arXiv subjects

E. Bertone

Publications and source records attributed to E. Bertone.

29 records · Page 2Linked to original sources

arXiv2006

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 [T_eff,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 FeI3000, BL3096 and MgI2852 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 6-10 angstrom 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 (FeI3000, 2110/2570, 2828/2921, S2850, and S2850L) display a remarkable good correlation with observations. The rest of the indices are either underestimated or overestimated, however, two of them, MgWide and BL3096, 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.

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arXiv2005

New Candidate EHB Stars in the Open Cluster NGC 6791: Looking Locally into the UV-upturn Phenomenon

Relying on U,B imagery at the Italian Telescopio Nazionale Galileo (TNG), we report here the discovery of a sample of 13 new UV-bright post-HB candidate stars in the field of the galactic open cluster NGC 6791. Owing to its super-solar metal content ([Fe/H]>0.2 dex) and estimated age (t> 8 Gyr), this cluster represents the nearest and ideal stellar aggregate to match the distinctive properties of the evolved stellar populations possibly ruling the UV-upturn phenomenon in elliptical galaxies and bulges of spirals. Our ongoing spectroscopic follow-up of this unique UV-bright sample will allow us to assess - once cluster membership of the candidates is properly checked - the real nature (e.g. SdB, SdO, AGB-manque' or EHB stars) of these hot sources, and their link with the ultraviolet excess emerging from low-mass, metal-rich evolutionary environments of external galaxies.

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arXiv2005

UVBLUE: a new high-resolution theoretical library of ultraviolet stellar spectra

We present an extended ultraviolet-blue (850-4700 AA) library of theoretical stellar spectral energy distributions (SEDs) computed at high resolution, R= 50,000. The UVBLUE grid, as we named the library, is based on LTE calculations carried out with ATLAS9 and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800 entries that cover a large volume of the parameter space. It spans a range in effective temperature from 3000 to 50,000 K, the surface gravity ranges from log g= 0.0 to 5.0 with a step of 0.5 dex, while seven chemical compositions are considered: [Fe/H]= -2.0, -1.5, -1.0, -0.5, +0.0, +0.3 and +0.5 dex. For its coverage across the H-R diagram, this library is the most comprehensive one ever computed at high resolution in the short-wavelength spectral range, and useful application can be foreseen both for the study of single stars and in population synthesis models of galaxies and other stellar systems.

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arXiv2005

Nearby early-type galaxies with ionized gas. Characterization of the underlying stellar population

We present a preliminary analysis of the sample of early type galaxies of Rampazzo et al. (2005), selected to build a data-set of spectral properties of well studied early-type galaxies showing emission lines. Because of the presence of emission lines, the sample is biased toward objects that might be expected to have ongoing and recent star formation. We have compared the line-strength indices presented in Rampazzo et al. (2005) with Simple Stellar Populations (SSPs) in order to characterize the underlying stellar population of the galaxies. We have derived ages, metallicities and [alpha/Fe] ratios. The positive trend of the sigma-metallicity and sigma-[alpha/Fe] relations is reproduced. The bulk of the galaxies span a range in metallicity from ~ solar to ~ twice solar and a range in [alpha/Fe] from ~0.2 to ~0.4. Furthermore the comparison of the derived parameters at different galactocentric distances shows the presence of negative metallicity gradients from the center outwards.

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arXiv2004

ATLAS vs. NextGen model atmospheres: a combined analysis of synthetic spectral energy distributions

We carried out a critical appraisal of the two theoretical models, Kurucz' ATLAS9 and PHOENIX/NextGen, for stellar atmosphere synthesis. Our tests relied on the theoretical fit of SEDs for a sample of 334 target stars along the whole spectral-type sequence. The best-fitting physical parameters of stars allowed a calibration of the temperature and bolometric scale. The main conclusions of our analysis are: i) the fitting accuracy of both theoretical libraries drastically degrades at low Teff; ii) comparing with empirical calibrations, both ATLAS and NextGen fits tend to predict slightly warmer Teff, but ATLAS provides in general a sensibly better fit; iii) there is a striking tendency of NextGen to label target stars with an effective temperature and surface gravity in excess with respect to ATLAS. This is a consequence of some ``degeneracy'' in the solution space, partly induced by the different input physics and geometry constraints. A different T(τ) vertical structure of stellar atmosphere seems also required for NextGen synthetic SEDs in order to better account for limb-darkening effects in cool stars, as supported by the recent observations of the EROS BLG2000-5 microlensing event.

