Transcript Does Transparent Hidden Matter Generate Optical Scintillation?

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