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Photo: J. Fanson (JPL)
Planet Forming Disks ‐ What We Can Learn by Combining the World's Largest Telescopes Rafael Millan-Gabet
Caltech / Michelson Science Center
Outline
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Some context: our place in the Universe
How stars and planets are believed to form
Why is it important to study the inner disk
regions?
Limitations of “normal” telescopes
Ingenious solutions …
What we have learned
Exciting prospects!
2 Context: Our place in the Universe …
A minuscule area on the sky (dime thickness viewed from 75 feet!) contains 1500 galaxies! Big Dipper
3 Our Galaxy
Looking toward the center of the Galaxy ‐ 10x8 degrees area. Artistic sketch
~ 100.000 milion stars Real data …
(2MASS – NASA) ~ 10 million stars 4 Star and planetary system
formation in our neighborhood
Eagle nebula The “Pillars of CreaUon” HST (WFPC) ‐ NASA Credit: T. Greene (NASA Ames) 5 An old hypothesis …
Immanuel Kant (1724-1804)
Marquis Pierre Simon de Laplace
(1749-1827)
6 Observational evidence (I)
•  collect all the light from a young stellar object
•  disperse it by wavelength
=> Looks very different for a naked star vs. a star + disk
credit: NASA/JPL-Caltech/T. Pyle (SSC)
BUT: very different spatial distributions of the circumstellar material
7 - even not disk-like at all - produce similar observable …
Observational evidence (II)
Taurus
Amazing Hubble Space
Telescope images
HH30
8 So what’s the problem?
9 What about planetary system scales?
HST/WFPC2 (C. Burrows STScI)
This is not going to do it …
10 What is limiting the “resolution”?
Let’s review how a telescope works …
11 Telescope as a light bucket
  Telescopes collects number of
photons proportional to area of
primary mirror α Diameter2
  big telescopes are good
because they are more
sensitive.
  can see very faint things,
or things that appear faint
because they are very far
away.
D = 10m
12 Distant star
(point-like)
Image formation
(a bit more difficult …)
Telescope
primary mirror
Image of a
“point” …
Not a point, but a
spread of energy!
13 Resolution
the image of the point source is
narrower (good, better resolution!)
for:
  shorter wavelength (λ)
  larger telescope (D)
Example:
Keck D = 10m
say, λ = 2 μm
 0.2 millionths of a radian
(human hair from 1500 feet)
14 …
Fabulous, but not enough
But, it is technologically not possible to
construct arbitrarily large telescope mirrors …
Current record from the ground: D=10 m
Current record from space: D=2.4 m
Next generation ground telescopes 30-100m under study
Next generation space telescope (JWST): 6.5 m
What if I need better resolution than that??
15 Give up and learn to surf …
Waiting for
the big one …
16 Or …
17 A creative solution …
Coherently combine the light from 2 or more separated telescopes
D = 10m
18 How does this work?
Single
telescope
image:
Interferometer
image:
sum
one star
two stars
19 The amazing general relation
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Point to an object and form the
interference pattern.
Measure its contrast (and position,
or phase).
Repeat for many pairs of telescopes.
Perform a mathematical operation
(Fourier transform)
Reconstructed object map!
(with very high resolution)
20 Too good to be true? …
21 Limitations
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Most of the “synthetic
aperture” is missing.
Earth rotation helps a little …
Atmosphere corrupts the
fringe phases
Would need LOTS of telescopes.
More typically, instead of performing the
“blind” image reconstruction, only a few
measurements are obtained and one uses
some a priori knowledge about the object to
infer some important properties.
22 A real life example
R
Fringe contrast
Our idea for these objects
Dominant component
for these observations
Ring too small
Ring just right!
(R=0.3 AU)
Ring too big
Baseline length
Toy model
We have just
determined a
simple, but very
important,
property of this
object, that
cannot be
obtained any
other way!
23 First steps toward true imaging
http://www.nsf.gov/news/news_summ.jsp?cntn_id=109612&org=NSF
24 Radio-interferometry
Very Large Array (New Mexico) VLBI   much longer wavelengths (less resolution)
o  less stringent optical tolerances (mm-m)
o  probes different physical conditions (i.e. disk regions)
  detectors can respond to electric field directly
o  record telescope signals and do “digital interference”
  less severe atmospheric effects
25 A bit of history …
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[1868 – H. Fizeau] stellar interferometry first suggested.
[1872-73 – E. Stephan] first implementation.
[1920 – A. Michelson] first stellar diameters measured.
[1920s – 30s] the technique runs into practical limitations of the time.
[1933 – K. Jansky] radio astronomy.
[1940s] radio interferometry.
[1950s – 60s] intensity interferometry.
[1974 – A. Labeyrie] first fringes from separated optical telescopes.
[1980s – 90s] first generation of fully scientifically productive instruments.
20 ft beam on
the 100 inch Mt
Wilson telescope
(1920s)
26 Some interferometers around the world
GI2T (France)
IOTA (Mt Hopkins, AZ)
PTI (Palomar Mt, CA)
SUSI
(Australia)
27 CHARA (Mt Wilson, CA)
Find Waldo …
KI (Mauna Kea, HI)
VLTI (Paranal, Chile)
28 Proto-planetary disks at high resolution
Credit: Roy van Boekel
And very hot
gas plunging
onto the
young star
Measured the
location of
inner gas
component
Measured the
size & shape of
inner dust front
Measured the size
& shape of outer
disk (1-20AU) and
also dust
29 chemistry
Exo-zodiacal disks
Consider now the later
evolutionary stage:
Our solar system zodiacal light Most luminous component a]er the Sun! In addition to the fully formed
planets, there is a very tenuous
disk of dust …
30 Why is it important to study exo-zodi disks?
1. 
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Features in the dust disk may be signatures of unseen
planets.
They are a “noise” source for the planned direct planet
finding missions (TPF/DARWIN).
X300 brighter than exo-planet signal at IR wavelengths!
Vega
Credit: D. Wilner (CfA)
31 “Nulling” at the Keck Interferometer
ConstrucUve interference DestrucUve interference Keck Interferometer, Mauna Kea, Hawaii 32 Many exciting things to look forward to!
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Continue developing and improving ground based facilities.
  e.g OHANA: combine all the Mauna Kea telescopes!!
Coming soon: ALMA, Herschel space telescope.
Coming later: JWST space telescope.
Even later - space planet finders: SIM, TPF/DARWIN
An “optical ALMA”?
Probe all spatial scales in preplanetary disks, providing a
global understanding of how
stars and planets form, and
the conditions for habitability
in those planets.
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