Rene Rohrmann - Academia.edu (original) (raw)
Papers by Rene Rohrmann
Astronomy & astrophysics, May 22, 2024
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Astronomy & Astrophysics
The nonrelativistic cross section from Rayleigh scattering by hydrogen atoms in the ground state ... more The nonrelativistic cross section from Rayleigh scattering by hydrogen atoms in the ground state was calculated over a wide range of photon energies (<0.8 keV). Evaluations were performed in terms of the real and imaginary components of the atomic polarizability. The sum over intermediate states that characterizes this second-order radiative process was performed using exact analytic expressions for oscillator strengths of bound and continuum states. Damping terms associated with the finite lifetimes of excited states and their splitting into two fine-structure levels (p1/2 and p3/2) are taken into account in resonance cross sections. Fitting formulas required for cross-section evaluation are presented for incident photon energy (i) redward of the first resonance (Lyman-α1/2), (ii) in the spectral region corresponding to resonances (for an arbitrary number of them), and (iii) above the ionization threshold.
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El trabajo apunta a determinar el origen y naturaleza de las estrellas de C deficientes en H y po... more El trabajo apunta a determinar el origen y naturaleza de las estrellas de C deficientes en H y por lo tanto, su relacion con las restantes estrellas de nuestra galaxia. Apunta a determinar la edad y el estado evolutivo de estas estrellas. En lo que se refiere a las atmosferas, se trata de desarrollar una rutina que calcule en forma automatica un modelo, tomando debidamente en cuenta las dificultades del problema (apartamiento de LTE, transporte convectivo, etc.), de modo de poderlo aplicar al estudio e interpretacion de los espectros de las estrellas HdC. Objetivos Contribuye a clarificar la naturaleza de las estrellas HdC: 1. Una correcta identificacion de las estrellas de esta clase por las caracteristicas que presentan sus espectros. 2. Determinar la abundancia de los distintos elementos en sus atmosferas. 3. Determinar su luminosidad intrinseca. 4. Poder decidir cuales son las caracteristicas cinematicas del grupo. Dentro del tema de las atmosferas estelares, es de interes contar con una rutina que sobre la base de ciertos datos basicos (temperatura efectiva, composicion quimica y gravedad superficial) calcule un modelo con un minimo de intervencion externa. Con este elemento disponible, hay temas interesantes que se estan investigando dentro del grupo, como formacion de lineas del He en estrellas B con atmosferas extendidas (en presencia de un viento estelar y de una cromosfera) y la distinta estructura que se debe esperar de una atmosfera de una estrella HdC, a causa de ser el C el siguiente elemento en importancia despues del He.
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From the paper of Martin & Wickramasinghe (1979) we have developed a program to calculate the Sto... more From the paper of Martin & Wickramasinghe (1979) we have developed a program to calculate the Stokes parameters for different models of stellar atmospheres. Particularly, we applied it in B stars with magnetic fields. The Stokes parameters are a function of the value of the source function in each point of the atmosphere. We analyze how the magnetic fields affect the source function and how the latter influences the behavior of the Stokes parameters. FULL TEXT IN SPANISH
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The atmosphere structure and emergent radiation of white dwarfs perturbed by external radiation f... more The atmosphere structure and emergent radiation of white dwarfs perturbed by external radiation fluxes are studied through the use of detailed atmosphere models. The analysis corresponds to stellar conditions in short-period (P < 10 h) double degenerates (systems that consist of two close white dwarfs). Our results show that the irradiation effect in a close degenerate binary can have a significant influence on the temperatures and spectral energy distribution of the irradiated stellar surfaces.
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Se presentan resultados de modelos de atmósferas para estrellas tempranas que poseen una estructu... more Se presentan resultados de modelos de atmósferas para estrellas tempranas que poseen una estructura cromosférica y una atmósfera extendida en expansión. Se analiza con rigurosidad el acople del campo de radiación en el continuo de Lyman y su relación con las poblaciones en equilibrio estadístico del H y He.
