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We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the determination of ages and metallicities old stellar systems on a quantitative basis. Our program will provide the tools to analyze their... more
We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the determination of ages and metallicities old stellar systems on a quantitative basis. Our program will provide the tools to analyze their composite integrated-light spectra in the near ultraviolet (UV). We will calculate an extensive grid of stellar spectra from 2280A to 3120A from first
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The goal of our Hubble Treasury Program is to provide observational and theoretical spectral templates for interpreting the age metallicity and population of old stellar systems. To this end we are calculating stellar mid-UV and optical... more
The goal of our Hubble Treasury Program is to provide observational and theoretical spectral templates for interpreting the age metallicity and population of old stellar systems. To this end we are calculating stellar mid-UV and optical spectra from first principles for comparison with observed spectra. Here we present the Peterson et al. (2003) comparison of coadditions of such spectral calculations to archival mid-UV and optical spectra of the globular cluster G1 in Andromeda (M31). Our composite spectra were generated by coadding the fluxes calculated for four to six individual stellar models weighted by the model radius squared times the relative number of stars represented by the model. We attain an excellent match to the Ponder et al. (1998) FOS observations but only when the calculations include stars on the cool blue horizontal branch (BHB) and hot extreme horizontal branch (EHB). Our metallicity [Fe/H] = -0.8 and BHB fraction both agree well with the results of Rich et al. (1996) from the G1 color-magnitude diagram. Our G1 turnoff temperature is 6000K - 6100K that of the Galactic halo. We conclude that G1 is old and mildly metal-poor but nonetheless harbors both BHB and EHB stars. Only the UV can discern them.
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The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution considers a criterion for the existence of a partial mixing zone in a general stellar model. This criterion implies that there must be a zone... more
The present semianalytical treatment for aspects of horizontal branch (HB) star core evolution considers a criterion for the existence of a partial mixing zone in a general stellar model. This criterion implies that there must be a zone with a composition gradient. Attention is given to the consequences of the assumption for the behavior of the core close to the central He exhaustion; with the degree of mixing implied by the constraint on the temperature gradient, core opacity tends to a limit late in evolution, and then declines. The central opacity and flux decreases give the convective region a strong tendency to shrink.
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ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provide both theoretical and observational spectral templates for the mid-ultraviolet, from 2200Å to 3150Å and beyond, for a quantitative... more
ABSTRACT This is a progress report for our Hubble Treasury program GO-10094. Our goal is to provide both theoretical and observational spectral templates for the mid-ultraviolet, from 2200Å to 3150Å and beyond, for a quantitative interpretation of the spectrum of a stellar system such as a globular cluster or elliptical galaxy. This is aimed at determining the age and metallicity of systems as young as 1 Gyr. We are now calculating grids of theoretical spectral templates for stars all across the color-magnitude diagram, from red giants to turn-off stars to blue stragglers to extreme blue horizontal branch stars. The grids span a wide range of metallicities, and incorporate both the solar mix of elemental abundances and mixes with enhanced abundances of light elements such as magnesium. At the same time we are calculating stellar tracks and isochrones for these metallicities and mixtures. We will then generate theoretical spectral templates for single-age, single-metallicity systems by coadding the stellar spectra with weights derived from the isochrones. We will make public the stellar and composite grids, as well as broad-band indices and colors derived from them. We present examples of the excellent matches between observed and theoretical spectra achieved to date, in both the ultraviolet and the optical spectral regions. We also illustrate the application of these theoretical spectra to several problems, from the determination of very heavy elements produced by the s- and r-process in individual stars, to the age and metallicity of extragalactic globular clusters and remote galaxies.
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The effect of higher than solar oxygen-to-iron abundance ratios on horizontal branch stellar evolution is reviewed. It is found that the evolutionary masses of pulsating stars may be reduced quite significantly as a result of changes in... more
The effect of higher than solar oxygen-to-iron abundance ratios on horizontal branch stellar evolution is reviewed. It is found that the evolutionary masses of pulsating stars may be reduced quite significantly as a result of changes in nucleosynthesis resulting from higher CNO abundance, whereas an increase in the opacity alone causes much smaller changes in the mass at fixed color. The implications of the results for the forthcoming revision in metal opacities in a narrow range of stellar envelope temperatures are discussed.
