arXiv subjects

Paolo Molaro

Publications and source records attributed to Paolo Molaro.

68 records · Page 4Linked to original sources

arXiv2000

First UVES observations of beryllium in very metal-poor stars

We report on a attempt to detect beryllium in two dwarf stars of the Galactic halo with metallicities below one thousandth solar. The data were obtained during the Commissioning of the Ultraviolet and Visible Echelle Spectrograph (UVES) mounted on the ESO VLT Kueyen telescope, and show the potential of UVES for studies in the UV-optical domain. We claim a beryllium detection in LP 815--43 ([Fe/H]~-2.95,[Be/H]=-13.09) at the 99.7% confidence level, while only an upper limit can be set for the second target CD -24 17504 ([Fe/H]~-3.30, [Be/H]~-13.39). These results suggest that the trend of beryllium with metallicity keeps decreasing as lower metallicities are probed, with no evidence for flattening. In CD -24 17504 we also analyzed the Li I line at 6708A, and derived a lithium abundance close to the Spite plateau.

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arXiv2000

Intrinsic colour calibration for F, G, K stars

We derive an intrinsic colour calibration for F--K stars using broad band Johnson colours and line indices KP and HP2. Through this calibration we can determine E(B-V) of an individual star within 0.03 mag. The E(B-V) values thus derived are in excellent agreement with those derived from Stromgren photometry through the Schuster & Nissen (1989) calibration. The agreement is also good with the reddening maps of Burstein & Heiles (1982) and Schlegel et al (1998), although in this case there exists a small offset of about 0.01 mag. This calibration may be applied to the large body of data of the HK survey extension which will be published in the near future.

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arXiv2000

Zinc as a tracer of metallicity evolution of Damped Lyman alpha systems

Zinc is a good indicator of metallicity in Damped Lyman alpha (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 z_abs > 3 obtained by means of the UVES spectrograph at the VLT prompted us to re-analyse the full sample of zinc abundances present in the literature to search for a metallicity-redshift relation in DLA systems. The study of the metallicities of individual systems shows evidence for an anti-correlation 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 <~ z_abs <~ 3.5. This rate is in good agreement with that found by Savaglio, Panagia & Stiavelli (2000) 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, the analysis of the column-density weighted metallicity of the sample, , does not show a clear evidence for redshift evolution, consistent with previous studies of zinc abundances. We propose that the apparent lack of evolution of is due to the combination of selection bias effects together with the extreme sensitivity of to low-number statistics.

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arXiv2000

First results of UVES at VLT: abundancesin the Sgr dSph

Two giants of the Sagittarius dwarf spheroidal have been observed with the UVES spectrograph on the ESO 8.2m Kueyen telescope, during the commissioning of the instrument. Sgr 139 has [Fe/H]=-0.28 and Sgr 143 [Fe/H]=-0.21, these values are considerably higher than photometric estimates of the metallicity of the main population of Sgr. We derived abundances for O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Y, Ba, La, Ce, Nd and Eu; the abundance ratios found are essentially solar with a few exceptions: Na shows a strong overdeficiency, the heavy elements Ba to Eu, are overabundant, while Y is underabundat. The high metallicity derived implies that the Sgr galaxy has experienced a high level of chemical processing. The stars had been selected to be representative of the two main stellar populations of Sagittarius, however, contrary to what expected from the photometry, the two stars show a very similar chemical composition. We argue that the most likely explanation for the difference in the photometry of the two stars is a different distance, Sgr 143 being about 2Kpc nearer than Sgr 139. This result suggests that the interpretation of colour -- magnitude diagrams of Sgr is more complex than previously thought and the effect of the line of sight depth should not be neglected. It also shows that spectroscopic abundances are required for a correct interpretation of Sgr populations.

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arXiv2000

UVES observations of QSO 0000-2620: oxygen and zinc abundances in the Damped Ly-alpha galaxy at z_abs=3.3901

Observations of the QSO 0000-2620 with UVES spectrograph at the 8.2m ESO KUEYEN telescope are used for abundance analysis of the damped Ly-alpha system at z_{abs}=3.3901. Several Oxygen lines are identified in the Ly_alpha forest and a measure for the oxygen abundance is obtained at [O/H]=-1.85 +/- 0.1 by means of the unsaturated OI 925 A and OI 950 A lines. This represents the most accurate O measurement in a damped Ly_alpha galaxy so far. We have also detected ZnII 2026 A and CrII 2056, 2062 A redshifted at about 8900 A and found abundances [Zn/H] = -2.07 +/- 0.1 and [Cr/H]=-1.99 +/- 0.1. Furthermore, previous measurements of Fe, Si, Ni and N have been refined yielding [Fe/H]=-2.04 +/- 0.1, [Si/H]=-1.90 +/- 0.1, [Ni/H]=-2.27 +/- 0.1, and [N/H]=-2.68 +/- 0.1. The abundance of the non-refractory element zinc is the lowest among the damped Ly-alpha 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 Ly-alpha 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 [alpha-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.

