1 Astronomical and Meteoritic Evidence for the Nature of Interstellar Dust and its Processing in Protoplanetary Disks C. M. O’D. Alexander Carnegie Institution of Washington A. P. Boss Carnegie Institution of Washington L. P. Keller NASA Johnson Space Center J. A. Nuth NASA Goddard Space Flight Center A. Weinberger Carnegie Institution of Washington Here we compare the astronomical and meteoritic evidence for the nature and origin of interstellar dust, and how it is processed in protoplanetary disks. The relative abundances of circumstellar grains in meteorites and interplanetary dust particles (IDPs) are broadly consistent with most astronomical estimates of Galactic dust production, although graphite/amorphous C is highly underabundant. The major carbonaceous component in meteorites and IDPs is an insoluble organic material (IOM) that probably formed in the interstellar medium, but a solar origin cannot be ruled out. GEMS (glass with embedded metal and sulfide) that are isotopically solar within error are the best candidates for interstellar silicates, but it is also possible that they are Solar System condensates. No dust from young stellar objects has been identified in IDPs, but it is difficult to differentiate them from Solar System material or indeed some circumstellar condensates. The crystalline silicates in IDPs are mostly solar condensates, with lesser amounts of annealed GEMS. The IOM abundances in IDPs are roughly consistent with the degree of processing indicated by their crystallinity if the processed material was ISM dust. The IOM contents of meteorites are much lower suggesting that there was a gradient in dust processing in the Solar System. The microstructure of much of the pyroxene in IDPs suggests that it formed at temperatures >1258 K and cooled relatively rapidly (~1000 K/hr). This cooling rate favors shock heating rather than radial transport of material annealed in the hot inner disk as the mechanism for producing crystalline dust in comets and IDPs. Shock heating is also a likely mechanism for producing chondrules in meteorites, but the dust was probably heated at a different time and/or location to chondrules. 1. INTRODUCTION There are two sources of information on protoplanetary disk evolution: astronomical observations, and for our Solar System, primitive chondritic meteorites, interplanetary dust particles (IDPs) and comets. Astronomical observations are largely confined to the surfaces of disks and have relatively low spatial resolution. Primitive meteorites, IDPs and comets retain a complex record of processes that occurred throughout the early solar protoplanetary disk (solar nebula). This record is still being deciphered. How complete it is and how representative it is of disk evolution in general are open questions. Silicate dust in the interstellar medium is observed to be largely amorphous (e.g., Mathis, 1990; Kemper et al., 2004). One of the most striking observations of protoplanetary disks is that their dust has a significant crystalline component (e.g., Meeus et al., 1998; Bouwman et al., 2001; van Boekel et al., 2004) and it tends to be coarser grained than interstellar dust. Both observations suggest that dust has been thermally processed and has aggregated in these disks even at large radial distances from the central stars. These observations imply either extensive transport of dust from the hot inner regions of the protoplanetary disks (Nuth et al., 2000b; Gail, 2004), or perhaps more localized heating mechanisms that operate over large2 portions of disks (e.g., Chick and Cassen, 1997; Harker and Desch, 2002). Comets that formed at large radial distances from the Sun, including Halley (Swamy et al., 1988) and 9P/Tempel 1 (Harker et al., 2005) have a large crystalline component in their silicate dust. Thus, the Solar System appears to have at least one process in common with other protoplanetary disks. Some IDPs may come from comets, and components of chondrites share some features in common with IDPs and comets. This includes presolar material inherited from the protosolar molecular cloud. Thus, it is likely that we can study in the laboratory unprocessed and processed dust that may help constrain the conditions and mechanism of thermal processing. Here we review astronomical observations of dust in the interstellar medium (ISM), compare them to observations of annealed dust analogs, cometary dust, chondrites and IDPs. Components of meteorites and IDPs retain evidence of several distinct thermal processes that operated in the solar nebula, and we discuss which, if any, may have been responsible for the processing of dust observed in protoplanetary disks. Finally, we discuss the implications and challenges these observations have for the dynamics of protoplanetary disks. 2. CIRCUMSTELLAR, INTERSTELLAR AND PROTOPLANETARY DUST 2.1 Sources of ISM dust - evolved stars and YSOs The relative importance of ISM dust sources is very uncertain. Most estimates find that red giant (RGB) and asymptotic giant branch (AGB) stars are the main stellar sources of O-rich and C-rich dust (e.g., Jones, 2001). Kemper et al. (2004) suggest that M supergiants may be as important sources of silicate dust as AGB stars. On the other hand, Tielens et al. (2005) estimate that most silicate dust comes in roughly equal quantities from O-rich AGB stars and from young stellar objects (YSO). They also suggest that supernovae could be important sources of dust, but are only able to set upper limits. The crystalline fractions of silicates from O-rich AGB stars and M supergiants are ~10-20% (Kemper et al., 2004). The principle crystalline components are very Mg-rich pyroxene (MgxFe1-xSiO3) and olivine (Mg2xFe2-2xSiO4), with on average 3-4 times as much pyroxene as olivine (Molster et al., 2002). The crystallinity of the young stellar o0bject (YSO) dust is unknown, but it is likely to be dominated by high temperature condensates and annealed material since it is ejected in winds/jets that are generated in the hot inner
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