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We present deep ultraviolet observations of a field containing the cluster Abell 2246 (z=0.225) which provide far-ultraviolet (FUV) images of some of the faintest galaxies yet observed in that bandpass. Abell 2246 is within the field of... more
We present deep ultraviolet observations of a field containing the cluster Abell 2246 (z=0.225) which provide far-ultraviolet (FUV) images of some of the faintest galaxies yet observed in that bandpass. Abell 2246 is within the field of view of Ultraviolet Imaging Telescope (UIT) observations of the quasar HS1700+64, which, as a high-priority target for the Hopkins Ultraviolet Telescope, accumulated over
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The blue horizontal branch population consists of extreme horizontal branch (EHB) stars and the cooler intermediate blue horizontal branch (IBHB) stars. EHB stars have very tiny hydrogen envelopes (<= 0.05M_sun) as a result of... more
The blue horizontal branch population consists of extreme horizontal branch (EHB) stars and the cooler intermediate blue horizontal branch (IBHB) stars. EHB stars have very tiny hydrogen envelopes (<= 0.05M_sun) as a result of extreme mass loss during the red giant branch (RGB) phase. They occupy a small range in mass and evolve at high effective temperatures after core He
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In the last two decades, considerable progress has been made in our understanding of the physical properties and evolutionary status of hot, hydrogenrich subdwarfs B stars (see, e.g., Saffer et al. 1994). It is now currently believed... more
In the last two decades, considerable progress has been made in our understanding of the physical properties and evolutionary status of hot, hydrogenrich subdwarfs B stars (see, e.g., Saffer et al. 1994). It is now currently believed (e.g., Heber et al. 1984) that sdB stars are ~ 0.5 M ⊙ objects belonging to the so-called extended horizontal branch (EHB), which never evolve toward the asymptotic giant branch (AGB) after core helium exhaustion (their hydrogen envelope masses being too small) (see, e.g., Dorman 1995 for a review). They remain at high effective temperatures (T e f f ≥ 20000K) throughout their core-burning evolution and ultimately contribute to a small fraction of the total white dwarf population (Bergeron et al. 1994).
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Theoretical models are calculated for stars on the hydrogen-burning lower main sequence having masses less than or equal to 0.55 solar mass. Particular attention has been paid to the treatment of the atmosphere and envelope and to the... more
Theoretical models are calculated for stars on the hydrogen-burning lower main sequence having masses less than or equal to 0.55 solar mass. Particular attention has been paid to the treatment of the atmosphere and envelope and to the choice of the radiative opacities and equation of state. The dependence of the model properties on the outer boundary conditions is examined, with a detailed comparison of models constructed using two independent determinations of the equation of state. It is found that the theoretically inferred properties of stars of mass less than about 0.10 solar mass can differ noticeably depending on the assumed input physics.
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... HORIZONTAL-BRANCH SEQUENCES BEN DORMAN Department of Physics and Astronomy, University ofVictoria; and Department of Astronomy, University of ... calculations by Sweigart ( 1987, hereafter 587 ), computations by the Italian groups at... more
... HORIZONTAL-BRANCH SEQUENCES BEN DORMAN Department of Physics and Astronomy, University ofVictoria; and Department of Astronomy, University of ... calculations by Sweigart ( 1987, hereafter 587 ), computations by the Italian groups at and Frascati (Bencivenni et al. ...
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... An astrometric solution was determined for each frame by matching detected sources with those of Dickens et al.(1988). ... The data are dereddened and the tracks shifted by the distance modu-lus, derived from fitting HB evolutionary... more
... An astrometric solution was determined for each frame by matching detected sources with those of Dickens et al.(1988). ... The data are dereddened and the tracks shifted by the distance modu-lus, derived from fitting HB evolutionary models of Dickens et al.(1988) optical data. ...
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We present a quantitative analysis of the UV radiation from evolved low mass hot stars. We use a new, extensive grid of horizontal branch (HB) and post-HB models (Dorman, Rood, & O'Connell 1993, preprint), which concentrates on... more
We present a quantitative analysis of the UV radiation from evolved low mass hot stars. We use a new, extensive grid of horizontal branch (HB) and post-HB models (Dorman, Rood, & O'Connell 1993, preprint), which concentrates on extreme HB stars and their progeny, post-Early AGB and AGB-Manque stars, and post-AGB evolutionary sequences. The models cover the metallicity range -2.3 < [Fe/H] 0. We also present sample UV spectra from our synthetic stellar populations and demonstrate the variation of the spectral energy distribution with the stellar mass distribution.
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During the second flight of the Astro observatory in March 1995, we used the Ultraviolet Imaging Telescope (UIT) to obtain ultraviolet ( ~ 1600 Angstroms) images of five old open clusters. Most stars in an old open cluster are too cool to... more
During the second flight of the Astro observatory in March 1995, we used the Ultraviolet Imaging Telescope (UIT) to obtain ultraviolet ( ~ 1600 Angstroms) images of five old open clusters. Most stars in an old open cluster are too cool to be detected in the ultraviolet, but among the possible UV-bright sources are blue stragglers, hot white dwarfs, and sdB or sdO stars. In NGC 188, we detect only the hot subdwarf and possible member II-91, and the blue straggler E43 (from Caputo et al. 1990, AJ, 99, 261). In M67, we detect ten blue stragglers, two white dwarf candidates (including the ROSAT source G152) and, surprisingly, the red giant S1040 from the catalogue of Sanders (1977). The star S1040 (V = 11.52, B--V = 0.82) is a single-lined spectroscopic binary with a period of 42.8 days, and an astrometric and radial-velocity member of M67 (Mathieu et al. 1990, AJ, 100, 1859). It occupies a ``red straggler'' position in the color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. Our detection of S1040 in the ultraviolet implies that (1) the secondary is likely a hot ( ~ 25,000 K) white dwarf, and (2) the unusual position of S1040 in the CMD is likely due to previous mass transfer from the white dwarf progenitor. We discuss the implications our observations for the study of mass transfer binaries, and the origin of the blue stragglers in M67.
