A new method, based on simulated annealing technique and aimed at the inverse problem in the analysis of intergalactic or interstellar complex spectra of hydrogen and metal lines, is outlined. We consider the process of line formation in clumpy stochastic media accounting for fluctuating velocity and density fields self-consistently. Two examples of the analysis of ‘H+D’-like absorptions seen at za = 3.514 and 3.378 towards APM 08279+5255 are presented.

Accurate measurements of the D/H ratio in our Galaxy provide critical tests of Galactic chemical evolution and constrain the primordial value of D/H. Very high quality ultraviolet spectra from the GHRS and STIS instruments on HST have been analyzed for lines of sight toward both early and late-type stars and hot white dwarfs. We will summarize the results that are being obtained for D/H along sightlines through the Local Interstellar Cloud (LIC) and other nearby warm clouds. All sightlines through the LIC are consistent with D/H = (1.53 ± 0.18) × 10−5. Whether or not significantly different values of D/H are present in other clouds within 100 pc of the Sun is not yet settled, but there is evidence that D/H is significantly lower in Orion (500 pc). We will describe the likely sources of systematic errors in determining D/H that must be understood and quantified when analyzing such ultraviolet spectra.

Galactic D/H evaluations from observations completed in the far UV with first the Copernicus satellite then followed by IUE and the GHRS on the HST studies already suggest that D/H variations may be present in the interstellar medium. More recent IMAPS (the Interstellar Medium Absorption Profile Spectrograph) observations confirm that conclusion while new STIS (which replaced the GHRS on board HST) studies seem to indicate the contrary. The situation is discussed here.

Hopefully FUSE (the Far Ultraviolet Spectroscopic Explorer, launched the 24th of June 1999) will give access to the D/H ratio in many different galactic sites. This will help us reach a better global view of the evolution of that key element, and thus better constrain any evaluation of its primordial abundance.

Space Telescope Imaging Spectrograph (STIS) observations of the D/H ratio in the two velocity components towards G191-B2B are consistent with 1.5 ± 0.1 × 10−5 and do not agree with the values derived using the Goddard High Resolution Spectrograph (GHRS) data. We present some new work on the G191-B2B sightline, and the results we obtain are consistent with those of Sahu et al. (1999).

We confirm that deuterium exists in the Galactic Center (GC) and estimate that D/H = 3 × 10−6 using a new 5192-chemical reaction model. This is the lowest D/H ratio observed in the Galaxy, five times lower than the local ISM D/H = 1.5 × 10−5 but 106 × larger than D/H predicted by GC models. We detected DCN in the GC Sgr A 50 km/s molecular cloud located 10 pc from the GC with the NRAO 12m telescope and obtained T*R = 0.061 ± 0.007 K and 0.04 ± 0.02 K for the J = 1-0 and 2-1 lines. The most likely source of the GC D is continuous injection from the infall of primordial matter with D/H = 5 × 10−5 with the D/H determined by astration and mixing. Thus there are no significant Galactic sources of D and no recent quasar or AGN activity in the GC. This primordial D/H implies that the baryon density is less than the density necessary to close the Universe; most of the baryons are in dark matter; and there are fewer than four ν families.

Blue compact galaxies (BCG) are ideal objects in which to derive the primordial 4He abundance because they are chemically young and have not had a significant stellar He contribution. We discuss a self-consistent method which makes use of all the brightest He I emission lines in the optical range and solves consistently for the electron density of the He II zone. We pay particular attention to electron collision and radiative transfer as well as underlying stellar absorption effects which may make the He I emission lines deviate from their recombination values. Using a large homogeneous sample of 45 low-metallicity H II regions in BCGs, and extrapolating the Y-O/H and Y-N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.2443±0.0015, in excellent agreement with the weighted mean value Yp = 0.2452±0.0015 obtained from the detailed analysis of the two most metal-deficient BCGs known, I Zw 18 and SBS 0335-052. The derived slope dY/dZ = 2.4±1.0 is in agreement with the value derived for the Milky Way and with simple chemical evolution models with homogeneous outflows. Adopting Yp = 0.2452±0.0015 leads to a baryon-to-photon ratio of (4.7+1.0-0.8) × 10−10 and to a baryon mass fraction in the Universe ωbh250 = 0-068+0.015-0.012, consistent with the value derived from the primordial D abundance of Burles &Tytler (1998).

