Infrared Signatures of Planetary Systems

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Infrared Signatures of Planetary Systems
Amaya Moro-Martin
Department of Astrophysical Sciences, Princeton
University
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Quick Tour to Star and Planet Formation
Stars form in clouds of dust and gas. Local
density increase occurs within these clouds
that portion of the cloud contracts in on itself
under its own gravitational pull
a protostar is formed (no fusion yet).
By conservation of angular momentum, what is left
of the cloud rotates with the protostar and
begins to flatten into a circumstellar disk.
Some of this dust and gas accretes onto the
protostar adding to its mass.
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The disk is very dense. The grains are subject to
many forces and collide with each other often.
Some grains begin to stick together.
Grains grow until they form planetesimals
(asteroid-size bodies) some of them grow even
further into small planets. The terrestrial
planets in our solar system are large
accumulations of these bodies. Further from the
central star, some of these large rocky cores
accrete gas, forming giant gas planets like
Jupiter and Saturn.
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The outcome resembles our solar system.
Unfortunately, it is the only planetary system we
can observe in detail, so our view of planetary
formation is biased.
Observations in the infrared can help us study
other systems
Studying the evolution of disk properties
(mass, radial structure ) and dust properties
(size, composition). Looking for warm
molecular gas (H2). For more mature systems,
we can trace evolution of dust disks generated
through collisions of planetesimals and infer
location and mass of giant planets.
Define the timescales over which terrestrial and
gas giant planets are built.
Why the IR and not the optical? - In the optical,
the light from the star overpowers that of the
planet. - The disk is completely dark, but it
glows brightly in the infrared.
Lets see this in more detail
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With time, the remaining dust in the disk
dissipates, its either Blown away by the
star due to radiation pressure, or Drifts
all the way into the star due to
Poynting-Robertson drag where it sublimates
(timescale 105 -106 yrs)
?
However, many stars older than 107 yrs are still
surrounded by dust disks (1-10M ) ?!
Our Sun has a dust disk too of 10-4 M
This dust is not primordial but must be
replenished by a reservoir of undetected
planetesimals producing dust by mutual
collisions. This is why we call them debris
disks. Debris disks are indirect evidence of
planetary formation!!
..and for a long time it was the only evidence we
had
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Do debris disks harbor massive planets?
As dust particles spiral inward (due to PR drag),
they can get trapped in Mean Motion Resonances
with the planets. I.e. massive planets shepherds
the dust grains in the disks.
Without planets
with Solar System planets
minimum at Neptunes position (to avoid resonant
planet)
Neptune
ring-like structure along Neptunes orbit
(trapping into Mean Motion Resonances)
clearing of dust from inner 10 AU (due to
gravitational scattering by Jupiter and Saturn)
Uniform density disk
Massive planets may scatter and eject dust
particles out of a planetary system creating gaps.
Massive planets sculpt the debris disks in which
they are embedded
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Gaps and asymmetries observed in high-resolution
observations suggest giant planets may be
present. Structure is sensitive to long period
planets complementary to radial velocity
and transit surveys.
Needed to determine stability of orbits in
habitable zones (TPF)
We can learn about the diversity of planetary
systems from the study of debris disks structure!
e-Eri 850?m (emitted light Greaves et al. 98)
HR4796A 1.6 ?m (scattered light Schneider et al.
99)
H141569 1.1?m (scattered light Weinberger et al.
99)
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Looking for planets in spatially unresolved disks
Many disks are too far away to be spatially
resolved in most cases we wont be
able to look for planets by studying
debris disk structure directly.
Infrared excess
But the structure carved by the planets affects
the shape of the Spectral Energy Distribution
(SED) of the disk
we can study the debris disk structure indirectly.
Lets see some modeled SEDs of debris disks with
embedded planets in different configurations.
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No planet
Carbonaceous grains
Fe-rich silicate grains
LogF(mJy)
Fe-poor silicate grains
Log???m)
Planetesimals (Kuiper Belt)
star
50AU
1AU
5AU
30AU
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1 MJup at 5 AU
Carbonaceous grains
Fe-rich silicate grains
LogF(mJy)
Fe-poor silicate grains
Log?(?m)
Planetesimals (Kuiper Belt)
star
50AU
1AU
5AU
30AU
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3 MJup at 1 AU
Carbonaceous grains
Fe-rich silicate grains
LogF(mJy)
Fe-poor silicate grains
Log?(?m)
Planetesimals (Kuiper Belt)
star
50AU
1AU
5AU
30AU
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3 MJup at 5 AU
Carbonaceous grains
Fe-rich silicate grains
LogF(mJy)
Fe-poor silicate grains
Log?(?m)
Planetesimals (Kuiper Belt)
star
50AU
1AU
5AU
30AU
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3 MJup at 30AU
Carbonaceous grains
Fe-rich silicate grains
LogF(mJy)
Fe-poor silicate grains
Log?(?m)
Planetesimals (Kuiper Belt)
star
50AU
1AU
5AU
30AU
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What could we learn from the Spectral Energy
Distributions?
