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Register a new userAbundances of Deuterium, Oxygen, and Nitrogen in the Local Interstellar Medium: Overview of First Results from the Far Ultraviolet Spectroscopic Explorer Mission
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Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D I, O I, and N I along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 pc to 179 pc. Five of the sight lines are within the Local Bubble and two penetrate the surrounding H I wall. Reliable values of N(H I) were determined for five of the sight lines from HST data, IUE data, and published EUVE measurements. The weighted mean of D I/H I for these five sight lines is (1.52 +/- 0.08) x10-5 (1 sigma uncertainty in the mean). It is likely that the D I/H I ratio in the Local Bubble has a single value. The D I/O I ratio for the five sight lines within the Local Bubble is (3.76 +/- 0.20) x10-2. It is likely that the O I column densities can serve as a proxy for H I in the Local Bubble. The weighted mean for O I/H I for the seven FUSE sight lines is (3.03 +/-0.21) x10-4, comparable to the weighted mean (3.43 +/- 0.15) x10-4 reported for 13 sight lines probing larger distances and higher column densities (Meyer et al. 1998, Meyer 2001). The FUSE weighted mean of N I/H I for the five sight lines is half that reported by Meyer et al. (1997) for seven sight lines with larger distances and higher column densities. This result combined with the variability of O I/N I (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O I, N I cannot be used as a proxy for H I or as a metallicity indicator in the LISM. Subject Headings: cosmology: observations- ISM: abundances- ISM: evolution - Galaxy:abundances-Ultraviolet:ISM
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High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187A, were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total HI, DI, OI and NI column densities along the line of sight. Previous determinations of N(DI) based upon GHRS and STIS observations were controversial due to the saturated strength of the DI Lyman-alpha line. In the present analysis the column density of DI has been measured using only the unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are: log N(DI) = 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07 quoted with 2-sigma uncertainties. The measurement of the HI column density by profile fitting of the Lyman-alpha line has been found to be unsecure. If additional weak hot interstellar components are added to the three detected clouds along the line of sight, the HI column density can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Lyman-alpha are excellent. The new estimate of N(HI) toward G191-B2B reads: log N(HI) = 18.18 +/-0.18 (2-sigma uncertainty), so that the average (D/H) ratio on the line of sight is: (D/H) = 1.66 (+0.9/-0.6) *10^-5 (2-sigma uncertainty).
We present a deuterium abundance analysis of the line of sight toward the white dwarf WD2211-495 observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Numerous interstellar lines are detected on the continuum of the stellar spectrum. A thorough analysis was performed through the simultaneous fit of interstellar absorption lines detected in the four FUSE channels of multiple observations with different slits. We excluded all saturated lines in order to reduce possible systematic errors on the column density measurements. We report the determination of the average interstellar D/O and D/N ratios along this line of sight at the 95% confidence level: D/O = 4.0 +/-1.2 *10^-2; D/N = 4.4 +/-1.3 *10^-1. In conjunction with FUSE observations of other nearby sight lines, the results of this study will allow a deeper understanding of the present-day abundance of deuterium in the local interstellar medium and its evolution with time.
We present an analysis of interstellar absorption along the line of sight to the nearby white dwarf star HZ43A. The distance to this star is 68+/-13 pc, and the line of sight extends toward the north Galactic pole. Column densities of OI, NI, and NII were derived from spectra obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE), the column density of DI was derived from a combination of our FUSE spectra and an archival HST GHRS spectrum, and the column density of HI was derived from a combination of the GHRS spectrum and values derived from EUVE data obtained from the literature. We find the following abundance ratios (with 2-sigma uncertainties): DI/HI = (1.66 +/- 0.28) x 10^-5, OI/HI = (3.63 +/- 0.84) x 10^-4, and NI/HI = (3.80 +/- 0.74) x 10^-5. The NII column density was slightly greater than that of NI, indicating that ionization corrections are important when deriving nitrogen abundances. Other interstellar species detected along the line of sight were CII, CIII, OVI, SiII, ArI, MgII, and FeII; an upper limit was determined for NIII. No elements other than HI were detected in the stellar photosphere.
We report results from a FUSE survey of interstellar molecular hydrogen (H2) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes $|b| < 10^{circ}$, with updated photometric and parallax distances. The H2 absorption is measured using the far-ultraviolet Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels $J = 0$ and $J = 1$ and excited states up to $J = 5$ (sometimes $J = 6$ and 7). For each sight line, we report column densities $N_{H2}$, $N_{HI}$, $N(J)$, $N_H = N_{HI} + 2N_{H2}$, and molecular fraction, $f_{H2} = 2N_{H2}/N_H$. Our survey extends the 1977 Copernicus H2 survey up to $N_H sim 5times10^{21}$ cm$^{-2}$. The lowest rotational states have mean excitation temperatures and rms dispersions, $T_{01} = 88pm 20$ K and $T_{02} = 77pm18$ K, suggesting that J = 0,1,2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures, $T_{24} = 237pm91$ K and $T_{35} = 304pm108$ K, produced primarily by UV radiative pumping. Correlations of $f_{H2}$ with E(B-V) and N_H show a transition to $f_{H2} geq 0.1$ at $N_ H geq 10^{21}$ cm$^{-2}$ and $E(B-V) > 0.2$, interpreted with an analytic model of H2 formation-dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H2 in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values $langle f_{H2} rangle geq 0.20$ and $langle N_H/E(B-V) rangle = (6.07pm1.01)times10^{21}$ cm$^{-2}$ mag$^{-1}$.
The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a large sample (> 100) of active galactic nuclei in the low-redshift universe (z < 1). Its response at short wavelengths makes it possible to measure directly the far ultraviolet spectral properties of quasistellar objects (QSOs) and Seyfert 1 galaxies at z < 0.3. Using archival FUSE spectra, we form a composite extreme ultraviolet (EUV) spectrum of QSOs at z < 0.67. After consideration of many possible sources of systematic error in our analysis, we find that the spectral slope of the FUSE composite spectrum, alpha= -0.56^+0.38_-0.28 for F_ u propto u^alpha, is significantly harder than the EUV (lambda lesssim 1200 A) portion of the composite spectrum of QSOs with z > 0.33 formed from archival Hubble Space Telescope spectra, alpha=-1.76 pm 0.12. We identify several prominent emission lines in the fuse composite and find that the high-ionization O VI and Ne VIII emission lines are enhanced relative to the HST composite. Power law continuum fits to the individual FUSE AGN spectra reveal a correlation between EUV spectral slope and AGN luminosity in the FUSE and FUSE + HST samples in the sense that lower luminosity AGNs show harder spectral slopes. We find an anticorrelation between the hardness of the EUV spectral slope and AGN black hole mass, using estimates of this quantity found in the literature. We interpret these results in the context of the well-known anticorrelation between AGN luminosity and emission line strength, the Baldwin effect, given that the median luminosity of the FUSE AGN sample is an order of magnitude lower than that of the HST sample.
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