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Using a high-resolution spectrum of the secondary star in the neutron star binary Cen X-4, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a kh2 minimization procedure to explore a grid of 1,500,000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni, and Al. These element abundances are supersolar ([Fe/H] = 0.23 +- 0.10), but only the abundances of Ti and Ni appear to be moderately enhanced ([?]1 s) as compared with the average values of stars of similar iron content. These element abundances can be explained if the secondary star captured a significant amount of matter ejected from a spherically symmetric supernova (SN) explosion of a 4 M He core progenitor and if we assume solar abundances as primordial abundances in the secondary star. The kinematic properties of the system indicate that the neutron star received a natal kick velocity through an aspherical SN and/or an asymmetric neutrino emission. The former scenario might be ruled out, since our model computations cannot produce acceptable fits to the observed abundances. We have also examined whether this system could have formed in the Galactic halo, and our simulations show that this possibility seems unlikely. We also report a new determination of the Li abundance, consistent with previous studies, that is unusually high and close to the cosmic Li abundance in the Galactic disk.

We have detected in high-resolution spectra of the quasar Q0347-3819 obtained with the UV-Visual Echelle Spectrograph (UVES) at the VLT/Kueyen telescope over 80 absorption features in the Lyman and Werner H2 bands at the redshift of a damped Lya (DLA) system at zabs = 3.025. At the same redshift, numerous absorption lines of atoms and low ions (H I, D I, C II, C II*, N I, O I, Si II, P II, Ar I, Cr II, Fe II, and Zn II) were identified. The zabs = 3.025 system spans over ~80 km s-1 and exhibits a multicomponent velocity structure in the metal lines. The main component at zabs = 3.024855 shows a total H2 column density N(H2) = (4.10 +- 0.21) x 1014 cm-2 and a fractional molecular abundance fH2 = (1.94 +- 0.10) x 10-6 derived from the H2 lines arising from the J = 0-5 rotational levels of the ground electronic vibrational state. In the second strong component at zabs = 3.024684 we were able to set a stringent upper limit to fH2 of 3.3 x 10-9 (3 s c.l.). The population of the J levels can be represented by a single excitation temperature of Tex = 825 +- 110 K. The ortho-H2 : para-H2 ratio equals 3.1 +- 0.3. This ratio is comparable to that calculated from hot H2 formation upon grain surfaces (ortho-H2 : para-H2 = 3 : 1) but is higher than the "freezeout" quantity (ortho-H2 : para-H2 = 1 : 4) predicted for cosmological epochs at z < 20, implying recent formation of the hydrogen molecules following photodissociation of pristine H2. In the main component the properties of the neutral atoms and low ions can be described with the same broadening parameter b [?] 3 km s-1 of the H2 cloud. The rate at which UV photons are absorbed in the Lyman and Werner bands is found to be b0 [?] 2 x 10-9 s-1. Taking into account that similar b0 values are observed in the Galactic diffuse molecular clouds, it shows that the UV radiation fields in the DLA and in the Milky Way are very much alike. We also found the H2 formation rate of RnH [?] 3 x 10-16 s-1 at zabs = 3.025, which is consistent with observations in the Galactic disk diffuse clouds where nH2 is low. Comparing the relative rotational population ratios of H2 with the measured C II*/C II ratio, we can infer that nH [?] 6 cm-3 in the main component, which gives a size of D [?] 14 pc along the line of sight. The mean value of the dust-to-gas ratio estimated for the whole zabs = 3.025 system from the [Cr/Zn] ratio is equal toimg2.gif = 0.032 +- 0.005 (in units of the mean Galactic interstellar medium value). For the first time we unambiguously reveal a pronounced [a-element/iron peak] enhancement of [O,Si/Zn] = 0.6 +- 0.1 (1 s c.l.) at high redshift. The simultaneous analysis of metal and hydrogen lines leads to D/H = (3.75 +- 0.25) x 10-5. This value is consistent with standard big bang nucleosynthesis if the baryon-to-photon ratio, e, lies within the range 4.37 x 10-10 [?] e [?] 5.32 x 10-10, implying 0.016 [?] Obhimg3.gif [?] 0.020.

Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derived from measurements of the oxygen triplet at 7771-5 A and OH lines in the near-UV performed in high-resolution and high signal-to-noise ratio spectra obtained with WHT/UES, Keck I/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE and then corrected for non-LTE (NLTE) effects. The new oxygen abundances confirm previous findings for a progressive linear rise in the oxygen-to-iron ratio with a slope -0.33 +- 0.02 from solar metallicity to [Fe/H] ~ -3. A slightly higher slope would be obtained if the Fe NLTE corrections were not considered. Below [Fe/H] = -2.5 our stars show [O/Fe] ratios as high as ~1.17 (G64-12), which can be interpreted as evidence for oxygen overproduction in the very early epoch of the formation of the halo, possibly associated with supernova events with very massive progenitor stars. We show that the arguments against this linear trend given by Fulbright & Kraft in 1999, based on the LTE Fe analysis of two metal-poor stars, cannot be sustained when an NLTE analysis is performed. We discuss how the Fulbright & Kraft LTE ionization balance of Fe lines underestimates the gravity of the very metal-poor star BD +23deg3130 ([Fe/H] = -2.43) and how this leads to an underestimation of the oxygen abundance derived from the forbidden line. Gravities from Hipparcos appear to be in good agreement with those determined in NLTE, giving higher values than previously assumed, which reduces the discrepancies between the oxygen abundances determined from OH, triplet, and forbidden lines. Using one-dimensional models, our analysis of three oxygen indicators available for BD +23deg3130 gives an average [O/Fe] ratio of 0.78img1.gif. The high oxygen abundances at very low metallicities do not pose a problem to theoretical modeling since there is a range of parameters in the calculations of nucleosynthesis yields from massive stars at low metallicities that can accommodate our results.

The UV resonance transitions of neutral argon, Ar I?1066, and of singly ionized phosphorus, P II?963, originated in the damped Ly? system (DLA) at zabs= 3.3901 toward QSO 0000-2620, have been detected by means of the Ultraviolet-Visual Echelle Spectrograph (UVES) spectrograph at the 8.2 m ESO KUEYEN telescope. So far, this is the first measurement of Ar Iand the second of P IIever performed in damped galaxies and in high-redshift objects. This DLA is well known for having one of the lowest metal abundances and dust content and the lowest fractional abundance of molecular hydrogen H2. The measured Ar abundance is [Ar/H] = -1.91 ± 0.09, which is equal to the abundances of the other ?-chain elements (O, S, and Si). The similarity of the Ar abundance with the other ?-chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sight line throughout the damped Ly? system. Both log(Ar/O) and log(Ar/S) ratios are found close to those measured in the extragalactic H IIregions and in blue compact galaxies where O is more abundant by at least 1 order of magnitude. This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The abundance of the nonrefractory element phosphorus [P/H] = -2.31 ± 0.10 confirms the low amount of chemical evolution in the DLA. This is the measurement of P in the most metal-poor material and shows a subsolar [P/Fe] = -0.27 value. The measured ratios [P/Si] = -0.40 ± 0.13 and [P/S] = -0.40 ± 0.13 provide evidence for a mild odd-even effect. Finally, a stringent upper limit to the population of the 3P1level in the ground state of O Iis derived, which provides a lower limit to the physical dimensions of the zabs= 3.3901 system of L> 7 pc.

Zinc is a good indicator of metallicity in damped Lya (DLA) systems because it is almost unaffected by dust depletion. However, the use of zinc as a tracer of metallicity evolution has been hampered by the difficulty of detecting the Zn II resonance lines at high redshift. The measurement of zinc abundance in a DLA system at zabs > 3 obtained by means of the UVES spectrograph at the VLT prompted us to reanalyze the full sample of zinc abundances present in the literature to search for a metallicity-redshift relation in DLA systems. A study of the metallicities of individual systems shows evidence for an anticorrelation between [Zn/H] and redshift, supported by different types of statistical tools. The zinc metallicity decreases by -0.3 +- 0.1 dex per unit redshift interval in the range 0.5 [?] zabs [?] 3.5. This rate is in good agreement with that found by Savaglio, Panagia, & Stiavelli in their recent study of DLA abundances corrected for dust depletion. The present result does not require a knowledge of the dust depletion pattern(s) in DLA systems. On the other hand, analysis of the column-density-weighted metallicity of the sample,img1.gif , does not show a clear evidence for redshift evolution, consistent with previous studies of zinc abundances. We propose that the apparent lack of evolution ofimg1.gif is due to the combination of selection bias effects together with the extreme sensitivity ofimg1.gif to low-number statistics.

