Unreddened Stars Research Articles

Ion-atom charge-transfer reactions and a hot intercloud medium

An investigation is conducted concerning the ionization equilibrium of carbon in a hot intercloud medium (ICM), taking into account various charge-transfer reactions. Attention is given to problems related to observations of carbon along the lines of sight to several unreddened stars. It is pointed out that the observed underabundance of C III and overabundance of C I can be consistent with the presence of a hot, partially ionized ICM, provided that two of the charge-transfer reactions considered are rapid at thermal energies.

Interstellar molecules

Scans were made at the absorption wavelengths of 14 molecules resulting in the detection of three molecules: H 2 , HD and CO. Lines of H 2 are measured in all 11 reddened stars ( E B-V > 0.10) analysed with f , the fraction of hydrogen in molecular form, exceeding 10 -1 . In eight of nine unreddened stars ( E B-V < 0.05) no H 2 lines are detected limiting f to less than 10 -7 . Ratios of orthohydrogen ( J = 1) to parahydrogen ( J = 0) correspond to an average excitation temperature of 81 K. Two HD lines have been measured in nine stars yielding a ratio of HD to H 2 of about 10 -6 . Consideration of the differences in self-shielding between H 2 and HD for disruption by ultraviolet photons indicates that one HD molecule is formed and dissociated for about every 200 H 2 molecules. Column densities for CO for four reddened stars have been determined. Ratios of the J = 1 to J = 0 rotational levels in CO yield an excitation temperature of 5 K for CO toward three stars.

Physical state of interstellar atoms

A brief survey is given of the physical conditions along the lines of sight to reddened and unreddened stars, as determined from Copernicus observation of interstellar lines between 95 and 300 nm. Differences in ionization structure and density between clouds and the local intercloud medium are discussed. Some new data for β Centauri is used to supple­ment the previously available data.

Spectrophotometric Results from the Copernicus Satellite.IV. Molecular Hydrogen in Interstellar Space

Strong H/sub 2/ lines were measured in all 11 reddened stars STAE(B-V)> 0.101 observed; the fraction f of hydrogen gas in molecular form exceeded 10/sup - 1/. In eight out of nine unreddened stars STAE(B-V) < 0.05! there was no trace of H/sub 2/ absorption, with f less than 10/sup -7 In two stars of intermediate reddening f was between 10/sup -5/ and 10/sup -6/. The relatively large column densities in higher rotational levels, up to J = 5 or 6, corresponded to excitation temperatures mostly between 150 deg and 200 deg ; the ratios of ortho- hydrogen (J = 1) to para-hydrogen (J = 0) corresponded to lower temperatures, averaging about 80 deg . Measures of two HD lines in nine stars indicated a ratio of HD to H/sub 2/ equal to about 10/sup -6/; correction for the more rapid disruption of HD molecules, in the absence of effective optical shielding by many other such molecules, indicated that one HD molecule is formed and dissociated for about every 200 H/sub 2/ molecules. (auth)

Spectrophotometric Results from the Copernicus Satellite. III. Ionization and Composition of the Intercloud Medium

Interstellar lines have been studied in the unreddened stars lambda Sco, upsilon Sco, alpha Leo, and alpha Eri, which are located between 20 and 150 pc from the sun. The lines are on the linear and saturated Doppler portions of the curve of growth. Preliminary analysis shows that processes producing large amounts of highly ionized species are not dominant. Doubly ionized C, N, Si, and S have been detected, but are all quite weak relative to the singly ionized species. Abundances relative to nitrogen are in the cosmic ratios or slightly lower, with some indication of general depletion of the heavier elements. Hydrogen number densities range from a measured value of 0.22 per cu cm for lambda Sco to a derived value of 0.02 per cu cm for alpha Leo assuming nitrogen to have its solar abundance relative to hydrogen.

