Chemical Evolution of Protoplanetary Disks-The Effects of Viscous Accretion, Turbulent Mixing and Disk Winds

Draft version June 5, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 CHEMICAL EVOLUTION OF PROTOPLANETARY DISKS – THE EFFECTS OF VISCOUS ACCRETION, TURBULENT MIXING AND DISK WINDS D. Heinzeller1 and H. Nomura Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan and C. Walsh and T. J. Millar Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK Draft version June 5, 2018 ABSTRACT We calculate the chemical evolution of protoplanetary disks considering radial viscous accretion, vertical turbulent mixing and vertical disk winds. We study the effects on the disk chemical structure when different models for the formation of molecular hydrogen on dust grains are adopted. Our gas- phase chemistry is extracted from the UMIST Database for Astrochemistry (Rate06) to which we have added detailed gas-grain interactions. We use our chemical model results to generate synthetic near- and mid-infrared LTE line emission spectra and compare these with recent Spitzer observations. Our results show that if H2 formation on warm grains is taken into consideration, the H2O and OH abundances in the disk surface increase significantly. We find the radial accretion flow strongly influences the molecular abundances, with those in the cold midplane layers particularly affected. On the other hand, we show that diffusive turbulent mixing affects the disk chemistry in the warm molecular layers, influencing the line emission from the disk and subsequently improving agreement with observations. We find that NH3, CH3OH, C2H2 and sulphur-containing species are greatly enhanced by the inclusion of turbulent mixing. We demonstrate that disk winds potentially affect the disk chemistry and the resulting molecular line emission in a similar manner to that found when mixing is included. Subject headings: astrochemistry; protoplanetary disks; accretion, accretion disks; turbulence; infrared: planetary systems 1. INTRODUCTION The observation of molecular line emission from disks Protoplanetary disks are a natural and active environ- at (sub)mm wavelengths is limited by the low spatial res- ment for the creation of simple and complex molecules. olution and sensitivity of existing facilities and target the Understanding their physical and chemical evolution is rotational transitions of abundant molecules in the outer ( 50 AU) regions of the disk (see, e. g., Dutrey et al. key to a deeper insight into the processes of star and ≥ planetary system formation and corresponding chemi- 2007, for a review). Planet formation, however, most cal evolution. Protoplanetary disks are found encom- likely occurs at much smaller radii (Armitage 2007). passing young stars in star-forming regions and typically The superior sensitivity and spatial and spectral reso- disperse on timescales of less than a few million years lution of the Atacama Large Millimeter Array (ALMA), (e.g., Haisch et al. 2001, 2006; Watson et al. 2007). Ob- currently under construction, is required to observe the servations of thermal dust emission from these systems (sub)mm emission lines from the inner regions of proto- date back to the 1980s (see, e. g., Lada & Willking 1984; planetary disks in nearby star-forming regions to enable direct study of the physical and chemical structure of the arXiv:1102.3972v1 [astro-ph.EP] 19 Feb 2011 Beckwith et al. 1990) and gave important insight into the structure and evolution of disks (see, e. g., Natta 2007, inner disk. for a review), however, inferring the properties of pro- Near- and mid-infrared observations, on the other toplanetary disks from dust continuum observations has hand, can probe the warm planet-forming regions (see, several shortcomings: the dust component is only 1% e. g., Najita et al. 2007, for a review). The 4.7µm of the total mass contained in the disk and its proper-≈ CO line emission has been detected at radii < 1 AU, ties are expected to change with dust growth and planet using, for example, Keck/NIRSPEC, Gemini/Phoenix formation. Also, dust spectral features are too broad to and VLT/CRIRES (e.g., Carr 2007; Pontoppidan et al. study the disk kinematics (see, e. g., Carmona 2010), and 2008; Brittain et al. 2009; van der Plas et al. 2009). The observations suggest that the gas and dust temperatures Spitzer Space Telescope has been used to spectroscopi- can differ considerably, particularly in the UV- and X- cally study the inner disk at mid-infrared wavelengths. ray heated surface layers of the inner disk (e. g., Qi et al. These observations show a rich spectrum of emission lines 2006; Carmona et al. 2007). from simple organic molecules (e. g., Carr & Najita 2008; Salyk et al. 2008; Pontoppidan et al. 2010) as well as from complex polycyclic aromatic hydrocarbons (PAHs) [email protected] (e.g., Habert et al. 2006; Geers et al. 2006). They sug- 1 Current address: Meteorological Service of New Zealand gest high abundances of H2O, HCN, C2H2 and pro- Ltd., 30 Salamanca Rd, Kelburn, Wellington 6012, New Zealand 2 Heinzeller et al. vide evidence that the disk supports an active chem- culated the effect of accretion along streamlines in steady istry. C2H2 and HCN have also been detected in absorp- 1+1-dimensional α-disk models for chemical networks tion towards IRS 46 and GV Tauri (Lahuis et al. 2006; containing more than 200 species. They found that the Gibb et al. 2007). A particularly interesting case is CO2, chemical evolution is influenced globally by the inward which has been reported to be abundant in the circum- accretion process, resulting in, e. g., enhanced methanol stellar disk of AA Tauri at 1.2 AU (Carr & Najita abundances in the inner disk. Ilgner et al. (2004) also 2008), in IRS46 (Lahuis et al.