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Mon. Not. R. Astron. Soc. 000, 1{?? (2010) Printed 4 June 2018 (MN LATEX style file v2.2) The effect of the regular solution model in the condensation of protoplanetary dust F. C. Pignatale1?, S. T. Maddison1;2, V. Taquet1;2;3, G. Brooks4, K. Liffman5 1Centre for Astrophysics & Supercomputing, Swinburne University, H39, PO Box 218, Hawthorn, VIC 3122, Australia 2Laboratoire d'Astrophysique de Grenoble, UMR 5571 Universit´eJoseph Fourier/CNRS, BP 53, 38041 Grenoble cedex 9, France 3Magistere de Physique Fondamentale d'Orsay, Universite Paris-11, France 4Mathematics Discipline, FEIS, Swinburne University, H38, PO Box 218, Hawthorn, VIC 3122, Australia 5CSIRO/MSE, PO Box 56, Heightt, VIC 3190, Australia Accepted 2011 February 16. Received 2011 February 14; in original form 2010 July 22 ABSTRACT We utilize a chemical equilibrium code in order to study the condensation process which occurs in protoplanetary discs during the formation of the first solids. The model specifically focuses on the thermodynamic behaviour on the solid species assuming the regular solution model. For each solution, we establish the relationship between the activity of the species, the composition and the temperature using experimental data from the literature. We then apply the Gibbs free energy minimization method and study the resulting condensation sequence for a range of temperatures and pressures within a protoplanetary disc. Our results using the regular solution model show that grains condense over a large temperature range and therefore throughout a large portion of the disc. In the high temperature region (T > 1400 K) hibonite and gehlenite dominate and we find that the formation of corundum is sensitive to the pressure. The mid-temperature region is dominated by Fe(s) and silicates such as Mg2SiO4 and MgSiO3. The chemistry of forsterite and enstatite are strictly related, and our simulations show a sequence of forsterite{enstatite{forsterite with decreasing temperature and the abundance of the first high temperature peak of forsterite is also pressure sensitive. In the low temperature regions (T 6 600 K) a range of iron compounds (FeS, Fe2SiO3, FeAl2O3) form. We find that all the condensation sequences move towards lower temperature as the pressure decreases. We also run simulations using the ideal solution model and see clear differences in the resulting condensation sequences with changing solution model. In particular, we find that the turning point in which forsterite replaces enstatite in the low temperature region is sensitive to the solution model. In this same temperature region, fayalite is arXiv:1102.5602v1 [astro-ph.EP] 28 Feb 2011 the most stable compound for the regular solution, while magnetite replaces fayalite in the ideal solution model at the lowest values of temperature. Our results show that the ideal solution model is often a poor approximation to experimental data at most temperatures important in protoplanetary discs. We find some important differences in the resulting condensation sequences when using the regular solution model, and suggest that this model should provide a more realistic condensation sequence. Key words: astrochemistry, planetary systems: protoplanetary discs, star: pre-main sequence, circumstellar matter 1 INTRODUCTION Studying protoplanetary discs, and more particularly their dust content, is crucial for understanding the creation of ? E-mail: [email protected] the solar system. Solar system bodies such as planets, me- c 2010 RAS 2 Pignatale et al. teorites and comets are all created from solids during the free energy minimization method to solve for the condensa- protoplanetary disc phase. tion sequence and the steps followed to determine our final The quantity and the quality of observations of proto- thermodynamic model. In section 3 we discuss the behavior planetary discs around young stars has increased substan- of solid solutions and present a range of activity-composition tially in the last years: Spitzer Space Telescope has provided relations derived from laboratory experiments. In section 4, a large amount of useful data whose interpretation provides we present the results of our simulations and we compare information on the composition, growth and thermal pro- them with the ideal solution models (section 5) and with cessing of dust grains in discs (Olofsson et al. 2009; Meeus previous modeling work and recent observations (section 6). et al. 2009; Bouwman et al. 2008; Kessler-Silacci et al. 2006; Conclusions are presented in section 7. Geers et al. 2006). The processes of grain formation are in- trinsically related to the chemistry of gas and dust. Indeed, the material in the discs is a mixture of condensates with 2 THE MODEL different chemical composition and properties (Henning & Meeus 2009; Bergin 2009). Protoplanetary discs results from conservation of the angu- The study of objects in our solar system, including as- lar momentum of a collapsing, rotating cloud of gas and teroids, meteorites and comets provides more