I. Hubeny

The University of Arizona, Tucson, Arizona, United States

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Publications (366)1003.15 Total impact

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    ABSTRACT: There has been growing observational evidence for the presence of condensates in the atmospheres and/or comet-like tails of extrasolar planets. As a result, systematic and homogeneous tables of dust properties are useful in order to facilitate further observational and theoretical studies. In this paper we present calculations and analysis of non-isotropic phase functions, asymmetry parameter (mean cosine of the scattering angle), absorption and scattering opacities, single scattering albedos, equilibrium temperatures, and radiative accelerations of dust grains relevant for extrasolar planets. Our assumptions include spherical grain shape, Deirmendjian particle size distribution, and Mie theory. We consider several species: corundum/alumina, iron, olivines with 0% and 50% iron content, pyroxenes with 0%, 20% and 60% iron content, carbon at two different temperatures, water ice, liquid water, and ammonia. The presented tables cover the wavelength range of 0.2 to 500 micron and modal particle radii from 0.01 micron to 100 micron. Equilibrium temperatures and radiative accelerations assume irradiation by a non-black-body source of light with temperatures from 7000K to 700K seen at solid angles from 2$\pi$ to $10^{-6}$ sr. The tables are provided to the community together with a simple code which allows for an optional, finite, angular dimension of the source of light (star) in the phase function.
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    ABSTRACT: The presence of a white dwarf in a resolved binary system, such as Sirius, provides an opportunity to combine dynamical information about the masses, from astrometry and spectroscopy, with a gravitational red-shift measurement and spectrophotometry of the white dwarf atmosphere to provide a test of theoretical mass-radius relations of unprecedented accuracy. We demonstrated this with the first Balmer line spectrum of Sirius B to be obtained free of contamination from the primary, with STIS on HST. However, we also found an unexplained discrepancy between the spectroscopic and gravitational red-shift mass determinations. With the recovery of STIS, we have been able to revisit our observations of Sirius B with an improved observation strategy designed to reduce systematic errors on the gravitational red-shift measurement. We provide a preliminary report on the refined precision of the Sirius B mass-radius measurements and the extension of this technique to a larger sample of white dwarfs in resolved binaries. Together these data can provide accurate mass and radius determinations capable of testing the theoretical mass-radius relation and distinguishing between possible structural models.
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    Sally Heap · Jean-Claude Bouret · Ivan Hubeny
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    ABSTRACT: Ultraviolet and 21-cm observations suggest that the extremely low-metallicity galaxy, I Zw 18, is a stream-fed galaxy containing a "pocket" of pristine stars responsible for producing nebular He II recombination emission observed in I Zw18-NW. Far-UV spectra by Hubble/COS and the Far Ultraviolet Spectroscopic Explorer (FUSE) make this suggestion conclusive by demonstrating that the spectrum of I Zw 18-NW shows no metal lines like O VI 1032, 1038 of comparable ionization as the He II recombination emission.
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    ABSTRACT: The paper deals with the issue of assessing approximate models used for the treatment of radiation coupled with hydrodynamics equations. Radiation plays a key role in many astrophysical structures, such as accretion shocks on classical T Tauri stars, but also in experimental flows, such as plasmas generated in laser driven radiative shocks. It is therefore crucial to test the accuracy of the approximate radiation moment models used in radiation hydrodynamics (RHD) calculations. Based on a laboratory shock simulation test case, we present comparisons of approximate radiation quantities calculated with the gray M1 model with the three-dimensional (3D) RHD code HERACLES, and reference radiation quantities obtained after solving the radiative transfer equation with the 3D radiative transfer code IRIS by post-processing a HERACLES structure. Our results indicate that radiation quantities are correctly calculated by M1 in regions of the computational domain far from lateral boundaries, from which photons can freely escape, and through which no photon can enter from the outside. However, M1 fails to reproduce correct quantities in the vicinity of these boundaries. We suggest to implement an improved version of the M1 model: a half-moment model that makes it possible to distinguish between incoming flux and outgoing flux at boundaries.
    High Energy Density Physics 11/2014; DOI:10.1016/j.hedp.2014.10.006 · 1.52 Impact Factor
