A PDF file should load here. If you do not see its contents
the file may be temporarily unavailable at the journal website
or you do not have a PDF plug-in installed and enabled in your browser.
Alternatively, you can download the file locally and open with any standalone PDF reader:
https://link.springer.com/content/pdf/10.1007%2Fs11214-016-0325-5.pdf
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)