the o ptical s cintillation by e xtraterrestrial r efractors project

$: The O ptical S cintillation by E xtraterrestrial R efractors Project$
The Optical

Scintillation by

Extraterrestrial

Refractors

Project

ESO-Santiago 28/06/2006

La matière cachée

Fait-elle scintiller

Les étoiles?

Marc MONIEZ, IN2P3

Galactic hidden gas

OverviewIntroduction

•Where are the hidden baryons?

•The difficulty to detect H2

Diffraction through a refringent mediumObservabilityAn experimental schemeTests

The Milky-Way rotation curve

WM

AP

Hidden baryons

• visible = 0.006 (c unit)

• Big-Bang Nucleosynthesis=> bh2 > 0.01

• WMAP : bh2 = 0.0224 => b = 0.044 • A factor 8 missing: This factor fits the galactic missing mass factor• Essentially made of H + 25% He in mass

• 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).

Where are the hidden baryons?

Orders of magnitude

• Assuming a spherical isothermal dark halo

Orders of magnitude


• Made of H2 clouds

• Question:column densitytowards LMC?

Orders of magnitude


• 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

These clouds refract light

• Elementary process involved: polarizability – far from resonance

=> classical forced oscillator formalism

– close to initial propagation direction=> collective effect even with low moleculardensity ~ 109 cm-3 (<1/3)

• Extra optical path due to H2 medium– On average ~800 @ =500nm

=> varies from 0 (99% of the sky) to 80,000 (1%)

•Fresnel approximation•Stationnary phase approximation•Point-like source on axis at ∞•Phase screen described by (x1,y1)

Huyghens-Fresnel diffraction after

crossing a frozen phase screen

A few 1000 km at = 500 nmif z0 = a few kparsecs

Sphericwave

Scintillation through a strongly diffusive screen


Propagation of distorted wave surface driven by:Fresnel diffraction

+« global » refraction


Pattern moves at the

speed of the screen

Example : step of optical path

• Pattern as a function of

• Path step =/4

extra optical Path over 1/2 plane

Contrast is severely limited by the source size => spatial coherence

• Depression width ~ RS

=> Info on source size

• Contrast ~ RF/ RS

• Also depends on (time coherence), but not critically:<0.1 => RF/RF<0.05

Screen = /2 step

z1 z0

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

Light-curve of an A5V-LMC star(integral in the sliding disk)

Diffraction image ofa point-like source

through this cloud @1 kpc

Simulation of a turbulent cloud

Rdiff : Statistical characterization of a

stochastic screenSize of domain where(phase)= 1 radian• Or equivalently(column density) = 1.6x1018 molecules/cm2

• This corresponds to- n/n ~ 10-6 for disk/halo clumpuscule- n/n ~ 10-4 for Bok globule (NTT search)

Along this section

Key parameter: Rdiff separation such that:

[(r+Rdiff)-(r)] = 1 radian

• Rdiff >>RF Weakly structured mediumWeak diffractive mode

• Rdiff ≤RF

Strongly structured mediumStrong diffractive modeRefractive mode if largescale structure (Rref)

Remark: Rdiff ~RF natural scale as ||d(r)/dr||screen ~ 1 radian/RF

Scintillation modes

Simulation : modulation index of the light received on Earth, as a function of Rdiff (=500nm)

Illumination on earth from a LMC A5V star behind a

screen@1kpc

Rdiff separation such that:

[(r+Rdiff)-(r)] = 1 radian

scintillation modes and characteristicsDiffractiveB and R NOT

correlated

RefractiveB and R

correlated

SourceScreen

positiontscint

Contrast scale with 1/2

tscint contrast

LMC A5 stars

rS=1.7rSun, mv=20.5

OR

SNIa@max (z=0.2)

Thin disc (300pc)

Minute

~10%

Hour or

moreFew %

Thick disc (1kpc) ~ 5%

Gal. halo (10kpc) ~ 2%

LMC B8V stars

rS=3 rSun, mv=18.5

Thin disc (300pc)

10 min.

~5%

Thick disc (1kpc) ~ 2%

Gal. halo (10kpc) ~ 1%

for a star seen through a clumpuscule with column density fluctuations of 10-6 in a few 103km at = 500nm

Refractive scintillation simulation

21600

21800

22000

22200

22400

-80000 -60000 -40000 -20000 0 20000 40000 60000 80000

x (km)

flu

x (

arb

itra

ry u

nit

)

Refractive scintillation regimeof a B8V star in LMC (G=18.5)Lambda= 500 nmPhotometric precision: 0.5%

V = 30 km/s

RS projected stellar radius

1 hour

simulatedlight-curve

simulatedmeasurements

Rdiff

21500

21700

21900

22100

22300

-80000 -60000 -40000 -20000 0 20000 40000 60000 80000

x (km)

flu

x (

arb

itra

ry u

nit

)

Lambda= 900 nm

2.5%

Fraction of scintillating stars

• 1 star/100 is behind a molecularcloud if 100% gaseous halo

• Let the fraction ofhalo into molecular gas

• Optical depth – Max for all modes

.10-2

– Min for diffractive mode(better signature)

