Transience of hot dust around Sun-like stars

In this paper a simple model for the steady state evolution of debris disks due to collisions is developed and confronted with the properties of the emerging population of seven Sun-like stars that have hot dust at < 10 AU. The model shows that there is a maximum possible disk mass at a given age, since more massive primordial disks process their mass faster. The corresponding maximum dust luminosity is f(max) = 0.16; 10(-3)r(7/3)t(age)(-1) age, where r is disk radius in AU and t(age) is system age in Myr. The majority (4/7) of the hot disks exceed this limit by >> 1000 and so cannot be the products of massive asteroid belts; rather, the following systems must be undergoing transient events characterized by an unusually high dust content near the star: eta Corvi, HD 69830, HD 72905, and BD + 20 307. It is also shown that the hot dust cannot originate in a recent collision in an asteroid belt, since there is also a maximum rate at which collisions of sufficient magnitude to reproduce a given dust luminosity can occur. The planetesimal belt feeding the dust in these systems must be located farther from the star than the dust, typically at >> 2 AU. Other notable properties of the four hot dust systems are as follows: two also have a planetesimal belt at > 10 AU (eta Corvi and HD 72905); one has three Neptune mass planets at < 1 AU (HD 69830); all exhibit strong mid-IR silicate features. We consider the most likely origin for this transient dust to be a dynamical instability that scattered planetesimals inward from a more distant planetesimal belt in an event akin to the late heavy bombardment in our own system, the dust being released from such planetesimals in collisions and sublimation.