IV. Other Stars

At least 15% of nearby normal main sequence stars of all spectral types have cold dust populations with spatial scales corresponding to our system's Kuiper Belt (30-100 AU) (Backman and Paresce 1993) (Figure 5a).

Figure 5a: IRAS 25/60 µm versus 12/25 µm color-color diagram for A, F, G and K main sequence stars in the Bright Star Catalog surrounded by planetary material. The excluded region contains the colors of free-free (plasma) emission. The color of a pure photosphere with no circumstellar dust would be off the diagram below. The trajectories trace colors of disks structurally identical to the ones around b Pic and a PsA but with varying total amounts of dust in contrast to the stellar photospheres.

These are sometimes called "Vega / Pic" disks after two nearby especially prominent prototypes.

Figure 5b
I-Band (0.9 micron) image of the beta Pictoris disk, a nearby exozodiacal dust system (Kalas and Jewitt 1995, A.J. 110, 794). False color represents surface brightness. The central "hole" is a software mask 10 arcsec (190 AU) in diameter, somewhat larger than the hardware coronagraph mask employed in these observations. The beta Pic disk is entirely optically thin, although some regions of the disk are 10^4 times denser than our local zodiacal cloud.

The IRAS detection limit for dust around nearby stars corresponds to a fractional dust luminosity limit of L_dust/L_star 10^-5, about 100x the value for our zodiacal cloud and also 100x a model upper limit on the amount of dust in our KB (Backman et al. 1995). There is no obvious correlation within the present small-number statistics between stellar type and average amount of dust. Only a few main sequence stars have warm circumstellar dust (i.e., at terrestrial temperatures) detectable with IRAS sensitivity (Aumann and Probst 1991). Three examples, Pic, Lep, and 51 Oph, have Ldust/Lstar values for warm dust in the range 10^-5 to several x 10^-4 (Fajardo-Acosta et al. 1998).

It is crucial to note that some models of the construction of the planets in our solar system indicate that the presence of an asteroid belt is not guaranteed. Numerical experiments by Wetherill (1992) yield one or more large bodies instead of an asteroid population in 50% of runs. This is counter to the common intuition that an asteroid belt must form just inside the orbit of the first ice giant due to gravitational disturbances by that planet. These model systems otherwise resemble ours, with a set of terrestrial planets and a set of Jovian planets. Thus, complementing the point raised in section III.C that planetary systems can have much larger asteroid and asteroidal dust populations than does our system, present knowledge indicates that a system with an earth-like planet could also have much less terrestrial-temperature dust than ours does.

Recent discovery of nearby planetary systems containing "hot Jupiters", Jovian-mass or larger objects at 0.05 to 2.5 AU from the stars, indicates the variety of planetary systems that is apparently possible (Butler et al. 1997). Of course such systems will show up first and most easily in radial velocity searches, so their true prevalence is unknown. However, recent theoretical work by Lin et al. (1997) and Tremaine (1997) explain these objects as results of processes of large-planet migration in the protoplanetary disk caused by drag from either remnant gas or dense planetesimal populations. In "hot Jupiter" systems, terrestrial material (planets, asteroids, and dust) would likely have been erased by the inward migration of the large planets.

In contrast, a KB-like zone of planetesimals seemingly is guaranteed beyond the planets in the region where the protoplanetary disk density was too low and encounter times too long to support construction of planets. It seems possible that cold dust like in the "Vega / Pic" systems may be detected eventually around most main sequence stars (Dominik et al. 1998).

KB-like systems with L_dust/L_star < 10^-5 would be expected to send grains via PR drag toward their central stars because the mutual grain collision time scale would be longer than the PR time scale. However, as noted above, few main sequence KB-systems have significant grain populations at temperatures above 150 K, indicating that central "voids" are common. This can be explained as being due to a combination of: a) erosion of inbound grains by interstellar dust, b) dynamical influence of large outer planets preventing grains from drifting close to the primary stars, and c) present inability to detect zodiacal systems with L_dust/L_star much below 10^-5 such that KB-supplied inner zodiacal clouds could have escaped notice to date.

Thus, it seems that the amount of cold dust in a system cannot be used to predict the amount of hot dust because the presence of hot dust is not guaranteed even if there is a planetary system, and the cold dust may not be able to drift close enough to the star to become hot dust. In terms of the planet-finding problem, observational and theoretical studies of both the "Vega / Pic" KB-like dust disks and of the dust population in our outer solar system should help us discover whether KB-like systems are necessarily remnants of the planet-formation process and thus signposts of planetary systems, and whether central "voids" require the influence of planets for their maintenance.

IRAS and ISO have not been able to detect hot dust at the 1-zodi level around nearby stars because their low spatial resolutions prevent separation of dust emission from stellar photospheres. The best these instruments can manage in the face of uncertainty about photospheric fluxes is to discover nearby dust systems down to the few x 100-zodi level. SIRTF, on the other hand, will have detector arrays with sub-diffraction pixels. With this resolution and higher sensitivity it should be better than its predecessors at detecting thin dust around some of the nearest stars (Backman et al. 1997). Unfortunately a Centauri, the nearest system (1.3 pc), lies at a galactic latitude of -0.7^o and is projected against very bright interstellar background emission, so it may not be easily investigated.