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arXiv2000

Chemical Evolution of Damped Ly alpha galaxies: The [S/Zn] abundance ratio at redshift z > 2

Relative elemental abundances, and in particular the alpha/Fe ratio, are an important diagnostic tool of the chemical evolution of damped Ly alpha systems (DLAs). The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the alpha/Fe ratio. We report 6 new determinations of sulphur abundance in DLAs at zabs greater than or equal to 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 [alpha/Fe] enhancement observed in metal-poor stars of the Milky Way at comparable level of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the alpha/Fe ratio in the course of chemical evolution of DLA systems. However, in contrast to what observed in our Galaxy, the alpha/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 behaviour of the alpha/Fe ratio challenges the frequently adopted hypothesis that high redshift DLAs are progenitors of spiral galaxies and favours instead an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low redshift DLAs, where the candidate DLA galaxies show a variety of morphological types including dwarfs and LSBs and only a minority of spirals.

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arXiv1999

Low deuterium abundance in the zabs=3.514 absorber towards APM 08279+5255

A high-resolution, high signal-to-noise HIRES-Keck spectrum of APM 08279+5255 reveals a feature in the HI Ly alpha profile of the zabs=3.514 absorber at the expected position of the corresponding DI Lyalpha line. The absorber shows a relatively simple velocity structure, with a major component detected in several metallic lines including CII, CIV, SiII, SiIII and Si IV. Modeling of the hydrogen column density with the minimum number of components yields D/H ~ 1.5 x 10E-5. However, a more complex structure for the hydrogen cloud with somewhat ad hoc components would allow a higher D/H. The system has a very low metallicity with [Si/H] ~ -3.5 and [C/H] ~ -4.0 and is therefore representative of essentially unprocessed material, with a deuterium abundance close to the primordial value. The present analysis favours the low value of D/H =3.39(+/-0.25) x 10E-5 measured towards QSO 1009+2956 and QSO 1937-1009 (Burles & Tytler 1998a, 1998b) as the fiducial value of deuterium at high redshift.

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arXiv1998

Detection of the Li I 6104 Angstroem transition in the Population II star HD 140283

Lithium is one of the few primordially produced elements. The value of the primordial Li is taken to be that observed in metal--poor dwarfs, where it is not contaminated by stellar Li sources which act on longer time scales. The atmospheric abundance is currently derived from the LiI 6707 Angstroem resonance transition and the validity of the models employed has been questioned (Kurucz 1995). In this letter we report the first detection of the Li I 6104 Angstroem 2^2P - 3^2D subordinate transition in the prototype population II star HD 140283. The same Li abundance of (Li/H) =1.4x10^{-10} is found consistent with both the resonance and subordinate lines. The two lines form at different depths in the atmosphere implying that the 1-D homogeneous atmospheric models used in the abundance determination are essentially correct. When coupled with the standard big bang yields, the Li in the halo dwarfs provides two solutions for the baryon-to-photon ratio η_{10}= n_{b}/n_γ x 10^{10}and for the present baryon density Ω_b h_{70}^2=0.0748η_{10}: a) a first solution at η_{10} about 1.8, consistent with the η_{10} implied by the high deuterium values D/H\approx 2 x 10^{-4}observed in some quasar absorption systems (Webb et al 1997) and b) a second solution at η_{10} about 4 which is consistent, within the errors, with the low deuterium D/H =3.4x10^{-5} measured in other quasar absorption systems (Burles & Tytler 1998).

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arXiv1997

Chemical abundances in the young galaxy at z=2.309 towards PHL 957

We present high-resolution UES spectra of the quasar PHL 957 obtained for studying the foreground Damped Ly-alpha galaxy at z=2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]= -2.0 (+/-0.1), [S/H]= -1.54 (+/-0.06) and [N/H]=-2.76 (+/-0.07). The ratio [Fe/Zn]=-0.44 provides evidence that about 74% of iron and about 28% of zinc are locked into dust grains with a dust-to-gas ratio of about 3% of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=-1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of SII allows us to measure the SII/ZnII ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0 (+/-0.1), at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]_{c} <+0.2, [O/Fe]_{c}< 0.4, [Mn/Fe]_{c}<+0.0 and [P/Fe]_{c}<-0.7. The [alpha/Fe] values do not support a Galactic halo-like abundances implying that the chemical evolution of this young galaxy is not reproducing our own Galaxy's evolution.

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arXiv1997

Abundances of Big Bang elements

In this paper we review the present status of observations of the Big Bang elements D, 3He, 4He and 7Li and of their extrapolation to the primordial values. It is shown that, within the errors, the abundances are consistent with the predictions of the standard Big Bang nucleosynthesis for 1 < eta_10 < 6, which corresponds to 0.004 < Omega_b h_{0}^2 < 0.02$. Narrower consistencies in eta_{10} are still possible at about 1.7 or about 4, but in this case some of the observations of D and 4He should be incorrect. Finally, it is noted that "extragalactic" Li may have already been detected in a star possibly accreted by the Galaxy.