∼ 2006) and in several other studied vertical mixing using the diffusion approximation sources (e. g., Pontoppidan et al. 2010), but which is not and concluded that vertical mixing changes the abun- detected in the majority of sources observed by Spitzer. dances of the sulphur-bearing species. Their disk model, The recently launched Herschel Space Observatory and however, assumed the gas and dust temperatures were future space telescopes (e.g., the Space Infrared telescope coupled everywhere in the disk and ignored stellar irradi- for Cosmology and Astrophysics (SPICA) and the James ation processes. Semenov et al. (2006) investigated gas- Webb Space Telescope (JWST)) will enable the observa- phase CO abundances in a 2D disk model, allowing for tion of molecular line emission in the mid- to far-infrared radial and vertical mixing using a reduced chemical net- with unprecedented sensitivity and spectral resolution work of 250 species and 1150 chemical reactions, and find and will help address these controversial findings. the mixing processes strongly affect the CO abundances Due to increasing computational power, the theoreti- in the disk. Willacy et al. (2006) studied the effects of cal study of the physical and chemical evolution of pro- vertical diffusive mixing driven by turbulence and found toplanetary disks is entering its Golden Age. As a conse- that this can greatly affect the column density of vari- quence of the observational limitations in the pre-Spitzer ous molecules and increase the size of the intermediate, era, many numerical models focused only on the outer warm molecular layer in the disk. disk (e. g., Aikawa et al. 2002; Willacy 2007). With the In this study, we consider the effects of inward accre- launch of Spitzer, however, the chemistry of the inner tion motion and vertical turbulent mixing to investigate disk has increased in importance (e. g., Woods & Willacy to what extent the abundances of molecules such as H2O, 2009). Unfortunately, complete models, including the CO2 and CH3OH are affected in the inner disk. As an physical and chemical evolution of the gas and the dust alternative driving force for vertical transport, we inves- in the disk, coupled with a consistent treatment of the tigate the effect of upward disk winds on the disk chem- radiative transfer, is still beyond reach and several com- istry. Several kinds of disk wind models, such as mag- promises have to be made. netocentrifugally driven winds (e. g., Blandford & Payne One possibility is to ignore the chemical evolution 1982; Shu et al. 1994) and photoevaporation winds (e. g., and assume chemical equilibrium holds throughout the Shu et al. 1993) have been suggested thus far. Recently, disk to investigate the physical structure in detail Suzuki & Inutsuka (2009) studied disk winds driven by (e. g., Kamp & Dullemond 2004; Jonkheid et al. 2004). MHD turbulence in the inner disk and found that large- Pioneering work by Gorti & Hollenbach (2004) and scale channel flows develop most effectively at 1.5–2 disk Nomura & Millar (2005) combined a basic disk chemistry scale heights, i. e., approximately in those layers where with a self-consistent description of the density and tem- molecular line emission is generated. The disk model perature structure taking into account the gas thermal in our study is from Nomura & Millar (2005) including balance, the stellar irradiation and the dust grain prop- X-ray heating as described in Nomura et al. (2007) and erties. Recently, Woitke et al. (2009a) calculated radi- accounts for decoupling of the gas and dust tempera- ation thermo-chemical models of protoplanetary disks, tures in the disk surface due to stellar irradiation. We including frequency-dependent 2D dust continuum ra- use a large chemical network and allow for adsorption diative transfer, kinetic gas-phase and photochemistry, and desorption onto and from dust grains. We also cal- ice formation and non-LTE heating and cooling. These culate mid-IR dust continuum and molecular line emis- studies show, in accordance with the observations, that sion spectra assuming local thermodynamic equilibrium the stellar UV and X-ray irradiation increases the gas (LTE) and compare our results to recent observations.

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