Searcharxiv⌕ Search

arXiv subjects

Ivan Hubeny

Publications and source records attributed to Ivan Hubeny.

At least 55 records · Page 3Linked to original sources
arXiv2010

The Blue Hook Populations of Massive Globular Clusters

We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the "blue hook" phenomenon previously observed in UV images of the globular clusters omega Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the blue hook stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the blue hook population with respect to metallicity.

astro-ph.SR↗
arXiv2010

Models of Neptune-Mass Exoplanets: Emergent Fluxes and Albedos

There are now many known exoplanets with Msin(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.

astro-ph.EP↗
arXiv2009

V3885 Sagittarius: a Comparison with a Range of Standard Model Accretion Disks

A $\widetildeχ^2$ analysis of standard model accretion disk synthetic spectrum fits to combined $FUSE$ and STIS spectra of V3885 Sagittarius, on an absolute flux basis, selects a model that accurately represents the observed SED. Calculation of the synthetic spectrum requires the following system parameters. The cataclysmic variable secondary star period-mass relation calibrated by Knigge in 2007 sets the secondary component mass. A mean white dwarf (WD) mass from the same study, that is consistent with an observationally-determined mass ratio, sets the adopted WD mass of $0.7M_{\odot}$, and the WD radius follows from standard theoretical models. The adopted inclination, $i=65{\arcdeg}$, is a literature consensus, and is subsequently supported by $\widetildeχ^2$ analysis. The mass transfer rate is the remaining parameter to set the accretion disk $T_{\rm eff}$ profile, and the $Hipparcos$ parallax constrains that parameter to $\dot{M}=5.0{\pm}2.0{\times}10^{-9} M_{\odot} {\rm yr}^{-1}$ by a comparison with observed spectra. The fit to the observed spectra adopts the contribution of a $57,000{\pm}5000$K WD. The model thus provides realistic constraints on $\dot{M}$ and $T_{\rm eff}$ for a large $\dot{M}$ system above the period gap.

astro-ph.SR↗
arXiv2008

Modeling UX Ursae Majoris: An abundance of challenges

We present a system model for optical and far UV spectra of the nova-like variable UX UMa involving a white dwarf, secondary star, gas stream, hot spot and accretion disk using our code BINSYN and based on an initially adopted system distance. Calculated SED intensity data successfully fit successive tomographically-extracted annuli longward of the Balmer limit but require a postulated `iron curtain' shortward of the Balmer limit that is applied to the annulus section closest to the secondary star, while postulated recombination emission fills in the model SED shortward of the Balmer limit and is applied to the annulus section more remote from the secondary star. The same model fits $UBV$ 1954 light curves by Walker and Herbig. Fits to $HST$ $FOS$ spectra are approximate but require assumed time-variable changes in the SED. Comparable effects, possibly involving variable absorption, afflict $FUSE$ spectra.

astro-ph↗
arXiv2008

Spectrum and atmosphere models of irradiated transiting extrasolar giant planets

We show that a consistent fit to observed secondary eclipse data for several strongly irradiated transiting planets demands a temperature inversion (stratosphere) at altitude. Such a thermal inversion significantly influences the planet/star contrast ratios at the secondary eclipse,their wavelength dependences, and, importantly, the day-night flux contrast during a planetary orbit. The presence of the thermal inversion/stratosphere seems to roughly correlate with the stellar flux at the planet. Such temperature inversions might caused by an upper-atmosphere absorber whose exact nature is still uncertain

astro-ph↗
arXiv2008

Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars

Argon abundances have been derived for a sample of B main-sequence stars in the Orion association. The abundance calculations are based on NLTE metal line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an updated and complete argon model atom. We derive an average argon abundance for this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent agreement with a recent analysis of the Orion nebula, it is significantly higher than the currently recommended solar value which is based on abundance measurements in the solar corona. Moreover, the derived argon abundances in the Orion B stars agree very well with a measurement from a solar impulsive flare during which unmodified solar photospheric material was brought to flare conditions. We therefore argue that the argon abundances obtained independently for both the Orion B stars and the Orion nebula are representative of the disk abundance value in the solar neighborhood. The lower coronal abundance may reflect a depletion related to the FIP effect. We propose a new reference value for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10, corresponding to Ar/O = 0.009.

