Introduction

Recently a significant progress toward the understanding of the possible composition and properties of dust grains and gas species in many astrophysical environments has been achieved. For instance, (sub)millimeter observations of molecular lines provided basic information about chemical composition and dynamical properties of the gas in discs around pre-main-sequence stars and young stellar objects (e.g., Bujarrabal et al. [1997]; Olofsson, Liseau, & Brandeker [2001]; Piétu et al. [2003]; Thi et al. [2001]). The infrared-to-millimeter continuum observations of such environments constrain the properties of dust grains and can be used to estimate masses and thermal structure of the objects (e.g., Boogert, Hoderheijde, & Blake [2002]; Bouwman et al. [2000]; Tuthill et al. [2002]). Finally, experimental studies on the formation and spectra of various gaseous species (e.g., Butler et al. [2001]; Sanz, McCarthy, & Thaddeus [2002]) as well as the composition and properties of meteoritic, cometary, and interplanetary dust together with their laboratory analogues (e.g., Chihara et al. [2002]; Mutschke et al. [2002]) form a basis for theoretical investigations.

On the other hand, the increase of computer power and new numerical methods stimulate the development of more sophisticated hydrodynamical models (e.g., D'Angelo, Henning, & Kley [2002]; Kley, D'Angelo, & Henning [2001]; Struck, Cohanim, & Willson [2002]). Many of such simulations need an accurate treatment of the energy transport within a medium (see e.g., Klahr, Henning, & Kley [1999]), which requires a detailed description of the radiative properties of matter. Consequently, the adopted opacity model is an important issue.

In this paper, we deal with physical conditions typical of protostellar nebulae and protoplanetary discs around low-mass young stellar objects. Virtually everywhere within the medium dust grains are the main opacity source, as they absorb radiation much more efficiently compared to the gas and because the temperature in these regions is low enough to prevent their destruction. However, for hotter domains ($T < 1\,500$ K), where even the most stable dust materials cannot survive, it is necessary to take absorption and scattering due to gaseous species into account.

Recently, several extensive models describing the properties and evolution of dust grains in protostellar cores and protoplanetary discs were proposed by Henning & Stognienko ([1996]), Schmitt, Henning, & Mucha ([1997]), and Gail ([2001], [2002]).

Henning & Stognienko ([1996]) studied the influence of particle aggregation on the dust opacity in the early evolutionary phases of protoplanetary discs. They concluded that distribution of iron within the particles affects their optical properties in a great respect. Schmitt et al. ([1997]) for the first time investigated collisional coagulation of dust grains in protostellar and protoplanetary accretion discs coupled with hydrodynamical evolution of these objects. They reported significant alteration of the thermal disc structure caused by the modification of the opacity due to dust growth. Gail ([2001,2002]) considered annealing and combustion processes leading to the destruction of silicate and carbon dust grains consistently with the evolution of a stationary accretion disc. He found that the modification of the dust composition in the inner regions due to these processes and its consequent transport toward outer disc domains affect the opacity and, eventually, the entire disc structure.

A number of papers deal with the computation of Rosseland or/and Planck mean gas opacities in atmospheres of cool stars, protostars, and stellar winds. Alexander & Ferguson ([1994]) computed a set of opacity tables for temperatures between $700$ K and $12\,500$ K for several compositions. They considered the condensation sequence of refractory materials with chemical equilibrium calculations and took into account absorption and scattering properties of these solids as well as various gas species. Helling et al. ([2000]) calculated gas-dominated opacities for wide ranges of density, temperature and various chemical compositions based on up-to-date spectral line lists of the Copenhagen data base and studied the importance of the molecular opacity for the dynamics of the stellar winds of cool pulsating stars.

In Appendix A, we give a brief overview of the most common opacity models and studies, where they have been applied. It can be clearly seen that there is a lack of papers which focus on calculations of both Rosseland and Planck mean opacities of grain and gas species for temperatures between several K and few thousands K in a wide range of densities based of both the best estimates on the dust composition and properties and recent improvements in molecular and atomic line lists. The goal of this paper is to define such a model and to describe it in detail.