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We present our concept for Iris, an ultraviolet (UV, lambda ~ 1000-3000 Angstroms ) imaging mission which will provide exciting new capabilities in both angular and spectral resolution. Iris will use a tuneable UV filter to provide... more
We present our concept for Iris, an ultraviolet (UV, lambda ~ 1000-3000 Angstroms ) imaging mission which will provide exciting new capabilities in both angular and spectral resolution. Iris will use a tuneable UV filter to provide continusly adjustable wavelength and variable bandwidth. Iris will provide subarcsecond resolution UV images over a ~ 10arcmin field. By taking advantage of both the extremely dark sky background in the far UV as well as the factor ~ 4 improvement in diffraction pattern size of the far UV relative to the optical, Iris will have a major impact on UV imaging capabilities using a modest sized ( ~ 1 m) telescope. Scientific problems to be tackled with Iris include: making detailed diagnoses of the hot interstellar medium; obtaining a more complete understanding of the origins and evolution of galaxies; measuring the energy balance of hot gas in galaxies and clusters of galaxies; and determining properties and dynamics of gas in and around active galaxies at redshifts up to z ~ 2.2. The Iris mission will facilitate major new scientific discoveries and will fill a unique and important role in the NASA Astrophysics Program.
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We present comparisons of theoretical and observational high-resolution spectra for a half-dozen stars of a wide range of temperature and abundance, from A star to K giant. These show the fits achieved to date by our ab initio spectral... more
We present comparisons of theoretical and observational high-resolution spectra for a half-dozen stars of a wide range of temperature and abundance, from A star to K giant. These show the fits achieved to date by our ab initio spectral calculations. These comparisons form the first phase of our three-year Hubble Treasury program GO-9455/9974, aimed at providing mid-ultraviolet spectral templates to improve the determination of the age and metallicity of old stellar systems. From matches such as these, we have modified the input atomic-line parameters and guessed the identifications of spectral lines missing from the calculations, as described by Peterson, Dorman, & Rood (2001, ApJ, 559, 372). With this new line list, we now match well the optical spectra of stars of all line strengths. We have begun to calculate a grid of optical indices from the theoretical spectra. In the mid-UV, while the fits at solar abundance are much improved, we are still missing very weak absorption lines near 2650Å and 2900Å. This will be addressed as additional mid-ultraviolet spectra are taken for a larger range of stellar targets during Cycle 13. Support for this work includes grants GO-9455 and GO-9974 from the Hubble Space Telescope Science Institute, and an award from the NASA-OSS Long Term Space Astrophysics program.
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XSPEC was designed to be a mission-independent general purpose X-ray data analysis program. In order for XSPEC to keep fulfilling this role well into the 21st Century we have re-implemented its code base in ANSI C++ using object oriented... more
XSPEC was designed to be a mission-independent general purpose X-ray data analysis program. In order for XSPEC to keep fulfilling this role well into the 21st Century we have re-implemented its code base in ANSI C++ using object oriented programming techniques. The abstract programming formulation used for XSPEC12 gives considerable advantages for the future. It will be possible to implement
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The UIT on Astro-2 in March 1995 obtained deep far-UV exposures of the clusters NGC 362 and NGC 104 (= 47 Tuc). Both of these clusters are famous for having red horizontal branches. We report the detection of UV sources in both of these... more
The UIT on Astro-2 in March 1995 obtained deep far-UV exposures of the clusters NGC 362 and NGC 104 (= 47 Tuc). Both of these clusters are famous for having red horizontal branches. We report the detection of UV sources in both of these clusters, some of which appear to be blue HB stars that are almost certain cluster members.
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Research Interests: Astrophysics and Stars
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... B. , Paltrinieri, B. , Ferraro, FR , Fusi Pecci, F., &: Rood, RT , 1997, in preparation Faulkner, J. 1966, ApJ, 144, 978 Faulkner. J. 1972, ApJ, 173, 401 Ferraro, FR, Paltrinieri, B., Fusi Pecci, F., et at. 1997a, A&_-A, 324,... more
... B. , Paltrinieri, B. , Ferraro, FR , Fusi Pecci, F., &: Rood, RT , 1997, in preparation Faulkner, J. 1966, ApJ, 144, 978 Faulkner. J. 1972, ApJ, 173, 401 Ferraro, FR, Paltrinieri, B., Fusi Pecci, F., et at. 1997a, A&_-A, 324, 915 Ferraro. ... L. & Renzini, A. 1990, ApJ, 364, 35 Heber. ...
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We discuss far-UV (1500 Å) surface photometry and FUV-B color profiles for six E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope during the Astro-2 mission. In three cases, the FUV radial profiles are more consistent... more
We discuss far-UV (1500 Å) surface photometry and FUV-B color profiles for six E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope during the Astro-2 mission. In three cases, the FUV radial profiles are more consistent with an exponential than a de ...