arXiv subjects

R. Faraggiana

Publications and source records attributed to R. Faraggiana.

15 recordsLinked to original sources

arXiv2015

Chemical composition of a sample of bright solar-metallicity stars

We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute-Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line-by-line analysis. Chromospheric emission-line fluxes from CaII are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested.

astro-ph.SR↗

arXiv2011

The Galactic evolution of phosphorus

As a galaxy evolves, its chemical composition changes and the abundance ratios of different elements are powerful probes of the underlying evolutionary processes. Phosphorous is an element whose evolution has remained quite elusive until now, because it is difficult to detect in cool stars. The infrared weak P I lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators of the presence of phosphorus. The availability of CRIRES at VLT has permitted access to this wavelength range in stellar spectra.We attempt to measure the phosphorus abundance of twenty cool stars in the Galactic disk. The spectra are analysed with one-dimensional model-atmospheres computed in Local Thermodynamic Equilibrium (LTE). The line formation computations are performed assuming LTE. The ratio of phosphorus to iron behaves similarly to sulphur, increasing towards lower metallicity stars. Its ratio with respect to sulphur is roughly constant and slightly larger than solar, [P/S]=0.10+- 0.10. We succeed in taking an important step towards the understanding of the chemical evolution of phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance ratio is steeper than predicted by Galactic chemical evolution model model developed by Kobayashi and collaborators. Phosphorus appears to evolve differently from the light odd-Z elements sodium and aluminium. The constant value of [P/S] with metallicity implies that P production is insensitive to the neutron excess, thus processes other than neutron captures operate. We suggest that proton captures on 30Si and alpha captures on $27Al are possibilities to investigate. We see no clear distinction between our results for stars with planets and stars without any detected planet.

astro-ph.GA↗

arXiv2010

The solar photospheric abundance of carbon.Analysis of atomic carbon lines with the CO5BOLD solar model

The use of hydrodynamical simulations, the selection of atomic data, and the computation of deviations from local thermodynamical equilibrium for the analysis of the solar spectra have implied a downward revision of the solar metallicity. We are in the process of using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. We determine the solar photospheric carbon abundance by using a radiation-hydrodynamical CO5BOLD model, and compute the departures from local thermodynamical equilibrium by using the Kiel code. We measure equivalent widths of atomic CI lines on high resolution, high signal-to-noise ratio solar atlases. Deviations from local thermodynamic equilibrium are computed in 1D with the Kiel code. Our recommended value for the solar carbon abundance, relies on 98 independent measurements of observed lines and is A(C)=8.50+-0.06, the quoted error is the sum of statistical and systematic error. Combined with our recent results for the solar oxygen and nitrogen abundances this implies a solar metallicity of Z=0.0154 and Z/X=0.0211. Our analysis implies a solar carbon abundance which is about 0.1 dex higher than what was found in previous analysis based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths with respect to the other work based on a 3D model), in part it is likely due to the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results. (Abridged)

astro-ph.SR↗

arXiv2009

The Solar Photospheric Nitrogen Abundance: Determination with 3D and 1D Model Atmospheres

We present a new determination of the solar nitrogen abundance making use of 3D hydrodynamical modelling of the solar photosphere, which is more physically motivated than traditional static 1D models. We selected suitable atomic spectral lines, relying on equivalent width measurements already existing in the literature. For atmospheric modelling we used the co 5 bold 3D radiation hydrodynamics code. We investigated the influence of both deviations from local thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities (granulation effects) on the resulting abundance. We also compared several atlases of solar flux and centre-disc intensity presently available. As a result of our analysis, the photospheric solar nitrogen abundance is A(N) = 7.86 +/- 0.12.

astro-ph.SR↗

arXiv2009

The solar photospheric nitrogen abundance. Analysis of atomic transitions with 3D and 1D model atmospheres

