Miriam Centurion

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

The lithium content of the globular cluster NGC 6397

We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-o stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li)= 2:34 with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1-0.2 dex, as predicted by recent models, cannot be ruled out, there is no compelling reason for it. This fact, together with the good agreement with the Li abundance observed in field stars, supports the primordial origin of lithium in metal-poor stars. Taking the above value as the primordial lithium abundance implies a cosmic baryonic density which is eitherbh 2 = 0:016 0:004 orbh 2 = 0:005 +0:0026 0:0006 , from the predictions of standard big bang nucleosynthesis. The high baryonic density solution is in agreement with recent results on the primordial abundance of deuterium and 3 He and on the estimates derived from the fluctuations of the cosmic microwave background.

Ionization Properties and Elemental Abundances in Damped Lyα Systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Lyα systems (DLAs) from the analysis of the ratio R(Al+2/Al+) ≡ N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.

Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman α systems ,

We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly α absorbers (DLAs) obtained by means of high resolution (FWHM � 7k m s −1 ) UVES/VLT spectra. In addition to these measurements we have compiled data from all DLAs with measurements of nitrogen and α-capture elements (O, S or Si) available in the literature, including all HIRES/Keck and UVES/VLT data for a total of 32 systems, i.e. the largest sample investigated so far. We find that (N/α) ratios are distributed in two groups: 75% of the DLAs show a mean value of (N/α) = −0.87 with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at (N/α) = −1.45 with an even lower dispersion of 0.05 dex. The high (N/α) �− 0.9 plateau is consistent with the one observed in metal-poor H  regions of blue compact dwarf (BCD) galaxies ((N/α) = −0.73 ± 0.13), while the (N/α) �− 1.5 values are the lowest ever observed in any astrophysical site. These low (N/α) ratios are real and are not due to ionization effects. They provide crucial evidence against the primary production of N by massive stars as being responsible for the plateau at −0.9/−0.7 dex observed in DLAs and BCD galaxies. The transition between the low-N ((N/α) �− 1.5) and high-N ((N/α) �− 0.9) DLAs occurs at a nitrogen abundance of (N/H) �− 2.8, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The (N/α) �− 1.5 values and their low dispersion are consistent with a modest production of primary N in massive stars; however, due to the limited sample, specially for the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed. Nitrogen is an important element when attempting to under- stand the chemical evolution of galaxies. The synthesis of N in the CNO cycle during hydrogen burning is reasonably well understood, but the characteristics of the stars that produce this element (range of masses, stage of evolution, etc.) are not completely clear. N is mostly a secondary element, produced in the CNO cycle from seed C and O nuclei created in ear- lier generation of stars. Primary N is produced when freshly synthesized C in the helium-burning shell penetrates into the hydrogen-burning shell where it will be converted in primary N by means of the CNO cycle. In evolution models of intermedi- ate mass stars, thermal pulses occurring during the asymptotic giant branch AGB phase are responsible for the transport of

UVES Observations of QSO 0000–2620: Oxygen and Zinc Abundances in the Damped Lyα Galaxy at zabs = 3.3901*

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 Lyα system at zabs = 3.3901. Several oxygen lines are identified in the Lyα forest, and a measure for the oxygen abundance is obtained at [O/H] = -1.85 ± 0.15 by means of the unsaturated O I λ925 and O I λ950 lines. This represents the most accurate O measurement in a damped Lyα galaxy so far. We have also detected Zn II λ2026 and Cr II λλ2056, 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 Lyα 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 Lyα 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 [α-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.

Chemical Evolution of Damped Lyα Galaxies: The [S/Zn] Abundance Ratio at Redshift ≥2*

Relative elemental abundances, and in particular the α/Fe ratio, are important diagnostic tools of the chemical evolution of damped Lyα (DLA) systems. The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the α/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 [α/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 α/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is observed in our Galaxy, the α/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 α/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.

UVES Observations of QSO 0000–2620: Argon and Phosphorus Abundances in the Dust-free Damped Lyα System at zabs = 3.3901*

The UV resonance transitions of neutral argon, Ar I j1066, and of singly ionized phosphorus, P II j963, originated in the damped Lya system (DLA) at toward QSO 0000[2620, have been z abs \ 3.3901 detected by means of the Ultraviolet-Visual Echelle Spectrograph (UVES) spectrograph at the 8.2 m ESO KUEYEN telescope. So far, this is the —rst measurement of Ar I and the second of P II ever 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 The measured Ar abundance is [Ar/H] \[ 1.91 ^ 0.09, which is equal to the abundances of the H 2 . other a-chain elements (O, S, and Si). The similarity of the Ar abundance with the other a-chain elements implies the absence of signi—cant photoionization by either UV background or stellar sources along the sight line throughout the damped Lya system. Both log (Ar/O) and log (Ar/S) ratios are found close to those measured in the extragalactic H II regions 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 con—rms 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 eUect. Finally, a stringent upper limit to the population of the level in the ground state of O I is 3P 1 derived, which provides a lower limit to the physical dimensions of the system of L [ 7p c. z abs \ 3.3901

