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CALTECH GE 133 - Gaseous Inner Disks

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Gaseous Inner DisksJoan R. NajitaNational Optical Astronomy ObservatoryJohn S. CarrNaval Research LaboratoryAlfred E. GlassgoldUniversity of California, BerkeleyJeff A. ValentiSpace Telescope Science InstituteAs the likely birthplaces of planets and an essential conduit for the buildup of stellarmasses, inner disks are of fundamental interest in star and planet formation. Studies of thegaseous component of inner disks are of interest because of their ability to probe the dynamics,physical and chemical structure, and gas content of this region. We review the observationaland theoretical developments in this field, highlighting the potential of such studies to, e.g.,measure inner disk truncation radii, probe the nature of the disk accretion process, and chart theevolution in the gas content of disks. Measurements of this kind have the potential to provideunique insights on the physical processes governing star and planet formation.1. INTRODUCTIONCircumstellar disks play a fundamental role in the for-mation of stars and planets. A significant fraction of themass of a star is thought to be built up by accretion throughthe disk. The gas and dust in the inner disk (r <10 AU)also constitute the likely material from which planets form.As a result, observations of the gaseous component of in-ner disks have the potential to provide critical clues to thephysical processes governing star and planet formation.¿From the planet formation perspective, probing thestructure, gas content, and dynamics of inner disks is ofinterest, since they all play important roles in establish-ing the architectures of planetary systems (i.e., planetarymasses, orbital radii, and eccentricities). For example, thelifetime of gas in the inner disk (limited by accretion ontothe star, photoevaporation, and other processes) places anupper limit on the timescale for giant planet formation (e.g.,Zuckerman et al., 1995).The evolution of gaseous inner disks may also bear onthe efficiency of orbital migration and the eccentricity evo-lution of giant and terrestrial planets. Significant inwardorbital migration, induced by the interaction of planets witha gaseous disk, is implied by the small orbital radii of extra-solar giant planets compared to their likely formation dis-tances (e.g., Ida and Lin, 2004). The spread in the orbitalradii of the planets (0.05–5AU) has been further taken to in-dicate that the timing of the dissipation of the inner disk setsthe final orbital radius of the planet (Trilling et al., 2002).Thus, understanding how inner disks dissipate may impactour understanding of the origin of planetary orbital radii.Similarly, residual gas in the terrestrial planet region mayplay a role in defining the final masses and eccentricities ofterrestrial planets. Such issues have a strong connection tothe question of the likelihood of solar systems like our own.An important issue from the perspective of both star andplanet formation is the nature of the physical mechanismthat is responsible for disk accretion. Among the proposedmechanisms, perhaps the foremost is the magnetorotationalinstability (Balbus and Hawley, 1991) although other pos-sibilities exist. Despite the significant theoretical progressthat has been made in identifying plausible accretion mech-anisms (e.g., Stone et al., 2000), there is little observationalevidence that any of these processes are active in disks.Studies of the gas in inner disks offer opportunities to probethe nature of the accretion process.For these reasons, it is of interest to probe the dynami-cal state, physical and chemical structure, and the evolutionof the gas content of inner disks. We begin this Chapterwith a brief review of the development of this field and anoverview of how high resolution spectroscopy can be usedto study the properties of inner disks (Section 1). Previ-ous reviews provide additional background on these top-ics (e.g., Najita et al., 2000). In Sections 2 and 3, we re-view recent observational and theoretical developments inthis field, first describing observational work to date on thegas in inner disks, and then describing theoretical modelsfor the surface and interior regions of disks. In Section 4,we look to the future, highlighting several topics that can beexplored using the tools discussed in Sections 2 and 3.11.1 Historical PerspectiveOne of the earliest studies of gaseous inner disks wasthe work by Kenyon and Hartmann on FU Orionis objects.They showed that many of the peculiarities of these sys-tems could be explained in terms of an accretion outburst ina disk surrounding a low-mass young stellar object (YSO;cf. Hartmann and Kenyon, 1996). In particular, the varyingspectral type of FU Ori objects in optical to near-infraredspectra, evidence for double-peaked absorption line pro-files, and the decreasing widths of absorption lines fromthe optical to the near-infrared argued for an origin in anoptically thick gaseous atmosphere in the inner region of arotating disk. Around the same time, observations of COvibrational overtone emission, first in the BN object (Scov-ille et al., 1983) and later in other high and low mass objects(Thompson, 1985; Geballe and Persson, 1987; Carr, 1989),revealed the existence of hot, dense molecular gas plausiblylocated in a disk. One of the first models for the CO over-tone emission (Carr, 1989) placed the emitting gas in anoptically-thin inner region of an accretion disk. However,only the observations of the BN object had sufficient spec-tral resolution to constrain the kinematics of the emittinggas.The circumstances under which a disk would produceemission or absorption lines of this kind were explored inearly models of the atmospheres of gaseous accretion disksunder the influence of external irradiation (e.g., Calvet etal., 1991). The models interpreted the FU Ori absorptionfeatures as a consequence of midplane accretion rates highenough to overwhelm external irradiation in establishinga temperature profile that decreases with disk height. Atlower accretion rates, the external irradiation of the disk wasexpected to induce a temperature inversion in the disk atmo-sphere, producing emission rather than absorption featuresfrom the disk atmosphere. Thus the models potentially pro-vided an explanation for the FU Ori absorption features andCO emission lines that had been detected.By PPIV (Najita et al., 2000), high-resolution spec-troscopy had demonstrated that CO overtone emissionshows the dynamical signature of a rotating


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