Transience of Hot Dust around Sun‐like Stars

Abstract

There is currently debate over whether the dust content of planetary systems is stochastically regenerated or originates in planetesimal belts evolving in quasi-steady state. 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 7 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 −1age , 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 a factor ≫ 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: η Corvi, HD69830, HD72905 and BD+20307. 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 in a disk of a given age. For the 4 transient disks, there is at best a 1:10 5 chance of witnessing such an event compared with 2% of stars showing this phenomenon. Further it is shown that the planetesimal belt feeding the dust in these systems must be located further from the star than the dust, typically at ≫ 2 AU. Other notable properties of the 4 hot dust systems are: two also have a planetesimal belt at > 10 AU (η Corvi and HD72905); one has 3 Neptune mass planets at < 1 AU (HD69830); all exhibit strong silicate features in the mid-IR. We consider the most likely origin for the dust in these systems to be a dynamical instability which scattered planetesimals inwards 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 possibly also sublimation. Further detailed study of the planet, planetesimal and dust populations in these rare objects has the potential to uncover the chaotic evolutionary history of these systems and to shed light on the history of the solar system.

Insights into Planet Formation from Debris Disks

“…While f 12gi is not well known, it can be said to be constrained to be less than 100 %. One might be mistaken for thinking that this is close to 100 % from the literature, since mid-IR emission is usually interpreted as the result of giant impacts, but there are not many cases where it is possible to rule out that the dust is simply the bottom end of the collisional cascade of a massive asteroid belt, at least for systems that are in this age range for which collisional processes have yet to have a significant effect on depleting any remnant asteroid belt (Wyatt et al 2007a).…”

“…(1) and assuming that all the debris is in fragments of the same size. While more accurate calculations are readily available (see Wyatt et al 2007a), this illustrates how the collisional depletion rate is proportional to the mass of largest fragments. This leads to the mass remaining constant until the largest objects have come to collisional equilibrium, following which the mass decays inversely with time, with dynamical processes further accelerating the depletion process.…”

Modelling the huge,Herschel-resolved debris ring around HD 207129

“…During the inside-out stirring, different annular regions in a disk brighten up and enter their erosive debris disk phase. Their individual fractional luminosities decay more slowly than 1/t (Dominik & Decin 2003;Wyatt et al 2007;Trilling et al 2008;Löhne et al 2008). However, this decay is even more rapid than the outward migration of the stirring front.…”

“…For example, dust has been detected from well within one AU (Beichman et al 2005;Absil et al 2006), out to hundreds of AU (e.g., Su et al 2005). While the former cases seem to be rare (Wyatt et al 2007), the latter are more commonly reported, corresponding to significantly higher detection rates at longer wavelengths. In addition, from a purely observational point of view, a huge population of fairly massive, but cold disks A&A 537, A110 (2012) could still be hidden below current sensitivity limits.…”

ALMA and Herschel observations of the prototype dusty and polluted white dwarf G29-38

“…While the ALMA observations were best suited to outer regions analogous to evolved Kuiper belt analogs, the Herschel data were uniquely sensitive to an evolved, asteroid-like belt at G29-38 and in general to dust at temperatures and orbital regions intermediate to the relatively warm dust seen at polluted white dwarfs and the cooler dust often detected at main sequence stars. The non-detection at G29-38 is not wholly unexpected, as disk evolution models supported by observations of main sequence stars predict that the available mass in both dust and parent bodies decreases significantly over timescales of several hundred Myr (Wyatt et al 2007), and this depletion is likely enhanced during the post-main sequence (Bonsor & Wyatt 2010).…”