The SED of a dust disk with embedded planets is
fundamentally different from that of the disk
without planets. Significant decrease of
the near/mid-IR flux due to the clearing
of dust inside the planets orbit.
It may be possible to diagnose the location of
the planet and the absence/presence of planets
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Spitzer Space Telescope observations of debris
disks
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Debris Disks and planets co-exist! (Beichman et
al. 2005)
Spitzer has identify the first stars with
well-confirmed planetary systems and
well-confirmed IR excess!! Study of 26 FGK stars
with confirmed radial velocity planets (average
age 1 Gyr) 6/26 show 70 ?m excess (average
age 4 Gyr). none with 24 ?m excess upper
limit of warm dust Ldust/Lstar
5x10-5 (compared to Ldust/Lsun 10-7 for

the solar
systems asteroid belt dust).
Similar to Kuiper Belt dust disk Tlt100K gt10AU
100 x surface emitting area of the solar systems
KB dust.
Potential correlation of planets with IR excess
4/5 of the largest 70 ?m detections are for stars
with RV planets, even though the planet bearing
stars make up lt1/3 of the sample.
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Cold KB-like disks appear to be more common than
AB-like disks (Hines et al. 2005)
Only 1 out of 33 stars (with ages between 10 Myr
and 2 Gyr) have warm excesses - Are these
excesses short lived events connected with the
formation of terrestrial planets? or... - Is
dust production in terrestrial planet-building
zones rare? HD12039 (30 Myr). Strong emission at
24 ?m AB-like disk in terrestrial planet region
(T100-300K). LIR /Lstar 10-4 Not detected at
70 ?m rule out KB-like dust between 10-30AU. No
prominent spectral features grain size gt 3-10 ?m
located between 4-6AU. Lifetime (due to PR) lt 2
Myr (ltstellar age) dust is being regenerated.
Either there is a huge reservoir of material or
the dust is due to a recent collisional event.
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Individual collisional events can dominate the
properties of debris disks over Myr timescales (A
star survey)
(Rieke et al. 2005, Su et al. 2005)
Overall decay in the maximum 24 ?m excess with
age. 50 of young stars have no 24 ?m excess (in
some cases there is very little material between
10 and 60 AU after proto-planetary disk is
cleared). Stars of a similar age show substantial
differences in the amount of dust!
For Vega a dust production rate of 1015g/s over
the age of Vega (350Myr) would produce 6MJup of
dust (very unlikely!).
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Inner gaps appear to be common in cold KB-like
disks (Kim et al. 2005, Meyer et al. 2004)
70 ?m excesses Tmax lt 100K, Ringt10AU No 24 ?m
excesses Upper limit of warm dust inside Rin
10-6-10-6.5 MEarth 2-3 orders of magnitude
below the lower limits for the masses in the cold
disk. Large depletion inside Rin Lifetimes (due
to PR) 106 yr - Replenishment of dust - PR
would erase the density contrast inside and
outside Rin
What is stopping the particles from drifting all
the way toward the star?
(Kim et al. 2005)
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Sublimation of icy grains? No, Tlt100K. Blowout
by radiation pressure? No, dust grains are large
enough to be on bound orbits. An interesting
possibility scattering by a massive planet. If
the planet is in a circular orbit the models
predict the planet to be located (0.8- 1.25)xRin,
with a mass significantly larger that Neptune
and probably larger than Jupiter.
Inner gap radius
(Kim et al. 2005)
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Summary
Debris Disks are evidence of planetary formation
(because planetesimals are needed to generate the
dust). Massive planets create structure in debris
disks and high resolution observations show that
structure is indeed present. Structure is
sensitive to long period planets, complementing
radial velocity and transit surveys.
Debris disk help us learn about diversity of
planetary systems. The clearing of dust inside
the planets orbit has a clear signature in the
disk SED SEDs are sensitive to the presence
and location of massive planets. Spitzer Space
Telescope observations of debris disks
Debris disks and planets co-exist. Cold
KB-like disks are more common than AB-like
disks. Individual collisional events may
dominate disk properties. Inner gaps appear
to be common in cold KB-like disks May
indicate that massive long-period planets are
also common!
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Astrobiology link By studying these disks we can
Study frequency and timescale of
terrestrial planet formation, constraining
theories of planetary formation. Study the
diversity of planetary systems, allowing us to
put our solar system into context by comparing it
to other planetary systems.
Is the late bombardment epoch in the early
Solar System common among other stars? Is its
intensity below or above average?
Consequences for the survival of Life in the
terrestrial planets.
Is our solar system (in its evolution and
planetary configuration) common or rare?