arXiv subjects

F. Castelli

Publications and source records attributed to F. Castelli.

At least 55 records · Page 3Linked to original sources

arXiv2007

Metallicity estimates of Galactic Cepheids based on Walraven photometry

We present new empirical and theoretical calibrations of two photometric metallicity indices based on Walraven photometry. The empirical calibration relies on a sample of 48 Cepheids for which iron abundances based on high resolution spectra are available in the literature. They cover a broad range in metal abundance (-0.5 < [Fe/H] < +0.5) and the intrinsic accuracy of the Metallicity Index Color (MIC) relations is better than 0.2 dex. The theoretical calibration relies on a homogeneous set of scaled-solar evolutionary tracks for intermediate-mass stars and on pulsation predictions concerning the topology of the instability strip. The metal content of the adopted evolutionary tracks ranges from Z=0.001 to Z=0.03 and the intrinsic accuracy of the MIC relations is better than 0.1 dex.

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arXiv2007

Isotopic Anomalies in CP Stars: Helium, Mercury, Platinum, and Calcium

We review the classical observational results for isotopic abundance variations for several elements in CP stars. We concentrate on the "newest" anomaly, in calcium. The cosmically very rare isotope, Ca-48 can rival and even dominate the more common, alpha nuclide, Ca-40. Relevant examples are found in the hot, non-magnetic HgMn stars, and the field horizontal-branch star, Feige 86. The calcium anomaly is also present in cool, magnetic stars, including the notorious HD 101065, Przybylski's star.

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arXiv2007

A refined analysis of the remarkable Bp star HR 6000

UVES spectra of the very young (~10^7 years) peculiar B-type star HR 6000 were analyzed in the near-UV and visual spectral regions (3050-9460 A) with the aim to extend to other spectral ranges the study made previously in the UV using IUE spectra. Stellar parameters Teff=12850K, logg=4.10, and xi=0km/s, as determined from H_beta, H_gamma, H_delta Balmer profiles and from the Fe I, Fe II ionization equilibrium, were used to compute an individual abundances ATLAS12 model. We identified spectral peculiarities and obtained final stellar abundances by comparing observed and computed equivalent widths and line profiles. The adopted model fails to reproduce the (b-y) and c color indices. The spectral analysis has revealed: the presence of emission lines for Mn II, Cr II, and Fe II; isotopic anomalies for Hg, Ca; the presence of interstellar lines of Na I at lambda lambda 3302.3, 3302.9, 5890, 5896 A, and of K I at 7665, 7699 A; the presence of a huge quantity of unidentified lines, which we presume to be mostly due to Fe II transitions owing to the large Fe overabundance amounting to [+0.7]. The main chemical peculiarities are an extreme overabundance of Xe, followed by those of Hg, P, Y, Mn, Fe, Be, and Ti. The most underabundant element is Si, followed by C, N, Al, S, Mg, V, Sr, Co, Cl, Sc, and Ni. The silicon underabundance [-2.9] is the lowest value for Si ever observed in any HgMn star. The observed lines of He I can not be reproduced by a single value of the He abundance, but they require values ranging from [-0.8] to [-1.6]. Furthermore, when the observed and computed wings of He I lines are fitted, the observed line cores are much weaker than the computed ones. From the present analysis we infer the presence of vertical abundance stratification for He, Mn, and possibly also P.

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arXiv2007

High resolution spectroscopy of HgMn stars: a time of surprises

We present the results of a high spectral resolution study of a few spectroscopic binaries with HgMn primary stars. We detect for the first time in the spectra of HgMn stars that for many elements the line profiles are variable over the rotation period. The strongest profile variations are found for the elements Pt, Hg, Sr, Y, Zr, Mn, Ga, He and Nd. The slight variability of He and Y is also confirmed from the study of high resolution spectra of another HgMn star, alpha And.

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arXiv2007

Heavy Calcium in CP Stars

Large wavelength shifts of infrared triplet lines of Ca II have been observed in the spectra of HgMn and magnetic Ap stars. They have been attributed to the heavy calcium isotopes, including Ca-48. One member of the triplet, $λ$8542, had been either unavailable, or of poor quality in earlier spectra. The present material shows conclusively that the stellar $λ$8542 shifts are consistent with an interpretation in terms of Ca-48. We find no relation between isotopic shifts of the Ca II triplet lines, and those of Hg II $λ$3984. There is a marginal indication that the shifts are {\it anticorrelated} with the surface field strengths of the magnetic stars. We see sparse evidence for Ca-48 in other chemically peculiar stars, e.g. Am's, metal-poor stars, or chemically peculiar red giants. However, the sample is still very small, and the wavelengths of all three triplet lines, including those in the Sun, show slight positive shifts with respect to terrestrial positions. Some profiles of the Ca II infrared triplet in the magnetic stars show extensive wings beyond a well-defined core. We can obtain reasonable fits to these profiles using a stratified calcium abundance similar to that used by previous workers. There is no indication that either the stratification or the Zeeman effect significantly disturbs the measurement of isotope shifts.

