Mariagrazia Franchini

Synthetic mid-UV spectroscopic indices of stars

Using the UVBLUE library of synthetic stellar spectra we have computed a set of mid-UV line and continuum spectroscopic indices. We explore their behavior in terms of the leading stellar parameters (Teff, log g). The overall result is that synthetic indices follow the general trends depicted by those computed from empirical databases. Separately we also examine the index sensitivity to changes in chemical composition, an analysis only feasible under a theoretical approach. In this respect, lines indices Fe I 3000, BL 3096, and Mg I 2852 and the continuum index 2828/2921 are the least sensitive features, an important characteristic to be taken into account for the analyses of integrated spectra of stellar systems. We also quantify the effects of instrumental resolution on the indices and find that indices display variations up to 0.1 mag in the resolution interval between 6 and 10 A of FWHM. We discuss the extent to which synthetic indices are compatible with indices measured in spectra collected by the International Ultraviolet Explorer (IUE). Five line and continuum indices (Fe I 3000, 2110/2570, 2828/2921, S2850, and S2850L) display a remarkably good correlation with observations. The rest of the indices are either underestimated or overestimated; however, two of them, Mg Wide and BL 3096, display only marginal discrepancies. For 11 indices we give the coefficients to convert synthetic indices to the IUE system. This work represents the first attempt to synthesize mid-UV indices from high-resolution theoretical spectra and foresees important applications for the study of the ultraviolet morphology of old stellar aggregates.

Synthetic Lick Indices and Detection of α-Enhanced Stars

Synthetic Lick indices computed with solar scaled abundances and with α-element enhancement are presented and compared with predictions from both theoretical computations (Tripicco & Bell; Thomas, Maraston, & Bender; Barbuy et al.) and empirical fitting functions (de Freitas Pacheco). We propose selected combinations of indices capable of singling out α-enhanced stars without requiring previous knowledge of their main atmospheric parameters. By applying this approach to the 460 stars in the Worthey et al. catalog, we detected a list of 82 candidate α-enhanced stars. The confirmation of α-enhancement was obtained by searching the literature for individual element abundance determinations from high-resolution spectroscopy for a subsample of 34 stars. Preliminary discussion of the properties of the detected α-enhanced stars with respect to their [Fe/H] values and kinematics is presented.

Metallicity Determinations from Ultraviolet-Visual Spectrophotometry. I. The Test Sample

New visual spectrophotometric observations of non-supergiant solar neighborhood stars are combined with IUE Newly Extracted Spectra (INES) energy distributions in order to derive their overall metallicities, [M/H]. This fundamental parameter, together with effective temperature and apparent angular diameter, is obtained by applying the flux-fitting method while surface gravity is derived from the comparison with evolutionary tracks in the theoretical H-R diagram. Trigonometric parallaxes for the stars of the sample are taken from the Hipparcos Catalogue. The quality of the flux calibration is discussed by analyzing a test sample via comparison with external photometry. The validity of the method in providing accurate metallicities is tested on a selected sample of G-type stars with well-determined atmospheric parameters from recent high-resolution spectral analysis. The extension of the overall procedure to the determination of the chemical composition of all the INES non-supergiant G-type stars with accurate parallaxes is planned in order to investigate their atmospheric temperature structure.

Far-Ultraviolet Continuum of G-Type Stars: A Signature of the Temperature Minimum Region

The main results of a program of systematic comparison between observed and computed UV spectral energy distributions of field G-type stars are illustrated. We constructed the UV observed energy distributions for 53 G stars, starting from the IUE Uniform Low Dispersion Archive (ULDA) and computed the corresponding theoretical fluxes by using the atmospheric parameters from the Catalogue of [Fe/H] Determinations (1996 edition) and a Kurucz grid of model fluxes. From the comparison between observations and classical models, a UV excess shortward of 2000 A is evident for all the program stars. The UV continuum in the region 1600-2000 A can be described by synthetic fluxes computed from semiempirical models based on the temperature minimum concept. Values for the Tmin/Teff ratio on the order of 0.80 are suitable for the interpretation of the observed fluxes. The residual discrepancies shortward of 1600 A are suggested to be effects of the chromosphere, on the basis of a comparison with the Maltby et al. semiempirical model of the Sun.

