1 Introduction

Low mass protostars in their earliest stages of evolution are deeply embedded in large amounts of dust and gas. The nature of the emission from such star-forming regions makes them ideal to study in the infrared to radio wavelength regime. In the last decade, the sensitivity of receivers operating at these wavelengths has increased dramatically and the resulting high quality observations, supplemented by careful modelling, have provided most of the current day knowledge about the chemistry and physics, and their rapidly changing properties, in early stellar evolution. While the general scenario of low mass stellar evolution is reasonably well understood (e.g., Shu et al. 1993; Evans 1999; André et al. 2000) many uncertainties remain. For example, the nature of the hot ($T \ga90$ K) and dense ( $n_{{\rm H}_2} \ga 10^7$ cm^-3) regions of gas observed towards some low mass protostars is not yet fully established and the possible link with the so-called hot cores observed towards most high mass protostars needs to be investigated further.

In the case of high mass star formation, it has become clear that hot cores represent one of the earliest phases (Walmsley 1992). Chemically, hot cores are characterized by high abundances of fully hydrogenated molecules such as water (H₂O), ammonia (NH₃) and hydrogen sulfide (H₂S), along with a rich variety of complex organic molecules ranging from methanol (CH₃OH) and ethanol (CH₃CH₂OH) to methyl cyanide (CH₃CN), dimethyl ether (CH₃OCH₃), methyl formate (HCOOCH₃) and ethyl cyanide (C₂H₅CN) (Walmsley & Schilke 1993; Kuan & Snyder 1996; Hatchell et al. 1998; Schilke 2000). The chemical richness is explained by evaporation of the ice mantles above $\sim$90 K, followed by rapid gas-phase ion-molecule reactions leading to more complex species for a period of $\ga$10⁴ yrs (Charnley et al. 1995; Millar et al. 1997; Rodgers & Charnley 2001, see van Dishoeck & Blake 1998 and Langer et al. 2000 for overviews). Low mass protostars are less luminous and less massive, but a similar physical structure is expected except for a scale factor (Ceccarelli et al. 1996; Ivezic & Elitzur 1997). On the other hand, shocks due to the interaction of the outflows with the envelope can also liberate ice mantles and drive a high-temperature chemistry; such shocks may be relatively more important for low-luminosity objects than for high-mass protostars (e.g., van Dishoeck et al. 1995). It is of considerable interest to establish if low mass protostars also have hot and dense regions, and if so, whether a similarly complex organic chemistry to that found in the case of high-mass protostars has ensued and whether passive heating by the accretion luminosity or active shocks dominate the liberation of grain mantles. Since it is the material in the warm inner envelope that will be incorporated into circumstellar disks, it is important to know the level of chemical complexity as it relates to forming planetary systems.

IRAS 16293-2422 is by far the best candidate for investigating a low mass hot core (e.g., Blake et al. 1994; van Dishoeck et al. 1995; Ceccarelli et al. 2000a,b). IRAS 16293-2422 is a deeply embedded low mass protostellar object located within the $\rho$ Ophiuchus molecular cloud complex. Due to the relative proximity of this source (160 pc; Whittet 1974) a wealth of molecular lines has been detected, in spite of its relatively low luminosity (27 $L_{\odot}$; Mundy et al. 1986), and this has made IRAS 16293-2422 one of the best studied young stellar objects. Interferometer observations of radio and millimetre continuum emission reveal two compact sources in the center of its circumstellar envelope (Wootten 1989; Mundy et al. 1990; Mundy et al. 1992; Looney et al. 2000), likely to be accretion disks through which matter is fed onto the central stars. The separation of the two protostars is approximately 800 AU (Looney et al. 2000). IRAS 16293-2422 is thought to be in one of the earliest stages of formation; the observed spectral energy distribution (SED) can be fitted by a modified blackbody of $\sim$40 K (e.g., Walker et al. 1986; André et al. 2000) and has a high ratio of submillimetre to bolometric luminosity, suggesting a large amount of envelope mass. This places IRAS 16293-2422 among a family of deeply embedded and recently formed hydrostatic stellar objects known as, in the traditional evolutionary sequence of low mass protostellar objects (e.g., André et al. 1993), "class 0'' protostars.

The circumstellar surroundings of this protobinary star were extensively studied in a large molecular line survey presented in Blake et al. (1994) and van Dishoeck et al. (1995). It was found that molecular line emission is potentially a very powerful tool to probe both the physics and chemistry of the circumstellar environment; however, a full radiative transfer analysis was not performed and the derived abundances have significant uncertainties. Due to the complexity of molecular excitation and its sensitivity to the environment, various species - and even different lines of the same species - probe different parts of the circumstellar material. At least three physically and chemically distinct parts were identified including a circumbinary envelope, circumstellar disk(s), and outflow components. The latter component was thought to be a small and warm region of a few arcsec in size where the bipolar outflow(s) interact with the inner part of the circumbinary envelope. Recently, Ceccarelli et al. (2000a,b) used deep JCMT observations of H₂CO combined with a physical-chemical collapse model to argue that IRAS 16293-2422 does in fact have a hot-core-like region in which the liberation of ices is consistent with heating by the accretion luminosity.

We present here spherically-symmetric radiative transfer modelling of the dust and gas components constituting the material in the circumstellar envelope of IRAS 16293-2422. The dust parameters are constrained by the observed continuum emission in the form of submillimetre brightness maps and the SED over a large wavelength region. The resulting temperature and density structures are a prerequisite to chemical studies of the molecular gas present in the envelope. The approach taken here is different from that adopted by Ceccarelli et al. (2000a,b) in that the physical parameters of the envelope are derived empirically from the analysis of the dust emission. Once the physical structure of the envelope is known, a detailed excitation analysis of molecular millimetre line emission is performed aimed at obtaining accurate abundances. This provides valuable insight into the complex chemistry occurring in this proto-stellar envelope. In particular, the derived abundances allow for direct comparison with other sources and comets (e.g., Bockelée-Morvan et al. 2000). Moreover, searches for evidence of abundance changes, e.g., due to evaporation of ices ("jump models'') is of considerable interest. In addition to providing constraints on the chemistry, the molecular line observations give further information on the physical structure, e.g., kinematic information. Similar strategies have been adopted by van der Tak et al. (1999,2000b) and Hogerheijde & Sandell (2000) and have proven to be powerful tools when determining the physics and chemistry of star-forming regions.