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arXiv2007

On the effect of helium enhancement on bolometric corrections and Teff-colour relations

We evaluate the effects that variations in He content have on bolometric corrections and Teff-colour relations. To this aim, we compute ATLAS9 model atmospheres and spectral energy distributions for effective temperatures ranging from 3500 K to 40000 K for dwarfs and from 3500 K to 8000 K for giants, considering both ``He-non enhanced'' and ``He-enhanced'' compositions. The considered variations in He content are of DeltaY = +0.1 and +0.2 for the metallicity [M/H]=+0.5 and DeltaY=+0.1 for [M/H]=-0.5 and -1.5. Then, synthetic photometry is performed in the UBVRIJHK system. We conclude that the changes in bolometric corrections, caused by the adopted He-enhancements are in general too small (less than 0.01 mag), for both dwarfs and giants, to be affecting present-day tables of bolometric corrections at a significant level. The only possible exceptions are found for the U-band at Teff between 4000 K and 8000 K, where |Delta BC_U| amounts to ~0.02 mag, and for Teff equal to 3500 K, where |Delta BC_lambda| values become clearly much higher (up to 0.06 mag for passbands from U to V). However, even in the latter case the overall uncertainty caused by variations in the He content may be not so significant, because the ATLAS9 results are still approximative at their lowest temperature limit.

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arXiv2006

Predicted properties of RR Lyrae stars in the SDSS photometric system

The luminosities and effective temperatures, as well as the whole bolometric lightcurves of nonlinear convective RR Lyrae models with 0.0001<Z<0.006 are transformed into the SDSS photometric system. The obtained ugriz lightcurves, mean magnitudes and colors, pulsation amplitudes and color-color loops are shown and analytical relations connecting pulsational to intrinsic stellar parameters, similarly to the ones currently used in the Johnson-Cousins filters, are derived. Finally the behaviour in the color-color planes is compared with available observations in the literature and possible systematic uncertainties affecting this comparison are discussed.

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arXiv2006

A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an alpha-enhanced metal distribution

[Abridged] We present a large, new set of stellar evolution models and isochrones for an alpha-enhanced metal distribution typical of Galactic halo and bulge stars; it represents a homogeneous extension of our stellar model library for a distribution already presented in Pietrinferni et al.(2004). The effect of the alpha-element enhancement has been properly taken into account in the nuclear network, opacity, equation of state and, for the first time, the bolometric corrections, and color transformations. This allows us to avoid the inconsistent use - common to all alpha-enhanced model libraries currently available - of scaled-solar bolometric corrections and color transformations for alpha-enhanced models and isochrones. We show how bolometric corrections to magnitudes obtained for the U,B portion of stellar spectra for T_{eff}<=6500K, are significantly affected by the metal mixture, especially at the higher metallicities. We also provide complete sets of evolutionary models for low-mass, He-burning stellar structures covering the whole metallicity range, to enable synthetic horizontal branch simulations. We compare our database with several widely used stellar model libraries from different authors, as well as with various observed color magnitude and color-color diagrams (Johnson-Cousins BVI and near infrared magnitudes, Stromgren colors) of Galactic field stars and globular clusters. We also test our isochrones comparing integrated optical colors and Surface Brightness Fluctuation magnitudes with selected globular cluster data. We find a general satisfactory agreement with the empirical constraints.

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arXiv2005

BASTI: An interactive database of updated stellar evolution models

We present a new database of stellar evolution models for a large range of masses and chemical compositions, based on an up-to-date theoretical framework. We briefly discuss the physical inputs and the assumptions adopted in computing the stellar models. We explain how to access to the on-line archive and briefly discuss the interactive WEB tools that can be used to compute user-specified evolutionary tracks/isochrones/luminosity functions. The future developments of this database are also outlined.