astro-ph↗
arXiv2008

Synthetic Spectrum Constraints on a Model of the Cataclysmic Variable QU Carinae

Neither standard model SEDs nor truncated standard model SEDs fit observed spectra of QU Carinae with acceptable accuracy over the range 900Åto 3000Å. Non-standard model SEDs fit the observation set accurately. The non-standard accretion disk models have a hot region extending from the white dwarf to $R=1.36R_{\rm wd}$,a narrow intermediate temperature annulus, and an isothermal remainder to the tidal cutoff boundary. The models include a range of $\dot{M}$ values between $1.0{\times}10^{-7}M_{\odot} {\rm yr}^{-1}$ and $1.0{\times}10^{-6}M_{\odot} {\rm yr}^{-1}$ and limiting values of $M_{\rm wd}$ between $0.6M_{\odot}$ and $1.2M_{\odot}$. A solution with $M_{\rm wd}=1.2M_{\odot}$ is consistent with an empirical mass-period relation. The set of models agree on a limited range of possible isothermal region $T_{\rm eff}$ values between 14,000K and 18,000K. The model-to-model residuals are so similar that it is not possible to choose a best model. The Hipparcos distance, 610 pc, is representative of the model results. The orbital inclination is between $40\arcdeg$ and $60\arcdeg$.

astro-ph↗
arXiv2007

Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands

We find that a theoretical fit to all the HD 209458b data at secondary eclipse requires that the dayside atmosphere of HD 209458b have a thermal inversion and a stratosphere. This inversion is caused by the capture of optical stellar flux by an absorber of uncertain origin that resides at altitude. One consequence of stratospheric heating and temperature inversion is the flipping of water absorption features into emission features from the near- to the mid-infrared and we see evidence of such a water emission feature in the recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere optical absorber may help explain both the weaker-than-expected Na D feature seen in transit and the fact that the transit radius at 24 $μ$m is smaller than the corresponding radius in the optical. Moreover, it may be a factor in why HD 209458b's optical transit radius is as large as it is. We speculate on the nature of this absorber and the planets whose atmospheres may, or may not, be affected by its presence.

astro-ph↗
arXiv2007

A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects

We present a systematic study of the spectral consequences of departures from chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler dwarfs. The temperature/pressure profiles of the non-equilibrium models are fully consistent with the non-equilibrium chemistry. Our grid of non-equilibrium models includes spectra for effective temperatures from 200 K to 1800 K, three surface gravities, four possible values of the coefficient of eddy diffusion in the radiative zone, and three different CO/CH$_4$ chemical reaction prescriptions. We find that the non-equilibrium overabundance of CO translates into flux suppressions in the M ($\sim4-$5 $μ$m) band of at most $\sim$40% between effective temperatures of 600 and 1800 K. The effect is largest around $T_{\rm eff} \approx 1100$ K. The underabundance of ammonia due to non-equilibrium chemistry translates into flux enhancements of no more than $\sim$20% for the $T_{\rm eff}$ range from 300 to 1800 K, with the largest effects at the lowest values of $T_{\rm eff}$. The magnitude of the departure from chemical equilibrium increases with decreasing gravity, with increasing eddy diffusion coefficient, and with decreasing speed of the CO/CH$_4$ reaction. Though these effects are modest, they lead to better fits with the measured T dwarf spectra. Furthermore, the suppression in the M band due to non-equilibrium enhancements in the CO abundance disappears below $\sim$500 K, and is only partial above $\sim$500 K, preserving the M band flux as a useful diagnostic of cool atmospheres and maintaining its importance for searches for brown dwarfs cooler than T dwarfs.

astro-ph↗
arXiv2006

MV Lyrae in Low, Intermediate, and High States

We use analysis results from a low state of the VY Sculptoris system MV Lyrae, considered in an earlier paper (Hoard et al., 2004, ApJ, 604, 346), to study archival IUE spectra taken during an intermediate state and a HST spectrum taken during a high state. The intermediate state spectrum can be best fit by an isothermal accretion disk extending half way to the tidal cutoff radius. The high state spectrum can be best fit by a standard model extending from an inner truncation radius to an intermediate radius and an isothermal accretion disk beyond.

astro-ph↗
arXiv2006

A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars

We have constructed a comprehensive grid of 1540 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to early B-type stars. The BSTAR2006 grid considers 16 values of effective temperatures, 15,000 K <= Teff <= 30,000 K with 1,000 K steps, 13 surface gravities, 1.75 <= log g <= 4.75 with 0.25 dex steps, 6 chemical compositions, and a microturbulent velocity of 2 km/s. The lower limit of log g for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions range from twice to one tenth of the solar metallicity and metal-free. Additional model atmospheres for B supergiants (log g <= 3.0) have been calculated with a higher microturbulent velocity (10 km/s) and a surface composition that is enriched in helium and nitrogen, and depleted in carbon. This new grid complements our earlier OSTAR2002 grid of O-type stars (Lanz & Hubeny, 2003, ApJS, 146, 417). The paper contains a description of the BSTAR2006 grid and some illustrative examples and comparisons. NLTE ionization fractions, bolometric corrections, radiative accelerations, and effective gravities are obtained over the parameter range covered by the grid. By extrapolating radiative accelerations, we have determined an improved estimate of the Eddington limit in absence of rotation between 55,000 and 15,000 K. The complete BSTAR2006 grid is available at the TLUSTY website (http://nova.astro.umd.edu).