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White dwarf (WD) stars are the most common end-product of stellar evolution. Due to the high degr... more White dwarf (WD) stars are the most common end-product of stellar evolution. Due to the high degree of electron degeneracy in the interior of WDs, the non-degenerate outer layers and specially their atmospheres, control the loss of energy into outer space and therefore the cooling of these objects. Detailed atmosphere models provide the adequate surface boundary condition for the computation of evolutionary stellar models. Furthermore, atmosphere models are necessary to evaluate the emergent flux from the star, which is required to interpret the spectra, luminosities, and colors of observed WDs. Progress in our understanding of cool WDs and their observational appearances is closely related to the advances of atmosphere physics. Analysis of cool WD atmospheres demands the use of numerical codes dealing with radiative transfer, convection, non-ideal gas effects, pressure-induced opacities, as well as pressure ionization, partial degeneration, and electron thermal conduction in ultrac...
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arXiv (Cornell University), Mar 7, 2023
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Monthly Notices of the Royal Astronomical Society
We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space... more We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space Telescope optical imaging. Its position in the colour–magnitude diagrams and its spectrum are consistent with this star being a helium-core white dwarf (He WD) in a binary system. The optical light curve shows a periodicity at 18.5 h. We argue that this periodicity is due to the rotation of the WD and possibly due to magnetic spots on the surface of the WD. This would make this object the first candidate magnetic He WD in any GC, and the first candidate magnetic WD in a detached binary system in any GC and one of the few He WDs with a known rotation period and of magnetic nature. Another possibility is that this system is an He WD in a binary system with another WD or another degenerate object, which would make this object one of the few candidate non-accreting double-degenerate binaries in any GC.
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We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO ... more We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO white dwarfs, PG 1159 stars, and DB white dwarfs. White dwarf sequences are computed for a wide range of stellar masses and helium envelopes on the basis of a complete treatment of the evolutionary history of progenitors stars, including the core hydrogen and helium burning phases, the thermally pulsing asymptotic giant branch phase, and the born-again episode that is responsible for the hydrogen deficiency. We also provide colors and magnitudes for the new sequences for Teff < 40,000 K, where the NLTE effects are not dominant. These new calculations provide a homogeneous set of evolutionary tracks appropriate for mass and age determinations for both PG 1159 stars and DO white dwarfs. The calculations are extended down to an effective temperature of 7000 K. We applied these new tracks to redetermine stellar masses and ages of all known DO white dwarfs with spectroscopically determined...
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arXiv (Cornell University), Sep 30, 2012
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ABSTRACT Outer boundary conditions (temperature, pressure, outer stellar mass and geometrical dep... more ABSTRACT Outer boundary conditions (temperature, pressure, outer stellar mass and geometrical depth) at a Rosseland mean optical depth 25.1189, for effective temperatures between 2000K and 40000K, and 5.5<logg<9.0. These calculations, based on detailed stellar atmosphere models, are appropriated for computing the evolution of cool white dwarfs with hydrogen envelopes. (1 data file).
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Fil: Vera Rueda, Gustavo Matias. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Cent... more Fil: Vera Rueda, Gustavo Matias. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - San Juan. Instituto de Ciencias Astronomicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronomicas, de la Tierra y del Espacio; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Fisicas y Naturales; Argentina
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Astronomy & Astrophysics, 2020
We assess the partition function and ionization degree of magnetized hydrogen atoms at thermodyna... more We assess the partition function and ionization degree of magnetized hydrogen atoms at thermodynamic equilibrium for a wide range of field intensities, B ≈ 105–1012 G. Evaluations include fitting formulae for an arbitrary number of binding energies, the coupling between the internal atomic structure and the center-of-mass motion across the magnetic field, and the formation of the so-called decentered states (bound states with the electron shifted from the Coulomb well). Non-ideal gas effects are treated within the occupational probability method. We also present general mathematical expressions for the bound state correspondence between the limits of zero-field and high-field. This let us evaluate the atomic partition function in a continuous way from the Zeeman perturbative regime to very strong fields. Results are shown for conditions found in atmospheres of magnetic white dwarf (MWD) stars, with temperatures T ≈ 5000–80 000 K and densities ρ ≈ 10−12–10−3 g cm3. Our evaluations sh...
