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Journal articles on the topic 'Helium hydrogen atmospheres'

Author: Grafiati

Published: 4 June 2021

Last updated: 29 July 2025

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1

Liebert, James. "Abundance Patterns in White Dwarfs." Symposium - International Astronomical Union 145 (1991): 411–19. http://dx.doi.org/10.1017/s007418090022754x.

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Various evidence indicates that the atmospheric compositions of white dwarfs may switch from hydrogen- to helium- dominated, and vice versa, over wide ranges of surface temperature (luminosity). This envelope evolution may be understood only by assuming that the hydrogen (DA) sequence retains only a tiny residual mass of hydrogen, many orders of magnitude less than that predicted by stellar evolution theory. A corresponding problem may exist for the helium envelopes of some lot stars with helium-rich atmospheres.

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Cherubim, Collin, Robin Wordsworth, Renyu Hu, and Evgenya Shkolnik. "Strong Fractionation of Deuterium and Helium in Sub-Neptune Atmospheres along the Radius Valley." Astrophysical Journal 967, no. 2 (2024): 139. http://dx.doi.org/10.3847/1538-4357/ad3e77.

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Abstract We simulate atmospheric fractionation in escaping planetary atmospheres using IsoFATE, a new open-source numerical model. We expand the parameter space studied previously to planets with tenuous atmospheres that exhibit the greatest helium and deuterium enhancement. We simulate the effects of extreme-ultraviolet-driven photoevaporation and core-powered mass loss on deuterium–hydrogen and helium–hydrogen fractionation of sub-Neptune atmospheres around G, K, and M stars. Our simulations predict prominent populations of deuterium- and helium-enhanced planets along the upper edge of the r

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Salmi, Tuomo, Serena Vinciguerra, Devarshi Choudhury, et al. "Atmospheric Effects on Neutron Star Parameter Constraints with NICER." Astrophysical Journal 956, no. 2 (2023): 138. http://dx.doi.org/10.3847/1538-4357/acf49d.

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Abstract We present an analysis of the effects of uncertainties in the atmosphere models on the radius, mass, and other neutron star parameter constraints for the NICER observations of rotation-powered millisecond pulsars. To date, NICER has applied the X-ray pulse profile modeling technique to two millisecond-period pulsars: PSR J0030+0451 and the high-mass pulsar PSR J0740+6620. These studies have commonly assumed a deep-heated, fully ionized hydrogen atmosphere model, although they have explored the effects of partial ionization and helium composition in some cases. Here, we extend that exp

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Tian, Meng, and Kevin Heng. "Atmospheric Chemistry of Secondary and Hybrid Atmospheres of Super Earths and Sub-Neptunes." Astrophysical Journal 963, no. 2 (2024): 157. http://dx.doi.org/10.3847/1538-4357/ad217c.

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Abstract The atmospheres of small exoplanets likely derive from a combination of geochemical outgassing and primordial gases left over from formation. Secondary atmospheres, such as those of Earth, Mars, and Venus, are sourced by outgassing. Persistent outgassing into long-lived, primordial, hydrogen–helium envelopes produces hybrid atmospheres of which there are no examples in the solar system. We construct a unified theoretical framework for calculating the outgassing chemistry of both secondary and hybrid atmospheres, where the input parameters are the surface pressure, oxidation, and sulfi

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Heber, U., K. Hunger, T. Rauch, and K. Werner. "Improved Non-LTE Model Atmospheres for Subluminous O-Stars." Symposium - International Astronomical Union 132 (1988): 117–22. http://dx.doi.org/10.1017/s0074180900034872.

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Improved non-LTE model atmospheres designed for the analysis of very hot subluminous O stars are presented. The calculations are based on the new method of the accelerated lambda iteration (ALI) which proves capable of treating up to 100 levels of various ions. Presented here are improved calculations for (i) pure hydrogen model atmospheres including Stark broadening, (ii) for hydrogen- and helium-composed model atmospheres and (iii) first preliminary models which include in addition a detailed carbon model atom. These models remove an apparent mismatch of Balmer line profiles noted previously

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Radmilović-Radjenović, Marija, Martin Sabo, and Branislav Radjenović. "Transport Characteristics of the Electrification and Lightning of the Gas Mixture Representing the Atmospheres of the Solar System Planets." Atmosphere 12, no. 4 (2021): 438. http://dx.doi.org/10.3390/atmos12040438.

