Laboratory astrophysics under the ultraviolet, visible, and gravitational astrophysics research program: Oscillator strengths for ultraviolet atomic transitions

The conditions within astrophysical environments can be derived from observational data on atomic and molecular lines. For instance, the density and temperature of the gas are obtained from relative populations among energy levels. Information on populations comes about only when the correspondence between line strength and abundance is well determined. The conversion from line strength to abundance involves knowledge of meanlives and oscillator strengths. For many ultraviolet atomic transitions, unfortunately, the necessary data are either relatively imprecise or not available. Because of the need for more and better atomic oscillator strengths, our program was initiated. Through beam-foil spectroscopy, meanlives of ultraviolet atomic transitions are studied. In this technique, a nearly isotopically pure ion beam of the desired element is accelerated. The beam passes through a thin carbon foil (2 mg/cu cm), where neutralization, ionization, and excitation take place. The dominant process depends on the energy of the beam. Upon exiting the foil, the decay of excited states is monitored via single-photon-counting techniques. The resulting decay curve yields a meanlife. The oscillator strength is easily obtained from the meanlife when no other decay channels are presented. When other channels are present, additional measurements or theoretical calculations are performed in order to extract an oscillator strength. During the past year, three atomic systems have been studied experimentally and/or theoretically; they are Ar, I, Cl I, and N II. The results for the first two are important for studies of interstellar space, while the work on N II bears on processes occurring in planetary atmospheres

Laboratory Astrophysics Under the Ultraviolet, Visible, and Gravitational Astrophysics Research Program

Space-borne facilities, such as the Hubble Space Telescope, the recent ORFEUS-SPAS II Shuttle mission, and the soon-to-be launched Far Ultraviolet Spectroscopic Explorer, are providing data at ultraviolet wavelengths of unprecedented quality for spectroscopic studies of many astronomical environments. The first step in the analysis of these data involves the derivation of abundances. Obtaining accurate abundances is possible only when the correspondence between line strength and abundance is well known. The conversion of line strength to abundance relies on knowledge of transition probabilities and oscillator strengths, often obtained from mean lives branching fractions. For many ultraviolet transitions, the necessary atomic and molecular data are either relatively imprecise or not available. Our program addresses this need for accurate oscillator strengths; our focus is on transitions that probe the nature and composition of the interstellar medium

THE TRANSITION FROM DIFFUSE ATOMIC GAS TO MOLECULAR CLOUD IN TAURUS

We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions in the direction to stars behind the Taurus Molecular Cloud. New high-resolution spectra at visible and near infrared wavelengths of interstellar K~{\small I}, CH, CH$^+$, C$_2$, CN, and CO toward HD~28975 and HD~29647 are combined with published results for HD~27778 and HD~30122. Gas densities and temperatures are inferred from analyses of C$_2$, CN, and CO excitation. Our results for HD~29647 are noteworthy in that the CO column density is 10$^{18}$ cm$^{-2}$, our analysis of CO and C$_2$ excitation reveal a temperature of 10 K and densities of about 1000 cm$^{-3}$, and the CO excitation and radiation temperatures are the same, more like emission-line studies of dark molecular clouds. Similar results arise from our chemical analysis leading to CN through reactions involving the observed species CH and C$_2$. The other directions are typical of molecule-rich diffuse clouds and can be considered CO-dark gas

Atomic and Molecular Data for Interstellar Studies: A Status Report

Most interstellar species have a large fraction of their electronic transitions at far ultraviolet wavelengths. Observations at these wavelengths reveal spectra rich in absorption lines seen against the continuum of a background source, such as a hot star in our Galaxy, a supernova in a nearby galaxy, or even a bright nucleus in an active galaxy. Most of the observations continue to be made with space-borne instruments, but recent work includes measurements of extragalactic material at large redshifts obtained at high resolution with large ground-based telescopes (e.g., the Keck Telescope). The combination of precise experimental oscillator strengths, large-scale computations, and astronomical spectra with high signal-to-noise ratios are yielding a set of self-consistent-values that span a range in strength in excess of 100 for more and more species. The large range is important for studies involving the different environments probed by the various background sources. This review highlights recent work on the atomic species. Si II, S I, and Fe II, and on the molecules, CO and C2

OSCILLATOR STRENGTHS AND PREDISSOCIATION RATES FOR W_X BANDS AND THE 4P5P COMPLEX IN 13C18O

In our ongoing experiments on the DESIRS beam-line at the SOLEIL Synchrotron, we are acquiring the necessary data on oscillator strengths and predissociation rates for modeling CO photochemistry in astronomical environments. A VUV Fourier Transform Spectrometer with a resolving power of about 350,000 allows us to discern individual lines in electronic transitions. Here we focus on results obtained from absorption spectra of $^{13}$C$^{18}$O, for the $W~^1Pi~-~X~^1Sigma^+$ bands with $v^{prime}=0, 2, {rm and} 3$ and $v^{primeprime}=0$ and three resolved bands involving transitions to the upper levels 4p$pi$(2), 5p$pi$(0), and 5p$sigma$(0) of the 4p(2) and 5p(0) complexes. We compare our results with earlier determinations for this isotopologue of CO, as well as with our SOLEIL measurements on $^{12}$C$^{16}$O, $^{13}$C$^{16}$O, and $^{12}$C$^{18}$O