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arXiv2002

New insights in high-resolution spectroscopy: a wide theoretical library of R=500000 stellar spectra

We present an extended theoretical library of over 800 synthetic stellar spectra, covering energy distribution in the optical range (lambda = 3500-7000 angstrom), at inverse resolution R=500000. The library, based on the ATLAS9 model atmospheres, has been computed with the SYNTHE code developed by R.L. Kurucz. The grid spans a large volume in the fundamental parameter space (i.e. effective temperature, surface gravity, metallicity), and can be profitably applied to different research fields dealing both with the study of single stars and stellar aggregates, through population synthesis models. A complementary project, in progress, will extend the wavelength range to the ultraviolet, down to 850 angstrom, at an inverse resolution of R=50000.

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arXiv2001

Multiwavelenght analysis of Gl 355 (LQ Hya)

We discuss ROSAT, ASCA, {\it Beppo}SAX and optical observations of the young active star Gl 355. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. We have modeled the flare and derived a loop semi--length of the order of $\sim 1.5$ stellar radii. ROSAT, ASCA and {\it Beppo}SAX data suggest that the coronal abundance of Gl 355 is subsolar, in the range $0.1 ÷0.3 Z/Z_\odot$. A preliminary analysis of optical spectra allows us to compare the photospheric and coronal metal abundances.

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arXiv2001

A critical appraisal of ATLAS9 and NextGen 5 model atmospheres

The fitting atmosphere parameters (Teff, g, and [Fe/H]) for over 300 stars in the Gunn & Striker and Jacoby et al. catalogs have been obtained relying on the Kurucz (1992) ATLAS9 and Hauschildt et al (1999) NextGen5 synthesis models. The output results are compared, and a critical appraisal of both theoretical codes is performed.

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arXiv2001

Spectral classification of stars using synthetic model atmospheres

We devised a straightforward procedure to derive the atmosphere fundamental parameters of stars across the different MK spectral types by comparing mid-resolution spectroscopic observations with theoretical grids of synthetic spectra.The results of a preliminary experiment, by matching the Gunn and Stryker and Jacoby et al. spectrophotometric atlases with the Kurucz models, are briefly discussed. For stars in the A-K spectral range, effective temperature is obtained within a 1-2% relative uncertainty (at 2 sigma confidence level). This value raises to 4-5% for the hottest stars in the samples (O-B spectral types). A poorer fit is obtained throughout for stars cooler than 4000 K mainly due to the limiting input physics in the Kurucz models.

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arXiv2000

ISO observational campaign of the BL Lac object PKS 2155-304

The BL Lacertae object PKS 2155-304 was observed by the Infrared Space Observatory in May and June 1996, during a multiwavelength campaign. These are the first observations in the mid- and far-infrared bands since IRAS. In the observing period, the source showed no detectable time variability at 4.0, 14.3, 60 and 90 micron. The spectrum from 2.8 to 100 micron is well fitted by a single power law with energy spectral index alpha = 0.4, intermediate between the flatter radio spectrum and the steeper simultaneous optical spectrum. The overall infrared to X-ray spectral energy distribution can be well explained by optically thin synchrotron emission, with negligible contributions from thermal sources. We also show that the host galaxy flux is negligible in this spectral range.

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arXiv1999

ISO observations of the BL Lac object PKS 2155-304

The Infrared Space Observatory (ISO) observed the BL Lac object PKS 2155-304 16 times from 1996, May 7 to June 8, with both the ISOCAM camera and the ISOPHOT photometer, as part of a more general multiwavelength campaign. Two additional observations were performed on 1996, November 23 and 1997, May 15. This is the first time that there are simultaneous mid and far infrared data in a multiwavelength monitoring of a BL Lac object. We obtained four light curves at 4.0, 14.3, 60 and 90 microns, and a broad-band filter spectrum from 2.8 to 170 micron. No variability was detected in the infrared, although the source was varying at shorter wavelengths. The IR spectrum can be fitted by a single power law with an energy spectral index alfa = 0.40 +/- 0.06 and it can be explained as due to synchrotron emission only, with no noticeable contributions from thermal sources. Using the simultaneous data, we constructed the SED of PKS 2155-304.

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