Observations of the QSO 0000-2620 with the Ultraviolet-Visual Echelle Spectrograph at the 8.2 m ESO Kueyen telescope are used for abundance analysis of the damped Lya system at zabs = 3.3901. Several oxygen lines are identified in the Lya forest, and a measure for the oxygen abundance is obtained at [O/H] = -1.85 +- 0.15 by means of the unsaturated O I l925 and O I l950 lines. This represents the most accurate O measurement in a damped Lya galaxy so far. We have also detected Zn II l2026 and Cr II ll2056, 2062 redshifted at [?]8900 A and found abundances [Zn/H] = -2.07 +- 0.10 and [Cr/H] = -1.99 +- 0.09. Furthermore, previous measurements of Fe, Si, Ni, and N have been refined, yielding [Fe/H] = -2.04 +- 0.09, [Si/H] = -1.90 +- 0.08, [Ni/H] = -2.27 +- 0.09, and [N/H] = -2.68 +- 0.12. The abundance of the nonrefractory element zinc is the lowest among the damped Lya systems, showing that the associated intervening galaxy is indeed in the early stages of its chemical evolution. The fact that the Zn abundance is identical to that of the refractory elements Fe and Cr suggests that dust grains have not formed yet. In this damped Lya system the observed [O, S, Si/Zn, Fe, Cr] ratios, in whatever combination are taken, are close to solar (i.e., 0.1-0.2 dex) and do not show the [a-element/Fe] enhancement observed in Milky Way stars of comparable metallicity. The observed behavior supports a galaxy evolution model characterized by either episodic or low star formation rate rather than a Milky Way-type evolutionary model.

Relative elemental abundances, and in particular the ?/Fe ratio, are important diagnostic tools of the chemical evolution of damped Ly? (DLA) systems. The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the ?/Fe ratio. We report six new determinations of sulphur abundance in DLA systems at zabs? 2 with already-known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high-redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [?/Fe] enhancement observed in metal-poor stars of the Milky Way at a comparable level of metallicity ([Zn/H] ? -1). The behavior of DLA data is consistent with a general trend of a decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the ?/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is observed in our Galaxy, the ?/iron peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] ? -1) and to decrease below solar values at higher metallicities. The behavior of the ?/Fe ratio challenges the frequently adopted hypothesis that high-redshift DLA systems are progenitors of spiral galaxies and instead favors an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low-redshift DLA systems, where the candidate DLA galaxies show a variety of morphological types, including dwarf and low surface brightness galaxies and only a minority of spirals.

Nitrogen lines of the N I l1134 and l1200 multiplets in the damped Lya (DLA) galaxies at zabs = 2.309, 2.827, and 3.025 toward the QSOs 0100+1300, 1425+6039, and 0347-3819, respectively, have been detected by means of high-resolution spectra (R ~ 2 x 104) obtained with 4 m class telescopes at ESO (La Silla, Chile) and the Observatorio del Roque de los Muchachos (La Palma, Spain). The two N I multiplets offer a considerable range in oscillator strengths and the possibility of disentangling Lya interlopers. The derived nitrogen abundances for the three damped systems are [N/H] = -2.68 +- 0.11, -1.57 +- 0.09, -2.07 +- 0.13, respectively. The behavior of nitrogen relative to iron-peak and a elements has been investigated by considering all the extant N I determinations for a total of nine DLA galaxies. We have estimated the fraction of iron locked into dust grains to convert the observed [N/Fe] ratios into overall (dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/a] have been mostly determined by using sulphur as a tracer of a elements that is unaffected by dust. The [N/Fe] and [N/a] ratios show high dispersions, of 1 order of magnitude or more, which have no equivalent in other element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and [N/a] ratios are at variance with the values measured in Galactic halo stars of similar metallicity, suggesting that part of the DLA galaxies do not follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and their dispersion show some similarities with those observed in dwarf galaxies. Comparison with chemical evolution models shows that the lowest [N/Fe]corr and [N/a] DLA values are close to what would be expected for a pure secondary origin of nitrogen, whereas higher values are mostly consistent with a primary component. The behavior of nitrogen abundance ratios can be ascribed, in general, to the delayed release of nitrogen in the course of evolution. However it is difficult to conciliate this interpretation with the lowest [N/a] values measured, since an expected enhancement of a elements with respect to the iron-peak elements is not observed simultaneously in these DLA galaxies. In two cases, relatively high [N/a] values are observed that require also a more complex chemical evolution to be explained.