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Electrification represents a fundamental process in planetary atmospheres, widespread in the Solar System. The atmospheres of the terrestrial planets (Venus, Earth, and Mars) range from thin to thick are rich in heavier gases and gaseous compounds, such as carbon dioxide, nitrogen, oxygen, argon, sodium, sulfur dioxide, and carbon monoxide. The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) have thick atmospheres mainly composed of hydrogen and helium involving. The electrical discharge processes occur in the planetary atmospheres leading to potential hazards due to arcing on landers an

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Allart, R., V. Bourrier, C. Lovis, et al. "Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet." Science 362, no. 6421 (2018): 1384–87. http://dx.doi.org/10.1126/science.aat5879.

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Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature—neutral hydrogen at ultraviolet wavelengths—is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b by using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08 ± 0.05%. Interpreting absorption spectra with three-dime

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8

Liebert, James. "The Origin and Evolution of Helium-Rich White Dwarfs." International Astronomical Union Colloquium 87 (1985): 367–85. http://dx.doi.org/10.1017/s0252921100090965.

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AbstractWhite dwarfs with helium-rich atmospheres constitute about one fifth of the white dwarfs hotter than 12,000 K. They appear to have a mass distribution similar to the hydrogen atmosphere (DA) stars, and are similar in other properties. However, the temperature distribution exhibits a deficiency of DB/DO stars in the interval 25,000-45,000 K, which implies evolution in the dominant surface composition as the stars cool. The hottest group of transition DO white dwarfs are the pulsating objects of the PG1159 class. The central star of Kl-16 is a related object, as may be the newly discover

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Cunningham, Tim, Pier-Emmanuel Tremblay, Evan B. Bauer, et al. "Horizontal spreading of planetary debris accreted by white dwarfs." Monthly Notices of the Royal Astronomical Society 503, no. 2 (2021): 1646–67. http://dx.doi.org/10.1093/mnras/stab553.

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ABSTRACT White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code co5bold, we present the first transport coefficients in degenerate star atmospheres that describe the advection–diffusion of a passive scalar across the surface plane. We couple newly derived horizontal diffusion coefficients with previously published vertical d

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Cukanovaite, E., P.-E. Tremblay, B. Freytag, et al. "Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs." Monthly Notices of the Royal Astronomical Society 490, no. 1 (2019): 1010–25. http://dx.doi.org/10.1093/mnras/stz2656.

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ABSTRACT We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the co5bold radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range −10.0 ≤ log (H/He) ≤−2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0, and effective temperatures in the range 12 000

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Zhilkin, A. G., Y. G. Gladysheva, V. I. Shematovich, and D. V. Bisikalo. "AERONOMIC MODEL OF HYDROGEN-HELIUM UPPER ATMOSPHERES OF HOT GIANT EXOPLANETS." Астрономический журнал 100, no. 12 (2023): 1190–209. http://dx.doi.org/10.31857/s0004629923120149.

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The paper presents 1D aeronomic model of hydrogen-helium upper atmospheres of hot giant exoplanets based on the approximation of single-fluid multicomponent hydrodynamics. Chemical reactions and heating-cooling processes are taken into account. Typical hot Jupiter and warm Neptune are considered as an example of the application of the model. Calculations were carried out for various values of gas pressure at the photometric radius of the planet. In the solutions obtained, a transonic planetary wind is formed, leading to a hydrodynamic outflow of the atmosphere with mass loss rates of the order

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Vennes, S., G. Fontaine, and F. Wesemael. "Strrtified Model Atmospheres for Hot Da White Dvhrfs." International Astronomical Union Colloquium 114 (1989): 368–72. http://dx.doi.org/10.1017/s0252921100099899.

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Observations of hot DA white dwarfs in the EUV/soft X-ray range have revealed that, in a majority of cases, the detected flux is less than that expected from pure hydrogen atmospheres. This implies an extra opacity source which must be due to the presence of small traces of heavier elements. These elements are generally not spectroscopically detected in hot DA white dwarfs, but the large sensitivity of the EUV/soft X-ray broad-band flux to the presence of extra absorbers can be used with profit to Infer their abundances. For simplicity, it has been assumed that only helium provides the require

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Hosokawa, Ko, Takayuki Kotani, Hajime Kawahara та ін. "Measurement of Methane Line Broadening in Hot Hydrogen/Helium Atmospheres at λ = 1.60–1.63 μm for Substellar Object Spectroscopy". Astrophysical Journal 984, № 1 (2025): 92. https://doi.org/10.3847/1538-4357/adc455.

