The Mid-Infrared Spectra of Normal Galaxies.

The mid-infrared spectra (2.5-5 and 5.7-11.6 µm) obtained by ISOPHOT reveal the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6, and 11.3 µm. Additional emission appears in between the features, and an underlying continuum is clearly evident at 3-5 µm. This continuum would contribute about a third of the luminosity in the 3-13 µm range. The features together carry 5%-30% of the 40-120 µm far-infrared (FIR) luminosity. The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. The dip at 10 µm is unlikely to result from silicate absorption since its shape is invariant among galaxies. The continuum component has a fnu~nu+0.65 shape between 3 and 5 µm and carries 1%-4% of the FIR luminosity; its extrapolation to longer wavelengths falls well below the spectrum in the 6-12 µm range. This continuum component is almost certainly of nonstellar origin and is probably due to fluctuating grains without aromatic features. The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies and could thus be used to obtain redshifts of highly extincted galaxies up to z=3 with SIRTF.

The Detached Dust Shell around the Massive Star HD 179821.

We have used the Keck I telescope to resolve at three mid-IR wavelengths the emission from HD 179821 (= RAFGL 2343), a G-type supergiant of perhaps 30 M middle dot in circle with a detached dust shell. The shell is very approximately circular in shape with an inner diameter of approximately 3&farcs;3, corresponding to 3.0x1017 cm. We estimate that the star was losing approximately 4x10-4 M middle dot in circle yr-1 until about 1800 yr ago, when the mass loss slowed dramatically. During the past approximately 104 yr, the star has lost approximately 10% of its initial mass. The star lies about 0&farcs;35 off center and is closer to the brighter, northern hemisphere of the nebula, which can be explained if the outflow velocity Vinfinity deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate M&d2;(straight theta) may have been inversely correlated with the outflow velocity so that the momentum outflow p&d2; was isotropic during the mass-loss phase. It also seems that M&d2;totalVinfinity was within a factor of 2 of L*/c, where L* is the current luminosity of the star; the mass loss may have been driven by radiation pressure. These results may help characterize the asymmetric circumstellar winds into which supernova explosions propagate.