We report 4.5 µm luminosities for 27 nearby (D 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 µm luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 µm L-Z relation is 12+log(O/H) = (5.78 ± 0.21) + (−0.122 ± 0.012) M [4.5] , where M [4.5] is the absolute magnitude at 4.5 µm. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M * -Z) relation is 12+log(O/H) = (5.65 ± 0.23) + (0.298 ± 0.030) log M * , and extends the SDSS M * -Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M * -Z relation is similar over five orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M * -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.

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Classical novae, explosions that result from thermonuclear runaways (TNRs) on the surfaces of white dwarfs (WDs) accreting hydrogen-rich matter in close binary systems, are sporadically injecting material processed by explosive hydrogen-burning nucleosynthesis into the interstellar medium (ISM). Although novae probably have processed less than ∼0.3% of the interstellar matter in the Galaxy, both theoretical and observational evidence suggests that they may be important sources of the nuclides 7 Li, 15 N, and 17 O, as well as the radioactive isotopes 22 Na and 26 Al. The latter nuclides are astrophysically important in that they may have been involved in the production of the 22 Ne (Ne-E) and 26 Mg enrichments identified in meteoritic inclusions, the composition of which is thought to be representative of the chemical and mineral contents of the primitive solar nebula. These inclusions may be partially composed of dust condensed in nova outbursts. We review theoretical expectations for the yields of various isotopes in nova outbursts and conclude that any of the heavy isotope anomalies attributable to novae are most likely produced by the approximately 25%-33% of novae that occur in systems containing massive ( M , ) oxygen-neon-magnesium (ONeMg) WDs. We attempt to place quantitative constraints M 1 1.2 * on the degree to which classical novae participate in the production of chemical anomalies, both in the primitive solar system and on a Galactic scale. Diffuse Galactic g-ray fluxes provide particularly important clues to and constraints on the 22 Na and 26 Al yields from novae. Ultraviolet (UV), optical, and infrared (IR) emission-line spectra of classical novae reveal the abundances of some of the gas-phase elements present in the ejecta; recent results are reviewed. We describe how IR observations of novae reveal dust formation and gas-phase line emission and how they distinguish the temporal development of nova explosions on carbon-oxygen (CO) WDs (CO novae) from those on ONeMg WDs (ONeMg or "neon" novae). Recent studies show that the ejecta in some novae can be strongly cooled by near-and mid-IR forbidden-line radiation from highly ionized ("coronal") atomic states. We compare the abundances deduced from recent UV, optical, and IR observations with theoretical predictions, and we suggest that future studies of IR coronal emission lines may provide additional key information. Novae produce only about 0.1% of the Galactic "stardust" (dust condensed in stellar outflows), but IR observations show that it may be some of the more interesting dust. Novae appear capable of producing astrophysical dust of virtually every known chemical and mineral composition. We summarize recent IR observations of the dust production scenario in novae and argue that neon novae may lead to the formation of dust grains that carry the Ne-E and 26 Mg anomalies.

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Mid-infrared observations of the Andromeda galaxy, M31, obtained with the Infrared Array Camera on board the Spitzer Space Telescope are presented. The image mosaics cover areas of approximately and include 3Њ .7 # 1Њ .6 the satellite galaxies M32 and NGC 205. The appearance of M31 varies dramatically in the different mid-infrared bands, from the smooth bulge and disk of the old stellar population seen at 3.6 mm to the well-known "10 kpc ring" dominating the 8 mm image. The similarity of the 3.6 mm and optical isophotes and the nearly constant optical-mid-infrared color over the inner 400Љ confirm that there is no significant extinction at optical wavelengths in M31's bulge. The nuclear colors indicate the presence of dust but not an infrared-bright active galactic nucleus. The integrated 8 mm nonstellar luminosity implies a star formation rate of 0.4 M , yr Ϫ1 , consistent with other indicators that show M31 to be a quiescent galaxy.

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