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Abstract Recent high-dispersion spectroscopy from ground-based telescopes and high-precision spectroscopy from space observatories have enabled atmospheric observations of substellar objects, such as brown dwarfs and hot gaseous exoplanets, with sufficient precision to make ambient gas differences in molecular line broadening a significant factor. In this paper, we experimentally measured the pressure broadening of methane in a high-temperature hydrogen–helium background atmosphere in the H band, which had not been previously measured. The experiment used glass cells, inserted in a tube furnac

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Schindewolf, Markus, Peter Németh, Ulrich Heber, Tiara Battich, Marcelo M. Miller Bertolami, and Marilyn Latour. "Chemical fingerprints of He-sdO stars." Open Astronomy 27, no. 1 (2018): 27–34. http://dx.doi.org/10.1515/astro-2018-0010.

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Abstract The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31° 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10° 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with

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15

Kubát, Jiří. "The sphericity effects in model atmospheres of central stars of Planetary Nebulae." Symposium - International Astronomical Union 180 (1997): 117. http://dx.doi.org/10.1017/s0074180900130037.

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The model atmospheres of central stars of planetary nebulae are often calculated under the assumption of the plane-parallel geometry. This assumption seems to be reasonable for stars with relatively thin atmospheres like white dwarfs, where the thickness of the atmosphere is only few kilometers while the corresponding radius is about several thousands of kilometers. Nevertheless, calculations of a grid of pure hydrogen model atmospheres of hot white dwarfs demonstrated that this assumption may fail even for thin atmospheres (Kubát 1995). We found small differences between line profiles (about

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González-Caniulef, Denis, Sebastien Guillot, and Andreas Reisenegger. "Neutron star radius measurement from the ultraviolet and soft X-ray thermal emission of PSR J0437−4715." Monthly Notices of the Royal Astronomical Society 490, no. 4 (2019): 5848–59. http://dx.doi.org/10.1093/mnras/stz2941.

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ABSTRACT We analysed the thermal emission from the entire surface of the millisecond pulsar PSR J0437−4715 observed in the ultraviolet and soft X-ray bands. For this, we calculated non-magnetized, partially ionized atmosphere models of hydrogen, helium, and iron compositions and included plasma frequency effects that may affect the emergent spectrum. This is particularly true for the coldest atmospheres composed of iron (up to a few per cent changes in the soft X-ray flux). Employing a Markov chain Monte Carlo method, we found that the spectral fits favour a hydrogen atmosphere, disfavour a he

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McWilliams, R. Stewart, D. Allen Dalton, Zuzana Konôpková, Mohammad F. Mahmood, and Alexander F. Goncharov. "Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors." Proceedings of the National Academy of Sciences 112, no. 26 (2015): 7925–30. http://dx.doi.org/10.1073/pnas.1421801112.

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The noble gases are elements of broad importance across science and technology and are primary constituents of planetary and stellar atmospheres, where they segregate into droplets or layers that affect the thermal, chemical, and structural evolution of their host body. We have measured the optical properties of noble gases at relevant high pressures and temperatures in the laser-heated diamond anvil cell, observing insulator-to-conductor transformations in dense helium, neon, argon, and xenon at 4,000–15,000 K and pressures of 15–52 GPa. The thermal activation and frequency dependence of cond

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Löbling, L. "NLTE spectral analysis of the intermediate helium-rich subdwarf B star CPD−20°1123." Monthly Notices of the Royal Astronomical Society 497, no. 1 (2020): 67–80. http://dx.doi.org/10.1093/mnras/staa1686.

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ABSTRACT Subdwarf B (sdB) stars are core helium-burning stars with stratified atmospheres. Their atmospheres are dominated by hydrogen (H) while the helium (He) and metal abundances are shaped by an interplay of gravitational settling and radiative levitation. However, a small fraction of these show spectra dominated by He i absorption lines. In between these groups of He-deficient and extreme He-rich sdBs, some are found to have intermediate surface He abundances. These objects are proposed to be young ‘normal’ (He-deficient) sdBs for which the dynamical stratification of the atmosphere is st

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Liebert, James. "White Dwarfs and Planetary Central Stars." Symposium - International Astronomical Union 131 (1989): 545–54. http://dx.doi.org/10.1017/s0074180900139166.

