The Gas Disk: Evolution and Chemistry

Space Science Reviews, Dec 2016

Protoplanetary disks are the birthplaces of planetary systems. The evolution of the star-disk system and the disk chemical composition determines the initial conditions for planet formation. Therefore a comprehensive understanding of the main physical and chemical processes in disks is crucial for our understanding of planet formation. We give an overview of the early evolution of disks, discuss the importance of the stellar high-energy radiation for disk evolution and describe the general thermal and chemical structure of disks. Finally we provide an overview of observational tracers of the gas component and disk winds.

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The Gas Disk: Evolution and Chemistry

S. Basu, E.I. Vorobyov, Astrophys. J. The Gas Disk: Evolution and Chemistry Christian Rab 0 1 Carla Baldovin-Saavedra 0 1 Odysseas Dionatos 0 1 Eduard Vorobyov 0 1 Manuel G?del 0 1 0 Department of Astrophysics, University of Vienna , Tu?rkenschanzstr. 17, 1180 Vienna , Austria 1 Research Institute of Physics, Southern Federal University , Stachki 194, Rostov-on-Don, 344090 , Russia Protoplanetary disks are the birthplaces of planetary systems. The evolution of the star-disk system and the disk chemical composition determines the initial conditions for planet formation. Therefore a comprehensive understanding of the main physical and chemical processes in disks is crucial for our understanding of planet formation. We give an overview of the early evolution of disks, discuss the importance of the stellar high-energy radiation for disk evolution and describe the general thermal and chemical structure of disks. Finally we provide an overview of observational tracers of the gas component and disk winds. Stars; pre-main sequence; Stars; formation; Protoplanetary disks; Accretion; accretion disks; Planet-disk interactions; ISM; jets and outflow; Astrochemistry 1 Introduction Disks around young stellar objects are the birthplaces of planetary systems. A comprehensive knowledge of their evolution, structure and chemical composition is therefore crucial for our understanding of planet formation. Low mass stars like our Sun are formed with a disk component. Observations show that the fraction of disk-bearing stars in clusters with an age of ?1 Myr is usually 80 %. However, the disk fraction drops quite rapidly with cluster age. In clusters with ages of 2?3 Myr still ?50 % of the stars show disks, whereas in older (?10 Myr) clusters the fraction of disk-bearing stars is lower than 5 % (e.g. Hern?ndez et al. 2008; Fedele et al. 2010). This tells us that disks have a typical lifetime of a few million years. The lifetime of disks defines the timescale on which formation of massive (gaseous) planets can occur. Disks evolve on timescales of a few million years. The evolutionary stages are determined by the balance of accretion onto the disk and different disk dispersal/removal mechanisms (see Williams and Cieza 2011 for a review) . There is observational evidence that disks form already within 104?105 yr after the gravitational collapse of the parental cloudcore (e.g. Murillo et al. 2013) at the phase where the star is still deeply embedded (Class-0 protostellar phase; see review of Li et al. 2014). After the formation disks are growing due to the continuous inflow of mass from their surrounding envelopes during the Class-0 and Class-I (the star becomes visible) protostellar phases. Once this reservoir is drained and the star reaches its Class-II phase (T Tauri star), disk erosion processes due to accretion onto the star, disk photoevaporation, jet acceleration, and planet formation lead to the disappearance of dust and gas disks within typically 2?3 Myr (although in some rare cases disks are still present at ages of ?10 Myr). Observations of disks with large inner holes/gaps, the so called transitional disks, indicate that disk dispersal works from inside out. From the fraction of observed transition disks a timescale of 105 yr is derived for this period (see Espaillat et al. 2014 for a review) . The final result of the gas disk dispersal are debris disks. These disks are mainly made up of asteroid/comet/planet like bodies and dust but contain no or only very little amount of gas. This evolutionary phase can last for Gyr. However, it is still unclear if every gaseous disks ends up as a debris disk (see Matthews et al. 2014 for a review) . During these evolutionary stages also the dust component of the disk evolves. Recent discoveries like the horseshoe-shaped structures of larger dust particles in transition disks (e.g. van der Marel et al. 2013; P?rez et al. 2014a) or the spectacular dark rings in the thermal dust emission of HL Tau (ALMA Partnership et al. 2015; Pinte et al. 2016) and TW Hya (Andrews et al. 2016) clearly show this. The dust component also influences the gas disk. Dust is a strong opacity source in the optical and ultraviolet and therefore absorbs most of the stellar radiation. This significantly affects the disk thermal structure and chemistry (see Sects. 5 and 6). However, our focus here is on the gas disk and we refer the reader to Chap. 3 of this book and to the review by Testi et a l. (2014 ) for more details on dust evolution. We now briefly summarize the essential steps of disk evolution from formation to dispersal due to various erosion processes. Three major processes: accretion, winds/outflows/jets and planet formation contribute to the slow removal of disk material, determining the typical lifetime of disks. The first and best studied process is the continuous accretion of gas and dust through the disk onto the star. There are several possible mechanisms which can dri (...truncated)


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Christian Rab, Carla Baldovin-Saavedra, Odysseas Dionatos, Eduard Vorobyov, Manuel Güdel. The Gas Disk: Evolution and Chemistry, Space Science Reviews, 2016, pp. 3-40, Volume 205, Issue 1-4, DOI: 10.1007/s11214-016-0325-5