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Astron. Astrophys. 337, 85-95 (1998)

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1. Introduction

In the mid-fifties Platt (1955) investigated the optical properties of tiny interstellar dust grains ([FORMULA] Å). Almost two decades later Greenberg & Hong (1974) and Purcell (1976) showed that such small grains undergo temperature fluctuations in the interstellar environment. Andriesse (1977) proposed these temperature fluctuation to be the reason for the observed near- and mid- ([FORMULA]) infrared excess observed in M17. Since then, temperature fluctuations of small grains were made responsible for the near- and mid- infrared excess in various other kinds of objects, i.e. in planetary nebulae (Sellgren 1984), diffuse HI regions and dark clouds (Draine & Anderson 1985), and in the circumstellar environment of Herbig Ae/Be stars (Natta et al. 1993, Natta & Krügel 1995).

Furthermore, it turned out that many astrophysical objects show infrared emission lines in their spectral energy distributions. Since their discovery by Gillett et al. (1973) these "unidentified" IR features were found in many different classes of objects. They include planetary nebulae, emission and reflection nebulae (Geballe et al. 1985, Cohen et al. 1986, Russell et al. 1977, Bregman et al. 1989), proto-planetary nebulae (Buss et al. 1990), the environment of Be and Herbig Ae/Be stars (Tokunaga et al. 1991, Schutte et al. 1990, Brooke et al. 1993), active galactic nuclei (Gillett et al. 1975, Willner et al. 1977, Acosta-Pulido et al. 1996), and the general interstellar medium (Giard et al. 1988a,b, Mattila et al. 1996). Léger & Puget (1984) and Allamandola et al. (1989) proposed the IR features to be emission lines of polycyclic aromatic hydrocarbons (PAHs). This hypothesis was recently strengthened by discovering PAH molecules by probing carbonaceous chondrites (Clemett & Messenger 1996).

Up to now, many authors reported methods to calculate the emission from small particles (radii [FORMULA] Å), i.e. Greenberg & Hong (1974), Purcell (1976), Draine & Anderson (1985), Dwek (1986), Léger et al. (1989), and Guhathakurta & Draine (1989). But only little progress was made in considering temperature fluctuations of small particles in radiative transfer calculations. In fact, many radiative transfer codes were developed to handle spherically symmetric (Yorke 1979, Rowan-Robinson 1980), axisymmetric (Efstathiou & Rowan-Robinson 1990, Spagna et al. 1991, Pier & Krolik 1992, Granato & Danese 1994, Sonnhalter et al. 1995, Men'shchikov & Henning 1997) or, in principle, arbitrarily shaped dusty envelope configurations (Yorke 1986, Stenholm 1995). However, all these codes consider only thermal emission of "classical" grains. As calculating the emission of small grains is much more time-consuming, temperature fluctuations were taken into account only for spherically symmetric dust clouds (Lis & Leung 1991, Siebenmorgen et al. 1992, Szczerba et al. 1997). However, the availability of an approximate, but very efficient 2D radiative transfer code (Men'shchikov & Henning 1997, Manske et al. 1997) enabled us, for the first time, to include a treatment of temperature fluctuations of small dust grains in 2D radiative transfer calculations.

In Sect. 2, we describe in detail a numerically stable and efficient algorithm to calculate the emission of small dust particles. In Sect. 3, the radiative transfer code is described as well as the implementation of the emission from small grains. In Sect. 4, we apply our code to the starburst galaxy NGC 6090 to show the effect of temperature fluctuations on spectral energy distributions and intensity maps. We finish the paper in Sect. 5 with the conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998