The UVES Large Program for testing fundamental physics I. Bounds on a change in alpha towards quasar HE 2217-2818

Context. Absorption-line systems detected in quasar spectra can be used to compare the value of the fine-structure constant, , measured today on Earth with its value in distant galaxies. In recent years, some evidence has emerged of small temporal and also spatial variations in on cosmological scales. These variations may reach a fractional level of 10 ppm (parts per million). Aims. To test these claims we are conducting a Large Program of observations with the Very Large Telescope’s Ultraviolet and Visual Echelle Spectrograph (UVES), and are obtaining high-resolution (R 60 000) and high signal-to-noise ratio (S=N 100) UVES spectra calibrated specifically for this purpose. Here we analyse the first complete quasar spectrum from this programme, that of HE 2217 2818. Methods. We applied the many multiplet method to measure in five absorption systems towards this quasar: zabs = 0:7866, 0.9424, 1.5558, 1.6279 , and 1.6919. Results. The most precise result is obtained for the absorber at zabs = 1:6919 where 3 Feii transitions and Alii 1670 have high S/N and provide a wide range of sensitivities to . The absorption profile is complex with several very narrow features, and it requires 32 velocity components to be fitted to the data. We also conducted a range of tests to estimate the systematic error budget. Our final result for the relative variation in in this system is = = +1:3 2:4stat 1:0sys ppm. This is one of the tightest current bounds on -variation from an individual absorber. A second, separate approach to the data reduction, calibration, and analysis of this system yielded a slightly di erent result of 3.8 2:1stat ppm, possibly suggesting a larger systematic error component than our tests indicated. This approach used an additional 3 Feii transitions, parts of which were masked due to contamination by telluric features. Restricting this analysis to the Feii transitions alone and using a modified absorption profile model gave a result that is consistent with the first approach, = = +1:1 2:6stat ppm. The four other absorbers have simpler absorption profiles, with fewer and broader features, and o er transitions with a narrower range of sensitivities to . They therefore provide looser bounds on = at the > 10 ppm precision level.

Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT

We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ∼ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C IV doublet absorption systems. Six objects were observed with resolutions �27 and 34 km s −1 in the visual, and 37.5 and 53.5 km s −1 in the near-infrared. We detected 102 C IV lines in the range: 4.35 <z< 6.2 of which 27 are above z ∼ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si IV doublets associated with the C IV at z 5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z 5.3, with respect to the normalization defined by low-redshift (1.5 <z <4) C IV lines. This behaviour is reflected in the redshift evolution of the C IV cosmic mass density, � C IV, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of ∼ 2f or z 5.3. Considering only the stronger C IV lines (13.8 < log N(C IV) < 15), � C IV gently rises by a factor of ∼10 between z � 6.2 and z � 1.5 with a possible flattening towards z ∼ 0. The increase is well fitted by a power law: � C IV = (2 ± 1) × 10 −8 [(1 + z)/4] −3.1±0.1 .A n insight into the properties of the C IV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C IV ,S iIV and C II absorptions with the output of a set of CLOUDY photoionization models. As already claimed by cosmological simulations, we find that C IV is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z ∼ 5−6 while at z ∼ 3 it arises in gas with overdensity δ ∼ 100.

VLT/UVES constraints on the cosmological variability of the fine-structure constant

We propose a new methodology for probing the cosmological variability of alpha from pairs of FeII lines (SIDAM, single ion differential alpha measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to (i) ionization inhomogeneities in the absorbers and (ii) non-zero offsets between different exposures. Applied to the FeII lines of the metal absorption line system at zabs = 1.839 in the spectrum of Q1101-264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides da/a = (2.4+/-3.8) 10^{-6}. The zabs = 1.15 FeII system toward HE0515-4414 has been re-analyzed by this method thus obtaining for the combined sample da/a = (0.4+/-1.5) 10^{-6}. These values are shifted with respect to the Keck/HIRES mean da/a = (-5.7+/-1.1) 10^{-6} (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the da/a measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of 10 times the UVES dispersion coupled to a 100m class telescope the present Oklo level (da/a>= 4.5 10^{-8}) can be achieved along cosmological distances with differential measurements of da/a.We propose a new methodology for probing the cosmological variability of α from pairs of Fe  lines (SIDAM, single ion differential α measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to (i) ionization inhomogeneities in the absorbers; and (ii) non-zero offsets between different exposures. Applied to the Fe  lines of the metal absorption line system at zabs = 1.839 in the spectrum of Q 1101-264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides ∆α/α = (2.4 ± 3.8stat) × 10 −6 .T hezabs = 1.15 Fe  system toward HE 0515-4414 has been re-analyzed by this method thus obtaining for the combined sample ∆α/α = (0.4 ± 1.5stat) × 10 −6 . These values are shifted with respect to the Keck/HIRES mean ∆α/α = (−5.7 ± 1.1stat) × 10 −6 (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the ∆α/α measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of ∼10 times the UVES dispersion coupled to a 100 m class telescope the present Oklo level (∆α/α ≥ 4.5 × 10 −8 )