Pr Wesselius

THE INFRARED ASTRONOMICAL SATELLITE (IRAS) MISSION

The Infrared Astronomical Satellite (IRAS) consists of a spacecraft and a liquid helium cryostat that contains a cooled IR telescope. The telescopes focal plane assembly is cooled to less than 3 K, and contains 62 IR detectors in the survey array which are arranged so that every source crossing the field of view can be seen by at least two detectors in each of four wavelength bands. The satellite was launched into a 900 km-altitude near-polar orbit, and its cryogenic helium supply was exhausted on November 22, 1983. By missions end, 72 percent of the sky had been observed with three or more hours-confirming scans, and 95 percent with two or more hours-confirming scans. About 2000 stars detected at 12 and 25 microns early in the mission, and identified in the SAO (1966) catalog, have a positional uncertainty ellipse whose axes are 45 x 9 arcsec for an hours-confirmed source.

DISCOVERY OF A SHELL AROUND ALPHA-LYRAE

IRAS observations of Alpha Lyrae reveal a large infrared excess beyond 12 microns. The excess over an extrapolation of a 10,000 K blackbody is a factor of 1.3 at 25 microns, 7 at 60 microns, and 16 at 100 microns. The source of 60 microns emission has a diameter of about 20 arcsec. This is the first detection of a large infrared excess from a main-sequence star without significant mass loss. The most likely origin of the excess is thermal radiation from solid particles more than a millimeter in radius, located approximately 85 AU from Alpha Lyr and heated by the star to an equilibrium temperature of 85 K. These results provide the first direct evidence outside of the solar system for the growth of large particles from the residual of the prenatal cloud of gas and dust.

Infrared cirrus - New components of the extended infrared emission

Extended sources of far-infrared emission superposed on the zodiacal and galactic backgrounds are found at high galactic latitudes and near the ecliptic plane. Clouds of interstellar dust at color temperatures as high as 35 K account for much of this complex structure, but the relationship to H I column density is not simple. Other features of the extended emission show the existence of warm structures within the solar system. Three bands of dust clouds at temperatures of 150-200 K appear within 10 deg on both sides of the ecliptic plane. Their ecliptic latitudes and derived distances suggest that they are associated with the main asteroid belt. A third component of the 100-micron cirrus, poorly correlated with H I, may represent cold material in the outer solar system or a new component of the interstellar medium.

IRAS OBSERVATIONS NEAR YOUNG OBJECTS WITH BIPOLAR OUTFLOWS - L1551 AND HH 46-47

A 6-solar luminosity dust-embedded precursor of a low-mass (about 1 solar mass) pre-main-sequence star has been discovered with IRAS near the northeast lobe of the bipolar outflow region in the dust cloud L1551 and designated L1551 NE. Star formation is proceeding in at least two locations in L1551, reminiscent of the situation in regions of more massive star formation. If the position of NE in the flow from IRS 5 is indicative of the flow having initiated star formation in NE, then the object can be only about 24,000 years old. Alternatively, NE could appear by chance to lie in the flow from IRS 5. In the globule ES 0210-6A, a 12 solar luminosity dust-embedded precursor of a low-mass (about 1 solar mass) pre-main-sequence star is found which drives the bipolar flow responsible for the string of Herbig-Haro objects HH 46-47 A-D. In this globule, there is only one region of active star formation.

High-sensitivity IRAS observations of the Chamaeleon I dark cloud

Very sensitive IRAS observations of a region of 0.8 sq deg in the Chamaeleon I cloud have revealed 70 compact sources. Hot sources are field stars; warm sources are associated with pre-main-sequence (PMS) stars in the cloud center; others may be in an even earlier phase of gravitational collapse. Cool sources, detected only at the long wavelengths, surround the main cloud and appear to be associated with small globules. Only a small fraction (less than 20 percent) of the total luminosity of the known PMS objects is emitted in the IRAS bands. This has important implications for the classification of the newly discovered embedded objects.

An Infrared Study Of 3 Wolf-rayet Ring Nebulae

We have studied the IRAS colors of the ring nebula RCW 58 surrounding the Wolf-Rayet star HD 96548 (= WR 40; type WN 8) by analyzing the IRAS survey data with the Groningen Exportable High-Resolution Analysis system (GEISHA) and by using the Chopped Photometric Channel high-resolution imaging at 50-mu-m. Additional JHKLM photometry of the exciting star of RCW 58, and also of HD 50896 (WR 6, type WN 5), exciting the bubble S308, and of HD 192163 (WR 136, type WN 6), exciting the bubble nebula NGC 6888, have been obtained. We have estimated the emission from dust in all three bubbles by subtracting the stellar contributions to the IRAS images and correcting the IRAS fluxes for atomic emissions. Except for the 12-mu-m filter, the atomic contributions do not comprise the bulk of the observed fluxes. As compared to the interstellar medium (ISM) and clouds, the dust emissions are very weak in 12-mu-m relative to 25-mu-m but have F(60)/(100) > 1, compared to less-than-or-equal-to 0.2 for the ISM, showing that the grains are heated by the enhanced radiation field in the vicinity of the central star. The dust emission is analyzed by means of several models. For RCW 58, the diffuse dust model of Mathis, Rumpl, & Nordsieck, with optical constants as modified by Draine & Lee and grains in the size range 0.005-0.25-mu-m, produces fairly good agreement with the observations, except that the required radiation field is higher (almost-equal-to 100 eV cm-3) than one would expect from the mean distance of the bubble from the star (almost-equal-to 44 eV cm-3) with the stellar temperature we adopt. It is possible to fit the spectrum with the expected radiation field if a combination of small grains (0.002-mu-m-0.008-mu-m) are in the inner part of the nebula and somewhat larger ones (0.005-mu-m-0.05-mu-m) farther out. For S308 and NGC 6888, if there is enough radiation to produce the proper F(25)/F(60) from MRN grains the F(60)/F(100) flux ratio is too large. However, small grains transiently heated by random absorptions of stellar photons produce too large a F(60)/F(100) ratio at the observed F(25)/F(60). For these objects we find fits to both IRAS ratios by assuming that there are small grains transiently heated in the inner parts of the bubbles and standard interstellar grains in the outer portions. The masses of material in the nebulae are consistent with most of the matter being stellar for RCW 58 and a substantial amount of material being contributed by the swept-up ISM for S308 and NGC 6888. However, the masses are very sensitive to both the photometry and to the assumptions of the nature of the small grains near the star. Quantitative estimates of the mass are only order of magnitude. Graphs are given which allow an easy estimation of the nebular mass for other situations for nebular excitation. The principal parameter is the dilution factor of the stellar radiation.

Spectral Classification Using ANS Photometric Data

In this contribution we will discuss the extension of visual photoelectrical photometry into the ultraviolet and its potential impact on stellar classification. As is the case in the visual, classification by photoelectrical photometry in the ultraviolet has some important advantages over classification by inspection of individual spectra. Studies that require global characteristics of stellar properties — such as studies of stellar distribution, extinction properties of interstellar dust, galactic structure — often require large numbers of observations to derive statistical properties. In photoelectric photometry such a large number of observations can usually be obtained in a reasonable amount of time and relatively conveniently processed without subjective criteria being applied to the data. Comparatively, the classical way to obtain stellar classification is a complex process that requires many steps.