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International Astronomical Union Colloquium, 2000
We undertook a non-LTE radiative transfer calculation to find out the possible conditions under w... more We undertook a non-LTE radiative transfer calculation to find out the possible conditions under which observed H and He I line profiles arise from some Be stars.
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Astronomy & Astrophysics, 2019
Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asy... more Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asymptotic giant branch (AGB), type Ia supernova explosions, and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these white dwarfs, there are few evolutionary studies in the literature devoted to them. Here we present new ultra-massive white dwarf evolutionary sequences that constitute an improvement over previous ones. In these new sequences we take into account for the first time the process of phase separation expected during the crystallization stage of these white dwarfs by relying on the most up-to-date phase diagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution during the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditions for our evolving models are provided by detailed non-gray white dwarf model atm...
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Astronomy & Astrophysics, 2019
A selection of 17 stars in the Sloan Digital Sky Survey, previously identified as DC-class white ... more A selection of 17 stars in the Sloan Digital Sky Survey, previously identified as DC-class white dwarfs (WDs), has been reported to show spectra very close to blackbody radiation in the wavelength range from ultraviolet to infrared. Because of the absence of lines and other details in their spectra, the surface gravity of these objects has previously been poorly constrained, and their effective temperatures have been determined by fits to the continuum spectrum using pure helium atmosphere models. We computed model atmospheres with pure helium and H/He mixtures and used Gaia DR2 parallaxes that are available for 16 of the 17 selected stars to analyze their physical properties. We find that the atmospheres of the selected stars are very probably contaminated with a trace amount of hydrogen of −6 ≤ log(NH/NHe) ≤ −5.4. For the 16 stars with Gaia parallaxes, we calculate a mean stellar mass 0.606 ± 0.076 M⊙, which represents typical mass values and surface gravities (7.8 < logg <...
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Astronomy & astrophysics, May 22, 2024
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Astronomy & Astrophysics
The nonrelativistic cross section from Rayleigh scattering by hydrogen atoms in the ground state ... more The nonrelativistic cross section from Rayleigh scattering by hydrogen atoms in the ground state was calculated over a wide range of photon energies (<0.8 keV). Evaluations were performed in terms of the real and imaginary components of the atomic polarizability. The sum over intermediate states that characterizes this second-order radiative process was performed using exact analytic expressions for oscillator strengths of bound and continuum states. Damping terms associated with the finite lifetimes of excited states and their splitting into two fine-structure levels (p1/2 and p3/2) are taken into account in resonance cross sections. Fitting formulas required for cross-section evaluation are presented for incident photon energy (i) redward of the first resonance (Lyman-α1/2), (ii) in the spectral region corresponding to resonances (for an arbitrary number of them), and (iii) above the ionization threshold.
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Bookmarks Related papers MentionsView impact
El trabajo apunta a determinar el origen y naturaleza de las estrellas de C deficientes en H y po... more El trabajo apunta a determinar el origen y naturaleza de las estrellas de C deficientes en H y por lo tanto, su relacion con las restantes estrellas de nuestra galaxia. Apunta a determinar la edad y el estado evolutivo de estas estrellas. En lo que se refiere a las atmosferas, se trata de desarrollar una rutina que calcule en forma automatica un modelo, tomando debidamente en cuenta las dificultades del problema (apartamiento de LTE, transporte convectivo, etc.), de modo de poderlo aplicar al estudio e interpretacion de los espectros de las estrellas HdC. Objetivos Contribuye a clarificar la naturaleza de las estrellas HdC: 1. Una correcta identificacion de las estrellas de esta clase por las caracteristicas que presentan sus espectros. 2. Determinar la abundancia de los distintos elementos en sus atmosferas. 3. Determinar su luminosidad intrinseca. 4. Poder decidir cuales son las caracteristicas cinematicas del grupo. Dentro del tema de las atmosferas estelares, es de interes contar con una rutina que sobre la base de ciertos datos basicos (temperatura efectiva, composicion quimica y gravedad superficial) calcule un modelo con un minimo de intervencion externa. Con este elemento disponible, hay temas interesantes que se estan investigando dentro del grupo, como formacion de lineas del He en estrellas B con atmosferas extendidas (en presencia de un viento estelar y de una cromosfera) y la distinta estructura que se debe esperar de una atmosfera de una estrella HdC, a causa de ser el C el siguiente elemento en importancia despues del He.