.10-7

Looking for clumpuscules with (Nl)~10-7 in 1000km

« Event » rate = t

• Diffractive mode : phases of few % fluctuation at the minute scale, during a few minutes

>1 event per 106/ starxhour

• All modes : assumed quasi-permanent, few % fluctuations at the hour scale

1 scintillating star per ~ 100/* Short time scale fluctuations

=> continuity of observations is NOT criticalAny event is fully included in an observation session

Telescope > 2 metersFast readout Camera

2 camerasWide field

Detection requirements on Earth

• Refractive modeSlower, detectable with the same setup. Signature not as strong (B and R variations correlated)

• Diffractive mode => small stars (105/deg2)Smaller than A5 type in LMC => MV~20.5Characteristic time ~ 1 min. => few sec. exposuresPhotometric precision required ~1%

Dead-time < few sec. =>B and R fringes not correlated =>106/ starxhour for one event =>

Possible experimental setup

2 camerasWide field

2-4m telescope few hundreds hours

Dichroicseparator

tip/tiltcompensation

Focal planeMosaic of frame-

tranfert CCDs

QuickTime™ et undécompresseur TIFF (LZW)

sont requis pour visionner cette image.

10cm

Frame transfer E2V CCD47-20

• 1024x1024 pixels of 13• 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 I/I < 1% at any time scale in a big telescope

BECAUSE speckle with 3cm length scale is averaged in a >1m aperture

• High altitude cirrusesWould induce easy-to-detect collective absorption on neighbour stars.

• Gas at ~10pcScintillation would also occur on the biggest stars

• Intrinsic variabilityRare 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 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 cloudsPlus structuration of the clouds (inverse problem)

QuickTime™ et undécompresseur GIF


• 2873 stars monitored

• ~ 1000 exposures/night

• Search for few % variability

• Signal if n/n ~ 10-4 per ~1000 km

• Mainly test for back-ground and feasibility

Test towards Bok globule B68NTT IR (2 nights in june 2004)

Test towards Bok globule B68NTT 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 time-scale survey…

• Call for telescope (few 100’s hours, 2-4m)

Biblio : A&A 412, 105-120 (2003); Proc. 21rst IAP Colloquium (2005)

QuickTime™ et undécompresseur GIF


The end

Optical Scintillation by Extraterrestrial RefractorsMore info in astro-ph/0302460

Illumination sur Terredue 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

• Kolmogorov turbulence -> realistic

• Other power laws under study, but small sensitivity expected

This is a real storm cloud!

Illumination on earth from a LMC A5V star behind a screen@1kpc

Patterns- Show measurable contrasts- Move with the relative transverse speed of screen/line of sight- Could show inner variation?

Rdiff=1000km Rdiff=10 000km

Test towards Bok globule B68

Test towards Bok globule B68

Only 4 fluctuating stars(other fluctuations dueto identified artifacts)

« variable » objectsNot easy to conclude without complementary data

Time coherence

The bandwidths of the standart astronomical filters have small impact on the contrast

Illu

min

atio

n @

450n

m

Illu

min

atio

n @

550n

m

Conditions to get contrasted diffraction patterns

• Non-zero second deriva-tive of the optical path 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

80,000 in 5AU

– Average gradient is 1x per 10,000km (~RF)

Ex.: Diffraction pattern producedby a prism of gradient

1x per transverse distance RF

• Zones where secondary sources contribute for a positive amplitude (white) or negative (black) at observing point.

Stationnary phase approximation: Fresnel zones

Contributions without diffusor

2RF

Diffusor: phase delay spot ()

Simulation of a turbulent cloud

Diffraction image of a point-like source through

this cloud @10kpc

z0=10kpc, Rdiff=10 000km, Rstar=1.7Rsun, Vt=200km/s

9,04

9,06

9,08

9,1

9,12

9,14

9,16

9,18

9,2

9,22

1 31 61 91 121 151 181 211 241 271 301 331 361 391 421 451 481

Time (s)

Inte

nsit

y

1%

1 minute

Light-curve of an A5V-LMC star(integral in the sliding disk)

Configurations producing diffractive scintillation for Rdiff ~ RF at = 500nm

Source Screen position RF VTTime scale Contrast

All stars Atmosphere (10km) 3cm 1m/s 30ms ~ 1

Nearby stars (10pc) Solar system (1AU) 100m 10km/s 10ms

< or << 1

LMC/M31 stars ~ 1

LMC/M31 stars Sun suburbs (10pc) 150km 20km/s 8s 5-100%

LMC A5 stars (rS=1.7 rSun)

OR

SNIa@max (z=0.2)

Thin disc (300pc) 900km 30km/s 30s ~13% 40%

Thick disc (1kpc) 1600km 40km/s 40s ~ 7% 22%

Gal. halo (10kpc) 5000km 200km/s 30s ~ 2% 7%

M31 B0 stars (rS=7.4 rSun)

t and contrast scale with 1/2

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

the o ptical s cintillation by e xtraterrestrial r efractors project

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