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Studies of hot white dwarf samples constrain the properties and evolution of planetary nuclei and the nebulae. In particular, the white dwarf and planetary nebulae formation rates are compared. I discuss the overlap of the sequences of white dwarfs having hydrogen (DA) and helium-rich (DO) atmospheres with known central stars of high surface gravity. There is evidence that the hydrogen atmosphere nuclei have “thick” outer hydrogen layers (≳ 10−4 M⊙), but that DA white dwarfs may have surface hydrogen layers orders of magnitude thinner. Finally, a DA planetary nucleus is discussed (0950+139) wh

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Serenelli, Aldo, René D. Rohrmann, and Masataka Fukugita. "Nature of blackbody stars." Astronomy & Astrophysics 623 (March 2019): A177. http://dx.doi.org/10.1051/0004-6361/201834032.

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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 tha

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Blouin, Simon, and Patrick Dufour. "The spectral evolution of cool white dwarfs." Proceedings of the International Astronomical Union 15, S357 (2019): 166–69. http://dx.doi.org/10.1017/s1743921320000277.

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AbstractEmpirically characterizing the spectral evolution of cool white dwarfs is a prerequisite to understanding the physical processes that shape the evolution of these old objects. However, the high photospheric densities of cool helium-rich atmospheres seriously complicate the study of those stars. We have recently developed an updated atmosphere code that is appropriate for high densities and that can model any cool white dwarf (including DZs and DQpecs). Here, we present recent advances in our understanding of the spectral evolution of cool white dwarfs that were made possible thanks to

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Chaudhary, Rishabh, Robert D. White, Zarina Kosherbayeva, et al. "Acoustic signatures of helium abundance in hydrogen for planetary science." Journal of the Acoustical Society of America 155, no. 3_Supplement (2024): A345. http://dx.doi.org/10.1121/10.0027773.

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The aim of this work is to model and measure the acoustic signatures of varying mixtures of helium, ortho-hydrogen, and para-hydrogen. In particular, we hope to demonstrate a sensor for helium abundance and hydrogen ortho:para ratio for planetary science missions to Saturn or Uranus. Gas composition as a function of depth is important for understanding atmospheric dynamics and planetary formation of the gas giants but is not well known. In the presence of molecular relaxation, sound speed and absorption in polyatomic gas mixtures become dispersive due to frequency-dependent heat capacities. Th

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Plavec, Mirek J. "Hydrogen-Poor Binary Stars." International Astronomical Union Colloquium 87 (1985): 230–43. http://dx.doi.org/10.1017/s0252921100090837.

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AbstractHydrogen-poor and helium-rich stars are easy to produce in interacting binaries. Thus they should be found among Population I binaries, in which a large-scale mass transfer has occurred between the components (possibly associated with mass loss from the system). For in such cases, those layers are now on the surface of the “loser” (and, most likely, also on the surface of the “gainer”) that were subject to hydrogen burning and the associated mixing of processed material. Helium overabundance in these objects will be accompanied by an overabundance of nitrogen and underabundance of carb

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Koester, D., S. O. Kepler, and A. W. Irwin. "New white dwarf envelope models and diffusion." Astronomy & Astrophysics 635 (March 2020): A103. http://dx.doi.org/10.1051/0004-6361/202037530.

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Context. Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10 000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group poses several problems regarding their origin and also an unexpected correlation of effective temperature with mass. Aims. We propose to study the envelopes of these objects to determine the t

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Zhilkin, A. G., Y. G. Gladysheva, V. I. Shematovich, G. N. Tsurikov, and D. V. Bisikalo. "The effect of star flare activity on the structure of the hydrogen-helium upper atmosphere of hot Jupiter." Astronomičeskij žurnal 101, no. 9 (2024): 796–809. https://doi.org/10.31857/s0004629924090015.

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In the paper using 1D aeronomic model the impact of a stellar flare on the upper atmosphere of hot Jupiter is investigated. The atmosphere is assumed to have a hydrogen-helium chemical composition, and calculations were carried out for the hot Jupiter HD 209458b. We examined single and repeated flares in which the flux of hard UV radiation increases by 10, 100 and 1000 times compared to the quiescent state of the star. The active phase of the dynamic response of the atmosphere lasts 12–15 hours after the flare, and the characteristic period of relaxation to the initial state is about a day. Fr

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Allart, R., V. Bourrier, C. Lovis, et al. "High-resolution confirmation of an extended helium atmosphere around WASP-107b." Astronomy & Astrophysics 623 (March 2019): A58. http://dx.doi.org/10.1051/0004-6361/201834917.