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    ABSTRACT: Arguably, the best method for determining the effective temperature ($T_{\mathrm{eff}}$) and surface gravity (log $g$) of a DA white dwarf is by fitting the Hydrogen Lyman and Balmer absorption features. However, as has been shown for white dwarfs with $T_{\mathrm{eff}}$>50,000K, the calculated value from the Lyman and Balmer lines are discrepant, which worsens with increasing temperature. Many different solutions have been suggested, ranging from the input physics used to calculate the models, to interstellar reddening. We will focus on the former, and consider three variables. The first is the atomic data used, namely the number of transitions included in line blanketing treatments and the photoionization cross sections. The second is the stark broadening treatment used to synthesise the Lyman and Balmer line profiles, namely the calculations performed by Lemke (1997) and Tremblay & Bergeron (2009). Finally, the third is the atmospheric content. The model grids are calculated with a pure H composition, and a metal polluted composition using the abundances of Preval et al. (2013). We present the preliminary results of our analysis, whereby we have determined the $T_{\mathrm{eff}}$ for a small selection of white dwarfs. We plan to extend our analysis by allowing metallicity to vary in future model grids.
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    ABSTRACT: The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres.
    Proceedings of the International Astronomical Union 08/2014; 9(S307). DOI:10.1017/S1743921314006334
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    ABSTRACT: We present a series of systematic abundance measurements for 89 hydrogen atmosphere (DA-type) white dwarfs with temperatures spanning 16000-77000K drawn from the FUSE spectral archive. This is the largest study to date of white dwarfs where radiative forces are significant, exceeding our earlier work, based mainly on IUE and HST data, by a factor three. Using heavy element blanketed non-LTE stellar atmosphere calculations, we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and their error ranges using a \c{hi}2 fitting technique. We are able to establish the broad range of abundances seen in a given temperature range and establish the incidence of stars which appear, in the optical, to be atmospherically devoid of any material other than H. We compare the observed abundances to predictions of radiative levitation calculations, revealing little agreement. We propose that the supply of heavy elements is accreted from external sources rather than being intrinsic to the star. These elements are then retained in the white dwarf atmospheres by radiative levitation, a model that can explain both the diversity of measured abundances for stars of similar temperature and gravity, including cases with apparently pure H envelopes, and the presence of photospheric metals at temperatures where radiative levitation is no longer effective.
    Monthly Notices of the Royal Astronomical Society 02/2014; 440(2). DOI:10.1093/mnras/stu216 · 5.23 Impact Factor
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    ABSTRACT: We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our "gray LTE approach", i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared.
    The European Physical Journal Conferences 12/2013; 64. DOI:10.1051/epjconf/20136404005
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    ABSTRACT: Theory and simulations predict Quasi-Periodic Oscillations of shocks which develop in magnetically driven accretion funnels connecting the stellar disc to the photosphere of Young Stellar Objects (YSO). X-ray observations however do not show evidence of the expected periodicity. We examine here, in a first attempt, the influence of radiative transfer on the evolution of material impinging on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code ASTROLABE. The mechanical shock heating mechanism of the chromosphere only slightly perturbs the flow. We also show that, since the impacting flow, and especially the part which penetrates into the chromosphere, is not treated as a purely radiating transparent medium, a sufficiently efficient coupling between gas and radiation may affect or even suppress the oscillations of the shocked column. This study shows the importance of the description of the radiation effects in the hydrodynamics and of the accuracy of the opacities for an adequate modeling.
    The European Physical Journal Conferences 12/2013; 64. DOI:10.1051/epjconf/20136404002
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    Albert P. Linnell · Paul DeStefano · Ivan Hubeny
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    ABSTRACT: This paper extends synthetic photometry to components of binary star systems. The paper demonstrates accurate recovery of single star photometric properties for four photometric standards, Vega, Sirius, GD153, and HD209458, ranging over the HR diagram, when their model synthetic spectra are placed in fictitious binary systems and subjected to synthetic photometry processing. Techniques for photometric distance determination have been validated for all four photometric standards.
    The Astronomical Journal 08/2013; 146(3):68. DOI:10.1088/0004-6256/146/3/68 · 4.05 Impact Factor
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    ABSTRACT: We present a grid of optical (3200--7200 \AA) synthetic spectra calculated with Tlusty/Synspec. The new NLTE model atmospheres include the most recent hydrogen Stark broadening profiles; were calculated in opacity sampling and limited to pure H/He composition. The grid covers the observed parameter space of (He-)sdB and (He-)sdO stars, therefore it is suitable for the homogeneous spectral analyses of such evolved stars.
