Exploring Planetary Formation Through Infrared Signatures

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). 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. Let’s see this in more detail

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 minimum at Neptune’s position (to avoid resonant planet) Neptune ring-like structure along Neptune’s 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

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. 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. Let’s see some modeled SEDs of debris disks with embedded planets in different configurations.

No planet Carbonaceous grains Fe-rich silicate grains Log[F(mJy)] Fe-poor silicate grains Log[m)] Planetesimals (Kuiper Belt) star 50AU 1AU 5AU 30AU

1 MJup at 5 AU Carbonaceous grains Fe-rich silicate grains Log[F(mJy)] Fe-poor silicate grains Log[(m)] Planetesimals (Kuiper Belt) star 50AU 1AU 5AU 30AU

3 MJup at 30AU Carbonaceous grains Fe-rich silicate grains Log[F(mJy)] Fe-poor silicate grains Log[(m)] Planetesimals (Kuiper Belt) star 50AU 1AU 5AU 30AU

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 mexcess (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) (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!)].

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 (0.8- 1.25)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?

Exploring Planetary Formation Through Infrared Signatures

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