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Context. Probing the evaporation of exoplanet atmospheres is key to understanding the formation and evolution of exoplanetary systems. The main tracer of evaporation in the UV is the Lyman-α transition, which can reveal extended exospheres of neutral hydrogen. Recently, the near-infrared (NIR) metastable helium triplet (10 833 Å) revealed extended thermospheres in several exoplanets. This opens a new window into evaporation. Aims. We aim at spectrally resolving the first helium absorption signature detected in the warm Saturn WASP-107b with the Wide Filed Camera 3 on board the Hubble Space Tel

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Cunningham, Tim, Pier-Emmanuel Tremblay, Nicola Pietro Gentile Fusillo, Mark Hollands, and Elena Cukanovaite. "From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing." Monthly Notices of the Royal Astronomical Society 492, no. 3 (2020): 3540–52. http://dx.doi.org/10.1093/mnras/stz3638.

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ABSTRACT We present a study of the hypothesis that white dwarfs undergo a spectral change from hydrogen- to helium-dominated atmospheres using a volume-limited photometric sample drawn from the Gaia-DR2 catalogue, the Sloan Digital Sky Survey (SDSS), and the Galaxy Evolution Explorer (GALEX). We exploit the strength of the Balmer jump in hydrogen-atmosphere DA white dwarfs to separate them from helium-dominated objects in SDSS colour space. Across the effective temperature range from 20 000 to 9000 K, we find that 22 per cent of white dwarfs will undergo a spectral change, with no spectral evo

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Kempton, Eliza M. R., Madeline Lessard, Matej Malik, et al. "Where are the Water Worlds?: Self-consistent Models of Water-rich Exoplanet Atmospheres." Astrophysical Journal 953, no. 1 (2023): 57. http://dx.doi.org/10.3847/1538-4357/ace10d.

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Abstract It remains to be ascertained whether sub-Neptune exoplanets primarily possess hydrogen-rich atmospheres or whether a population of H2O-rich water worlds lurks in their midst. Addressing this question requires improved modeling of water-rich exoplanetary atmospheres, both to predict and interpret spectroscopic observations and to serve as upper boundary conditions on interior structure calculations. Here, we present new models of hydrogen-helium-water atmospheres with water abundances ranging from solar to 100% water vapor. We improve upon previous models of high-water-content atmosphe

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Moldenhauer, T. W., R. Kuiper, W. Kley, and C. W. Ormel. "Steady state by recycling prevents premature collapse of protoplanetary atmospheres." Astronomy & Astrophysics 646 (February 2021): L11. http://dx.doi.org/10.1051/0004-6361/202040220.

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Context. In recent years, space missions such as Kepler and TESS have discovered many close-in planets with significant atmospheres consisting of hydrogen and helium: mini-Neptunes. This indicates that these planets formed early in gas-rich disks while avoiding the runaway gas accretion that would otherwise have turned them into hot-Jupiters. A solution is to invoke a long Kelvin-Helmholtz contraction (or cooling) timescale, but it has also been suggested that thermodynamical cooling can be prevented by hydrodynamical planet atmosphere-disk recycling. Aims. We investigate the efficacy of the r

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Filatov, E. Yu, A. V. Zadesenets, S. V. Komogortsev, P. E. Plyusnin, A. A. Chepurov, and S. V. Korenev. "Study of CoxPt1−x nanoalloy formation mechanism via single-source precursors." Powder Diffraction 34, S1 (2019): S27—S31. http://dx.doi.org/10.1017/s0885715619000162.

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This paper is devoted to the study of formation mechanism of metal solid solutions during the thermolysis of single-source precursors in Co–Pt systems with a wide range of superstructural ordering. It is shown that the thermal decomposition of [Pt(NH3)4][Co(C2O4)2(H2O)2]·2H2O salt in helium is critically different from that under hydrogen atmospheres. Thermal degradation under the helium atmosphere is followed by a gradual reduction of platinum and cobalt, and at each thermolysis temperature only one phase is present. At 380 °C an equiatomic Co0.50Pt0.50 solid solution is formed (a = 3.749 (4)

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Liebert, James. "White Dwarf Stars: Evolution of the Envelope Composition." Highlights of Astronomy 9 (1992): 639–41. http://dx.doi.org/10.1017/s1539299600009904.