The FEROS-Lick/SDSS observational data base of spectral indices of FGK stars for stellar population studies

ABSTRACT We present FEROS–Lick/SDSS, an empirical database of Lick/SDSS spectral in-dices of FGK stars to be used in population synthesis projects for discriminating dif-ferent stellar populations within the integrated light of galaxies and globular clusters.From about 2500 FEROS stellar spectra obtained from the ESO Science Archive Facil-ity we computed line–strength indices for 1085 non–supergiant stars with atmosphericparameter estimates from the AMBRE project.Two samples of 312 dwarfs and of 83 subgiants with solar chemical compositionand no significant α–element abundance enhancement are used to compare their ob-servational indices with the predictions of the Lick/SDSS library of synthetic indices.In general, the synthetic library reproduces very well the behaviour of observationalindices as a function of temperature, but in the case of low temperature (T eff .5000K)dwarfs; low temperature subgiants are not numerous enough to derive any conclusion.Several possible causes of the disagreement are discussed and promising theoreticalimprovements are presented.Key words: stars: late-type – stars: fundamental parameters – astronomical databases: miscellaneous.

Synthetic Lick Indices and Detection of α-enhanced Stars. III. F, G, and K Stars with [Fe/H] > 0.00

A sample of 119 F, G, and K solar neighborhood stars, selected under the condition [Fe/H] > 0.00, is investigated in order to detect which of them, if any, present α-enhanced characteristics. According to the kinematics, the sample represents stars of the thin-disk component of the Galaxy. The search of α-enhanced characteristics is performed by adopting an already tested procedure that does not require previous knowledge of the stellar main atmospheric parameters. The analysis is based on the comparison of spectral indices in the Lick IDS system, coming from different observational data sets, with synthetic ones computed with solar-scaled abundances and with α-element enhancement. The main result of the analysis is the extreme paucity (likely just one in 119) of α-enhanced stars in our sample, thus suggesting [α/Fe] = 0.0 for thin-disk stars with [Fe/H] > 0.00. This result, which is in agreement with the standard evolutionary picture of the disk of the Galaxy, is compared with recent results from high-resolution analysis reported in the literature. The role of the atmospheric parameter assumptions in the analysis of high-resolution spectroscopic data is discussed, and a possible explanation of discrepant results about α-enhancement for stars with [Fe/H] > 0.00 is presented.

Design Study of an Adaptive Optics Visual Echelle Spectrograph and Imager for the VLT

We present a preliminary design study for an adaptive optics visual echelle spectrograph and imager/coronograph for use as parallel instrument of the Nasmyth Adaptive Optics System (NAOS) on unit UT3 of the VLT. The spectrograph is intended for intermediate resolution spectroscopy of faint sources. It could be used for observations of late-type dwarfs in distant Galactic clusters and in galaxies of the local group as well as for spectroscopy of extra galactic objects like quasars and Lyman break galaxies down to a limiting magnitude of V equals 22.5. The implementation of an imaging gand coronograph mode increases the versatility of the instrument and its scientific objectives. The instrument takes advantage of Adaptive Optics at visible wavelengths both for imaging and spectroscopy. With NAOS at the VLT, the light concentration in these bands will be above approximately 60 percent of the flux in a 0.3 arcsec aperture for typical Paranal conditions. Simulations show that a gain of more than one magnitude with respect to compatible non-adaptive optical spectrography will be possible for sky- and/or detector limited observations. In addition, the smaller diffraction limit in the optical than in the IR will allow a significant gain in imaging and coronography as well. Finally, the instrument will allow gathering unprecedented experience on the performances of AO at visible wavelengths, which will be fundamental for further development of AO systems, in particular for very large telescopes.

Search for α‐enhanced stars in the spectroscopic SDSS stellar database

We analyze a sample of about 8000 stars extracted from the Sloan Digital Sky Survey with Teff in the range 4500–7000 K and log g greater than 2.0 dex as estimated by comparing measured and synthetic Lick indices. We present preliminary results on the dependence of the α‐enhancement phenomenon on the stellar metallicity and on the galactic position.

Spatial Resolution Improvement for an 8 Meter Class Telescope via Adaptive Optics and On‐Line Subaperture Selection

We present a technique for improving the image resolution achievable in the visible with an 8 m class telescope. The method is based on the combined use of an adaptive optics (AO) system and a multiaperture fast shutter. A real-time subaperture selection is used to discard those parts of the AO-corrected wave front that are still characterized by significant residual aberrations. Achievable gains in resolution and peak intensity are computed via numerical simulation adopting two different selection approaches. Depending on the selection rate and seeing conditions, different gains, reaching a maximum value of about 4, are obtained with respect to the atmospheric aberrated image. An application to the problem of detecting a faint companion close to a bright star is presented.

Online subaperture selection in AO closed-loop systems

The possibility of designing and running an AO system working in the visible for observations with a large diameter telescope is still under debate due to the large number of sub-pupils required to properly sample the incoming WaveFront. On the other hand, the situation is much more relaxed in the IR, where r0 is much larger, and several systems are forthcoming or already in operation. Morossi et al. proposed to support IR AO system with on-line multi-aperture selection devices. They showed that the image quality of a ground-based telescope in the visual could be improved by means of pupil segmentation and on-line multi- sub-aperture selection notwithstanding the large D/r0 ratio.