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Observational Diagnostics of Gas in Protoplanetary Disks

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Observational Diagnostics of Gas in Protoplanetary Disks Noname manuscript No. (will be inserted by the editor) Observational diagnostics of gas in protoplanetary disks Andr´esCarmona October 2008. Review written for the proceedings of the conference "Origin and Evolution of Planets 2008", Ascona, Switzerland, 2008. Abstract Protoplanetary disks are composed primar- disk last?, how much material is available for forming ily of gas (99% of the mass). Nevertheless, relatively giant planets?, how do the density and the temperature few observational constraints exist for the gas in disks. of the disk vary as a function of the radius?, what are In this review, I discuss several observational diagnos- the dynamics of the disk?. tics in the UV, optical, near-IR, mid-IR, and (sub)-mm Although we have learned important insights from wavelengths that have been employed to study the gas disks from dust observations (see for example the re- in the disks of young stellar objects. I concentrate in views by Henning et al. 2006 and Natta et al. 2007), diagnostics that probe the inner 20 AU of the disk, the dust presents several limitations: (i) dust spectral fea- region where planets are expected to form. I discuss the tures are broad; consequently, dust emission does not potential and limitations of each gas tracer and present provide kinematical information; (ii) dust properties are prospects for future research. expected to change during the planet formation pro- Keywords solar system formation · protoplanetary cess; therefore, quantities such as the gas-to-dust ra- disks · observations · gas · spectroscopy tio (needed to derive the disk mass from dust contin- uum emission in the (sub)-mm) are expected to strongly vary with respect to the conditions of the Interstellar Medium (ISM). In addition, dust signatures are strongly 1 Introduction related to dust size. In particular, as soon a dust parti- At the time when giant planets form, the mass (99%) cle reaches a size close to a decimeter it becomes prac- of the protoplanetary disk is dominated by gas. Dust tically "invisible". For example, a crucial quantity such is a minor constituent of the mass of the disk. Still, it as the disk dissipation time scale is normally deduced dominates the opacity; consequently, it is much easier from the decline of the fraction of the sources present- to observe. Therefore, most of the observational con- ing near-IR (JHKL) excess in clusters of increasing ages straints of disks have been deduced from the study of (e.g., Haisch et al. 2001). In reality, the time scale that dust emission (e.g., see chapter by Henning et al.). In we want to constrain is not the time scale in which the contrast, direct observational constraints of the gas in small warm dust particles disappear from the surface arXiv:0911.2271v1 [astro-ph.SR] 11 Nov 2009 the disk are relatively scarce. Nonetheless, to obtain layers in the inner disk, but the time scale in which the direct information from the gas content of the disk is gas in the disk disappears. crucial for answering fundamental questions in planet In summary, independent information of the gas is formation such as: how long does the protoplanetary crucial to understand protoplanetary disk structure and combined studies of gas and dust in the disk are needed. Andr´esCarmona In particular, we are interested in deriving constraints ISDC Data Centre for Astrophysics & Geneva Observatory, for the inner disk, the region where the planets are ex- University of Geneva chemin d'Ecogia 16 pected to form (R<20 AU). The observational study 1290 Versoix, Switzerland of the gas in the inner disk is a relatively new emerg- E-mail: [email protected] ing topic. Only with the advent of ground-based high- 2 Fig. 1 Line shape, inclination and the emitting region of a disk in Keplerian rotation. The double peaked profile indicates emission Fig. 2 Cartoon of the structure of an optically thick disk. The from a rotating disk. The line width and double peak separation disk has a hot inner region (R<20 AU) and a cold outer region depend on the inclination of the disk. The line profile wings de- (R>20 AU). Near-IR and mid-IR diagnostics probe the inner pend on the innermost radius of the emission. In this toy model, disk, (sub)-mm diagnostics probe the outer disk. The disk has it is assumed that the intensity is constant as a function of the a vertical structure: a dense mid-plane and a hotter less dense radius and a spectral resolution of 1 km/s. surface layer. Near-IR and mid-IR gas and dust emission features originate in the optically thin surface layer of the inner disk. Near- IR and mid-IR continuum originates from the optically thick