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Galactic hidden gas The Optical S cintillation by E xtraterrestrial R efractors Marc MONIEZ, IN2P3 Project ESO-Santiago 28/06/2006 Overview Introduction •Where are the hidden baryons? •The difficulty to detect H2 Diffraction through a refringent medium Observability An experimental scheme Tests The Milky-Way rotation curve • 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 WMAP Hidden baryons 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). Orders of magnitude • Assuming a spherical isothermal dark halo Orders of magnitude • Assuming a spherical isothermal dark halo • Made of H2 clouds • Question: column density towards LMC? Orders of magnitude • Assuming a spherical isothermal dark halo • Made of H2 clouds • Average mass column density towards LMC 250g/m2 or a column of 3m H2 (normal P and T) Clouds cover 1% of sky => concentration of 100 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 molecular density ~ 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%) Huyghens-Fresnel diffraction after crossing a frozen phase screen Spheric wave •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 nm if z0 = a few kparsecs Scintillation through a strongly diffusive screen Scintillation through a strongly diffusive screen Propagation of distorted wave surface driven by: Fresnel diffraction + « global » refraction Scintillation through a strongly diffusive screen Scintillation through a strongly diffusive screen Example : step of optical path d extra optical Path over 1/2 plane • Pattern as a function of d • Path step d=l/4 Contrast is severely limited by the source size => spatial coherence Screen = l/2 step z1 z0 • 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 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 Simulation of a turbulent cloud Light-curve of an A5V-LMC star (integral in the sliding disk) Diffraction image of a point-like source through this cloud @1 kpc Rdiff : Statistical characterization of a stochastic screen Size of domain where s(phase)= 1 radian • Or equivalently s(column density) = 1.6x1018 molecules/cm2 • This corresponds to - Dn/n ~ 10-6 for disk/halo clumpuscule - Dn/n ~ 10-4 for Bok globule (NTT search) Along this section 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 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 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 Screen Source position Diffractive Refractive B and R NOT correlated B and R correlated tscint LMC A5 stars Thin disc (300pc) rS=1.7rSun, mv=20.5 Thick disc (1kpc) Minute OR SNIa@max (z=0.2) Gal. halo (10kpc) Thin disc (300pc) LMC B8V stars Thick disc (1kpc) 10 min. rS=3 rSun, mv=18.5 Gal. halo (10kpc) Contrast t contrast scale with l1/2 scint ~10% ~ 5% ~ 2% ~5% ~ 2% ~ 1% Hour or more Few % Refractive scintillation simulation flux (arbitrary unit) 22400 V = 30 km/s 1 hour 22200 simulated light-curve 22000 simulated measurements Refractive scintillation regime of a B8V star in LMC (G=18.5) Lambda= 500 nm Photometric precision: 0.5% 21800 Rdiff RS projected stellar radius 21600 flux (arbitrary unit) -80000 -80000 -60000 -40000 -20000 0 22300 x (km) 20000 40000 60000 80000 22100 21900 2.5% 21700 Lambda= 900 nm 21500 -60000 -40000 -20000 0 x (km) 20000 40000 60000 80000 Refractive scintillation simulation flux (arbitrary unit) 22400 V = 30 km/s 1 hour 22200 simulated light-curve 22000 simulated measurements Refractive scintillation regime of a B8V star in LMC (G=18.5) Lambda= 500 nm Photometric precision: 0.5% 21800 Rdiff RS projected stellar radius 21600 flux (arbitrary unit) -80000 -80000 -60000 -40000 -20000 0 22300 x (km) 20000 40000 60000 80000 22100 21900 2.5% 21700 Lambda= 900 nm 21500 -60000 -40000 -20000 0 x (km) 20000 40000 60000 80000 Fraction of scintillating stars Looking for clumpuscules with d(Nl)~10-7 in 1000km • 1 star/100 is behind a molecular cloud if 100% gaseous halo • 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 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/deg2) Smaller