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Topics addressed include: star formation; galactic infrared emission; molecular clouds; OB star luminosity; dust grains; IRAS observations; galactic disks; stellar formation in Magellanic clouds; irregular galaxies; spiral galaxies; starbursts; morphology of galactic centers; and far-infrared observations.
Mapping of the CO emission line in 42 Virgo cluster galaxies reveals that the molecular gas contents and distributions are roughly normal in severaly HI-deficient Virgo spirals. The survival of the molecular component mitigates the impact of the HI-stripping on star formation and subsequent galactic evolution. For spirals which are deficient in HI by a factor of 10, far-infrared, H alpha line, and nonthermal radio continuum luminosities are lower by no more than a factor of 2. The fact that the inner galactic disks are stripped of HI, while CO is normal, suggests that the lifetime of the molecular phase is approximately one billion years in the inner regions of luminous spirals.
New data is presented that indicate that strong far infrared galaxies commonly have largescale emission line nebulae whose properties are suggestive of mass outflows (superwinds), presumably driven by the high supernova rate associated with the central starburst. These data include longslit spectra of M82 which show that the radial variation of the gas pressure in the emission line nebula is in excellent agreement with a previous wind model. The M82 nebula also has a LINER spectrum, consistent with shock heating. Morphologically and spectroscopically similar emission line nebulae were found in NGC253, and Arp 220 and NGC6240. A longslit spectroscopic investigation was conducted of 20 additional very powerful far-infrared galaxies and found that they generally have spatially extended emission line nebulae whose spectra closely resemble that of the M82 nebula. If the superwind interpretation is correct, it could have many important consequences in extragalactic astronomy.
Advances in the understanding of comet Halley's dust environment are highlighted. Spacecraft results on the dust composition, the detection of subfemtogram grains, and the particle-mass distribution; jets and the dust-emission pattern; the existence of discrete sources on the nucleus surface and the determination of their locations from day-to-day motions of coma features; CN jets; large-scale phenomena, including an antitail, streamers, and a sunward spike; infrared observations, such as the 3.4 microns emission feature, applications of array detectors, and rare far-infrared data; the chemical modification of low-temperature ices by UV and cosmic-ray irradiation processes, their laboratory simulations, and applications to the evolution of comets; the interstellar-connection models of comet origin; and the formation of a mantle made of refractory organic material on the nucleus surface.
The service life and/or reliability of far-infrared sensors on surveillance satellites is presently limited by the cryocooler. The life and/or reliability, however, can be extended by using redundant cryocoolers. To reduce parasitic heat leak, each stage of the inactive redundant cryocooler must be thermally isolated from the optical system, while each stage of the active cryocooler must be thermally connected to the system. The thermal break or the thermal contact can be controlled by heat switches. Among different physical mechanisms for heat switching, mechanically activated heat switches tend to have low reliability and, furthermore, require a large contact force. Magnetoresistive heat switches are, except at very low temperatures, of very low efficiency. Heat switches operated by the heat pipe principle usually require a long response time. A sealed gas gap heat switch operated by an adsorption pump has no mechanical motion and should provide the reliability and long lifetime required in long-term space missions. Another potential application of a heat switch is the thermal isolation of the optical plane during decontamination.
Graphite fiber-reinforced glass matrix composites are being developed for a variety of structural applications requiring excellent thermomechanical stability. These materials are ideally suited for lightweight, high strength, thermally stable infrared mirrors because of their low density, low thermal expansion, high strength and stiffness, and their ability to be machined, replicated and figured using standard polishing techniques. These properties are particularly promising for applications such as a 3-meter balloon-borne far-infrared and submillimeter telescope mirror which must be both very lightweight and able to retain its figure accuracy when cycled between room temperature and its operating temperature of -50 C. This paper presents the results of a set of low temperature optical tests conducted to determine the figure stability of a 30-cm diameter, frit-bonded graphite/glass mirror in the +20 to -60 C temperature range using a 10.6 micron laser interferometer. The results indicate that the residual change in figure was less than 0.3 microns, rms.
A heat trap, also called a Winston cone or compound parabolic concentrator, has been designed and built to serve as the beam defining optic for the far-infrared absolute spectrophotometer on the Cosmic Background Explorer satellite. This quasi-optical multimode antenna is coupled to a flared entrance section shaped like a trumpet bell to reduce response to off-axis radiation. The antenna has very low sidelobes over a two-decade frequency range from 1 to 100/cm. Fabrication is described, and stray light data are presented and compared to theory. A novel optical figure test for concentrators is presented.
Highlights of infrared observations of the dust are discussed and compared with first results from the space probes. An emission feature was detected at 3.4 microns; the 10 and 20 micron silicate features were well-observed; and far-infrared data out to 160 micron were obtained. Organic material seems to be abundant in grains and may explain the 3.4 micron emission. Calculations are presented for one example of organic material. A component of the grains may volatize at temperatures around 300 K.
In order to make an unbiased survey for high-velocity gas in a single molecular cloud, a sensitive and uniformly sampled map of CO (J = 1-0) emission over a large fraction of the Mon OB1 (NGC 2264) molecular cloud was obtained. High-velocity line wings were detected on profiles over about 10 percent of the region surveyed. Complete maps of the 12, 25, 60 and 100-micron far-infrared emission from the Mon OB1 cloud were constructed as well by coadding the appropriate IRAS survey fields.
