Dust around Main-Sequence Stars: Nature or Nurture by the Interstellar Medium?

Dust from the interstellar medium (ISM) can collide with and destroy particles in the circumstellar dust disks around main-sequence stars (Vega/β Pic stars). Two current theories tying the occurrence of the Vega/β Pic phenomenon to the erosive influence of the ISM are critically reconsidered here. Using the local standard of rest frame, we find little evidence for a correlated motion (streaming) of prominent disk systems, which one theory suggests would result from a passage about 10⁷ yr ago of these stars, but not the control A-type stars, through the nearby Lupus-Centaurus interstellar cloud complex. Moreover, the prototype system of β Pic could not have retained dust produced in such a passage for much longer than 10⁴ yr. We show theoretically that the ISM sandblasting of disks has minor importance for the structure and evolution of circumstellar disks, except perhaps in their outskirts (usually >400 AU from the stars), where under favorable conditions it may cause asymmetries in observed brightness and color. The ISM neither produces the disks (as in one theory) nor depletes and eliminates them with time (as in another theory), because typical ISM grains are subject to strong radiative repulsion from A- and F-type dwarfs (a few to 100 times stronger than gravity). Atypically large ISM grains are not repelled strongly, but are unimportant on account of their small number density.

Dust production and destruction in β Pic-type disks results mainly from their collisional nature enhanced by the radiatively produced eccentricities of particle orbits, rather than from nurture in a hostile ISM. The residence times of the few-micron dust grains predominant in the densest part of the β Pic disk is only 10⁴ yr, or a few dozen orbital periods. Submicronic debris is blown out as β meteoroids, carrying away from this system an equivalent of the solar system's total mass in solids (~120 Earth masses) in only ~65 Myr. This rate of collisional erosion exceeds almost 10⁸ times that of the zodiacal light disk of our own system. A massive and relatively young (≲10² Myr) planetesimal disk appears to surround β Pic, destined to decline in dust density over time comparable to its age. Other dust disks, like those around Fomalhaut and Vega, contain much less dust and may be much older than the β Pic disk, but like the β Pic disk they are also derived from and replenished many times during their lifetimes by unseen parent bodies.