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arXiv2005

An extensive library of 2500-10500 Ang synthetic spectra

We present a complete library of synthetic spectra based on Kurucz's codes that covers the 2500-10500 Ang wavelength range at resolving powers 20,000, 11,500 (GAIA), 8500 (RAVE), 2000 (SLOAN) and uniform dispersions of 1 and 10 Ang/pix. The library maps the whole HR diagram, exploring 51,288 combinations of atmospheric parameters spanning the ranges: 3500 <= Teff <= 47500 K, 0.0 <= log g <= 5.0, -2.5 <= [M/H] <= 0.5, [alpha/Fe] = 0.0,+0.4, xi =1,2,4 km/sec, 0 <= Vrot <= 500 km/sec. The spectra are available both as absolute fluxes as well as continuum normalized. Performance tests and foreseen spectroscopic applications of the library are discussed, including automatic classification of data from surveys like RAVE, SLOAN, GAIA. The entire library of synthetic spectra is accessible via http://archives.pd.astro.it/2500-10500/ or (for the 1 AA/pix version only) http://gaia.esa.int/spectralib/spectralib1A/SpectraLib1a.cfm

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arXiv2005

Spectroscopic Survey of the Galaxy with Gaia II. The expected science yield from the Radial Velocity Spectrometer

The Gaia mission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848 to 874 nm wavelength range, with a resolving power R = 11 500, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper (Katz et al. 2004) presents the characteristics of the RVS and its performance. This paper details the outstanding scientific impact of this important part of the Gaia satellite on some key open questions in present day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than V=15, the resolving power of the RVS will give information about most of the effects which influence the position of a star in the Hertzsprung-Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterisation of the many types of double, multiple and variable stars.

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arXiv2004

ATLAS and SYNTHE under Linux

We have successfully ported under GNU Linux ATLAS 9, the widely used stellar atmosphere modeling code, as well as both the SYNTHE suite of programs, its ``companion'' for spectral synthesis, and WIDTH, used to derive chemical abundances from equivalent widths of spectral lines. The porting has been realized by using the Intel Fortran Compiler. Our aim was to port the codes with the minimum possible amount of modifications: full compatibility with the VMS version has been maintained, along with all the codes functionalities. Dramatic improvement in calculation speed with respect to the VMS version has been achieved. The full suite of codes is intended to be freely available to anyone.

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arXiv2004

A large stellar evolution database for population synthesis studies. I. Scaled solar models and isochrones

We present a large and updated stellar evolution database for low-, intermediate- and high-mass stars in a wide metallicity range, suitable for studying Galactic and extragalactic simple and composite stellar populations using population synthesis techniques. The stellar mass range is between \sim0.5Mo and 10Mo with a fine mass spacing. The metallicity [Fe/H] comprises 10 values ranging from -2.27 to 0.40, with a scaled solar metal distribution. The initial He mass fraction ranges from Y=0.245, for the more metal-poor composition, up to 0.303 for the more metal-rich one, with Delta Y/Delta Z\sim 1.4. For each adopted chemical composition, the evolutionary models have been computed without and with overshooting from the Schwarzschild boundary of the convective cores during the central H-burning phase. The whole set of evolutionary models can be used to compute isochrones in a wide age range, from \sim30 Myr to \sim15Gyr. Both evolutionary tracks and isochrones are available in several observational planes, employing updated set of bolometric corrections and color-Te relations computed for this project. The number of points along the models and the resulting isochrones is selected in such a way that interpolation for intermediate metallicities not contained in the grid is straightforward. We compare our isochrones with results from different stellar evolution databases and perform some empirical tests for the reliability of our models. Since this work is devoted to scaled solar compositions, we focus our attention on the Galactic disk populations, employing multicolor photometry of unevolved field MS stars with precise Hipparcos parallaxes, well-studied open clusters and one eclipsing binary system with precise measurements of masses, radii and [Fe/H] of both components.

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arXiv2004

New Grids of ATLAS9 Model Atmospheres

New opacity distribution functions (ODFs) for several metallicities have been computed. The main improvements upon previous ODFs computed by Kurucz (1990) are: (1) the replacement of the solar abundances from Anders & Grevesse (1989) with those from Grevesse & Sauval (1998); (2) the replacement of the TiO lines provided by Kurucz (1993) with the TiO lines from Schwenke (1998), as distributed by Kurucz (1999a); (3) the addition of the H2O lines from Partridge & Schwenke (1997) as distributed by Kurucz (1999b); (4) the addition of the HI-HI and HI-H+ quasi-molecular absorptions near 1600 A and 1400 A computed according to Allard et al. (1998). Other minor improvements are related with some changes in a few atomic and molecular data. New grids of ATLAS9 model atmospheres for Teff from 3500K to 50000K and logg from 0.0 dex to 5.0 dex have been computed for several metallicities with the new ODFs. Preliminary comparisons of the results from the old and new models have shown differences in the energy distributions of stars cooler than 4500K, in the ultraviolet energy distribution of metal-poor A-type stars, in the U-B and u-b color indices for Teff<7000K and in all the color indices for Teff<4250K.

