Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of

The ultraviolet spectrum (1145-1720 Å) of the distant quasar Q 0302-003 (z = 3.286) was observed at 1.8 Å resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson He II absorption trough, produced by He II Lyα along the line of sight over the redshift range z = 2.78 − 3.28. Its interpretation was facilitated by modeling

We obtained high-resolution FUSE (R ∼ 20,000) and VLT (R ∼ 45,000) spectra of the quasar HE2347-4342 to study the properties of the intergalactic medium between redshifts z = 2.0 − 2.9. The high-quality optical spectrum allows us to identify approximately 850 H I absorption lines with column densities between N ∼ 5 × 10 11 and 10 18 cm −2 . The reprocessed FUSE spectrum extends the wavelength coverage of the He II absorption down to an observed wavelength -2of 920Å. Source flux is detected to restframe wavelengths as short as ∼ 237Å. Approximately 1400 He II absorption lines are identified, including 917 He II Lyα systems and some of their He II Lyβ, Lyγ, and Lyδ counterparts. The ionization structure of He II is complex, with approximately 90 absorption lines that are not detected in the hydrogen spectrum. These features may represent the effect of soft ionizing sources. The ratio η=N(He II)/N(H I) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indices of quasars. This provides evidence that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured AGN, which do not ionize helium. We find an evolution in η toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z > 2.7, the large but finite increase in the He II opacity, τ = 5 ± 1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He + ions. For line widths b He + = ξb H , we find ξ = 0.95 ± 0.12, indicating a velocity field in the intergalactic medium dominated by turbulence. At hydrogen column densities N < 3 × 10 12 cm −2 the number of forest lines shows a significant deficit relative to a power law, and becomes negligible below N = 10 11 cm −2 .

Artículo de publicación ISI.We present 13 spectra and 31 photometric observations covering the first 150 days of SN 1991bg in NGC 4374 (M 84). Although SN 1991bg was a type Ia supernova displaying the characteristic Si II absorption at 6150 angstrom near maximum and the Fe emission lines at late phases, it varied from the well-defined norm for SNe Ia in several important respects. The peculiarities include faster declines in the B and V light curves after maximum, a distinct color evolution, a very red B-V color near maximum, relatively faint peak luminosity, a distinct spectral evolution, and a short peak phase. The narrow peak of the luminosity and the rapid declines of the light curves suggest a smaller mass in the ejecta and larger energy losses than for most SNe Ia. The unusually red color at maximum is not a result of normal extinction, since SN 1991bg was as blue as other SNe Ia at late times and no narrow interstellar lines are observed in the spectra. The faint absolute magnitude of SN 1991bg is established beyond doubt by comparison with SN 1957B, another type Ia supernova in the same galaxy, which was approximately 2.5 magnitudes brighter than SN 1991bg. The spectral evolution reveals minor differences near maximum compared to other well-observed SNe Ia, mainly in relative line strengths. At later phases several wavelength regions display discrepancies when compared to spectra of normal SNe Ia. Although other SNe Ia, such as SN 1986G and SN 1939B, have light curves with fast decline rates, SN 1991bg is unique, with deviations in both light curves and spectra. In particular SN 1991bg is the only SN Ia observed to date with a distinct spectrum at approximately 40 days past maximum. Although SN 1991bg is an extreme case, with unusual photometric and spectroscopic properties, we believe it can be understood in the context of exploding white dwarf models, and is properly grouped with type Ia. SN 1991bg demonstrates the need for detailed observations of SNe Ia as part of their use as standard candles for cosmology. While there is a well-defined prototype with homogeneous properties, unusual cases like SN 1991bg must be identified and separated to avoid misleading results