direct and ac- dust. Detailed studies have been made to define the struc- cessible evidence of the chemistry of the young solar system. ture of protoplanetary discs (Brauer et al. 2008; D'Alessio Meteorites are characterized by heterogeneous compositions et al. 2004, 1999, 1998; Gail 2004, 2001, 1998) and the (Krot et al. 2005) and petrographic and mineral analysis physics involved in the evolution of the resulting gas{dust show mixtures of compounds that are the result of conden- system (Birnstiel et al. 2010; Dullemond et al. 2008). In this sation processes in different environments and times during work, we calculate the condensation sequence for an initial the protoplanetary disc phase (Scott & Krot 2005). Even chemistry over a range of temperatures and at fixed values comets, among the oldest and most distant objects of our of pressure which are appropriate for protoplanetary discs. solar system, show this heterogeneity (Brownlee 2005). The In this section, we describe the conditions within our Stardust mission (Brownlee et al. 2003) returned samples model, the chemical equilibrium code used to study the con- from comet Wild 2 and studies of their composition (Naka- densation sequence, along with assumptions and limitations mura et al. 2008; Zolensky et al. 2006) show complex ther- of the code. modynamic and dynamic scenarios where particles proba- bly formed in the inner regions of the solar system (Liffman 2.1 Physical properties of the disc 2006) which experienced high temperature processes were found close to other material produced by cold thermody- Different disc models return different temperature and pres- namics. sure distributions. These can be described as functions of This paper is the first in a series in which we aim to the disc radius (r) in 1D models and both the radius and determine the composition of both the gas and solid phases the distance from the mid-plane (r,z) for 2D models. In this in various regions of protoplanetary discs in order to un- model, we do not subscribe to any specific disc model, but derstand their chemistry and study the dust content. We instead chosen range of temperatures and specific pressure assume thermodynamic equilibrium and solve for the con- values that are relevant in protoplanetary discs. These val- densation sequence using the regular solution model (DeHoff ues are not meant to represent specific locations in the disc 1993) for the behaviour of solid solutions. In this paper we but to constrain the values of temperature and pressure we present the regular solution model and compare the conden- use in the our simulations and define three main regions sation results with those obtained using an ideal solution for (high, middle, low temperatures) in the disc in which con- a range of temperatures at fixed pressures. In our next pa- densation occurs. Furthermore, condensation sequences cal- pers, we will apply this technique to specific 1D and 2D discs culated over a range of temperatures at fixed values of pres- models. sure allow us to compare our results with previous work and The Gibbs energy minimization technique for equilib- thermodynamical predictions. rium calculation is a powerful method for understanding the For this work, we use the disc model of D'Alessio et al. evolution of chemistry in a complex system (DeHoff 1993). (1998) to guide us to find suitable values of T and P . However, the predictions using this technique are sensitive D'Alessio et al. (1998) studied the vertical structure, tem- to providing rigorous mathematical descriptions of solution perature, and surface density of a disc whose heating pro- behaviour, in particular, providing allowances for the effect cesses include viscous dissipation, radioactive decay, cosmic of concentration and temperature on the activity of species rays, and stellar irradiation. They chose a typical T Tauri dissolved into a solid state solution (DeHoff 1993). In this star with M∗ = 0:5 M , R∗ = 2 R , T∗ = 4000 K, and −8 −1 study, we include the effect of temperature of solution be- M_ = 10 M yr . haviour by using the regular solution model, which is su- We assume an irradiated disc with a constant accre- perior to the assumption used in previous studies in which tion rate and consider only the inner 5 AU. Most of the ideal mixing is assumed for many of the solid phases and dust mass is located near the mid-plane of the disc, so the the effect of temperature on solution behaviour is ignored study of temperature and pressure was done at the mid- (e.g. Pasek et al. 2005; Gail 1998). It is well established that plane. D'Alessio et al. (1998) calculated the radial temper- these effects are important in complex phase equilibria. ature profile at the mid-plane, the surface density, and the The outline of this paper is as follows: in section 2 we different vertical heights above the mid-plane. The tempera- describe the physical and chemical conditions assumed to be ture at the mid-plane, Tc, ranges from 60 to 1900 K for radii present in protoplanetary discs.

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