Proto-planetary disks

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

The above treatments provide a way of determining the surface density of a proto-planetary disk, which is a convenient value in terms of astronomical observations as it easily translates into quantities such as the optical depth. However, models of chemistry within protoplanetary disks require knowledege of the pressure of the gas and how it varies with time and location. Such a quantity is found by solving the equation of hydrostatic equilibrium. 

Figure 6.4 The effects of grain processing on the spectral appearance of proto-planetary disks in the infrared. The 10 pm features of several young stars are shown with various degrees of grain growth and crystallization. As indicated in the upper-right panel, the broadening of the feature indicates grain growth, while the appearance of sharp resonances indicates the presence of crystalline silicates. The spectra are taken from van Boekel el al. (2005).

Laboratory experiments show that micrometer-sized dust grains, like those in proto-planetary disks, readily stick together during collisions, forming porous aggregates up to 1 cm in size (Poppe et al. 2000 Marshall Cuzzi 2001 Krause Blum... 

Figure 10.4 A simulation of oligarchic growth in the inner region of a proto-planetary disk around a solar-mass star. In the inner disk, embryos grow to 0.1 Earth-masses in <106 years. Growth then slows dramatically. Embryos continue to grow larger beyond the snowline at 2.5 AU. The simulation uses the semi-analytic model of Chambers (2008) with Esom ccl/a.

Hewins R. H. and Connolly H. C., Jr. (1996) Peak temperatures of flash-melted chondrules. In Chondrules and the Proto-planetary Disk (eds. R. H. Hewins, R. H. Jones, and E. R. D. Scott). Cambridge University Press, Cambridge, pp. 197-204. 

Beckwith S. V. W., Henning T., and Nakagawa Y. (2000) Dust properties and assembly of large particles in proto-planetary disks. In Protostars and Planets IV (eds. V. Mannings, A. P. Boss, and S. S. Russell). University of Arizona Press, Tucson, pp. 533-558. 

Astronomical observations of molecular clouds and young stellar objects provide the basis for our understanding of the early solar system (Cameron, 1995 Alexander et al., 2001). The first stage in this process is when a fragment of an interstellar molecular cloud collapses to form a disk-like nebula, or proto-planetary disk. This process normally takes... 

The science datacube selected for the next simulations corresponds to a proto-planetary disk surrounding a Herbig Ae star 10,000 K). As presented earlier in this chapter, Herbig Ae/Be stars are pre-main-sequence stars. The main difference with T Tauri stars is the mass, this being Af+ > Mq. Spectrally, their SED shows strong infrared radiation excess due to the presence of the drcumstellar accretion disk (Hillenbrand et al. 1992), this is, the thermal emission of circumstellar dust. 

Abstract We discuss models that astrochemists have developed to study the chemical composition of the interstellar medium. These models aim at computing the evolution of the chemical composition of a mixture of gas and dust under astrophysical conditions. These conditions, as well as the geometry and the physical dynamics, have to be adapted to the objects being studied because different classes of objects have very different characteristics (temperatures, densities, UV radiation fields, geometry, history etc) e.g., proto-planetary disks do not have the same characteristics as proto-stellar envelopes. Chemical models are being improved continually thanks to comparisons with observations but also thanks to laboratory and theoretical work in which the individual processes are studied. 

Another type of parameter, to which the model can be sensitive, is the choice in the initial composition of species (initial conditions). Since the chemistry is not at steady-state in most objects, the chemical composition predicted by the model will depend on the assumed initial conditions. There does not exist yet any model able to follow the chemical composition of the gas and dust during a complete cycle of evolution starting with material ejected from stars and ending with the collapse of clouds to form new stars, mainly because the evolution between different stages of star formation (e.g., diffuse to dense clouds, proto-steUar envelopes to proto-planetary disks) is not fuUy... 

The geometry is also important for the treatment of the interaction with UV photons. Borders of dense molecular clouds are exposed to the interstellar UV field produced by massive stars. The penetration of these photons into the cloud has to be computed as a function of depth since they are absorbed and scattered by the dust (see Sect. 4.3.1.1). In proto-planetary disks, the young central star usually presents a strong emission of UV (and X-ray) photons so that the UV penetration has to be computed in two dimensions to take into account the interstellar UV radiation field and the one coming from the star. 

Such reduction methods have been applied by a number of groups for different cases. To the best of our knowledge. Ruffle et al. [35] were the first to apply such a technique (in a more complex version) to determine the minimum network for the computation of the gas-phase CO abundance in interstellar clouds of different density, temperature and visual extinction. Their network still contains more than two hundred reactions and more than 60 species. Reduced networks to compute the ionisation fraction in dense clouds [36] and in proto-planetary disks [37], and reduced networks for dense clouds [38] have also been proposed. 

In the Monte Carlo methods, the evolution of (discrete) number densities is followed in time by randomly selecting a sequence of processes. The probability of selecting a process is proportional to its rate, which is determined in a similar manner to the rate equation method. Because Monte Carlo methods use random numbers and probabilities instead of analytical expressions, coupling between the two methods (rate equations for the gas and a Monte Carlo procedure for the grain) is harder to achieve. Different (kinetic) Monte Carlo implementations are used and usually a distinction between macroscopic and microscopic Monte Carlo is made. In the macroscopic simulations, only the number density is followed in time in the microscopic simulations the exact positions of the species are also considered. Recently, macroscopic Monte Carlo simulations of both the gas phase and grain surface chemistry have been carried out for a proto-planetary disk [52]. 

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