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Infrared Signatures of
Planetary Systems
Amaya Moro-Martin
Department of Astrophysical Sciences, Princeton University
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.
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.
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).
Define the timescales
over which terrestrial
and gas giant planets
are built.
For more mature systems, we can trace
evolution of dust disks generated through
collisions of planetesimals and infer
location and mass of giant planets.
Let’s see this in more detail
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.
With time, the remaining dust in the disk dissipates, it’s 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…
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
Neptune
minimum at Neptune’s position (to
avoid resonant planet)
ring-like structure along Neptune’s orbit
(trapping into Mean Motion Resonances)
Uniform density disk
clearing of dust from inner 10 AU (due
to gravitational scattering by Jupiter
and Saturn)
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
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)
H141569 1.1m
(scattered light;
Weinberger et al. 99)
HR4796A 1.6 m (scattered
light; Schneider et al. 99)
Looking for planets in spatially unresolved disks
Many disks are too far away to be spatially resolved
in most cases we won’t be able to look for planets
by studying debris disk structure directly.
But the structure carved by
the planets affects the shape
of the Spectral Energy
Distribution (SED) of the disk
Infrared
excess
we can study the debris
disk structure indirectly.
Let’s see some modeled SEDs of debris disks with embedded
planets in different configurations.
No planet
Log[F(mJy)]
Carbonaceous grains
Fe-rich silicate grains
Fe-poor silicate grains
Log[m)]
Planetesimals (Kuiper Belt)
star
1AU 5AU
30AU
50AU
1 MJup at 5 AU
Log[F(mJy)]
Carbonaceous grains
Fe-rich silicate grains
Fe-poor silicate grains
Log[m)]
Planetesimals (Kuiper Belt)
star
1AU 5AU
30AU
50AU
3 MJup at 1 AU
Log[F(mJy)]
Carbonaceous grains
Fe-rich silicate grains
Fe-poor silicate grains
Log[m)]
Planetesimals (Kuiper Belt)
star
1AU 5AU
30AU
50AU
3 MJup at 5 AU
Log[F(mJy)]
Carbonaceous grains
Fe-rich silicate grains
Fe-poor silicate grains
Log[m)]
Planetesimals (Kuiper Belt)
star
1AU 5AU
30AU
50AU
3 MJup at 30AU
Log[F(mJy)]
Carbonaceous grains
Fe-rich silicate grains
Fe-poor silicate grains
Log[m)]
Planetesimals (Kuiper Belt)
star
1AU 5AU
30AU
50AU
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 planet’s orbit.
It may be possible to diagnose the location of the
planet and the absence/presence of planets
Spitzer Space Telescope
observations of debris disks
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 systems asteroid belt dust).
Similar to Kuiper Belt dust disk: T<100K; >10AU; 100 x
surface emitting area of the solar system’s 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 <1/3 of the sample.
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
(T=100-300K). LIR /Lstar ~ 10-4
Not detected at 70 m: rule out KB-like dust between 10-30AU.
No prominent spectral features: grain size > 3-10 m located
between 4-6AU.
Lifetime (due to PR) < 2 Myr (<stellar age): dust is being regenerated.
Either there is a huge reservoir of material or the dust is due to a
recent collisional event.
Individual collisional events can dominate the properties
of debris disks over Myr timescales (A star survey)
Overall decay in the maximum
24 m excess with age.
(Rieke et al. 2005, Su et al. 2005)
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!)].
Inner gaps appear to be
common in cold KB-like disks
(Kim et al. 2005, Meyer et al. 2004)
70 m excesses: Tmax < 100K, Rin>10AU
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)
Sublimation of icy grains? No, T<100K.
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 (0.81.25)xRin, with a mass significantly larger that
Neptune and probably larger than Jupiter.
Inner gap
radius
(Kim et al. 2005)
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 planet’s 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!
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 it’s evolution and planetary
configuration) common or rare?