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arXiv2004

The ubiquitous nature of the Horizontal Branch second U-jump: A link with the Blue Hook scenario?

In a previous paper we reported on a discontinuity in the extreme horizontal branch (EHB) of the Galactic globular cluster NGC6752, which we called the second U-jump. This feature was attributed to a combination of post zero-age horizontal branch evolution and diffusion effects. In this follow-up study we analyze other EHB clusters and show that the second U-jump is a common feature among EHB clusters reaching T_{eff}\ge 23,000K, and that its onset in different clusters converges around T_{eff}\sim 21,000\pm3,000K. We also present near-ultraviolet diagrams of ωCen and NGC2808, the only two objects with spectroscopically confirmed ``blue hook'' stars (T_{eff}\ge 35,000K). We confirm predictions of a photometric discontinuity separating late from early-helium flashers. Moreover, we present empirical evidence that the second U-jump population might be mainly composed by early-helium flashers. Lastly, we revisit the discussion on the ubiquitous nature of the gaps and jumps so far identified in the blue HB tails, suggesting a possible discrete nature of the distribution in temperature of the HB stars.

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arXiv2003

Why Hot Horizontal Branch Stars Can Appear Redder Than Red Giants

In this paper we report on a curious feature in the V, (U-B) color-magnitude diagrams of globular clusters. In our database, we find that a considerable fraction of blue horizontal branch stars, hotter than the instability strip and cooler than the Grundahl et al. (1999) jump (i.e., 6000 <~ T_eff K <~ 10000), have (U-B) colors redder than their red giant progenitors. This red incursion is not expected on theoretical grounds, as horizontal branch stars (whose convective regions are less extended than in red giant structures) should not ``appear'' cooler than a red giant. Analyzing data from different telescopes we show that: 1) the horizontal branch red incursion is strongly dependent on the shape of the adopted U filter and to a lesser extent, on the B filter; 2) the photometry done with U filters which do not encompass the Balmer jump, do not show the blue horizontal branch red incursion; 3) the occurrence of this feature is also due to the peculiar dependence of the U and B magnitudes on stars effective temperature, gravity, and metallicity; 4) theoretical tracks can reproduce the observed horizontal branch morphology, provided that the appropriate (i.e. exactly responding to the filters effectively used in the observations) transmission curve efficiencies are used for deriving color-T_eff transformations; 5) the red incursion extent depends on metallicity.

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arXiv2002

Some Aspects of the calculation of Balmer lines in the sun and stars

We compare the results of Balmer-line calculations using recent theory and improved computational algorithms with those from the widely-used SYNTHE and BALMER9 routines. The resulting profiles are mostly indistinguishable. Good fits to the normalized solar Balmer lines H$α$ through H$δ$ are obtained (apart from the cores) using the recent unified-broadening calculations by Barklem and his coworkers provided that some adjustment for the continuum is performed. We discuss a surprising linearity with temperature of the Balmer line profiles in dwarfs.

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arXiv2000

A spectroscopic study of field BHB star candidates

New spectroscopic observations (R=15000 and 40000) are given for 31 nearby (V < 11) BHB star candidates. IUE low-resolution spectra are available for most of them. New determinations of temperature, gravity, reddening and abundances were obtained from these data using models computed by Castelli with an updated version of the ATLAS9 code. A summary of the visual photometry for these stars (including new photometry obtained at Kitt Peak) is also given. All 31 candidates are halo stars. We classify 28 as BHB stars because: (1) They lie close to the ZAHB (in a similar position to globular cluster BHB stars) in the Teff vs. log g plot. (2) Their distribution of vsini (less than 40 km/s) is similar to that found for the BHB stars in globular clusters. (3) Their [Fe/H] lies between -0.99 and -2.95 (mean [Fe/H] -1.67, dispersion 0.42 dex) which is similar to that found for field halo RR Lyrae and red HB stars but more metal- poor than for halo globular clusters. Field BHB stars with Teff greater than 8500 K show the He I (4471 A) line corresponding to solar abundance. (4) They show a similar average enhancement of the alpha-elements ([Mg/Fe] = +0.43 +/- 0.04 and [Ti/Fe] = +0.44 +/- 0.02) to that found for other halo field stars of similar metallicity. Comments are given on a further 25 stars that have previously been suggested to be BHB stars; these were not included in our sample because they did not fulfil one or more of the above criteria.

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