CONTEXT: In recent years, the solar chemical abundances have been studied in considerable detail because of discrepant values of solar metallicity inferred from different indicators, i.e., on the one hand, the "sub-solar" photospheric abundances resulting from spectroscopic chemical composition analyses with the aid of 3D hydrodynamical models of the solar atmosphere, and, on the other hand, the high metallicity inferred by helioseismology. AIMS: After investigating the solar oxygen abundance using a CO5BOLD 3D hydrodynamical solar model in previous work, we undertake a similar approach studying the solar abundance of nitrogen, since this element accounts for a significant fraction of the overall solar metallicity, Z. METHOD: We used a selection of atomic spectral lines to determine the solar nitrogen abundance, relying mainly on equivalent width measurements in the literature. We investigate the influence on the abundance analysis, of both deviations from local thermodynamic equilibrium ("NLTE effects") and photospheric inhomogeneities ("granulation effects"). RESULTS: We recommend use of a solar nitrogen abundance of A(N)=7.86+-0.12 whose error bar reflects the line-to-line scatter. CONCLUSION: The solar metallicity implied by the CO5BOLD-based nitrogen and oxygen abundances is in the range 0.0145<= Z <= 0.0167. This result is a step towards reconciling photospheric abundances with helioseismic constraints on Z. Our most suitable estimates are Z=0.0156 and Z/X=0.0213.

astro-ph.SR↗

arXiv2005

Sulphur abundance in Galactic stars

We investigate sulphur abundance in 74 Galactic stars by using high resolution spectra obtained at ESO VLT and NTT telescopes. For the first time the abundances are derived, where possible, from three optical multiplets: Mult. 1, 6, and 8. By combining our own measurements with data in the literature we assemble a sample of 253 stars in the metallicity range -3.2 < [Fe/H] < +0.5. Two important features, which could hardly be detected in smaller samples, are obvious from this large sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H] ~ -1 ; 2) at low metallicities we observe stars with [S/Fe] ~ 0.4, as well as stars with higher [S/Fe] ratios. The latter do not seem to be kinematically different from the former ones. Whether the latter finding stems from a distinct population of metal-poor stars or simply from an increased scatter in sulphur abundances remains an open question.

astro-ph↗

arXiv2005

Sulphur abundances in Terzan 7

We present here the first measurements of sulphur abundances in extragalactic stars. We make use of high resolution spectra, obtained with UVES at the ESO 8.2 m Kueyen telescope, of three giants of the Globular Cluster Terzan 7, which belongs to the Sagittarius dwarf galaxy. We measure the sulphur abundances using the lines of S I multiplet 1. The S/Fe ratios for all three stars are nearly solar, thus considerably lower than what is found in Galactic stars of comparable iron content ([Fe/H] ~ -0.50). This finding is in keeping with the abundances of other alpha-chain elements in this cluster and in Sagittarius and other dSphs in general. These low alpha-chain elements to iron ratios suggest that Sagittarius and its Globular Clusters have experienced a low or bursting star-formation rate. Our sulphur abundances imply <\log (S/O)> = -1.61 which is comparable to what is found in many H II regions of similar oxygen content, and is slightly lower than the solar value (log (S/O)sun = -1.51). These are also the first measurements of sulphur abundances in a Globular Cluster, thus a direct comparison of Terzan 7 and Galactic Globular Clusters is not possible yet. However our analysis suggests that the lines of S I multiplet 1 should be measurable for other Globular Clusters at least down to a metallicity ~ -1.5.

astro-ph↗

arXiv2005

C,N,O in lambda Boo stars and in composite spectra

The selective abundance of C, N, O (almost solar) with respect to that of other elements (underabundant) in lambda Boo stars has been interpreted as a characteristic peculiarity of these objects, when considered as single stars. We show here that a similar selective abundance is predicted from the composite spectra resulting from two unresolved stars in the same temperature range as the lambda Boo stars.