astro-ph↗
arXiv2006

A New Algorithm for 2-D Transport for Astrophysical Simulations: I. General Formulation and Tests for the 1-D Spherical Case

We derive new equations using the mixed-frame approach for one- and two-dimensional (axisymmetric) time-dependent radiation transport and the associated couplings with matter. Our formulation is multi-group and multi-angle and includes anisotropic scattering, frequency(energy)-dependent scattering and absorption, complete velocity dependence to order v/c, rotation, and energy redistribution due to inelastic scattering. Hence, the "2D" realization is actually "6 1/2"-dimensional. The effects of radiation viscosity are automatically incorporated. Moreover, we develop Accelerated-Lambda-Iteration, Krylov subspace (GMRES), Discontinuous-Finite-Element, and Feautrier numerical methods for solving the equations and present the results of one-dimensional numerical tests of the new formalism. The virtues of the mixed-frame approach include simple velocity dependence with no velocity derivatives, straight characteristics, simple physical interpretation, and clear generalization to higher dimensions. Our treatment can be used for both photon and neutrino transport, but we focus on neutrino transport and applications to core-collapse supernova theory in the discussions and examples.

astro-ph↗
arXiv2006

Neon Abundances in B-Stars of the Orion Association: Solving the Solar Model Problem?

We report on non-LTE Ne abundances for a sample of B-type stellar members of the Orion Association. The abundances were derived by means of non-LTE fully metal-blanketed model atmospheres and extensive model atoms with updated atomic data. We find that these young stars have a very homogeneous abundance of A(Ne) = 8.27 +/- 0.05. This abundance is higher by ~0.4 dex than currently adopted solar value, A(Ne)=7.84, which is derived from lines produced in the corona and active regions. The general agreement between the abundances of C, N, and O derived for B stars with the solar abundances of these elements derived from 3-D hydrodynamical models atmospheres strongly suggests that the abundance patterns of the light elements in the Sun and B stars are broadly similar. If this hypothesis is true, then the Ne abundance derived here is the same within the uncertainties as the value required to reconcile solar models with helioseismological observations.

astro-ph↗
arXiv2006

A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries

Self-consistent vertical structure models together with non-LTE radiative transfer should produce spectra from accretion disks around black holes which differ from multitemperature blackbodies at levels which may be observed. High resolution, high signal-to-noise observations warrant spectral modeling which both accounts for relativistic effects, and treats the physics of radiative transfer in detail. In Davis et al. (2005) we presented spectral models which accounted for non-LTE effects, Compton scattering, and the opacities due to ions of abundant metals. Using a modification of this method, we have tabulated spectra for black hole masses typical of Galactic binaries. We make them publicly available for spectral fitting as an Xspec model. These models represent the most complete realization of standard accretion disk theory to date. Thus, they are well suited for both testing the theory's applicability to observed systems and for constraining properties of the black holes, including their spins.

astro-ph↗
arXiv2005

L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence with observed spectra and infrared colors for early- and mid-L dwarfs and for mid- to late-T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and, therefore, that gravity is the "second parameter" of the L/T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams upon various cloud properties, such as particle size and cloud spatial distribution. In the region of the L$\to$T transition, we find that no one cloud-particle-size and gravity combination can be made to fit all the observed data. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund (2002) produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L$\to$T transition region might be achieved.

astro-ph↗
arXiv2005

Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

We report the detection, from FUSE data, of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. Collectively, these objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We use a grid of homogeneous non-LTE synthetic spectra to measure abundances of[P/H]=-8.57 +0.09 -0.13, -8.70 +0.23 -0.37 and -8.36 +0.14 -0.19 in GD71, RE J1918+595 and RE J0605-482 respectively. At the observed level we find phosphorus has no significant impact on the overall energy distribution of GD71. We explore possible mechanisms responsible for the presence of this element in these stars, concluding that the most likely is an interplay between radiative levitation and gravitational settling but possibly modified by weak mass loss.

astro-ph↗