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AbstractOver the last several years, evidence has been unfolding that the surface abundances of white dwarfs may evolve as the stars cool. It is possible that some post-AGB stars enter the white dwarf sequence with far less hydrogen and helium than predicted by standard theory of the quenching of shell sources, and with atmospheres dominated by C and O. The interplay of diffusion and episodes of convective mixing may lead first to He-rich atmospheres, then H-rich, and finally He-rich again.

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Danev, P., H. Tonchev, Zh Stoyanov, and Y. Mutafchieva. "Diamagnetic corrections to low-excited electronic states of light molecular systems." Journal of Physics: Conference Series 2701, no. 1 (2024): 012046. http://dx.doi.org/10.1088/1742-6596/2701/1/012046.

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Abstract Compact astrophysical objects like white dwarfs and neutron stars generate very strong magnetic fields. In their atmosphere, accretion disks, and the interstellar medium surrounding them, the predominant matter consists of hydrogen, helium and few-particle atoms and molecules. In this work, we have studied the effects of strong magnetic fields on light atomic and molecular systems like hydrogen molecular ions and antiprotonic helium. The full energy of the investigated systems is calculated in the Born-Oppenheimer approximation for a few low-lying electronic states. We investigate the

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Abbott, David C. "3. Wolf-Rayet star atmospheres." Transactions of the International Astronomical Union 19, no. 1 (1985): 510–13. http://dx.doi.org/10.1017/s0251107x0000657x.

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Wolf-Rayet (WR) stars are the helium-burning remnants of massive stars (initial mass ≲30 M©), which have lost their outer hydrogen-rich layers through the processes of Roche lobe overflow to a companion or mass loss by a strong stellar wind. The characteristic emission-line spectrum which defines the WR spectral type is produced by a stellar wind that is so dense and opaque, that the radiation of all lines and continua arise from material in the wind. Because the wind completely screens any radiation emitted by the hydrostatic core of the star, the spectra of WR stars are nearly impossible to

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Dorsch, M., M. Latour, and U. Heber. "Heavy metals in intermediate He-rich hot subdwarfs: the chemical composition of HZ 44 and HD 127493." Astronomy & Astrophysics 630 (October 2019): A130. http://dx.doi.org/10.1051/0004-6361/201935724.

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Context. Hot subluminous stars can be spectroscopically classified as subdwarf B (sdB) and O (sdO) stars. While the latter are predominantly hydrogen deficient, the former are mostly helium deficient. The atmospheres of most sdOs are almost devoid of hydrogen, whereas a small group of hot subdwarf stars of mixed H/He composition exists, showing extreme metal abundance anomalies. Whether such intermediate helium-rich (iHe) subdwarf stars provide an evolutionary link between the dominant classes is an open question. Aims. The presence of strong Ge, Sn, and Pb lines in the UV spectrum of HZ 44 su

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Keek, L., A. Heger, and J. J. M. in 't Zand. "SUPERBURST MODELS FOR NEUTRON STARS WITH HYDROGEN- AND HELIUM-RICH ATMOSPHERES." Astrophysical Journal 752, no. 2 (2012): 150. http://dx.doi.org/10.1088/0004-637x/752/2/150.

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Colonna, Gianpiero, Lucia Daniela Pietanza, and Annarita Laricchiuta. "Ionization kinetic model for Hydrogen-Helium atmospheres in hypersonic shock tubes." International Journal of Heat and Mass Transfer 156 (August 2020): 119916. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2020.119916.

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Kubát, Jiří. "SPHERICALLY SYMMETRIC NLTE MODEL ATMOSPHERES OF HOT HYDROGEN-HELIUM FIRST STARS." Astrophysical Journal Supplement Series 203, no. 2 (2012): 20. http://dx.doi.org/10.1088/0067-0049/203/2/20.

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Zhilkin, A. G., Yu G. Gladysheva, V. I. Shematovich, and D. V. Bisikalo. "Aeronomic Model of Hydrogen–Helium Upper Atmospheres of Hot Giant Exoplanets." Astronomy Reports 67, no. 12 (2023): 1329–47. http://dx.doi.org/10.1134/s1063772923120144.

