5 Investigations beyond main goals and follow upThe SEDI data will be useful for many scientific purposes beyond the main goals of the project. In the following, we illustrate this statement on three different topics. 5.1 Dust SpectroscopyThe SEDI maps and catalogs will represent a vast data base from which specific regions or sources for follow-up IRS spectroscopy can be selected. Such observations will provide insight into the nature of small dust particles in the diffuse ISM. ISO observations opened a new perspective on the formation and evolution of dust in evolved stars and on its processing in deeply YSOs and circumstellar disks. A large number of new bands were discovered in these objects, such as the emission bands of crystalline silicates. However, there is still no clear evidence for these bands in the ISM. Also the PAH emission bands observed in young planetary nebulae (PNe) are remarkably similar to those observed in the ISM. However, in Asymptotic Giant Branch stars, which evolve into PNe, and in fully developed PNe such as the Helix nebula, the mid-infrared spectra are different. These ISO findings indicate that the link between interstellar dust particles and the dust released by stars is still unclear. With IRS, for the first time, spectroscopic observations of interstellar matter in radiation fields less than 1000 times the solar neighborhood radiation field will be obtained in the wavelength range 5-40 micron. These spectra will permit a search for new spectral features in the diffuse ISM which could reveal the nature of VSGs. This question is related to the origin of small dust particles. Are they clusters of PAHs or do they also include other types of grains, e.g. small silicates or any other type of grains seen in the spectra of evolved stars? This search will also clarify the link between interstellar and circumstellar dust and the extent to which dust processing erases the memory of its formation. 5.2 Extragalactic studiesThe SEDI observations cover the full range of environments contributing to the mid- and far-infrared emission of external galaxies including infrared luminous starbursts. They will quantify the dependence of the SIRTF dust SED on the radiation field intensity and the gas density which will help to understand and model the infrared SEDs of galaxies, a key to the interpretation of the infrared emission from galaxies and of cosmological counts. Galactic dust emission is a source of noise and contamination for the studies of the Cosmic Infra-Red and Microwave Backgrounds (CIRB and CMB). The statistical properties of the SEDI images and their correlation with maps of the interstellar gas are essential to understand how to best disentangle brightness fluctuations due to the ISM from those in the CIRB and CMB. This is relevant to quantify (1) the contamination of SIRTF point source catalogs by cirrus emission, (2) the contamination of CMB maps from present balloon experiments or future space projects (MAP and PLANCK) by emission from cold dust and by the microwave emission from small spinning dust grains (Draine and Lazarian 1998). This latter emission is thought to originate from the same particles as those responsible for the near- and mid-IR emission that will be probed by IRAC observations. 5.3 Serendipity Science with CatalogsA by-product of the SEDI program which covers ~50 square degrees will be an immense multifrequency catalog of point sources which will include a diversity of extra-galactic and stellar sources. Due to SIRTF's unparalleled sensitivity and the ancillary optical data, the SEDI catalogs will serve many communities. Based on existing models of galaxy evolution, the catalog will include about 10E6 galaxies. In the near and mid-IR, we will detect as many galaxies as a dedicated cosmological survey at high Galactic latitude because it is only in the far-IR that the sensitivity of the SEDI catalog is limited by cirrus contamination. From the 2MASS counts in our fields we estimate that in the L and M bands, the SEDI catalog will contain more than 2x10E6 stars in the Galactic disk and spheroid. The SEDI fields at intermediate latitudes (b~15 degrees) are close to sampling the optimal Galactic volume for stellar population studies since the expected density of sources in the L and M band is close to the sensitivity limit set by confusion. Interesting stellar populations include brown dwarfs, main sequence stars with debris disks and evolved stars. Finally, the size of the SEDI catalog will enable searches for rare objects. It is worth remembering perhaps the excitement generated by the excesses found by IRAS around Vega-like objects and we think there is a reasonable likelihood of such serendipitous discoveries coming from the SEDI point-source catalog.
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