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From the paper of Martin & Wickramasinghe (1979) we have developed a program to calculate the Sto... more From the paper of Martin & Wickramasinghe (1979) we have developed a program to calculate the Stokes parameters for different models of stellar atmospheres. Particularly, we applied it in B stars with magnetic fields. The Stokes parameters are a function of the value of the source function in each point of the atmosphere. We analyze how the magnetic fields affect the source function and how the latter influences the behavior of the Stokes parameters. FULL TEXT IN SPANISH
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The atmosphere structure and emergent radiation of white dwarfs perturbed by external radiation f... more The atmosphere structure and emergent radiation of white dwarfs perturbed by external radiation fluxes are studied through the use of detailed atmosphere models. The analysis corresponds to stellar conditions in short-period (P < 10 h) double degenerates (systems that consist of two close white dwarfs). Our results show that the irradiation effect in a close degenerate binary can have a significant influence on the temperatures and spectral energy distribution of the irradiated stellar surfaces.
Bookmarks Related papers MentionsView impact
Se presentan resultados de modelos de atmósferas para estrellas tempranas que poseen una estructu... more Se presentan resultados de modelos de atmósferas para estrellas tempranas que poseen una estructura cromosférica y una atmósfera extendida en expansión. Se analiza con rigurosidad el acople del campo de radiación en el continuo de Lyman y su relación con las poblaciones en equilibrio estadístico del H y He.
Bookmarks Related papers MentionsView impact
White dwarf (WD) stars are the most common end-product of stellar evolution. Due to the high degr... more White dwarf (WD) stars are the most common end-product of stellar evolution. Due to the high degree of electron degeneracy in the interior of WDs, the non-degenerate outer layers and specially their atmospheres, control the loss of energy into outer space and therefore the cooling of these objects. Detailed atmosphere models provide the adequate surface boundary condition for the computation of evolutionary stellar models. Furthermore, atmosphere models are necessary to evaluate the emergent flux from the star, which is required to interpret the spectra, luminosities, and colors of observed WDs. Progress in our understanding of cool WDs and their observational appearances is closely related to the advances of atmosphere physics. Analysis of cool WD atmospheres demands the use of numerical codes dealing with radiative transfer, convection, non-ideal gas effects, pressure-induced opacities, as well as pressure ionization, partial degeneration, and electron thermal conduction in ultrac...
Bookmarks Related papers MentionsView impact
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arXiv (Cornell University), Mar 7, 2023
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Monthly Notices of the Royal Astronomical Society
We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space... more We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space Telescope optical imaging. Its position in the colour–magnitude diagrams and its spectrum are consistent with this star being a helium-core white dwarf (He WD) in a binary system. The optical light curve shows a periodicity at 18.5 h. We argue that this periodicity is due to the rotation of the WD and possibly due to magnetic spots on the surface of the WD. This would make this object the first candidate magnetic He WD in any GC, and the first candidate magnetic WD in a detached binary system in any GC and one of the few He WDs with a known rotation period and of magnetic nature. Another possibility is that this system is an He WD in a binary system with another WD or another degenerate object, which would make this object one of the few candidate non-accreting double-degenerate binaries in any GC.
Bookmarks Related papers MentionsView impact
We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO ... more We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO white dwarfs, PG 1159 stars, and DB white dwarfs. White dwarf sequences are computed for a wide range of stellar masses and helium envelopes on the basis of a complete treatment of the evolutionary history of progenitors stars, including the core hydrogen and helium burning phases, the thermally pulsing asymptotic giant branch phase, and the born-again episode that is responsible for the hydrogen deficiency. We also provide colors and magnitudes for the new sequences for Teff < 40,000 K, where the NLTE effects are not dominant. These new calculations provide a homogeneous set of evolutionary tracks appropriate for mass and age determinations for both PG 1159 stars and DO white dwarfs. The calculations are extended down to an effective temperature of 7000 K. We applied these new tracks to redetermine stellar masses and ages of all known DO white dwarfs with spectroscopically determined...