When calculating the helium (4He) abundance in low metallicity H ii regions, the ionization correction factor (icf) for unseen neutral helium (and hydrogen) is usually assumed to be unity. In this paper, we explore this factor for H ii regions ionized by young stellar clusters. Our main result is that icf < 1 for homogeneous H ii regions and, that the effect of density condensations in the H ii regions is to further reduce the icf. For icf < 1, the primordial helium abundance inferred from observations of low-metallicity, extragalactic H ii regions is decreased.

A new determination of the pregalactic helium abundance based on the Magellanic Clouds H II regions is discussed. This determination amounts to Yp = 0.2345 ± 0.0030 and is compared with those derived from giant extragalactic H II regions in systems with extremely low heavy elements content. It is suggested that the higher primordial value derived by other authors from giant H II region complexes could be due to two systematic effects: the presence of neutral hydrogen inside the helium Strömgren sphere and the presence of temperature variations inside the observed volume.

Two aspects of the chemical evolution of 4He in the Galaxy are considered on the basis of a sample of disk planetary nebulae by the application of corrections due to the contamination of 4He from the progenitor stars. First, the He/H radial gradient is analyzed, and then, the helium to heavy element enrichment ratio is determined for metallicities up to the solar value.

We summarize the past 17 years of our efforts to determine the cosmic abundance of the 3He isotope. The vast majority of our 3He+ observations were made with the NRAO 140 Foot telescope in Green Bank, WV. The 140 Foot ceased operations in July 1999 so that NRAO could prepare to commission its replacement, the Green Bank Telescope (GBT). Our 3He experiment was the last scientific program at the 140 Foot. It is thus poignant and timely for us to reassess the astrophysical context of our 3He results. Here we argue that the existence of “The 3He Plateau” for our sample of simple sources and recent advances in the understanding of the evolution of solar analog stars together suggest that we can finally estimate the primordial abundance of 3He. Our current best estimate for the primordial abundance is 3He/H = (1.5 ±0.6) × 10−5 (s.e.) by number.

New measurements of the isotopic composition of helium in the solar wind obtained from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses are presented and compared with earlier SWICS results and previous mass spectrometric determinations of this ratio with the Apollo Solar Wind Composition (SWC) experiment and the Ion Composition Instrument (ICI) on the International Sun Earth Explorer 3 (ISEE 3). The new SWICS data from both the fast and slow solar wind are extrapolated to the photosphere to obtain a representative value of the present-day ratio of 3He/4He = (3.75 ± 0.27) × 10−4 in the Outer Convective Zone (OCZ) of the Sun. After corrections of this ratio for secular changes caused by diffusion, mixing and 3He production by incomplete H-burning (Vauclair 1998), we obtain (D + 3He)/H = (3.6 ± 0.38) × 10−5 for the Protosolar Cloud (PSC). Adopting the Jovian 3He/4He ratio = (1.66 ±0.05) × 10−4 measured by the Galileo Probe mass spectrometer (Mahaffy et al. 1998) as representative for the PSC, we obtain (D/H)protosolar = (1.94 ±0.39) × 10−5. Using results of galactic evolution studies (Tosi 1998, 2000) and the D and 3He abundances in the Protosolar Cloud and the Local Interstellar Cloud (Linsky 1998; Gloeckler &Geiss 1998), we estimate (D/H)primordial = (2.4 − 4.2) × 10−5. This range corresponds to a universal baryon/photon ratio of (5.9−4.2) × 10−10.

In this work we report new high quality spectroscopic data for a sample of PNe in the SMC, aiming to derive physical parameters and chemical abundances, in particular to settle the question concerning the oxygen discrepancy found for type I planetaries with respect to stars and HII regions.

We present observations of the binary Sirius A / Sirius B performed with HST-GHRS. Two interstellar clouds are detected on this sightline, one of them being identified as the Local Interstellar Cloud (LIC). Lyman α interstellar lines are also observed, but whereas the deuterium Lyman α line is well detected in the LIC with an abundance in agreement with that obtained by Linsky et al. (1995), no significant D i line is detected in the second cloud. The deuterium abundance which we measured in this cloud is 0 < (D/H)ism < 1-6 × 10−5. Despite the large error bar, this sightline appears consequently as a good candidate for a low (D/H)ism.