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Husfeld, C., U. Heber, and J. S. Drilling. "NLTE-Analysis of Three Extremely Helium-Rich O-Type Subdwarfs." International Astronomical Union Colloquium 87 (1985): 353–57. http://dx.doi.org/10.1017/s0252921100090941.

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AbstractThree extremely helium-rich sdO stars (LSE 153, LSE 259 and LSE 263) were analyzed spectroscopically by means of detailed NLTE model atmospheres. These stars are very hot, with effective temperatures ranging from 70000 to 75000 K and gravities between log g = 4.4 and 4.9. Upper limits for the hydrogen abundance were also derived. The evolutionary status of the sdO stars is discussed and it is concluded that they evolve from the asymptotic giant branch towards the white dwarf stage. A possible evolutionary link between these hot stars and the extreme helium stars of spectral type B is d

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Kayar, Susan R., and Erich C. Parker. "Oxygen pulse in guinea pigs in hyperbaric helium and hydrogen." Journal of Applied Physiology 82, no. 3 (1997): 988–97. http://dx.doi.org/10.1152/jappl.1997.82.3.988.

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Kayar, Susan R., and Erich C. Parker. Oxygen pulse in guinea pigs in hyperbaric helium and hydrogen. J. Appl. Physiol. 82(3): 988–997, 1997.—We analyzed O2 pulse, the total volume of O2 consumed per heart beat, in guinea pigs at pressures from 10 to 60 atmospheres. Animals were placed in a hyperbaric chamber and breathed 2% O2 in either helium (heliox) or hydrogen (hydrox). Oxygen consumption rate (V˙o 2) was measured by gas chromatographic analysis. Core temperature and heart rate were measured by using surgically implanted radiotelemeters. TheV˙o 2 was modulated over a fourfold range by vary

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Werner, Klaus, Nicole Reindl, Stephan Geier, and Max Pritzkuleit. "Discovery of hot subdwarfs covered with helium-burning ash." Monthly Notices of the Royal Astronomical Society: Letters 511, no. 1 (2022): L66—L71. http://dx.doi.org/10.1093/mnrasl/slac005.

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Abstract Helium-rich subdwarf O stars (sdOs) are hot compact stars in a pre-white dwarf evolutionary state. Most of them have effective temperatures and surface gravities in the range Teff = 40 000–50 000 K and log g = 5.5–6.0. Their atmospheres are helium dominated. If present at all, C, N, and O are trace elements. The abundance patterns are explained in terms of nucleosynthesis during single star evolution (late helium core flash) or a binary He-core white dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs (PG1654+322 and PG1528+025) that exhibit unusually stron

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Schmutz, W., W. R. Hamann, and U. Wessolowski. "NLTE Analysis of the Wolf-Rayet Star HD193077 (WN5+abs)." International Astronomical Union Colloquium 108 (1988): 143–44. http://dx.doi.org/10.1017/s0252921100093635.

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A model atmosphere code that accounts for the special physical conditions in Wolf-Rayet atmospheres (Hamann and Schmutz, 1986; Wessolowski et al., 1987) is used to analyse the spectrum of the Wolf-Rayet star HD193077 (WN5+abs). The stellar parameters are determined such that the profiles of the helium lines He I λλ4471, 5876, He II λ5412, and the absolute visual magnitude are reproduced.In order to estimate the systematic errors introduced by the model assumptions, we perform some test calculations. Instead of the velocity-law exponent of ß=1, another fit is obtained with ß=0.5 (Fig. 1). This

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Koskinen, Tommi T., Panayotis Lavvas, Chenliang Huang, Galen Bergsten, Rachel B. Fernandes, and Mitchell E. Young. "Mass Loss by Atmospheric Escape from Extremely Close-in Planets." Astrophysical Journal 929, no. 1 (2022): 52. http://dx.doi.org/10.3847/1538-4357/ac4f45.

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Abstract We explore atmospheric escape from close-in exoplanets with the highest mass-loss rates. First, we locate the transition from stellar X-ray and UV-driven escape to rapid Roche lobe overflow, which occurs once the 10–100 nbar pressure level in the atmosphere reaches the Roche lobe. Planets enter this regime when the ratio of the substellar radius to the polar radius along the visible surface pressure level, which aligns with a surface of constant Roche potential, is X/Z ≳ 1.2 for Jovian planets (Mp ≳ 100 M ⊕) and X/Z ≳ 1.02 for sub-Jovian planets (M p ≈ 10–100 M ⊕). Around a Sun-like s

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Hamann, W. R. "Model atmospheres and spectral analyses of Wolf-Rayet stars." Symposium - International Astronomical Union 163 (1995): 105–15. http://dx.doi.org/10.1017/s0074180900201769.