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    ABSTRACT: We study the evolution, rotation, and surface abundances of O-type dwarfs in the Small Magellanic Cloud. We analyzed the UV and optical spectra of twenty-three objects and derived photospheric and wind properties. The observed binary fraction of the sample is ~ 26%, which is compatible with more systematic studies, if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample excluded obvious spectroscopic binaries. The location of the fastest rotators in the H-R diagram indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity-class Vz, expected to best characterize extreme youth, is located on the ZAMS, the other two stars are more evolved. The distribution of nitrogen abundance of O and B stars suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depends only weakly on the star's mass. We confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that matches the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators. The N/O versus N/C ratios agree qualitatively well with those of stellar evolution models. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.
    Astronomy and Astrophysics 04/2013; DOI:10.1051/0004-6361/201220798 · 4.48 Impact Factor
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    ABSTRACT: Abridged. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The HI region provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. Our primary objective is to study the enrichment of the HI region and the interplay between star-formation history and metallicity evolution. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the abundances in the HII region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HI region through physical diagnostics drawn from the fine-structure level of C+. We find that HI region abundances are lower by a factor of ~2 as compared to the HII region. There is no differential depletion on dust between the HI and HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the HI envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the HI region by investigating the CII* absorption line. The cooling rate derived from CII* is consistent with collisions with H atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).
    Astronomy and Astrophysics 02/2013; 553. DOI:10.1051/0004-6361/201220948 · 4.48 Impact Factor
  • Ivan Hubeny
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    ABSTRACT: Basic concepts of the stellar atmospheres theory are briefly outlined. After discussing essential assumptions, approximations, and basic structural equations describing a stellar atmospheres, emphasis is given to describing efficient numerical methods developed to deal with the stellar atmosphere problem, namely the method of complete linearization and its recent variants, and the whole class of methods known by name Accelerated Lambda Iteration. The existing computer codes, and some of the most useful grids of model atmospheres that are publicly available, are briefly summarized. Some interesting properties of newly computed NLTE models atmospheres and their comparison to LTE models are shown. Finally, it is briefly shown how the model atmospheres are used to determine basic stellar parameters.
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    ABSTRACT: We show preliminary results of our numerical simulations of laboratory experiments of radiative shocks. Such experiments aim at understanding accretion shocks in young stellar objects. Three-dimensional non-stationary radiation hydrodynamics calculations were performed with the code HERACLES. X-UV spectra were then generated with the new three-dimensional radiative transfer code IRIS.
  • L. Ibgui · I. Hubeny · T. Lanz · C. Stehlé
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    ABSTRACT: We describe the implementation in our generic three-dimensional radiative transfer code, IRIS, of an algorithm that allows the modeling of periodic infinite media. We show how this algorithm has been validated by comparison with well-established 1D plane-parallel models. A particularly interesting astrophysical application will be the calculation of synthetic spectra of the fully three-dimensional solar atmosphere.
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    ABSTRACT: There are now many known exoplanets with M sin i within a factor of two of Neptune’s, including the transiting planets GJ436b and HAT-P-11b. Planets in this mass-range are different from their more massive cousins in several ways that are relevant to their radiative properties and thermal structures. By analogy with Neptune and Uranus, they are likely to have metal abundances that are an order of magnitude or more greater than those of larger, more massive planets. This increases their opacity, decreases Rayleigh scattering, and changes their equation of state. Furthermore, their smaller radii mean that fluxes from these planets are roughly an order of magnitude lower than those of otherwise identical gas giant planets. Here, we compute a range of plausible radiative equilibrium models of GJ436b and HAT-P-11b. In addition, we explore the dependence of generic Neptune-mass planets on a range of physical properties, including their distance from their host stars, their metallicity, the spectral type of their stars, the redistribution of heat in their atmospheres, and the possible presence of additional optical opacity in their upper atmospheres. Subject headings: equation of state – line: profiles – planetary systems – radiative transfer – stars: individual GJ436, HAT-P-11 – astrochemistry 1.
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    L. Ibgui · I. Hubeny · T. Lanz · C. Stehlé
