The Chemical Evolution of Protoplanetary DisksEdwin A. BerginUniversity of MichiganYuri AikawaKobe UniversityGeoffrey A. BlakeCalifornia Institute of TechnologyEwine F. van DishoeckLeiden ObservatoryIn this review we re-evaluate our observational and theoretical understanding of the chemi-cal evolution of protoplanetary disks. We discuss how improved observational capabilities haveenabled the detection of numerous molecules exposing an active disk chemistry that appearsto be in disequilibrium. We outline the primary facets of static and dynamical theoreticalchemical models. Such models have demonstrated that the observed disk chemistry arises fromwarm surface layers that are irradiated by X-ray and FUV emission from the central accretingstar. Key emphasis is placed on reviewing areas where disk chemistry and physics are linked:including the deuterium chemistry, gas temperature structure, disk viscous evolution (mixing),ionization fraction, and the beginnings of planet formation.1. INTRODUCTIONFor decades models of our own Solar nebular chemicaland physical evolution have been constrained by the chemi-cal record gathered from meteorites, planetary atmospheres,and cometary comae. Such studies have provided impor-tant clues to the formation of the sun and planets, but largequestions remain regarding the structure of the solar nebula,the exact timescale of planetary formation, and the chemi-cal evolution of nebular gas and dust. Today we are on theverge of a different approach to nebular chemical studies,one where the record gained by solar system studies is com-bined with observations of numerous molecular lines in amultitude of extra-solar protoplanetary disk systems track-ing various evolutionary stages.Our observational understanding of extra-solar proto-planetary disk systems is still in its infancy as the currentcapabilities of millimeter-wave observatories are limited bysensitivity and also by the small angular size of circumstel-lar disks, even in the closest star-forming regions (< 3−4′′).Nonetheless, numerous molecules have been detected inprotoplanetary disks, exposing an active chemistry (Dutreyet al., 1997; Kastner et al., 1997; Qi et al., 2003; Thi et al.,2004). Since the last Protostars & Planets review (Prinn,1993) these observations have led to a paradigm shift inour understanding of disk chemistry. For many years fo-cus was placed on thermochemical models as predictors ofthe gaseous composition, and these models have relevancein the high pressure, & 10−6bar, (inner) regions of thenebula (e.g., Fegley, 1999). However, for most of the diskmass, the observed chemistry appears to be in disequilib-rium and quite similar to that seen in dense regions of theinterstellar medium (ISM) that are directly exposed to radi-ation (Aikawa and Herbst, 1999; Willacy and Langer, 2000;Aikawa et al., 2002).In this review we focus on gains in our understanding ofthe chemistry that precedes and is contemporaneous withthe formation of planets from a perspective guided by ob-servations of other stellar systems whose masses are similarto the Sun. We examine both the observational and theo-retical aspects of this emerging field, with an emphasis onareas where the chemistry directly relates to disk physics.Portions of this review overlap with other chapters, such asthe observational summary of molecular disks, inner diskgas, and disk physical structure (see the chapters by Dutreyet al., Najita et al., and Dullemond et al., respectively). Forthis purpose we focus our review on the physics and chem-istry of the outer disk (r > 10 AU) in systems with ages of0.3-10 Myr. In support of theory we also present an obser-vational perspective extending from the infrared (IR) to thesubmillimeter (sub-mm) to motivate the theoretical back-ground and supplement other discussions.2. GENERAL THEORETICAL PICTURE2.1. Basic Physical and Chemical Structure of DiskChemical abundances are determined by physical condi-tions such as density, temperature, and the incident1iHeight from Miplane [AU]COiceHCO+H2D+H3+HCNCNC2HC+010-510-410-610-710-810-910-1110-1210-10100200300400n(i)/nHiiiiiProtosun/starmixing?Cosmic Ray penetration to disk midplane?<UV, optical, X-rayaccretion ii.warm mol. layeriii. midplane i.photon-dominated layerHeight from Midplane [AU]1002003004001020304050Temperature [K]n [cm-3]105106107108Tniii ii ipuffed-up inner rim disk in shadow?ISM abundances?external radiationCOiceCH4iceHCNiceH2COiceCOCO2iceC2H2iceCH40 20 40 60 80 10010-510-410-610-710-8Radial Distance from Star [AU]n(i)/nHFig. 1.— Chemical structure of protoplanetary disks. Vertically the disk is schematically divided into three zones: a photon-dominated layer, a warm molecular layer, and a midplanefreeze-out layer. The CO freeze-out layer disappears at r . 30 − 60 AU as the mid-plane temperature increases inwards. Various non-thermal inputs, cosmic ray, UV, and X-ray drivechemical reactions. Viscous accretion and turbulence will transport the disk material both vertically and radially. The upper panels show the radial and vertical distribution of molecularabundances from a typical disk model at the midplane (Aikawa et al., 1999) and r ∼ 300 AU (van Zadelhoff et al., 2003). A sample of the hydrogen density and dust temperature at the samedistance (D’Alessio et al., 1999) is also provided. In upper layers (& 150 AU) the gas temperature will exceed the dust temperature by & 25 K (Jonkheid et al. 2004).2radiation field. Recent years have seen significant progressin characterizing disk physical structure, which aids in un-derstanding disk chemical processes. Isolated disks can bequite extended with rout∼ one hundred to a few hun-dred AU (Simon et al. 2000), much larger than expectedfrom comparison with the Minimum Mass Solar Nebula(MMSN: Hayashi, 1981). Although, it should be stated thatwe have an observationalbias towards detecting larger disksdue to our observational limitations. The radial distributionof column density and midplane temperature have been es-timated by observing thermal emission of dust; they are fit-ted by a power-law Σ(r) ∝ r−pand T (r) ∝ r−q, withp = 0 − 1 and q = 0.5 − 0.75. The temperature at 1 AUis ∼ 1 00 − 200 K, while the surface density at 100 AU is0.1 − 10 g cm−2(e.g., Beckwith et al., 1990; Kitamura etal., 2002).The vertical structure is estimated by calculating the hy-drostatic equilibrium for the density and radiation transferfor the dust temperature (see the
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