astro-ph↗

arXiv2004

Lambda Boo stars with composite spectra

We examine the large sample of lambda Boo candidates collected in Table 1 of Gerbaldi et al. (2003) to see how many of them show composite spectra. Of the 132 lambda Boo candidates we identify 22 which definitely show composite spectra and 15 more for which there are good reasons to suspect a composite spectrum. The percentage of lambda Boo candidates with composite spectra is therefore > 17 and possibly considerably higher. For such stars the lambda Boo classification should be reconsidered taking into account the fact that their spectra are composite. We argue that some of the underabundances reported in the literature may simply be the result of the failure to consider the composite nature of the spectra. This leads to the legitimate suspicion that some, if not all, the lambda Boo candidates are not chemically peculiar at all. A thorough analysis of even a single one of the lambda Boo candidates with composite spectra, in which the composite nature of the spectrum is duly considered, which would demonstrate that the chemical peculiarities persist, would clear the doubt we presently have that the stars with composite spectra may not be lambda Boo at all.

astro-ph↗

arXiv2001

HD 174005: another binary classified as lambda Boo

We demonstrate that HD 174005, a star recently classified as belonging to the lambda Boo group, is in reality a double lined spectroscopic binary; at some phases, the observed composite spectrum may be similar to that of a single star with weak metal lines.

astro-ph↗

arXiv2001

Rotational velocities of A-type stars I. Measurement of vsini in the southern hemisphere

Within the scope of a Key Programme determining fundamental parameters of stars observed by HIPPARCOS, spectra of 525 B8 to F2-type stars brighter than V=8 have been collected at ESO. Fourier transforms of several line profiles in the range 4200-4500 A are used to derive vsini from the frequency of the first zero. Statistical analysis of the sample indicates that measurement error is a function of vsini and this relative error of the rotational velocity is found to be about 6% on average. The results obtained are compared with data from the literature. There is a systematic shift from standard values from Slettebak et al. (1975), which are 10 to 12% lower than our findings. Comparisons with other independent vsini values tend to prove that those from Slettebak et al. are underestimated. This effect is attributed to the presence of binaries in the standard sample of Slettebak et al., and to the model atmosphere they used.

astro-ph↗

arXiv2001

Spectra of binaries classified as lambda Boo stars

High angular resolution observations have shown that some stars classified as lambda Boo are binaries with low values of angular separation and magnitude difference of the components; therefore the observed spectrum of these objects is a combination of those of the two components. These composite spectra have been used to define spectroscopic criteria able to detect other binaries among stars classified as lambda Boo. The application of this method to HD 111786 is presented: the contribution of 5 components to the observed spectrum is demonstrated by the shape of the O I 7774 Angstrom feature. This result makes unreliable any attempt to perform an abundance analysis of this object which therefore must be definitely rejected from the class of the peculiar lambda Boo stars. This approach allowed us also to recognize that the SB2 star HD 153808 is in reality a triple system.

astro-ph↗

arXiv2001

A speckle interferometry survey of lambda Bootis stars

A search for duplicity of lambda Bootis stars has been made by using the speckle camera installed at the Telescopio Nazionale Galileo. The operation mode and the reduction procedure allow one to obtain not only the separation, but also the magnitude difference between the components; the latter parameter is fundamental for determining the degree of contamination from the secondary component of a binary system and thus the importance of the veiling effect that produces absorption lines weaker than normal. Two stars, HD 38545 and HD 290492, are close binaries with values of the separation and of the magnitude difference such that only a composite spectrum can be observed. For another 15 lambda Bootis candidates, observed with negative results, the upper limits of a possible companion separation are given.

astro-ph↗

arXiv1998

Definitions of the Lambda Boo stars

Since the identification of these stars by Morgan et al. in 1943, various definitions have been proposed for the stars of the Lambda Bootis group. We present here the various definitions which have been given to these objects in order to induce a general discussion on this topic.

astro-ph↗

arXiv1998

Determination of T_eff and log g for A0 type stars and its impact on the interpretation of the HR diagram with the HIPPARCOS results

The determination of the ``Fundamental Parameters'' T_eff and log g for a set of dwarf A0-type stars is discussed in terms of consistency when comparing these values determined through different methods. The position of these stars in the HR diagram are discussed, taking into account the HIPPARCOS data. A large number of binary stars with components of similar spectral types has been found from this spectroscopic survey.

astro-ph↗