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Stratified non-LTE models for expanding atmospheres have become available in the recent years. They are based on the idealized assumptions of spherical symmetry, stationarity and radiative equilibrium. The satisfactory agreement between calculated and observed Wolf-Rayet spectra suggests that this “standard model” is basically adequate for describing real WR atmospheres and hence can be applied for their quantitative spectral analyses. By the application of these models, the fundamental parameters have been determined meanwhile for the majority of the known Galactic WR stars. Most WN stars pop

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Wehrse, Rainer. "4. White Dwarf Atmospheres." Transactions of the International Astronomical Union 19, no. 1 (1985): 513–15. http://dx.doi.org/10.1017/s0251107x00006581.

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The main characteristic of white dwarf atmospheres is a pressure that is at least two orders of magnitude higher than in main sequence star atmospheres of the same effective temperature. This is due to the high gravity (log g ≈ 8), extreme metal under-abundances (Δlog εM ≳ 2.5), and (in many cases) the replacement of hydrogen by helium as the main constituent. As consequences the atmospheres are very thin (ΔR ≲ a few km) and the level populations of all species are given by the Boltzmann distribution (perhaps with the exception of the extreme outer layers, Greenstein 1973, Pilachowski 1984). T

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Misener, William, and Hilke E. Schlichting. "To cool is to keep: residual H/He atmospheres of super-Earths and sub-Neptunes." Monthly Notices of the Royal Astronomical Society 503, no. 4 (2021): 5658–74. http://dx.doi.org/10.1093/mnras/stab895.

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ABSTRACT Super-Earths and sub-Neptunes are commonly thought to have accreted hydrogen/helium envelopes, consisting of a few to ten percent of their total mass, from the primordial gas disc. Subsequently, hydrodynamic escape driven by core-powered mass-loss and/or photoevaporation likely stripped much of these primordial envelopes from the lower mass and closer-in planets to form the super-Earth population. In this work, we show that after undergoing core-powered mass-loss, some super-Earths can retain small residual H/He envelopes. This retention is possible because, for significantly depleted

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Owen, James E. "Atmospheric Escape and the Evolution of Close-In Exoplanets." Annual Review of Earth and Planetary Sciences 47, no. 1 (2019): 67–90. http://dx.doi.org/10.1146/annurev-earth-053018-060246.

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Exoplanets with substantial hydrogen/helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars. The extreme irradiation levels that these atmospheres experience cause them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and for lower-mass super-Earths and mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, a

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Heber, U. "Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects." Symposium - International Astronomical Union 145 (1991): 363–73. http://dx.doi.org/10.1017/s0074180900227502.

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Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars and PG 1159 stars are reviewed. These classes of stars trace stellar evolution from the horizontal branch towards the white dwarf cooling sequence. The impact of Non-LTE model atmospheres for the analyses of sdO and PG 1159 stars is outlined. The resulting atmospheric parameters and abundances are used to obtain constraints on the evolutionary status of the different classes of stars. The sdB stars form a homogeneous group and can be identified with models of the extended horizontal branch. Abundance anomalies (

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Greenstein, Jesse L. "White Dwarfs as Observed at High Signal to Noise." Symposium - International Astronomical Union 132 (1988): 175–83. http://dx.doi.org/10.1017/s0074180900034987.

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The goal is largely historical, 30 years of instrumental progress in a difficult new field, faint white dwarfs, and some results. High signal-to-noise spectrophotometry at 40–160 Å resolution revealed the separation between hydrogen- and helium-rich atmospheres, and provided a temperature scale from models. The white-dwarf color-luminosity relation proved narrow. Their simple spectra made brute-force averaging possible to 14th magnitude at good photographic resolution. Features as shallow as 5% and 200 Å wide included C2, but in magnetic stars some strong absorptions remain unidentified. Metal

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Barstow, Martin A., Ivan Hubeny, Thierry Lanz, Jay B. Holberg, and Edward M. Sion. "The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy." International Astronomical Union Colloquium 152 (1996): 203–10. http://dx.doi.org/10.1017/s0252921100035983.

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The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settl

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