Transient dust in warm debris disks

Abstract

Context. Debris disks trace remnant reservoirs of leftover planetesimals in planetary systems. In the past years, a handful of "warm" debris disks have been discovered in which emission in excess starts in the mid-infrared. An interesting subset of these warm debris disks shows emission features in mid-infrared spectra, which points towards the presence of μm-sized dust grains, with temperatures above hundreds K. Given the ages of the host stars, the presence of these small grains is puzzling, and raises questions about their origin and survival in time.Aims. This study focuses on determining the mineralogy of the dust around seven debris disks with evidence for warm dust, based on Spitzer/IRS spectroscopic data, to provide new insights into the origin of the dust grains. Methods. We developed and present a new radiative transfer code (Debra) dedicated to spectral energy distribution (SED) modeling of optically thin disks. The Debra code is designed such that it can simultaneously determine dust composition and disk properties. We used this code on the SEDs of seven warm debris disks, in combination with recent laboratory experiments on dust optical properties. Results. We find that most, if not all, debris disks in our sample are experiencing a transient phase, suggesting a production of small dust grains on relatively short timescales. Dust replenishment should be efficient on timescales of months for at least three sources. From a mineralogical point of view, we find that crystalline pyroxene grains (enstatite) have low abundances compared to crystalline olivine grains. The main result of our study is that we find evidence for Fe-rich crystalline olivine grains (Fe/[Mg + Fe] ∼ 0.2) for several debris disks. This finding contrasts with studies of gas-rich protoplanetary disks, where Fe-bearing crystalline grains are usually not observed. Conclusions. These Fe-rich olivine grains, and the overall differences between the mineralogy of dust in Class II disks compared to debris disks suggests that the transient crystalline dust in warm debris disk is of a new generation. We discuss possible crystallization routes to explain our results, and also comment on the mechanisms that may be responsible for the production of small dust grains.

The 69μm forsterite band in spectra of protoplanetary disks. Results from theHerschelDIGIT programme

“…It is interesting to note that only for a few debris disk sources is there solid evidence for olivine with a high (∼20%) iron fraction, suggesting that the olivine material in these debris disks originated from larger differentiated bodies (Olofsson et al 2012). This is also consistent with the recent detection of the 69 μm band in the young debris disk system β Pictoris, which shape and position is consistent with olivine with an iron fraction of at most 1% (de Vries et al 2012).…”

Survival of the impactor during hypervelocity collisions – I. An analogue for low porosity targets

“…Olivine and pyroxene minerals are the primary minerals in stony and stony-iron meteorites, 75% of chondrite meteorites and 50% of pallasites (Petrovic 2001;Gaffey et al 2002); (b) Mg-rich olivine (fosterite) has been detected in spectra of several cometary tails and is present in the majority of comet Wild 2 samples returned by NASA's Stardust Mission (Zolensky et al 2006); (c) parallel studies of the spectral features of the dust particles, observed in exo-planetary system β Pictoris (de Vries et al 2012), confirm similar abundance of Mg-rich olivine in respective areas (large heliocentric distances) to our Solar System; (d) Fe-rich olivine (fayalite) is mostly encountered in asteroid mineralogies (Nakamura et al 2011) and therefore in the warmer, inner parts, of planetary space (Olofsson et al 2012). Possible explanations for this distribution of the different types of olivine are: (1) the presence of water on comets which leads to aqueous alteration, as the fayalite may not survive in the presence of water (Olofsson et al 2012) and, (2) the higher abundance of heavier elements, such as Fe, in the inner Solar System; (e) Basalt is considered to be the main material on the surface of the differentiated asteroids. Differentiation, which leads to a multi-layered body with core, mantle and crust and the production of basalt, occurred in the early Solar System.…”

Scattering of small bodies by planets: a potential origin for exozodiacal dust?

“…Defrère et al 2011), MIDI/VISIR (e.g. Smith et al 2009b), or using their mid-infrared spectra with Spitzer (Olofsson et al 2012;Lisse et al 2012Lisse et al , 2009). Such sources have been compared to the solar system's exozodiacal cloud, but in general are several orders of magnitude brighter, for example η Corvi is 1250 ± 260 times brighter than the solar system's exozodiacal dust cloud at 10 μm (Millan-Gabet et al 2011).These observations can be modelled to determine estimates on the geometry, mass, position and grain properties.…”