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    ABSTRACT: We present IRIS, a new generic three-dimensional (3D) spectral radiative transfer code that generates synthetic spectra, or images. It can be used as a diagnostic tool for comparison with astrophysical observations or laboratory astrophysics experiments. We have developed a 3D short-characteristic solver that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise cubic, locally monotonic, interpolation technique that dramatically reduces the numerical diffusion effect. The code takes into account the velocity gradient effect resulting in gradual Doppler shifts of photon frequencies and subsequent alterations of spectral line profiles. It can also handle periodic boundary conditions. This first version of the code assumes Local Thermodynamic Equilibrium (LTE) and no scattering. The opacities and source functions are specified by the user. In the near future, the capabilities of IRIS will be extended to allow for non-LTE and scattering modeling. IRIS has been validated through a number of tests. We provide the results for the most relevant ones, in particular a searchlight beam test, a comparison with a 1D plane-parallel model, and a test of the velocity gradient effect. IRIS is a generic code to address a wide variety of astrophysical issues applied to different objects or structures, such as accretion shocks, jets in young stellar objects, stellar atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds, cosmological structures. It can also be applied to model laboratory astrophysics experiments, such as radiative shocks produced with high power lasers.
    Astronomy and Astrophysics 11/2012; 549. DOI:10.1051/0004-6361/201220468 · 4.48 Impact Factor
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    ABSTRACT: We present an online catalog containing spectra and supporting information for cataclysmic variables that have been observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). For each object in the catalog we list some of the basic system parameters such as (RA,Dec), period, inclination, white dwarf mass, as well as information on the available FUSE spectra: data ID, observation date and time, and exposure time. In addition, we provide parameters needed for the analysis of the FUSE spectra such as the reddening E(B-V), distance, and state (high, low, intermediate) of the system at the time it was observed. For some of these spectra we have carried out model fits to the continuum with synthetic stellar and/or disk spectra using the codes TLUSTY and SYNSPEC. We provide the parameters obtained from these model fits; this includes the white dwarf temperature, gravity, projected rotational velocity and elemental abundances of C, Si, S and N, together with the disk mass accretion rate, the resulting inclination and model-derived distance (when unknown). For each object one or more figures are provided (as gif files) with line identification and model fit(s) when available. The FUSE spectra as well as the synthetic spectra are directly available for download as ascii tables. References are provided for each object as well as for the model fits. In this article we present 36 objects, and additional ones will be added to the online catalog in the future. In addition to cataclysmic variables, we also include a few related objects, such as a wind accreting white dwarf, a pre-cataclysmic variable and some symbiotics.
    The Astrophysical Journal Supplement Series 10/2012; 203:29. DOI:10.1088/0067-0049/203/2/29 · 14.14 Impact Factor
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    Albert P. Linnell · Paul DeStefano · Ivan Hubeny
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    ABSTRACT: The BINSYN program suite, a collection of programs for analysis of binary star systems with or without an optically thick accretion disk, is available for download from a wiki. This article describes the package, including download instructions. BINSYN produces synthetic spectra of individual binary star components plus a synthetic spectrum of the system. If the system includes an accretion disk, BINSYN also produces a separate synthetic spectrum of the disk face and rim. A system routine convolves the synthetic spectra with filter profiles of several photometric standards to produce absolute synthetic photometry output. The package generates synthetic light curves and determines an optimized solution for system parameters. This article includes illustrative literature references that have used the suite, including mass transfer rates in several cataclysmic binary systems.
    Publications of the Astronomical Society of the Pacific 08/2012; 124(918):885-894. DOI:10.1086/667217 · 3.23 Impact Factor

Publication Stats

7k Citations
1,003.15 Total Impact Points

Institutions

  • 2004–2014
    • The University of Arizona
      • Department of Astronomy
      Tucson, Arizona, United States
  • 1994–2009
    • Universities Space Research Association
      Houston, Texas, United States
  • 2002–2008
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 2005
    • Columbia University
      • Columbia Astrophysics Laboratory
      New York City, New York, United States
  • 1999–2005
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom
  • 2002–2004
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2001
    • Villanova University
      노리스타운, Pennsylvania, United States
  • 1991–2000
    • NASA
      • Goddard Space Flight Centre
      Вашингтон, West Virginia, United States
  • 1993
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 1990
    • University of California, Los Angeles
      • Division of Astronomy & Astrophysics
      Los Ángeles, California, United States
  • 1988–1989
    • University of Colorado at Boulder
      Boulder, Colorado, United States
  • 1987
    • Université Libre de Bruxelles
      Bruxelles, Brussels Capital Region, Belgium
  • 1983
    • Collège de France
      Lutetia Parisorum, Île-de-France, France