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The O ptical S cintillation by E xtraterrestrial R efractors Project

Galactic hidden gas. The O ptical S cintillation by E xtraterrestrial R efractors Project. La matière cachée Fait-elle scintiller Les étoiles?. Marc MONIEZ, IN2P3. ESO-Santiago 28/06/2006. Overview. Introduction Where are the hidden baryons? The difficulty to detect H 2

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The O ptical S cintillation by E xtraterrestrial R efractors Project

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  1. Galactic hidden gas TheOpticalScintillationbyExtraterrestrialRefractorsProject La matière cachée Fait-elle scintiller Les étoiles? Marc MONIEZ, IN2P3 ESO-Santiago 28/06/2006

  2. Overview • Introduction • Where are the hidden baryons? • The difficulty to detect H2 • Diffraction through a refringent medium • Observability • An experimental scheme • Tests

  3. The Milky-Way rotation curve

  4. WMAP Hidden baryons • Wvisible = 0.006 (Wc unit) • Big-Bang Nucleosynthesis=> Wbh2 > 0.01 • WMAP :Wbh2 = 0.0224 => Wb = 0.044 • A factor 8 missing: This factor fits the galactic missing mass factor • Essentially made of H + 25% He in mass

  5. Where are the hidden baryons? • Compact Objects?===> NO (microlensing) • Gas? • Atomic H well known (21cm hyperfine emission) • Poorly known contribution: molecular H2(+25% He) • Cold (10K) => 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).

  6. Orders of magnitude • Assuming a spherical isothermal dark halo

  7. Orders of magnitude • Assuming a spherical isothermal dark halo • Made of H2 clouds • Question:column densitytowards LMC?

  8. Orders of magnitude • Assuming a spherical isothermal dark halo • Made of H2 clouds • Average mass column density towards LMC • 250g/m2 or a columnof 3m H2(normal P and T) • Clouds cover 1% of sky=> concentration of 100

  9. These clouds refract light • Elementary process involved: polarizability a • far from resonance=> classical forced oscillator formalism • close to initial propagation direction=> collective effect even with low moleculardensity ~ 109 cm-3 (<1/l3) • Extra optical path due to H2 medium • On average ~800l @ l=500nm=> varies from 0 (99% of the sky) to 80,000l (1%)

  10. Spheric wave Huyghens-Fresnel diffraction after crossing a frozen phase screen • Fresnel approximation • Stationnary phase approximation • Point-like source on axis at ∞ • Phase screen described by d(x1,y1) A few 1000 km at l = 500 nmif z0 = a few kparsecs

  11. Scintillation through a strongly diffusive screen

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

  13. Scintillation through a strongly diffusive screen Pattern moves at thespeed of the screen

  14. Scintillation through a strongly diffusive screen Pattern moves at the speed of the screen

  15. Example : step of optical path • d extra optical • Path over 1/2 plane • Pattern as a function of d • Path step d=l/4

  16. Contrast is severely limited by the source size => spatial coherence Screen = l/2 step • Depression width ~ RS=> Info on source size • Contrast ~ RF/ RS • Also depends on Dl (time coherence), but not critically:Dl/l<0.1 => DRF/RF<0.05 z1 z0

  17. Fresnel diffraction on stars has been observed • In radioastronomy: classical technique for interstellar medium studies • In optics:diffraction during lunar occultations, clearly distinct from atmospheric effects

  18. Diffraction image ofa point-like sourcethrough this cloud @1 kpc Light-curve of an A5V-LMC star (integral in the sliding disk) Simulation of a turbulent cloud

  19. Along this section Rdiff : Statistical characterization of a stochastic screen • Size of domain where • s(phase)= 1 radian • Or equivalentlys(column density) = 1.6x1018molecules/cm2 • This corresponds to • Dn/n ~ 10-6 for disk/halo clumpuscule • Dn/n ~ 10-4 for Bok globule (NTT search)

  20. Scintillation modes • Key parameter: Rdiff separation such that: • s[f(r+Rdiff)-f(r)] = 1 radian • Rdiff >>RF • Weakly structured medium • Weak diffractive mode • Rdiff≤RF • Strongly structured medium • Strong diffractive mode • Refractive mode if large • scale structure (Rref) • Remark: Rdiff ~RF natural scale as ||df(r)/dr||screen ~ 1 radian/RF

  21. Illumination on earth from a LMC A5V star behind a screen@1kpc Simulation: modulation index of the light received on Earth, as a function of Rdiff (l=500nm) Rdiff separation such that: s[f(r+Rdiff)-f(r)] = 1 radian

