1 The SEDI Legacy Program

1.1 Scientific Motivation

Understanding the nature and physics of interstellar dust is an essential aspect of many fundamental astrophysical problems: the evolution of galaxies, the formation of stars, the formation of planetary systems and the chemical evolution of matter in space, including the formation of complex organic molecules which may lead to life.

Space-based observations of the diffuse emission from interstellar dust have given us many exciting perspectives on the interstellar medium (ISM) which highlight the role of dust as an actor and a tracer of the ISM.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The IRAS images revealed the intricate morphology of infrared (IR) cirrus. The structure of the cold neutral medium is now known to extend to much smaller angular scales than the IRAS resolution. The physical conditions are inferred to be extremely inhomogeneous with large departures from mean values in numerous localized regions. Extreme physical conditions may trigger processes that drastically influence the gas and dust evolution.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The inference of interstellar Polycyclic Aromatic Hydrocarbons (PAHs) and very small grains has extended the dust size distribution to the molecular scale. These small dust particles are now considered to be the dominant heating source of diffuse gas. Questions relating to the origin of the small dust particles, and their role in gas-phase chemistry, are still open.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The changes observed in the spectral energy distribution (SED) of the dust emission across the ISM are a spectacular manifestation of the evolution of matter in space which impact the thermodynamic and chemical state of the ISM.

What makes SIRTF unique for ISM studies is its combination of high mapping efficiency, high angular resolution, broad wavelength coverage and high sensitivity. This combination of capabilities opens up the exciting possibility of building a coherent database which will include observations of the diffuse IR emission at small angular scales over the full range of interstellar environments and over the whole dust size distribution from molecular-sized PAHs to large grains. The study of the IR emission of the diffuse ISM is only achievable with cooled telescopes operating from space and SIRTF is the only planned facility to complete such a study in the foreseeable future.

The ambition of the SEDI program is to build a coherent data base to study the interstellar dust in the more general framework of the ISM structure. In doing this we will address fundamental questions relating to dust and the evolution of the ISM such as:

$\bgroup\color{bluesedi}$\triangleright$\egroup$ What physical processes regulate dust evolution? Where in the ISM and on what size scales are they active?

$\bgroup\color{bluesedi}$\triangleright$\egroup$ How closely is dust evolution correlated with the physical and chemical evolution of the gas? What is the impact of the gas dynamics and of the density structure upon the evolution of dust and, conversely, what is the impact of dust evolution on the chemical and thermodynamical evolution of the gas?

Beyond these questions the SEDI data base, in combination with follow-up ground-based, airborne and space-based observations, will have a lasting impact on ISM research of a similar magnitude to that triggered by the IRAS sky images. The results of the SEDI observations will constitute a lasting legacy of the SIRTF mission.

1.2 Observing Program

The main emphasis of the SEDI legacy proposal is to provide maps of the diffuse ISM at all SIRTF wavelengths in order to derive the SED of the dust emission. The main features of the program are optimized to produce the maximum scientific impact of SIRTF for studies of the diffuse ISM as follows.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The full SED potential of SIRTF will be exploited. The broad band IRAC and MIPS SEDs will be complemented by second-look spectroscopic observations on selected positions within the images in order to obtain template spectra for all types of observed SEDs. The combination of photometry and spectroscopy provides the necessary information to completely characterize dust evolution.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The SIRTF maps will be sufficiently large to be integrated into the larger scale images provided by the Diffuse Infrared Background Experiment (DIRBE) at the same wavelengths. This is required in order to include reference area outside of the targets. It will permit us to follow the dust SED and the ISM structure from the smallest angular scales available with SIRTF to the larger angular scales of the DIRBE data. The dust properties can then be related to the local properties as well as to the large scale environment.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The observations will be towards regions in the solar neighborhood away from the Galactic plane. This allows us to study specific environments where the excitation conditions are best understood and to optimize the spatial resolution of the observations. The fields selected for this program will sample the full diversity of Galactic ISM environments heated by starlight.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ The SIRTF observations will be complemented with CO and H I observations which will allow us to identify and separate structures in the IR images on the basis of their velocity and to relate these structures to the dust and gas properties. A by-product of the SEDI observations will be an immense point source catalog which will be complemented by ancillary optical observations.

For ISM research, the results of this program will constitute a richer data base than the IRAS survey both in terms of the number of resolution elements and the wavelength coverage.

1.3 Methodology

Our approach to the problem of dust evolution in the ISM relies on the dependence of the dust SED on the size distribution of small dust particles and the emission properties of the large grains (Fig. 1). The emission spectrum at long wavelengths is characterized by the emissivity and temperature of large grains. At wavelengths shorter than ~60 micron, the observed emission is attributed to particles small enough to be heated by the stochastic absorption of photons. From the mid-IR emission bands the smallest dust particles are inferred to be aromatic hydrocarbons (PAHs). Dust models show that an additional population of stochastically heated grains, intermediate in size between PAHs and large grains, is necessary to account for the observed emission from 15 to 60 micron. These intermediate size grains are referred to as very small grains (VSGs) the nature of which is unknown. The SED of small dust grains follows from the probability distribution of their temperatures which is directly related to their size distribution. This three-component dust description is the basis of a widely used dust model, developed for the interpretation of the IRAS observations of diffuse emission in the solar neighborhood (Désert et al. 1990).

The SEDI program includes second-look observations, with MIPS in the SED mode and IRS in the low spectral resolution mode, towards a limited set of regions which will be selected on the basis of the mapping results. These observations will fill a key wavelength range (15-100 micron) where no spectroscopic information is available outside of compact bright sources. The IRS spectrometer from 5 to ~40 micron covers the full wavelength range where emission features from small dust grains are expected. The MIPS SED from 55 to ~96 micron covers a range of wavelengths where the emission from small grains merges with that of large grains. For the interpretation of the dust SEDs we will develop a dust model which will take into account the spectroscopic constraints. This SIRTF dust model will be essential in the interpretation of SIRTF photometric observations well beyond the present proposal. It will play a similar role to that of the successful Désert et al. model in the interpretation of the IRAS colors from nearby Galactic diffuse emission and also from distant galaxies. The SIRTF dust model will be an important product of the SEDI Legacy Program. It will be made available to the general science community in a readily-useable form and in a timely manner.

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