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arXiv1996

Lithium observations in 47 Tuc

We present high resolution observations (FWHM=15 km/sec) of the Li region in 5 stars belonging to the old (14 Gyrs), metal rich ([Fe/H]=-0.7) globular cluster 47Tuc. At the ESO NTT telescope we obtained EMMI spectra for three V about 17.4 magnitude stars located at the turnoff with Teff on the Spite plateau. In two of the turnoff stars the lithium line is clearly detected and the mean lithium content [Li] = 2.37$+/- 0.08 +/- 0.07 is derived. This value is slightly higher than the Spite plateau at [Li]=2.19 +/- 0.016 for field halo stars obtained with the same Teff scale by Bonifacio and Molaro (1996). For the third turnoff star only an upper limit could be derived: [Li] < 2.19. This upper limit may suggest the presence of some dispersion on the plateau stars of 47Tuc as it is observed in field stars of similar metallicity. When compared with field dwarfs of [Fe/H] about -0.7 the Li content of 47Tuc turnoff stars is among the highest observed. Considering the extreme age of 47Tuc this suggests that no significant depletion affected these stars and it implies a mild Li Galactic enrichment between the epoch of Pop II formation and the formation of 47Tuc stars with metallicities up to about -0.7. Spallation reactions, as deduced from Be observations, can account for such an increase. The remaining two stars are evolved, and only upper limits could be placed at [Li]<0.7 and <-0.7. These very low values imply some extra depletion mechanism in addition to dilution as it is observed in evolved field stars by Pilachowki et al (1993) and in NGC6397 by Pasquini and Molaro (1996).

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arXiv1996

The primordial lithium abundance

Lithium abundances in a selected sample of halo stars have been revised by using the new accurate IRFM effective temperatures by Alonso, Arribas & Martinez-Roger (1996a). From 41 plateau stars (Teff > 5700 and [Fe/H] <= -1.5) we found no evidence for intrinsic dispersion, a tiny trend with Teff and no trend with [Fe/H]. The trend with the Teff is fully consistent with the standard Li isochrones of Deliyannis, Demarque & Kawaler (1990) implying a primordial value for Li of A(Li) = 2.238 +/- 0.012 {1 sigma} +/- 0.05{sys}. The present results argue against any kind of depletion predicted by diffusion, rotational mixing or stellar winds. Therefore the Li observed in Pop II stars provides a direct and reliable estimate of the baryonic density that can rival other baryonic indicators such as the deuterium in high redshift systems. The present upwards revision of primordial Li in the framework of SBBN gives at 1 sigma two solutions for the baryonic density: Omega_{B}h^2 = 0.0062 {+0.0018,-0.0011} or Omega_{B}h^2 = 0.0146 {+0.0029,-0.0033} .

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arXiv1996

Baryonic Density from Primordial Li

Lithium abundances in a selected sample of halo stars have been revised by using new accurate Teff. From 41 plateau stars we found no evidence for intrinsic dispersion, a tiny trend with Teff and no trend with [Fe/H]. These results argue against depletion by either stellar winds, diffusion or rotational mixing. Therefore the Li observed in PopII stars provides a reliable estimate of the baryonic density. A more detailed discussion can be found in Bonifacio & Molaro (1996).

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arXiv1994

Abundances of four very metal-poor stars of the BPS survey

High resolution CASPEC and EMMI spectra (R$\approx$10$^4$) of the faint metal- deficient candidates CS 22891--209, CS 22897--8, CS 22948--66 and CS 22968--14 taken from the survey of Beers, Preston and Shectman (1985, BPS) are analyzed for abundances. The fine analysis with LTE model atmospheres gives an iron metallicity for the four stars between $-$3.15 and $-$3.45 in good agreement with the abundances inferred from CaII K index by Beers et al. (1992). These four objects considerably augment the number of spectroscopically studied stars at the low metallicity extreme. We found the $α$-elements Mg, Ca, Si, and Ti enhanced, and the iron-peak elements Sc, Mn and Cr tracking closely iron with the typical pattern of the more metallic halo stars. Aluminum results deficient with respect both to iron and to magnesium showing a strong {\it odd-even\/} effect, but for [Fe/H] $\leq$ $-$3.0 [Al/Mg] levels out into a plateau around $-$1.0, showing a primary behaviour. [Na/Fe] is $\approx$ $-$0.2 and only compared to magnesium it shows a mild odd-even effect. The heavy elements Sr and Ba show a large scatter which is not observed for other elements and cannot be due to observational errors only. In the case of CS 22891--209 they are particularly overabundant with [Sr,Ba/Fe] $\approx$ 0.9. The spread might reflect the presence of strong inhomogeneities in the interstellar gas at the earliest stages of Galaxy formation. This might be an important signature of the still elusive processes of the early formation of these elements observed in old stars.

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