  22. scintillation modes and characteristics for a star seen through a clumpuscule with column density fluctuations of 10-6 in a few 103km at l = 500nm

  23. Refractive scintillation simulation

  24. Refractive scintillation simulation

  25. Fraction of scintillating stars Looking for clumpuscules with d(Nl)~10-7 in 1000km • 1 star/100 is behind a molecularcloud if 100% gaseous halo • Let a the fraction ofhalo into molecular gas • Optical deptht • Max for all modes t < a.10-2 • Min for diffractive mode(better signature) t > a.10-7

  26. « 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 criticalAny event is fully included in an observation session

  27. Telescope > 2 meters Fast readout Camera 2 cameras Wide field Detection requirements on Earth • Diffractive mode => small stars (105/deg2) • Smaller than A5 type in LMC => MV~20.5 • Characteristic time ~ 1 min. => few sec. exposures • Photometric precision required ~1% • Dead-time < few sec. => • B and R fringes not correlated => • 106/a starxhour for one event => • Refractive mode • Slower, detectable with the same setup. Signature not as strong (B and R variations correlated)

  28. Focal plane Mosaic of frame-tranfert CCDs 10cm Dichroic separator Possible experimental setup tip/tilt compensation 2-4m telescope few hundreds hours 2 cameras Wide field

  29. Frame transfer E2V CCD47-20 • 1024x1024 pixels of 13m • High quantum efficiency (~80%) • Allows a repetition rate without dead-time > 2 shots/minute

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

  31. 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 => complementaryobservations (large telescope photometry, spectroscopy, synchronized telescopes…)

  32. What could we learn from detection or non-detection? • Expect 1000a events after monitoring 105 stars during 100 hours if column density fluctuations > 10-7 within 1000km • 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)

  33. 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 • Simultaneous Rdiff and VT measurements • => positions and dynamics of the clouds • Plus structuration of the clouds (inverse problem)

  34. Test towards Bok globule B68NTT IR (2 nights in june 2004) • 2873 stars monitored • ~ 1000 exposures/night • Search for few % variability • Signal if Dn/n ~ 10-4per ~1000 km • Mainly test for back-ground and feasibility

  35. Test towards Bok globule B68NTT IR (2 nights in june 2004) 4 fluctating stars (other than known artifacts)

  36. Conclusions - perspectives • Opportunity to search for hidden transparent matter is technically accessible right now • Risky project but not worse than many others • Sensitive to clumpuscules with a structuration that induce column density fluctuations ≥ 10-7 (1017 molecules/cm2) per 1000 km • Alternatives to OSER: GAIA, LSST. But much longer time scale • Don’t forget the potential by-products of such a short time-scale survey… • Call for telescope (few 100’s hours, 2-4m) Biblio : A&A 412, 105-120 (2003); Proc. 21rst IAP Colloquium (2005)

  37. The end Optical Scintillation by Extraterrestrial Refractors More info in astro-ph/0302460

  38. Illumination sur Terre due à une étoile de type A5V du LMC

  39. Simulation: Fractal phase screen • Kolmogorov turbulence -> realistic • Other power laws under study, but small sensitivity expected

  40. Simulation: Fractal phase screen • Kolmogorov turbulence -> realistic • Other power laws under study, but small sensitivity expected This is a real storm cloud!

  41. Illumination on earth from a LMC A5V star behind a screen@1kpc Rdiff=1000km Rdiff=10 000km Patterns • Show measurable contrasts • Move with the relative transverse speed of screen/line of sight • Could show inner variation?

  42. Test towards Bok globule B68

  43. Test towards Bok globule B68 Only 4 fluctuating stars (other fluctuations due to identified artifacts)

  44. « variable » objects Not easy to conclude without complementary data

  45. Time coherence Illumination @450nm Illumination @550nm The bandwidths of the standart astronomical filters have small impact on the contrast

  46. Conditions to get contrasted diffraction patterns • Non-zero second deriva-tive of the optical path d within a RF size domain (Ex. Stochastic fluctuations) • These conditions have a good chance to occur in molecular clouds of 10AU • Optical path varies from 80,000l in 5AU • Average gradient is 1xl per 10,000km (~RF) Ex.: Diffraction pattern producedby a prism of gradient 1xl per transverse distance RF

  47. Stationnary phase approximation: Fresnel zones • Zones where secondary sources contribute for a positive amplitude (white) or negative (black) at observing point.

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