J. H. Elias

Infrared standard stars

The results of an observational program aimed at setting up a network of faint near-infrared standards of sufficient accuracy are reported. The network covers both northern and southern hemispheres and includes standards red enough to provide at least a limited check on color transformations. The standards are set up at J (1.2 micron), H (1.6 micron), K (2.2 microns), and L (3.5 microns), and their H2O and CO molecular absorption indices are determined. The problem of color transformations between observatories is discussed briefly. All magnitudes presented are transformed to the natural system defined by the CIT observations.

Type I supernovae in the infrared and their use as distance indicators

New infrared data for 11 Type I supernovae are presented. These results, when combined with other published data for Type I supernovae, show that the light curves fall into two well-defined groups. The first more common type - Type Ia - shows strong, variable, unexplained absorption at 1.2 μm and probably at 3.5 μm, while the second type - Type Ib - shows no such absorption and a slower decline after maximum. The light curves of the Type Ia supernovae appear to have a dispersion in color and absolute magnitude of ±0.2 mag or less, making them potentially valuable for distance determination within the Local Supercluster.

Carbon stars at high Galactic latitude

Photometry and kinematics are presented for a sample of objective prism selected carbon stars toward the north and south Galactic poles. Distances are determined by fitting the infrared colors to a giant branch. If these stars are like the carbon stars seen in dwarf spheroidal galaxies, the median distance of the sample is 28 kpc. If they are more like the carbon stars found recently in the Galactic bulge, they may be only half as distant. The surface density of carbon stars as a function of distance is remarkably consistent with an R exp 1/4 density profile for the Galactic halo. This density profile can be traced to about 15 scale radii and fills a volume similar to that occupied by globular clusters. The data yields an effective radius of either 7.0 or 3.5 kpc depending on choice of distance scale. The velocity dispersion of the sample is 96 + or {minus} 12 km/s. A kinematic model in which vertical velocity dispersion is independent of height above the Galactic plane seems in best accord with the data. 33 refs.

Infrared observations of Nova Cygni 1975

Infrared photometry from 1 to 20 microns and spectroscopy at about 2 microns are presented for Nova Cygni 1975 for the period from 2 days before to 1 yr after maximum light. The data can be explained by a simple model in which the expanding gas expelled during the explosion is always a plasma at approximately 10,000 K. Initially the gas is optically thick; this phase clearly defines the time of onset of the nova. Later, as the gas continues to expand, it becomes optically thin. The temporal dependence of the observed flux suggests that in this phase the expanding cloud is in the form of a shell. After about 300 days, long-wavelength emission which may be attributable to thermal reradiation from dust is observed.

The near-infrared variability of a sample of optically selected quasars

A complete sample of 108 optically selected quasars, the Palomar-Green quasars, has been studied at near-infrared wavelengths over an average time base of about 6 yr; in some cases the time base extends to 20 yr. The measurements show that about half the quasars have a high probability of having varied, but that half show little evidence of variability, in sharp contrast to published studies indicating that most quasars vary on comparable timescales at optical wavelengths. The maximum amplitude of variation is less than 1 mag and generally near 0.5 mag. During variations, the near-infrared colors are almost constant. The more luminous quasars show a smaller mean probability of having varied in the time frame of the study than the less-luminous quasars. The majority of the quasars with flat radio spectra have a high probability of having varied. Among those quasars that have a high probability of having varied, the rate is fairly independent of the time interval between the observations after the first year. The quasar 3C 273 shows definite variations at 10.1 µm which preclude the emission being thermal emission from heated dust grains for that quasar. The observations of the other quasars cannot be used to differentiate uniquely between thermal and nonthermal emission mechanisms.

1 millimeter continuum observations of extragalactic objects

Continuum observations of 23 extragalactic objects have been made at a wavelength of 1 mm; nine of the 23 have been definitely detected at this wavelength. The sources detected include seven from which the 1-mm emission appears to be nonthermal, and two from which the emission appears to be thermal radiation from dust. Repeated observations of the brightest nonthermal sources permitted a search for 1-mm variations on time scales of one month to three years. BL Lac was observed to vary by more than a factor of 2 in flux, and 3C 84, 3C 120, and 3C 273 showed possible variations; 3C 279 showed no variations greater than + or - 30%. The 1-mm variations of BL Lac appear to be correlated with variations at radio wavelengths. The other sources with nonthermal spectra detected were 3C 111 and M87; the two thermal sources detected were NGC 253 and M82. Comparison of the latter two 1-mm measurements with 2.6-mm CO measurements suggests that the CO line in these galaxies is not heavily saturated.

One-millimeter continuum emission studies of four molecular clouds

Maps with 1-arcmin resolution are presented of 1-mm thermal emission from dust grains in the central portions of the molecular clouds associated with four H II regions: W3, M42 (OMC-1), Sgr B2, and W49. The maps cover regions approximately 5 by 5 arcmin in extent. In each source, a sharp peak in the 1-mm surface brightness is seen at the position of a compact H II region and/or one or several luminous near-infrared sources. The radial density distributions in the center of the clouds have been estimated from the observed distribution of 1-mm surface brightness near the peaks. The results indicate that the central density gradients are quite steep. These gradients may have been established in the clouds by the collapse processes which led to the formation of the central luminous objects.

Far-infrared observations of IRC + 10216

Broad-band photometric observations of IRC + 10216 in five wavelength intervals from 50 to 1000 μ are reported. The observed radiation is interpreted as thermal emission from dust in the extended molecular cloud heated by the compact 2-20 μ source as the core of the cloud. The shape of the 50-1000 μ spectrum suggests that the emissivity of the dust particles varies approximately as λ^(-1) over this spectral interval. The mass of dust inferred from the far-infrared emission is comparable with the mass of heavy molecules in the cloud.

Further joint X-ray, infrared and radio observations of Cygnus X-3

Observations of Cygnus X-3 were carried out at 2.5--7.5 keV, 2.2 micron, 8.1 GHz, and 2.7 GHz over a two-week period. The x-ray data show the periodic structure which is typical of Cyg X-3. At times the x-ray and infrared measurements show very similar periodic structure, both in phase and shape, while at other times the infrared data show no periodic variability. The radio fluxes were usually low during the period of observation; both the daily average radio flux levels and spectral index remained nearly constant. (auth)

Observations of 1-millimeter continuum radiation from the DR 21 region

Continuum emission at 1 mm has been mapped with 1-min resolution over a 3-min by 8-min area including the compact H II region DR 21, the maser source W75(S)-OH, and the central portions of the surrounding molecular cloud. Peaks in the 1-mm intensity are observed at the positions of DR 21 and W75(S)-OH, and are roughly coincident with less marked peaks in the intensity of HCN and CO emission. An extended component of 1-mm emission is present as well. The principal source of 1-mm radiation is thought to be thermal emission from dust within the molecular gas. The characteristics of this radiation which make it a promising tool for the study of molecular clouds are briefly discussed.