Concept appears valid Studies in progress : coupling Zemax APre. S-MIDI with Zemax MIDI; photometric beams; test and alignment devices and procedures; room avalaible in the VLTI laboratory; simulations of a serie of images for different classes of sources. Phase A engineering study can start end 2004 (- end 2005) with good basis.

VLTI elements allowing the operation of a 4 beams instrument ? Switchyard : OK Delay lines : OK LEONARDO for co-alignment : 2 beams, usable for 3 and 4 beams FINITO : ‘ 3’ beams by now

MIDI/VLTI and other 10 µm interferometers Keck (2 x 10 m) 80 m (2002) ISI (3 x 1. 5 m) 72 m (2003) Large Binocular Telesccope (2 x 8 m) 15 m (2005) 1) The VLTI has a potential that is unique for producing images, in particular in the 9 -12 µm domain. 2) The N Spectral band is of importance: between the optical domain and mm (ALMA) 3) Preliminary step before Mid-IR imaging in space (DARWIN, TPF)