in- terior layer. At (sub)-mm wavelengths we observe line emission spectral resolution infrared spectrographs and space- from optically thick CO gas and optically thin emission of cold born infrared spectrographs has this research become dust located in the outer disk. possible. The main tool used to study the gas in the disk is 2 The gas emitting region. molecular spectroscopy. The gas in the disk is heated by collisions with dust, shocks or UV or X-rays from An important element that one should always bear in the central star. The heated molecules populate dif- mind when studying molecular emission from disks is ferent rotational and vibrational levels and when they the region where the emission observed is produced. de-excite emission lines are produced. The key is that As the disk has a radial and vertical temperature and these emission lines have the imprint of the physical density gradient (hotter closer to the star and higher conditions of the gas where they originated. Important in the disk, denser closer to the star and deeper in the constraints can be derived from emission lines: (i) line disk) we need to have clear three concepts: (i) inner and ratios of different transitions constrain the excitation outer disk, (ii) interior and surface layer, (iii) optical mechanism (shocks, UV or X-rays) and the temperature depth of the medium (i.e., optically thin or thick) and of the gas responsible of the emission; (ii) if the emit- its wavelength and molecular species dependency. ting gas is optically thin, then the measured line fluxes The first concept is related to the distance of the are proportionally related to the amount of molecules central star (i.e., the radius) and the radial tempera- present; therefore, line fluxes can be used to derive col- ture gradient. Different gas tracers probe different ra- umn densities and gas masses; (iii) line shapes and spa- dial regions of the disk. The hot and warm (T∼100- tial extended provide constraints on the size and geom- 2500 K) inner disk (R<20 AU) is traced by lines in the etry of emitting region as well as the dynamics of the UV, near- and mid-IR. The cold (T<100 K) outer disk emitting gas. Figure 1 displays a pedagogical example (R>20 AU) is probed by lines in the (sub)-mm. of how a line profile changes as a function of the incli- The second and third concepts are related to the nation and size of the emitting region in the disk. The vertical temperature and density gradient. As a first ap- line width and double peak separation depend on the proximation, we can understand a protoplanetary disk inclination . The line profile wings depend on the inner- at each radius as composed of two vertical layers: a most radius of the emission. For the interested reader, dense colder interior layer and less dense hotter sur- recent reviews by Najita et al. (2000 & 2007a), Blake face layer (Chiang and Goldreich 1997). At near-IR and (2003) and Carr (2005) cover the subject of observa- mid-IR wavelengths (that probe R<20 AU), the dust tional studies of gas in disks. and gas in the interior layer are optically thick and 3 the interior layer radiates as a black body (i.e., con- tinuum equal to the blackbody level). Gas lines -and dust emission features- in the UV, near-IR and mid-IR arise from the optically thin surface layer of the inner disk. They and are observed on top of the continuum of the interior layer. Emission is observed from the sur- face layer because the surface layer is hotter than the interior layer. The gas emission from the surface layer could be optically thin or optically thick depending on the gas density and the molecular species (for example CO emission is generally optically thick, and H2 emis- sion is optically thin). Optically thick gas lines from the surface layer of the inner disk are observable because the dust in the surface layer is optically thin. At (sub)-mm wavelengths (that probe R>20 AU), the dust become optically thin in the interior layer as Fig. 3 Summary of disk gas diagnostics and the disk region that they probe. well (i.e., continuum below the black-body level), and we can observe optically thick emission from - CO - gas from the interior layer on top of the optically thin 3.1 Hα in emission dust continuum. Optically thin gas emission can be ob- served from other less abundant molecular species such Properly speaking Hα in emission (6590 A)˚ from young as 13CO. Figure 2 presents a cartoon of the structure stars does not trace the gas in the disk. However, since of a disk and the observable properties of the medium. Hα emission is one of the primary indicators of accre- In addition to the surface layer, in particular cases 1 tion of gas from the disk onto the star (e.g., Hartmann such as transitional disks or disks in close binaries, gas et al.
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