Ultraviolet Spectra Of Stars Research Articles

Superheavy elements in the atmospheres of magnetic stars

The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly.

Fluorescence lines in ultraviolet spectra of stars

We discuss the presence of fluorescence lines that appear in stellar spectra in the satellite ultraviolet region between 1200 and 3200 Å. Strong lines in partial multiplets result from a selective excitation, which in most cases is due to accidental wavelength coincidences. The prototype of photoexcitation by accidental resonance (PAR) is the classical Bowen mechanism. Photoexcitation by continuum radiation (PCR) is another source of selective population.Besides a brief review of ultraviolet fluorescence lines identified in spectra of symbiotic stars and cool stars we discuss in detail two emission peaks at 2506–2508 Å. We give three different explanations for the appearance of this feature in IUE spectra of three different objects: RR Tel, η Car and KQ Pup. In connection to these lines we also discuss primary and secondary cascades in the near-infrared observed in high-resolution spectra. Comments are made on the strong Fe II line at 9997 Å.

Absolute monochromatic fluxes in the ultraviolet spectra of stars according to the data of orion-2

Absolute monochromatic fluxes in the ultraviolet spectra of stars according to the data of orion-2

Mission Design Implications of an Inclined Elliptical Geosynchronous Orbit (International Ultraviolet Explorer)

Mission Design Implications of an Inclined Elliptical Geosynchronous Orbit (International Ultraviolet Explorer)

A discussion of the theory for interpreting ultraviolet stellar spectra

The observed spectral distribution from a star will, in principle, give information about the parts of the atmosphere from which the radiation comes. Most detailed theoretical studies assume that the atmosphere may be represented by plane parallel layers, but in the case of very strong absorption lines and weak continuous absorption, a case which is encountered in the ultraviolet spectral region of B stars, the radiation received from the centers of the lines may come from an entirely different part of the star than does that from the far wings of the lines or from the continuous spectrum. In the case of strong lines, because stars are three dimensional objects and because a field of motion may occur in the outermost part of the atmosphere, the appearance of the ultraviolet line spectrum may be poorly predicted using the hypothesis of stationary plane parallel layers.Some numerical examples are presented demonstrating that with the UV resonance lines the opacity in the centre of a line may exceed the continuous opacity by a factor 106–108. A summary is given of the chief factors which should be taken into account in any theory of line formation when the hypothesis of LTE is not valid. A detailed description of the interactions between radiation and matter which can occur is necessary. Some examples of the distribution in energy of the lower energy levels of the ion are presented for typical ions of interest. This distribution and the presence of metastable levels are important factors in determining the significance of non-LTE physics. The paper concludes with remarks indicating that ultraviolet spectra of stars should be obtained with a spectral purity of at east 0.1 Å if progress is to be made in understanding the physical state of stars.

Is Interstellar Hydrogen capable of Maser Action at 21 Centimetres?

THREE sets of observations suggest that the density of interstellar neutral hydrogen may be less than the value usually inferred from 21 cm emission line measurements. This possible discrepancy is found in observations of (a) the incoming high velocity neutral hydrogen clouds which, according to Oort1, deposit material on to the galactic disk at an alarmingly large rate; (b) the ultraviolet spectra of stars in Orion which show an unexpectedly weak interstellar Lyman-α absorption line2,3; and (c) the X-ray background at ∼ 50 A which suggests that the opacity of the Galaxy is less than would be estimated from the observed column density of neutral hydrogen4.