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The ultraviolet spectra of 25 early-type stars have been obtained with the International Ultraviolet Explorer observatory. Bands of the 4th-positive system of interstellar CO are seen towards 12 of these stars. Spectra of HD46223 have been examined for interstellar lines of CH, C2, CH2, OH, HCl, and H2O.
The results of recent observations of the column densities of chlorine species in diffuse interstellar clouds are inconsistent with the column densities predicted by model calculations. The use of new atomic and molecular data fails to decrease the discrepancy. It is speculated that the exothermic reaction Cl(+) yields HCl(+) + H may not be rapid at the low temperatures prevailing in diffuse interstellar clouds.
Observational evidence is presented for the existence of interstellar medium electron density irregularities over a wide range of scale sizes. Radio scattering observations associated with the wavenumber range of about 10 to the -11th to 10 to the -6th/m are consistent with a density spectrum of the wavenumber form -3.7 + or - 0.6. At lower spatial wavenumbers (10 to the -16th to 10 to the -18th/m) the power spectrum can be estimated by (1) computation of the density fluctuations near the outer scale of the spectrum, (2) comparison with density irregularities predicted by theoretical models of the interstellar medium, and (3) comparison with observations of the velocity structure function.
A model is developed of the interstellar diffusion of galactic civilizations which takes into account the population dynamics of such civilizations. The problem is formulated in terms of potential theory, with a family of nonlinear partial differential and difference equations specifying population growth and diffusion for an organism with advantageous genes that undergoes random dispersal while increasing in population locally, and a population at zero population growth. In the case of nonlinear diffusion with growth and saturation, it is found that the colonization wavefront from the nearest independently arisen galactic civilization can have reached the earth only if its lifetime exceeds 2.6 million years, or 20 million years if discretization can be neglected. For zero population growth, the corresponding lifetime is 13 billion years. It is concluded that the earth is uncolonized not because interstellar spacefaring civilizations are rare, but because there are too many worlds to be colonized in the plausible colonization lifetime of nearby civilizations, and that there exist no very old galactic civilizations with a consistent policy of the conquest of inhabited worlds.
H2CO absorption observations against Sgr A are used to select the molecular material lying in front of the galactic center, and an analysis similar to that of Blitz and Shu (1980) is carried out. The ratio of visual extinction to equivalent width of the 6 cm line of H2CO is found to be similar for several nearby interstellar clouds. By considering only the molecular components in front of the infrared cluster, the estimate for the extinction is decreased to 15-46 mag. These results are consistent with those from infrared observations toward the galactic center. Extinction estimates based on other interstellar species are also considered. Atomic gas is seen as contributing less than 10 mag of extinction.
Five-band UV photometry is used to investigate the character of interstellar extinction toward 1367 stars of spectral type B3 and earlier, with observations producing a galactic average of 5.11, 4.78, 6.52, 4.10 and 1.95 at 1550, 1800, 2200, 2500, and 3300 A, respectively. Differences in the strengths of the UV extinction features of these stars appear to be independent, since objects with either strong or weak 2200 A extinction can have strong, normal or weak far-UV extinction. Further investigation reveals that some of the deviant stars are embedded in localized regions whose average extinction curves diverge dramatically from the galactic mean. The anomalous extinction of these regions attests to localized inhomogeneities in the UV characteristics of interstellar dust. Because the evaluation of UV extinction toward a particular star would be more accurate if based on the average curve of the region immediately surrounding that star rather than that of the entire Galaxy, it is recommended that the galactic average presented be used with caution to correct astronomical data of individual objects.
A few, specially selected interstellar absorption lines were measured in the high resolution, far ultraviolet spectra of 200 O and B type stars observed by the International Ultraviolet Explorer (IUE). For lines of sight extending beyond about 500 pc from the galactic plane, the abundance of singly ionized iron atoms increases relative to singly ionized sulfur. However, the relative abundances of singly ionized sulfur, silicon and aluminum do not seem to change appreciably. An explanation for the apparent increase of iron is the partial sputtering of material off the surfaces of dust grains by interstellar shocks. Another possibility might be that the ejecta from type I supernovae enrich the low density medium in the halo with iron.
Interstellar rubidium is detected through observations of the resonance line of Rb I at 7800 A towards zeta Oph. The abundance ratio of rubidium to potassium is estimated to be approximately solar, and if rubidium is generally found to have an abundance similar to potassium, it is indicated that the local interstellar medium is well mixed with a wide variety of the products of nucleosynthesis.
The abundance of interstellar neutral atomic carbon is investigated by means of its ground state fine-structure line emission at 492 GHz using the 91.5 cm telescope of NASAs Kuiper Airborne Observatory. Atomic carbon is found to be very abundant in dense interstellar molecular clouds with column densities of about 10 to the 19th per sq cm. Because the observations have considerably greater column densities than current theories of carbon chemistry, it is suggested that the physical conditions of these clouds are not as simple as assumed in the models. Various situations are discussed which would lead to large C I abundances, including the possibility that the chemical lifetimes of the clouds are relatively short.
