I. Hubeny

The University of Arizona, Tucson, Arizona, United States

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Publications (332)967.49 Total impact

  • [Show abstract] [Hide abstract]
    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.
    08/2014;
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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.
    02/2014; 440(2).
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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.
    12/2013;
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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.
    12/2013;
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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. · 4.97 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.
    08/2013;
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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; · 5.08 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; · 5.08 Impact Factor
  • 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.
    12/2012;
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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.
    12/2012;
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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; · 5.08 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. · 16.24 Impact Factor
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    ABSTRACT: Context. Hot post-AGB stars are luminous objects of low- and intermediate mass (0.8-8 M⊙) in the final stage of evolution, moving between the asymptotic giant branch (AGB) stars and planetary nebulae. The absorption lines observed in their spectra are typical of early-type stars and their abundance pattern may be associated with the occurence of one or more mixing processes during the previous AGB phase. Aims: To better constrain their observed chemical pattern and evolutionary status, we determined chemical abundances for a sample of hot post-AGB stars selected according to spectroscopic criteria. Methods: The observational data are high-resolution spectra obtained with the FEROS spectrograph. The stellar parameters and chemical composition were obtained from fully consistent non-LTE synthesis. Results: The general abundance pattern reveals relevant nitrogen enrichment, slight depletion in carbon and sulfur and mild excess in helium for most of the objects. One notable exception is LSE 148, with Z = 0.001, which is likely to be a metal-poor object at high galactic latitude. The atmospheric parameters and chemical abundances obtained are discussed in the context of evolutionary models. Mixing processes like the second/third dredged-up and "hot bottom-burning" are invoked to explain the obtained results. Based on observations made with the 2.2 m telescope at the European Southern Observatory (La Silla, Chile).
    Astronomy and Astrophysics 07/2012; · 5.08 Impact Factor
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    ABSTRACT: The BINSYN program suite is a collection of programs for analysis of binary star systems with or without an optically thick accretion disk. 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.
    Astrophysics Source Code Library. 07/2012;
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    ABSTRACT: I. Hubeny Today, the discussion will be open to the general audience. In Sessions C, D, and E, we have talked about models and modelling techniques so I expect the discussion will focus on these topics.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):341-348.
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    Ivan Hubeny
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    ABSTRACT: Most popular computer codes for calculating model stellar and planetary atmospheres are briefly reviewed. A particular emphasis is devoted to our universal computer program Tlusty (model stellar atmospheres and accretion disks), CoolTlusty (a variant of Tlusty for computing model atmospheres of substellar-mass objects such as giant planets and brown dwarfs), and Synspec (an associated spectrum synthesis code). We show the highlights of actual applications of these codes which include extensive grids of fully line-blanketed non-LTE model atmospheres of O and B stars, and grids of model atmospheres of extrasolar giant planets and L and T dwarfs.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):221-228.
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    ABSTRACT: The importance to stellar evolution of understanding the metal abundances in hot white dwarfs is well known. Previous work has found the hot DA white dwarfs REJ 1032+532, REJ 1614-085 and GD 659 to have highly abundant, stratified photospheric nitrogen, due to the narrow absorption-line profiles of the far-ultraviolet (FUV) N V doublet and the lack of extreme-ultraviolet (EUV) continuum absorption. A preliminary analysis of the extremely narrow, deep line profiles of the photospheric metal absorption features of PG 0948+534 suggested a similar photospheric metal configuration. However, other studies have found that REJ 1032+532, REJ 1614-085 and GD 659 can be well described by homogeneous models, with nitrogen abundances more in keeping with those of white dwarfs with higher effective temperatures. Here, a re-analysis of the nitrogen absorption features seen in REJ 1032+532, REJ 1614-085 and GD 659 is presented, with the aim of better understanding the structure of these stars, to test which models better represent the observed data and apply the results to the line profiles seen in PG 0948+534. A degeneracy is seen in the modelling of the nitrogen absorption-line profiles of REJ 1032+532, REJ 1614-085 and GD 659, with low-abundance, homogeneously distributed nitrogen models most likely being a better representation of the observed data. In PG 0948+534, no such degeneracy is seen, and the enigmatically deep line profiles could not be modelled satisfactorily.
    Monthly Notices of the Royal Astronomical Society 04/2012; 421(4):3222-3228. · 5.52 Impact Factor
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    ABSTRACT: I. Hubeny Welcome to the last panel meeting. We invite general comments either from the audience or from the panelists. V. Trimble Well, Mercedes started us with a vocabulary item and I think I would like to end with a vocabulary item. When they were first discovered, we called them `extra solar system planets' which was descriptive and fine, but it's just rather cumbersome. At some point they became `extra solar planets.' Now I have never seen a planet inside the Sun. And therefore `extrasolar' is not a good descriptor. `Exoplanets' is OK, but now that there are so many of them that perhaps they are simply `the planets.' When you want to specialize to ours, you could say `solar system planets.' Think how much ink it would save.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):551-553.
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    ABSTRACT: M. Richards: Several talks today have expressed fuzzy boundaries to describe the objects called ``stars.'' Is the following classification correct? Are stars restricted to objects that have masses greater than 0.089 solar masses and begin making energy with hydrogen burning? Do we include the stellar remnants: the white dwarfs and neturon stars? Do we include the brown dwarfs because they burn lithium or deuterium. We know that planets are not in this group since they have no energy production.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):145-152.
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    ABSTRACT: I. Hubeny Does anyone from the panel have a theme question to start with today? V. Trimble It's another one-liner: From an active galaxy meeting many years ago when people talked about spiral structure. I was reminded by Dr. Rucinski's talk of Lodewijk Woltjer's remark: ``The larger our ignorance, the stronger the magnetic field.''
    Proceedings of the International Astronomical Union 04/2012; 7(S282):501-506.

Publication Stats

5k Citations
967.49 Total Impact Points

Institutions

  • 1999–2014
    • The University of Arizona
      • Department of Astronomy
      Tucson, Arizona, United States
  • 1997–2009
    • Universities Space Research Association
      Houston, Texas, United States
  • 2003–2008
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 2007
    • New Mexico State University
      Las Cruces, New Mexico, United States
  • 1995–2005
    • Columbia University
      • Columbia Astrophysics Laboratory
      New York City, New York, United States
  • 2002–2004
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
    • Loyola University Maryland
      Baltimore, Maryland, United States
    • University of Sydney
      Sydney, New South Wales, Australia
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 2001
    • University of Hawaiʻi at Hilo
      Hilo, Hawaii, United States
  • 1988–1989
    • University of Colorado at Boulder
      Boulder, Colorado, United States