Transcript The invisible Kuiper Belt Explored by Stellar Occultations

The invisible Kuiper Belt explorated by
stellar occultations
Françoise Roques
Observatoire de Paris
A. Doressoundiram, M. Moncuquet
M. Auvergne, M. Chevreton, F. Colas, J. Lecacheux, S. Pau,
I. Belskaya, N. Peixinho, G.P. Tozzi, O. Mousis
1. a diffracting phenomenon
2. a strategy of research
3. the large telescopes research campaign
4. results about the Kuiper Belt
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
stellar occultations :a diffracting
phenomenon
What ?
• detection of the diffraction shadow of KBOs to scan invisible small
and far populations
Why ? :
 the perturbation of small KBO on the occulted star flux can be
detectable
 the diffraction shadow is larger than the geometrical shadow
How ?
• fast photometry
• large telescope
Where ? :
• target stars with small angular size
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
stellar occultations :
a diffracting phenomenon
The strategy :
• search for the smallers
KBO
• ponctual stars
• very few target stars
• high frequency
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The Fresnel scale
• R = 3 AU, F = 245 m
• R = 40 AU, F = 1.1 km
• R = 105 AU, F = 55 km
F = √(.R/2)
: wavelength
R: distance of the occultor
The Fresnel scale is a scaling factor
of the occultation
dF/F = 0.1 => r = 0.2
Roques et Moncuquet, 2000
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The distance of the occultor
For objects < 0.7, the size of
the shadow is almost constant
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The distance of the occultor
If the observations detect 0.2 radius occultors…
•
•
The shadow diameter is 3 (in Fresnel scale!)
The event duration dt drives to the distance of the occultor R:
hypothesis 1 : the impact parameter is 0 : 3. F = vo. dt
hypothesis 2 : the occultor orbit is circular : vo = vE .(cos()-R-1/2)
3. √(.R/2) = v E .(cos()-R-1/2). dt
 direct estimation of R
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The T-2M campaign
2000-2003
• Observations on Bernard Lyot Telescope with multi-objects
photometers
• field : 2 targets + 1 reference star
• 20 Hz
• 15h with S/N = 55
• strategy : search for dips in the
ligthcurve
=> first constraint on the size
one potential event (r=150m)
Distribution :q < 4.5
Roques et al., 2003
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The large telescopes campaign
2004-2006
The instrument : ULTRACAM
an ultra-fast, triple-beam CCD camera
(u’: 0.36 m, g’:0.48 m and i’:0.77 m)
Vik Dhillon (U. Sheffield)
Tom Marsh (U. Southampton)
pulsars, X-ray binaries stars…
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The large telescopes campaign
The may 2004 observations
WHT,Canary Islands (4 meters)
- 3 successive fields of 2 stars (O stars)
- 44 Hz
- 4 nights
The may 2005 observations
VLT (8 meters)
- 2 fields of 2 stars (O stars)
- 68 Hz
- 2 nights
Catania - 3-7 july 2006
The march 2006 observations
WHT,Canary Islands (4 meters)
- 3 successive fields of 2 stars (O stars)
-68 Hz
- 4 nights
(under analysis)
Trans Neptunian Objects
Dynamical and Physical properties
The data
Tycho 2
ref.
V
magn.
Radius
at 40AU
Ecliptic
latitude
281-906
12.29
80 m
+2.9°
281-864
12.15
250 m
“
6172-1154
12.70
90 m
-0.4°
6172-1103
11.81
270 m
“
6821-1317
12.25
70 m
-7.1°
6821-1425
12.40
100 m
“
data S/N :
• WHT : 40 to 72
• VLT : 50 to 160
Catania - 3-7 july 2006
=> dF/F =0.1 > 4.
Trans Neptunian Objects
Dynamical and Physical properties
The data analysis
Electronic problems
Atmospheric effect
(Dravins et al., 1977)
Examples of false events
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Trans Neptunian Objects
Dynamical and Physical properties
The data analysis
0.48 m
standard deviation (i)
0.77 m
A synthetic event: a 200 m
radius object at 40 AU
standard deviation (g)
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
The results
In 35 h of data, 3 events
with deviation larger than
5 :
5.6 
7.2 
5.3 
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Trans Neptunian Objects
Dynamical and Physical properties
The results
Fit with the hypothesis 1 and 2
R : 100 AU and 200 AU
 : 0 and 400 m
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320  20 meters radius
140  1 AU distance
Trans Neptunian Objects
Dynamical and Physical properties
The results
Several cheks have been run to asses the validity of the detections :
• Comparison star
• The color signature
• Check the star position
• Chech the sky glitches
• Search in randomized data
• Search for events in the ligthcurve of a large star (occultations by Uranus
and Neptune of stars with radius larger than 10 km at 40 AU)
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Dynamical and Physical properties
I - an object of 110m at 9-19 AU
More dark matter around Saturn and Uranus?
22 avril 1982 : during an occultation par Uranus, an event
has been observed simultaneously with two nearby telescopes.
compatible with a 1.4 km diameter object
More dark matter around Uranus and Neptune?, Sicardy et al., Nature 1986
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Trans Neptunian Objects
Dynamical and Physical properties
II - no object in the Kuiper Belt
30-60 AU
200 meters objects are detectables
Diffracting occultations scan a surface of : dS = 10 -9 R-1.5(AU) d°2. dt(h)
No detections in 35 h => N(200 m) < 4. 1013 at 40 AU
less than 0.2 earth mass in the KB
Catania - 3-7 july 2006
Trans Neptunian Objects
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III - 1-2 object farther than 100AU
115 - 225 AU
A cold extented disk composed of small objects ?
N(200m) : 10 14 (0.5 earth mass)
NB : serendipitous occultations is a statistical method,
The physical reality will com from “numbers”
Roques et al., AJ, in press
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties
Conclusions
• More observations
• more observations
• more observations …
and also
• more wavelengths
• more diffraction fringes !
Catania - 3-7 july 2006
Trans Neptunian Objects
Dynamical and Physical properties