We report on the detection and first identification of the deuterium Balmer lines dα and dβ, observed in emission in the Orion Nebula (M 42). The excitation mechanism is UV fluorescence from the Lyman(D i) lines at the interface between the H ii region and the molecular cloud. These lines may open the possibility to measure D/H in galactic H ii regions and, e.g., low-metallicity extragalactic H ii regions, using optical spectroscopy. Fluorescence provides an extremely sensitive way to detect deuterium. Thus, the non-detection of dα and dβ in the planetary nebula NGC 6572 leads to the stringent upper limit (D/H)ngc 6572 less than the order of 1 × 10−7.

We present far-UV spectra (910-1190Å) of Sk 80, an 07 supergiant in the Small Magellanic Cloud, observed by the FUSE satellite during its In-Orbit Checkout phase. The spectra reveal many interstellar absorption lines, including H2, and O VI, and several key stellar wind features.

Observations with IMAPS demonstrate that interstellar atomic D/H abundance ratios differ by a factor of 3 on the sightlines toward Δ Ori A, ζ Pup, and γ2 Vel, early-type stars within ˜ 500 pc of the Sun. The observed D/H differences are not inversely correlated with N/H, as would be expected if injection of CNO-processed material was the primary mechanism for the D/H abundance variations in the ISM.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched by a Delta II rocket on 24 June 1999 into a ˜768 km, 25° orbit. Following the In-Orbit Checkout phase, FUSE is obtaining high-resoluti on spectra (λ/Δλ ≳ 20,000) in the spectral range 905 −1187 Å. The sensitivity meets or exceeds the pre-launch predictions. Many QSOs and AGNs have already been observed.

A highly homogeneous study of 23 halo field dwarf stars has achieved a Li abundance accuracy of 0.033 dex per star. The work shows that the intrinsic spread of the Li abundances of these stars at a given metallicity is < 0.02 dex, and consistent with zero. That is, the Spite Li plateau for halo field dwarfs is incredibly thin. The thinness rules out depletion by more than 0.1 dex by a rotational-induced extra-mixing mechanism. Despite the thinness of the plateau, an increase of Li with [Fe/H] is seen, interpreted as evidence of Galactic chemical evolution (GCE) of Li, primarily due to Galactic cosmic ray (GCR) spallation reactions in the era of halo formation. The rate of Li evolution is concordant with: (1) observations of spallative 6Li in halo dwarfs; (2) GCE models; and (3) data on Li in higher metallicity halo stars. New data have also revealed four new ultra-Li-deficient halo dwarfs, doubling the number known. Based on their propensity to cluster at the halo main sequence turnoff and also to exist redward of the turnoff, we hypothesise that they are the products of binary mergers that ultimately will become blue stragglers. We explain their low Li abundances by normal pre-main-sequence (and possibly main-sequence) destruction in the low mass stars prior to their merging. If this explanation is correct, then such stars need no longer be considered an embarrassment to the existence of negligible Li destruction in the majority of field halo dwarfs.

Methods and accuracies of determining the lithium isotope ratio in stellar atmospheres from high resolution observations of the profile of the λ6708 Li i doublet are discussed, and recent results for metal-poor disk stars and halo stars are reviewed. The data are compared to models for the Galactic evolution of the 6Li abundance. It is concluded that a much larger data set is needed to understand properly the formation of 6Li in the interstellar medium as well as the depletion in stars.

The study of Li in Pop I stars has focussed in the last years on observations of open clusters, spanning a large range of ages and metallicities. So far the observational picture is quite complex: the data indicate several phenomena which are not well understood, from the 'Li dip', to the scatter in Li abundances found among (otherwise similar) stars both in young and old clusters. Models fail in reproducing most of the observed features; in particular the almost total lack of dependence of Li depletion with metallicity. The comparison between clusters of different ages show that no PMS depletion occurs, and that for a large fraction of old star no additional depletion occurs after the first ˜1.5 Gyrs, while the depletion is strong at earlier ages. These observations are crucial for our understanding of the relationship between the Pop II Li Plateau and the Li primordial abundance.