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2. Other worlds discovered. I. Dust and planetesimals in Vega-type systems

In 1983, IRAS discovered excessively IR-emitting stars, or Vega-type systems

Dust in Beta Pic

Many Vega-type systems turned out to be dusty disks like that around Beta Pictoris, here imaged in the scattered starlight:

Burrows et al (1995) found that the innermost observed regions of the disk appear warped, and proposed a giant planet inside the the disk gap as an explanation (see below).

Lagage and Pantin (1994,1996) obtained the first 12-micron maps showing clearly a gap and a large inner asymmetry of dust distribution

The left panel shows the 12-micron flux distribution. The right-hand side panel shows a simple 2-D model of dust distribution that would produce the observed flux distribution. The fairly uniform flux distribution corresponds to an almost empty inner disk gap, because of strong dependence of emissivity of dust on the distance from the star. These data are now showing directly the presence of the ~25 AU gap, whose presence but not the exact size was known for the last decade from analyses of IR spectrum of Beta Pic.

Planetesimals in Beta Pic

Comet-sized bodies are most likely the culprits in very frequent (up to a few hundreds a year) spectroscopic events, during which narrow circumstellar absorption lines are distorted, indicating the presence of gas clouds (comae) partially covering the star and moving toward it (rarely away from it) with velocity of dozens to hundreds km/s.

Planets?

Burrows et al. (1996) and Mouillet et al. (1997) proposed that the apparent slight warp in the innermost coronographically observed disk region (30-100 AU from the star) is caused by a Jovian-type planet orbiting inside that region on a slightly inclined (i=3o) orbit.

See the new ESO press release about the latter work.

Lecavelier des Etang et al (1996) found unusual brightening around Nov 10, 1981.
The control stars that same night (lower panel) behaved normally.

Dynamical theory of dust in Beta Pic

The theory of dust dynamics and processing in the disk requires the existence of a young planetesimal system (~100 Myr old), comparable in mass with all the solids in the solar system ( Artymowicz & Clampin 1997)

Beta Pictoris - a young solar system

Artymowicz (1997, Ann.Rev.Earth&Planet.Sci.)
Similarities Dissimilarities
  1. Dust-dominated thin disk
  2. Grey scattering, ~15% polarization, and
  3. Size distribution of grains (mostly by 2--20 microns)
  4. Composition (= comets,asteroids)
    • warm amorphous silicates (<40 AU)
    • warm crystalline silicates
    • cold main disk (bright olivine/pyroxene preferred)
    • ices photoevaporated?
  5. Nearly gas-free (by mass)
  6. Total solids >~100 Earth masses (~` Earth mass directly observed)
  7. Dust replenished in 104 yr << age (20-100 Myr)...
  8. ...by the same mechanisms (collisions of meteoroids, evaporation & photosputtering of icy planetsimals).
  9. Planetesimals:
    • FEB=Falling Evaporating Bodies, in large numbers --> like a young solar system
    • the largest maintain disk thickness(?)
    • release gas, produce slowly varying & stable absorpotion
  10. Planet(s) invoked to explain:
    • the inner clearing (40 AU)
    • its asymmetric dust distribution
    • the warp at r=30-80 AU
    • send planetesimals onto the star
    • send planetesimals to r>1000 AU
    • produce Nov 1981 brightening
  11. A clearing stage of a planetary system
  1. 104 more dust than in the Zodiacal Light and Kuiper belt
  2. up to 108 times larger dust grinding rate
  3. Larger than standard solar nebula
  4. More asymmetric than ZL
  5. Brighter dust (A>0.4),
  6. Dust contains little C and Fe (Fe:Mg < 0.3 while in the chondritic composition Fe:Mg ~0.8)
  7. Larger role of radiation pressure
  8. Dust avalanches may occur

Key resolved problems:

Key unresolved problems:

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