IUE observations toward 10 stars have shown that zinc is not depleted in the interstellar medium by more than a factor of two, suggesting that its abundance may serve as a tracer of the true metallicity in the gas. A result pertinent to the history of nucleosynthesis in the solar neighborhood is that the local interstellar medium has abundances that appear to be homogeneous to within a factor of two, when integrated over paths of about 500 pc.
It is argued that interstellar polycyanoacetylenes are formed not on dust grains by catalytic buildup or by dissociation of longer molecules, but rather by gas phase ion-molecule reactions. The primary evidence for this view is the detection of deuterated cyanoacetylene in an interstellar cloud. It is also argued that the relative abundance of successive homologs of polycyanoacetylenes rules out the grain catalysis theory.
Observations of 16 UV resonance lines of Fe II and six of Mn II in five stars are used to derive new f-values for the lines of these species at wavelengths lower than 1300 A. Values of forbidden lines Fe/H and Mn/H are derived. These new values are used to reassess mean depletions and range of variations in depletions for several lines of sight. On an integrated line-of-sight basis, depletions of Fe and Mn show larger variations than P, Cl, or Zn. The mean local depletion forbidden line Fe/H is 1.65, in interstellar gas. One Fe II line, 2366.864 A, has never been detected. Its f-value is shown to be much lower than previously thought. This line is therefore not useful for interstellar studies at the present time. It is suggested that the true wavelength of 1142 A of Fe II, from UV multiplet 10, is 1142.285 A.
Observations of interstellar absorption in the resonance doublet 7664, 7698 A of neutral potassium toward 188 early-type stars at a spectral resolution of 8 km/s are reported. The 7664 A line is successfully separated from nearly coincident telluric O2 absorption for all but a few of the 165 stars for which K I absorption is detected, making possible an abundance analysis by the doublet ratio method. The relationships between the potassium abundances and other atomic abundances, the abundance of molecular hydrogen, and interstellar reddening are investigated.
Attention is given to the 'B versus n dilemma' associated with the near constancy of magnetic field strength based on H I Zeeman data over a range of gas densities. The problem is examined in terms of preferential mass flow along magnetic field lines resulting from the low thermal energy of these regions. Approximate relations have been found to scale the magnetic field strength in interstellar clouds. It is noted that the fiducial gas density for scaling the increasing magnetic field strength is 2-3 orders of magnitude above the average interstellar density often used to estimate B.
The diffuse interstellar band at 5780 A has been observed in lines of sight where the column density of H2 is less than 10 to the 16th/sq cm. The strength of 5780 A toward Gamma Cas, Kappa Ori, and Rho Leo argues strongly against a molecular carrier of the 5780 A feature. The interstellar band at 5797 A also appears to be present in Kappa Ori and Rho Leo. Although the exact mechanism remains unclear, it is concluded that the diffuse features at 5780 A and 5797 A are carried by dust grains.
Neutral nitrogen and oxygen column densities or limits are derived for 53 path lengths through the diffuse interstellar medium and compared with column densities of neutral hydrogen. For neither N nor O is a systematic increase of depletion found as reddening increases. The value found for forbidden O/N is 8, and the abundances of N and O are both between 40% and 70% of the solar values. Implications of these results for models of interstellar grains are briefly discussed.
Individual high-dispersion International Ultraviolet Explorer (IUE) spectra of extragalactic stars are noisy because of the long exposure times required and the inherent limitations of the IUE detectors. A program has, therefore, been undertaken to obtain multiple ultraviolet spectra of the brightest Magellanic Cloud stars in order to produce composite spectra with higher signal-to-noise ratios than in individual spectra. The present investigation is concerned with results for HD 5980. The target star, HD 5980, is one of the brightest extragalactic far-ultraviolet sources in the sky. HD 5980 is located in the northeast quadrant of NGC 346, the largest H II region and OB star cluster in the SMC. The analysis of the interstellar line spectrum of HD 5980 provides a number of conclusions about the complex absorption characteristics of the interstellar gas toward the SMC.
A reexamination of the observed column densities of various elements in diffuse clouds suggests that almost all elements including oxygen, nitrogen, sulfur, and argon may be depleted with respect to hydrogen in interstellar clouds with large hydrogen column density. The amount of depletion varies from element to element and increases with increasing column density of hydrogen nuclei. This result is in qualitative agreement with the depletion of oxygen and sulfur independently inferred from the gas phase chemistry of sulfur in dense clouds. The rate of increase of depletion with hydrogen column density implied by the present study is large. It is possible that observational selection effects may have amplified the real dependence on N(H). A broad spectrum of C/O ratios ranging from values greater than unity to values less than unity appears possible for interstellar clouds, which would have the effect of a large variation in chemical composition from cloud to cloud.