Observations of interstellar extinction in the ultraviolet with the OAO satellite

We present a preliminary analysis of a number of spectrophotometric scans of reddened and unreddened early-type stars obtained with the OAO-A2 satellite. The principal findings are: (1) all the extinction curves show a ‘bump’ at 1/λ ≈ 4.5; (2) most extinction curves show a minimum at 1/λ ≈ 5.5–6.0; (3) all the extinction curves show a rapid rise in the far ultraviolet; and (4) there appear to be real differences in the far ultraviolet extinction. A brief comparison of our observed extinction curves is made with theoretical particle models.

Measurement of interstellar extinction in emission line stars

Usually selective interstellar extinction is derived by comparing the spectrum of a reddened star with an unreddened nearby star, preferably of the same spectral type. But in many cases this comparison is not possible, so that new methods have to be used.A direct measure of this quantity is possible when a spectrum shows a number of emission lines as for the peculiar stars (novae, nova-like objects, symbiotic stars, etc.) and for nebulae. From a statistical point of view there are a small number of objects but they are of very high interest as far as stellar evolution is concerned.

The Wavelength Dependence of Interstellar Extinction

view Abstract Citations (101) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Wavelength Dependence of Interstellar Extinction Whiteoak, J. B. Abstract The law of interstellar extinction between wavelengths 3448 and 10500 A has been investigated in five regions of the Milky Way, namely, in Perseus, Orion, Monoceros, Cygnus, and Cepheus. Reddened and virtually unreddened stars of early spectral types were observed at thirty wavelengths with a photoelectric spectrum scanner at Mount Wilson. By adoption of theoretical models for unreddened energy distributions, the mean wavelength dependence of interstellar extinction was calculated for each region. The results for Perseus and Cepheus are similar. In Orion, the wavelength dependence for stars outside the Orion Nebula is different from the dependence within the nebula. The law for Monoceros lies between that for Orion and those of the remaining regions. There is no correlation between the results and galactic structure. Publication: The Astrophysical Journal Pub Date: April 1966 DOI: 10.1086/148605 Bibcode: 1966ApJ...144..305W full text sources ADS |

A High-Dispersion Photoelectric Spectrophotometer.

view Abstract Citations (21) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A High-Dispersion Photoelectric Spectrophotometer. Rogerson, J. B. ; Spitzer, L. ; Bahng, J. D. Abstract Photoelectric equipment has been successfully used for high-dispersion spectrophotometry at the 100-inch telescope. The intensity in the spectrum was measured by the ratio of photon counts obtained by two photocells, one fixed, the other scanning the spectrum. Successful seeing compensation was achieved by using phototubes with a sensitivity that was relatively uniform spatially and by focusing a very small (1.3-mm) light-spot on the photocathode. With an exit slit 0.12 A wide, the intensity observed in repeated scans, with counts of 1 or 2 minutes' duration at each wave length, has a non-statistical fluctuation averaging 0.6 per cent under good conditions; the fluctuation in the two channels separately amounts to about 8 per cent. The statistical accuracy is limited by the low quantum efficiency of the telescop e-sp ectroscope-photometer combination, with only one photon counted in every 5000-a loss of 9.3 mag. A more efficient image slicer, a more sensftive phototube, and more careful attention to reflection losses could yield a gain of 3-4 mag. In Per a search was made for the interstellar Al I lines at N 39 .0 A, with negative results. The equivalent width probably does not exceed 3 mA, about the value predicted by Field and Michie. Equivalent widths of the CH+ line at N 4232.6 were measured in nine stars, with mean errors between 0.8 and 6 mA. For late-type stars the results are in general agreement with the Bates-Spitzer theory of circumstellar molecules; for early-type stars the equivalent widths differ by a factor of 6 for stars of the same color excess, although unreddened stars show no line. Measures of interstellar D lines were made to resolve the systematic difference of 30 per cent between Mount Wilson and Victoria results. The resultant equivalent widths agree with the earlier Mount Wilson determinations to within 2 per cent. Publication: The Astrophysical Journal Pub Date: November 1959 DOI: 10.1086/146788 Bibcode: 1959ApJ...130..991R full text sources ADS | data products SIMBAD (14)