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arXiv (Cornell University), Sep 30, 2012
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ABSTRACT Outer boundary conditions (temperature, pressure, outer stellar mass and geometrical dep... more ABSTRACT Outer boundary conditions (temperature, pressure, outer stellar mass and geometrical depth) at a Rosseland mean optical depth 25.1189, for effective temperatures between 2000K and 40000K, and 5.5<logg<9.0. These calculations, based on detailed stellar atmosphere models, are appropriated for computing the evolution of cool white dwarfs with hydrogen envelopes. (1 data file).
Bookmarks Related papers MentionsView impact
Bookmarks Related papers MentionsView impact
Fil: Vera Rueda, Gustavo Matias. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Cent... more Fil: Vera Rueda, Gustavo Matias. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - San Juan. Instituto de Ciencias Astronomicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronomicas, de la Tierra y del Espacio; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Fisicas y Naturales; Argentina
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Astronomy & Astrophysics, 2020
We assess the partition function and ionization degree of magnetized hydrogen atoms at thermodyna... more We assess the partition function and ionization degree of magnetized hydrogen atoms at thermodynamic equilibrium for a wide range of field intensities, B ≈ 105–1012 G. Evaluations include fitting formulae for an arbitrary number of binding energies, the coupling between the internal atomic structure and the center-of-mass motion across the magnetic field, and the formation of the so-called decentered states (bound states with the electron shifted from the Coulomb well). Non-ideal gas effects are treated within the occupational probability method. We also present general mathematical expressions for the bound state correspondence between the limits of zero-field and high-field. This let us evaluate the atomic partition function in a continuous way from the Zeeman perturbative regime to very strong fields. Results are shown for conditions found in atmospheres of magnetic white dwarf (MWD) stars, with temperatures T ≈ 5000–80 000 K and densities ρ ≈ 10−12–10−3 g cm3. Our evaluations sh...
Bookmarks Related papers MentionsView impact
International Astronomical Union Colloquium, 2000
We undertook a non-LTE radiative transfer calculation to find out the possible conditions under w... more We undertook a non-LTE radiative transfer calculation to find out the possible conditions under which observed H and He I line profiles arise from some Be stars.
Bookmarks Related papers MentionsView impact
Astronomy & Astrophysics, 2019
Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asy... more Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asymptotic giant branch (AGB), type Ia supernova explosions, and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these white dwarfs, there are few evolutionary studies in the literature devoted to them. Here we present new ultra-massive white dwarf evolutionary sequences that constitute an improvement over previous ones. In these new sequences we take into account for the first time the process of phase separation expected during the crystallization stage of these white dwarfs by relying on the most up-to-date phase diagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution during the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditions for our evolving models are provided by detailed non-gray white dwarf model atm...
Bookmarks Related papers MentionsView impact
Astronomy & Astrophysics, 2019
A selection of 17 stars in the Sloan Digital Sky Survey, previously identified as DC-class white ... more A selection of 17 stars in the Sloan Digital Sky Survey, previously identified as DC-class white dwarfs (WDs), has been reported to show spectra very close to blackbody radiation in the wavelength range from ultraviolet to infrared. Because of the absence of lines and other details in their spectra, the surface gravity of these objects has previously been poorly constrained, and their effective temperatures have been determined by fits to the continuum spectrum using pure helium atmosphere models. We computed model atmospheres with pure helium and H/He mixtures and used Gaia DR2 parallaxes that are available for 16 of the 17 selected stars to analyze their physical properties. We find that the atmospheres of the selected stars are very probably contaminated with a trace amount of hydrogen of −6 ≤ log(NH/NHe) ≤ −5.4. For the 16 stars with Gaia parallaxes, we calculate a mean stellar mass 0.606 ± 0.076 M⊙, which represents typical mass values and surface gravities (7.8 < logg <...
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