The materials of which dust is composed are not known very precisely, but a general theorem in physics (the Kramers-Kronig relations) provides sufficient restraints regarding the cross section of dust in the ionizing ultraviolet. The large far-infrared luminosity of H II regions shows that dust absorbs most of the ionizing photons in them. However, models show that dust has little effect on the nebular spectrum, although of course it does tie up some heavy elements in the solid phase. For nebulae with very high excitation parameters the effect is potentially larger, but dust is strongly depleted, relative to the solar neighborhood, in such objects. Furthermore, the effects of large densities quenching the forbidden cooling lines, and thereby affecting the optical emission lines, is probably more important.
Star formation and the ISM in active dwarf galaxies are studied based on data from the IRAS Point Source Catalog. The dwarf galaxies that show evidence of recent energetic star formation are generally also strong far-infrared emitters; thus, active current star formation is associated with a history of energetic star formation. A fraction of the galaxies considered here have a star formation rate that is significantly greater than the average rate in the recent past. Available gas is being consumed as efficiently in the presently active dwarfs as in the Milky Way. A primary difference between galaxies that are energetically forming stars and those that are not is the difference in gas mass, as well as the efficiency of star formation. The most important process may be the assembly of large amounts of gaseous material.
Far-infrared color-color diagrams have been constructed for over 100 of the brightest evolved stars in the IRAS Point Source Catalog. The diagrams are used to deduce average values of the dust grain emissivity index (p) between 12 and 100 microns. Grains in C-rich and O-rich environments have similar values of p between 12 and 25 microns and between 60 and 100 microns, but between 25 and 60 microns p is larger by approximately 0.4 for the O-rich stars. Dust grains in envelopes around S-type stars seem to have 25 to 60 micron emissivities more nearly like grains in O-rich rather than C-rich environments. CO and HCN emissions from various stars are used to reclassify several stars as oxygen or carbon rich.
New observations of Orion Molecular Cloud 2 have been made from 1 to 100 microns using the NASA Infrared Telescope Facility and the Kuiper Airborne Observatory. An extensive program of polarimetry, photometry, and spectrophotometry has shown that the extended emission regions associated with two of the previously known near-infrared sources, IRS 1 and IRS 4, are infrared reflection nebulae, and that the compact sources IRS 1 and IRS 4 are the main luminosity sources in the cloud. The constraints from the far-infrared observations and an analysis of the scattered light from the IRS 1 nebula show that OMC-2/IRS 1 can be characterized by L of 500 solar luminosities or less and T of roughly 1000 K. The near-infrared albedo of the grains in the IRS 1 nebula is greater than 0.08.
In the IRAS color-color diagram, galaxies powered only by star formation are distributed in a band such that as systems get cooler at 12/25 microns they get warmer at 60/100 microns. A simple phenomenology is proposed which describes the emission from each galaxy as a combination of two components: cool, relatively constant 'cirrus' emission from the neutral medium, and from 'active' regions warmer emission whose colors depend on mean physical parameters in these regions. The model accounts for the IRAS colors of all galaxies detected as point sources in all four bands. The model implies that a galaxy's far-infrared flux as measured by IRAS is not simply proportional to the recent star formation rate, because only a (variable) fraction of the infrared is due to massive young stars, and only part of these stars' luminosity is reemitted in the infrared.
The spatial distribution of the 158 micron forbidden C II emission in the central 1 kiloparsec of the infrared galaxy M82 is discussed, and the detection of two fine-structure lines of forbidden O I and forbidden O III at 146 and 52 microns is reported. Within 500 pc of the nucleus, the far-infrared emission lines contain about 200 million solar luminosities. The neutral gas in the inner disk is in dense, relatively small clouds, thin filaments, or sheets of very low volume filling factor, associated with a lower density ionized medium of moderately high filling factor. Within 500 pc of the nucleus, about 30 million solar masses is contained in H II regions and 30 million solar masses in photodissociation regions. There is a component of ionized and neutral gas near LSR of roughly 150 km/s centered near the compact radio source 41.9 + 58 and elongated along the minor axis of M82. The gas is especially dense in this region and appears to be associated with X-ray emission and the inner H-alpha filaments.
Far-infrared CW radiation of 1.25 watts has been obtained at the 119 micron methanol line with a CO2 pump power of 125 watts, and the maximum frequency fluctuation of the free running laser is measured to be less than + or - 100 kHz per hour. Reflecting optics have been used, when possible, to minimize CO2 degradation, and the frequency stability is ensured by cooling the input and output couplers. The input and output assemblies within the lasing medium are enclosed to minimize the external effects on the cavity length and to eliminate the mechanical instabilities associated with the use of bellows. The vibrational bottle-neck is broken by cooling the resonator wall to 5 deg and adding He as the buffer gas.
A far-infrared and submillimeter study of regions within 0.03 pc of the low-luminosity sources embedded in the clouds B335, L723, B361, L1455, and L1551 is presented along with CO molecular line results on the extended environments of these sources out to 0.3 pc. Three of the embedded objects are definite bipolar outflow sources, one may be associated with an observed outflow, and one has no observable outflow. Data on the outflows are combined with determination of the luminosity of the embedded objects, and of the temperature and matter distribution about the sources, to infer the evolutionary stage of the embedded sources.
The far-infrared and submillimeter absorption properties of random aggregates of conducting spheres have been studied as a possible source for the long-wavelength absorption of interstellar dust grains. The complex shapes of random aggregates have various fractal dimensions, depending on the process that forms them. The long wavelength absorption versus frequency is affected more by the shape of a conducting grain than by its composition.