Projet O ptical S La cintillation by Fa

Projet O ptical S La cintillation by Fa ma it- tiè e xtraterrestrial Le lle re c s é sci ac to nti hée efractors ile lle s? r Marc MONIEZ, IN 2 P 3 E R Moriond 2006 20/03/2006

Where are the hidden baryons? • Compact Objects? ===> NO (microlensing) • Gas? – Atomic H well known (21 cm hyperfine emission) – Poorly known contribution: molecular H 2 (+25% He) • Cold (10 K) => no emission. Very transparent medium. • In fractal structure covering 1% of the sky. Clumpuscules ~10 AU (Pfenniger & Combes 1994) • In the thick disc or/and in the halo • Thermal stability with a liquid/solid hydrogen core • Detection of molecular clouds with quasars (Jenkins et al. 2003, Richter et al. 2003) and indication of the fractal structure with clumpuscules from CO lines in the galactic plane (Heithausen, 2004).

$These clouds refract light • Elementary process involved: polarizability a – far from resonance$

These clouds refract light • Elementary process involved: polarizability a – far from resonance • Extra optical path due to H 2 medium – ~80, 000 l (on 1% of the sky) @ l=500 nm – Corresponding to a column of ~300 m H 2 (normal P and T)

Scintillation through a strongly diffusive screen Propagation of distorted wave surface driven by: Fresnel diffraction + « global » refraction

Scintillation through a strongly diffusive screen Pat tern spe ed move of t s he at the scr een

$Fresnel diffraction on pulsars and stars have been detected before • In radioastronomy Classical$

Fresnel diffraction on pulsars and stars have been detected before • In radioastronomy Classical technique to study interstellar medium • In optics – diffraction during lunar occultations – effects from the upper atmosphere of Saturn (Cooray & Elliot 2003)

$scintillation modes and characteristics for a star seen through a Diffractive Refractive clumpuscule with$

scintillation modes and characteristics for a star seen through a Diffractive Refractive clumpuscule with column B and R density fluctuations of 10 -6 in a B and R NOT correlated few 103 km at l = 500 nm Screen Contrast t Source tscint scale with l scint contrast 1/2 position LMC A 5 stars Thin disc (300 pc) r. S=1. 7 r. Sun, mv=20. 5 Thick disc (1 kpc) Minute OR SNIa@max (z=0. 2) Gal. halo (10 kpc) Thin disc (300 pc) LMC B 8 V stars Thick disc (1 kpc) 10 min. r. S=3 r. Sun, mv=18. 5 Gal. halo (10 kpc) ~10% ~ 5% ~ 2% ~ 1% Hour or Few % more

Simulation of a turbulent cloud => Phase screen Light-curve of an A 5 V-LMC star (integral in the sliding disk) Diffraction image of a point-like source through this cloud @1 kpc

Illumination on earth from a LMC A 5 V star behind a screen@1 kpc Simulation : modulation index of the light received on Earth, as a function of Rdiff (l=500 nm) Rdiff separation such that: s[d(r+Rdiff)-d(r)]=l/2 p

$Refractive scintillation simulation B 8 V « big » star in LMC, screen @$

Refractive scintillation simulation B 8 V « big » star in LMC, screen @ 1 kpc

Fraction of scintillating stars Looking for clumpuscules with d(Nl)~10 -7 in 1000 km • Let a the fraction of halo into molecular gas • Optical depth t – Max for all modes t < a. 10 -2 – Min for diffractive mode (better signature) t > a. 10 -7

$« Event » rate G = t/Dt • Diffractive mode : phases of$

« Event » rate G = t/Dt • Diffractive mode : phases of few % fluctuation at the minute scale, during a few minutes G >1 event per 106/a starxhour • All modes : assumed quasi-permanent, few % fluctuations at the hour scale 1 scintillating star per ~ 100/a * Short time scale fluctuations => continuity of observations is NOT critical Any event is fully included in an observation session

$Detection requirements on Earth • Diffractive mode => small stars (105/deg 2) ü Smaller$

Detection requirements on Earth • Diffractive mode => small stars (105/deg 2) ü Smaller than A 5 type in LMC => MV~20. 5 ü Characteristic time ~ 1 min. => few sec. exposures ü Photometric precision required ~1% Telescope > 2 meters ü Dead-time < few sec. => Fast readout Camera 2 cameras ü B and R fringes not correlated => Wide field ü 106/a starxhour for one event => • Refractive mode Slower, detectable with the same setup. Signature not as strong (B and R variations correlated).

Possible experimental setup tip/tilt compensation 2 -4 m telescope few 100’s hours Focal plane Dichroic separator 2 cameras Wide field 10 cm Mosaic of frametranfert CCDs

Fore and back-grounds • Atmospheric turbulence Prism effects, image dispersion, BUT DI/I < 1% at any time scale in a big telescope BECAUSE speckle with 3 cm length scale is averaged in a >1 m aperture • High altitude cirruses Would induce easy-to-detect collective absorption on neighbour stars. • Gas at ~10 pc Scintillation would also occur on the biggest stars • Intrinsic variability Rare at this time scale and only with special stars

Expected difficulties, cures • Blending (crowded field)=> differential photometry • Delicate analysis – Detect and Subtract collective effects – Search for a not well defined signal • VIRGO robust filtering techniques (short duration signal) • Autocorrelation function (long duration signal) • Time power spectrum, essential tool for the inversion problem (as in radio-astronomy) • If interesting event => complementary observations (large telescope photometry, spectroscopy, synchronized telescopes…)

What could we learn from detection or non-detection? • Expect 1000 a events after monitoring 105 stars during 100 hours if column density fluctuations > 10 -7 within 1000 km • If detection – Get details on the clumpuscule (structure, column density -> mass) through modelling (reverse problem) – Measure contribution to galactic hidden matter • If no detection – Get max. contribution of clumpuscules as a function of their structuration parameter Rdiff (fluctuations of column density)

Test towards Bok globule B 68 NTT IR (2 nights in 2004 + 2 coming in 2006) 4 fluctating stars (other than known artifacts)

Conclusions - perspectives • Opportunity to search for hidden transparent matter is technically accessible right now • Risky project but not worse than many others • Need clumpuscules with a structuration that induce column density fluctuations ≥ 10 -7 (1017 molecules/cm 2) per 1000 km • Alternatives to OSER: GAIA, LSSC. But much longer time scale • Call for telescope (few 100’s hours, 2 -4 m) Biblio : A&A 412, 105 -120 (2003); Proc. 21 rst IAP Colloquium (2005)

And for the future… A network of distant telescopes • Would allow to decorrelate scintillations from atmosphere and interstellar clouds • Snapshot of interferometric pattern + follow-up ü Simultaneous Rdiff and VT measurements ü => positions and dynamics of the clouds ü Plus structuration of the clouds (inverse problem)