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

Using Independent Component Analysis to detect exoplanet reflection spectrum from composite spectra of exoplanetary binary systems

The analysis of the wavelength-dependent albedo of exoplanets represents a direct way to provide insight of their atmospheric composition and to constrain theoretical planetary atmosphere modelling. Wavelength-dependent albedo can be inferred from the exoplanet's reflected light of the host star, but this is not a trivial task. In fact, the planetary signal may be several orders of magnitude lower ($10^{-4}$ or below) than the flux of the host star, thus making its extraction very challenging. Successful detection of the planetary signature of 51~Peg\,b has been recently obtained by using cross-correlation function (CCF) or autocorrelation function (ACF) techniques. In this paper we present an alternative method based on the use of Independent Component Analysis (ICA). In comparison to the above-mentioned techniques, the main advantages of ICA are that the extraction is \textit{"blind"} i.e. it does not require any \textit{a priori} knowledge of the underlying signals, and that our method allows us not only to detect the planet signal but also to estimate its wavelength dependence. To show and quantify the effectiveness of our method we successfully applied it to both simulated data and real data of an eclipsing binary star system. Eventually, when applied to real 51~Peg~+~51~Peg\,b data, our method extracts the signal of 51~Peg but we could not soundly detect the reflected spectrum of 51~Peg\,b mainly due to the insufficient $SNR$ of the input composite spectra. Nevertheless, our results show that with "ad-hoc" scheduled observations an ICA approach will be, in perspective, a very valid tool for studying exoplanetary atmospheres.Comment: 25 pages, 12 figures. Accepted to A

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

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

Gaia-ESO Survey: empirical and synthetic Lick/SDSS indices for stellar population studies

Stellar population study is fundamental for the understanding of the physical process involved in the formation and evolution of galaxies. Out of several approaches to get information about abundance patterns in stellar population the use of broad and narrow spectral features of indices (e.g Lick/IDS system) is one of the most widely used. The large amount of data available nowadays allow to compute empirical libraries which are a fundamental tool for studying integrated spectra of stellar systems. Here, we present an empirical library of Lick-like spectral indices (ELickSDSSv2), computed using the spectra of the Gaia-ESO survey (GES). The library includes also the spectral indices of a previous version which was computed from Elodie, Miles, Indo-US, and FEROS spectra. Since empirical libraries carry on the imprints of the local properties of the solar neighbourhood, it is mandatory to complement them with synthetic libraries to reproduce integrated spectra of system whose star formation histories are different from those of local systems. Preliminary results of a new synthetic Lick/SDSS library are also presented

INTRIGOSS: A new Library of High Resolution Synthetic Spectra

INTRIGOSS (INaf Trieste Grid Of Synthetic Spectra) is a new High Resolution (HiRes) synthetic spectral library designed for studying F, G, and K stars. The library is based on atmosphere models computed with specified individual element abundances via ATLAS12 code. Normalized SPectra (NSP) and surface Flux SPectra (FSP), in the 4800-5400 Å wavelength range, were computed by means of the SPECTRUM code. The synthetic spectra are computed with an atomic and bi-atomic molecular line list including "bona fide" Predicted Lines (PLs) built by tuning loggf to reproduce very high SNR Solar spectrum and the UVES-U580 spectra of five cool giants extracted from the Gaia-ESO survey (GES). The astrophysical gf-values were then assessed by using more than 2000 stars with homogenous and accurate atmosphere parameters and detailed chemical composition from GES. The validity and greater accuracy of INTRIGOSS NSPs and FSPs with respect to other available spectral libraries is discussed. INTRIGOSS will be available on the web and will be a valuable tool for both stellar atmospheric parameters and stellar population studies

VizieR Online Data Catalog: Lick indices for FGK stars (Franchini+, 2014)

PRIN MIUR 2010–2011 project ‘The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T.The stars observed by FEROS and studied by the AMBRE project (Worley et al., 2012A&A...542A..48W) constitute an ideal working data set for our purposes since they include a large number of non-supergiant FGK stars with individual estimates of Teff, log g, [M/H], and α-to-iron ratio ([alpha/Fe]). We searched the European Southern Observatory (ESO) Science Archive Facility and retrieved, through the FEROS/HARPS pipeline processed data Query Form, all the public available spectra of FGK stars with AMBRE atmospheric parameter values in the following ranges: 38003.5, and global metallicity [M/H]>-3.0. A list of 1085 stars, corresponding to 2511 available spectra, was obtained. Since AMBRE provides individual estimates of stellar parameters derived from each spectrum, we computed for 202 stars with more than one observed spectrum average atmospheric parameter values. In any case, the dispersion of values for the same object resulted to be less than the external errors associated with AMBRE results. (1 data file). <P /

Accretion and outflows in young stars with CUBES

The science case on studies of accretion and outflows in low-mass (<1.5M⊙) young stellar objects (YSOs) with the new CUBES instrument is presented. We show the need for a high-sensitivity, near-ultraviolet (NUV) spectrograph like CUBES, with a resolving power at least four times that of X-Shooter and combined with UVES via a fibrelink for simultaneous observations. Simulations with the CUBES exposure time calculator and the end-to-end software show that a significant gain in signal-to-noise can be achieved compared to current instruments, for both the spectral continuum and emission lines, including for relatively embedded YSOs. Our simulations also show that the low-resolution mode of CUBES will be able to observe much fainter YSOs (V ∼ 22 mag) in the NUV than we can today, allowing us extend studies to YSOs with background-limited magnitudes. The performance of CUBES in terms of sensitivity in the NUV will provide important new insights into the evolution of circumstellar disks, by studying the accretion, jets/winds and photo-evaporation processes, down to the low-mass brown dwarf regime. CUBES will also open-up new science as it will be able to observe targets that are several magnitudes fainter than those reachable with current instruments, facilitating studies of YSOs at distances of ∼ kpc scale. This means a step-change in the field of low-mass star formation, as it will be possible to expand the science case from relatively local star-forming regions to a large swathe of distances within the Milky Way