than A5 type in LMC => Characteristic time ~ 1 min. => Photometric precision required MV~20.5 few sec. exposures ~1% Telescope > 2 meters Dead-time < few sec. => Fast readout Camera 2 cameras B and R fringes not correlated => Wide field 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 2-4m telescope few hundreds hours Focal plane Dichroic separator 2 cameras Wide field 10cm Mosaic of frametranfert CCDs QuickTime™ et un décompresseur TIFF (LZW) sont requis pour visionner cette image. Frame transfer E2V CCD47-20 • 1024x1024 pixels of 13m • High quantum efficiency (~80%) • Allows a repetition rate without dead-time > 2 shots/minute 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 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 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 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) 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) QuickTime™ et un décompresseur GIF sont requis pour visionner cette image. Test towards Bok globule B68 NTT IR (2 nights in june 2004) • 2873 stars monitored • ~ 1000 exposures/night • Search for few % variability • Signal if Dn/n ~ 10-4 per ~1000 km • Mainly test for background and feasibility Test towards Bok globule B68 NTT IR (2 nights in june 2004) 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 • 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 timescale survey… • Call for telescope (few 100’s hours, 2-4m) Biblio : A&A 412, 105-120 (2003); Proc. 21rst IAP Colloquium (2005) QuickTime™ et un décompresseur GIF sont requis pour visionner cette image. The end Optical Scintillation by Extraterrestrial Refractors More info in astro-ph/0302460 Illumination sur Terre due à une étoile de type A5V du LMC Simulation: Fractal phase screen • Kolmogorov turbulence -> realistic • Other power laws under study, but small sensitivity expected Simulation: Fractal phase screen This is a real storm cloud! • Kolmogorov turbulence -> realistic • Other power laws under study, but small sensitivity expected 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? Test towards Bok globule B68 Test towards Bok globule B68 Only 4 fluctuating stars (other fluctuations due to identified artifacts) « variable » objects Not easy to conclude without complementary data Illumination @550nm Illumination @450nm Time coherence The bandwidths of the standart astronomical filters have small impact on the contrast Conditions to get contrasted diffraction patterns • Non-zero second derivative 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 produced by a prism of gradient 1xl per transverse distance RF Stationnary phase approximation: Fresnel zones • Zones where secondary sources contribute for a positive amplitude (white) or negative (black) at observing point. Contributions without diffusor 2pRF Diffusor: phase delay spot (p) Diffusor: phase delay spot (p) Diffusor: phase delay spot (p) Diffraction image of a point-like source through this cloud @10kpc Simulation of a turbulent cloud z0=10kpc, Rdiff=10 000km, Rstar=1.7Rsun, Vt=200km/s 9,22 9,2 Light-curve of an A5V-LMC star (integral in the sliding disk) Intensity 9,18 9,16 1% 9,14 9,12 9,1 1 minute 9,08 9,06 9,04 1 31 61 91 121 151 181 211 241 271 Time (s) 301 331 361 391 421 451 481 Configurations producing diffractive scintillation for Rdiff ~ RF at l = 500nm Source Screen position RF All stars Atmosphere (10km) 3cm Nearby stars (10pc) LMC/M31 stars LMC/M31 stars Solar system (1AU) 100m Sun suburbs (10pc) 150km Dt and contrast scale with l1/2 Time Contrast VT scale 1m/s 30ms 10km/s 10ms 20km/s 8s ~1 < or << 1 ~1 5-100% LMC A5 stars Thin disc (300pc) 900km 30km/s 30s ~13% 40% (rS=1.7 rSun) Thick disc (1kpc) 1600km 40km/s 40s ~ 7% 22% OR SNIa@max (z=0.2) Gal. halo (10kpc) 5000km 200km/s 30s ~ 2% 7% M31 B0 stars (rS=7.4 rSun) Other studies to be done • Play with the model of screen – Stationnary turbulent structures • • • • Filaments Bubbles Plumes Acoustic waves • Other scintillation configurations: Quasars or SuperNovæ behind galaxies