Transcript Does Transparent Hidden Matter Generate Optical Scintillation?

Projet
O
ptical
S
cintillation by
E
xtraterrestrial
R
efractors
Marc MONIEZ, IN2P3
Moriond 2006
20/03/2006
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).
These clouds refract light
• Elementary process involved: polarizability a
– far from resonance
• Extra optical path due to H2 medium
– ~80,000l (on 1% of the sky) @ l=500nm
– Corresponding to a column of ~300 m H2
(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
Scintillation through a strongly
diffusive screen
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
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 %
Simulation of a
turbulent cloud
=> Phase screen
Light-curve of an A5V-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
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[d(r+Rdiff)-d(r)]=l/2p
Refractive scintillation simulation
B8V « big » star in LMC, screen @ 1kpc
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
• 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 100’s 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.
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)
Test towards Bok globule B68
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/cm2) per 1000 km
• Alternatives to OSER: GAIA, LSSC. But
much longer time scale
• Call for telescope (few 100’s hours, 2-4m)
Biblio : A&A 412, 105-120 (2003); Proc. 21rst 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
 Simultaneous Rdiff and VT measurements
 => positions and dynamics of the clouds
 Plus structuration of the clouds (inverse problem)