scholarly journals Hydrodynamical interaction of stellar and planetary winds: effects of charge exchange and radiation pressure on the observed Ly α absorption

2019 ◽  
Vol 487 (4) ◽  
pp. 5788-5798 ◽  
Author(s):  
A Esquivel ◽  
M Schneiter ◽  
C Villarreal D’Angelo ◽  
M A Sgró ◽  
L Krapp
Keyword(s):  
Charge Exchange ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Extreme Ultraviolet ◽  
Thermal Profile ◽  
Velocity Shift ◽  
Absorbing Material ◽  
Hydrodynamical Interaction ◽  
Collisional Ionization ◽  
Single Set

ABSTRACT Lyman α observations of the transiting exoplanet HD 209458b enable the study of exoplanet exospheres exposed to stellar extreme ultraviolet (EUV) fluxes, as well as the interacting stellar wind properties. In this study we present 3D hydrodynamical models for the stellar–planetary wind interaction including radiation pressure and charge exchange, together with photoionization, recombination, and collisional ionization processes. Our models explore the contribution of the radiation pressure and charge exchange to the Ly α absorption profile in a hydrodynamical framework, and for a single set of stellar wind parameters appropriate for HD 209458. We find that most of the absorption is produced by the material from the planet, with a secondary contribution of neutralized stellar ions by charge exchange. At the same time, the hydrodynamic shock heats up the planetary material, resulting in a broad thermal profile. Meanwhile, the radiation pressure yields a small velocity shift of the absorbing material. While neither charge exchange nor radiation pressure provides enough neutrals at the velocity needed to explain the observations at −100 km s−1 individually, we find that the two effects combined with the broad thermal profile are able to explain the observations.

scholarly journals AGB winds in interacting binary stars

2020 ◽  
Vol 493 (2) ◽  
pp. 2606-2617 ◽  
Author(s):  
Luis C Bermúdez-Bustamante ◽  
G García-Segura ◽  
W Steffen ◽  
L Sabin
Keyword(s):  
Radiation Pressure ◽  
Stellar Wind ◽  
Binary Stars ◽  
Massive Star ◽  
Density Ratio ◽  
Asymptotic Giant Branch ◽  
Surface Layers ◽  
Outflow Velocity ◽  
Analytical Formulae ◽  
Set Up

ABSTRACT We perform numerical simulations to investigate the stellar wind from interacting binary stars. Our aim is to find analytical formulae describing the outflow structure. In each binary system the more massive star is in the asymptotic giant branch (AGB) and its wind is driven by a combination of pulsations in the stellar surface layers and radiation pressure on dust, while the less massive star is in the main sequence. Time averages of density and outflow velocity of the stellar wind are calculated and plotted as profiles against distance from the centre of mass and colatitude angle. We find that mass is lost mainly through the outer Lagrangian point L2. The resultant outflow develops into a spiral at low distances from the binary. The outflowing spiral is quickly smoothed out by shocks and becomes an excretion disc at larger distances. This leads to the formation of an outflow structure with an equatorial density excess, which is greater in binaries with smaller orbital separation. The pole-to-equator density ratio reaches a maximum value of ∼105 at Roche lobe overflow state. We also find that the gas stream leaving L2 does not form a circumbinary ring for stellar mass ratios above 0.78, when radiation pressure on dust is taken into account. Analytical formulae are obtained by curve fitting the two-dimensional, azimuthally averaged density and outflow velocity profiles. The formulae can be used in future studies to set-up the initial outflow structure in hydrodynamic simulations of common-envelope evolution and formation of planetary nebulae.

scholarly journals On the influence of magnetic fields in neutral planetary wakes

2016 ◽  
Vol 12 (S328) ◽  
pp. 192-197
Author(s):  
C. Villarreal D’Angelo ◽  
M. Schneiter ◽  
A. Esquivel
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Magnetic Moments ◽  
Radiative Processes ◽  
Planetary Magnetic Fields ◽  
Stellar Magnetic Fields

AbstractWe present a 3D magnetohydrodynamic study of the effect that stellar and planetary magnetic fields have on the calculated Lyα absorption during the planetary transit, employing parameters that resemble the exoplanet HD209458b. We assume a dipolar magnetic field for both the star and the planet, and use the Parker solution to initialize the stellar wind. We also consider the radiative processes and the radiation pressure.We use the numerical MHD code Guacho to run several models varying the values of the planetary and stellar magnetic moments within the range reported in the literature.We found that the presence of magnetic fields influences the escaping neutral planetary material spreading the absorption Lyα line for large stellar magnetic fields.

scholarly journals Charge-Exchange Emission from Hydrogen-Like Carbon Ions Colliding with Water Molecules

Atoms ◽  
2019 ◽  
Vol 7 (1) ◽  
pp. 17
Author(s):  
Dennis Bodewits ◽  
Ronnie Hoekstra
Keyword(s):  
Electron Capture ◽  
Charge Exchange ◽  
Cross Sections ◽  
Extreme Ultraviolet ◽  
Single Electron ◽  
Water Molecules ◽  
Carbon Ions ◽  
Ultraviolet Emission ◽  
Single Electron Capture ◽  
Hardness Ratio

Absolute Extreme Ultraviolet emission cross-sections have been measured for collisions between C 5 + and H 2 O in the range of 0.113 to 3.75 keV/amu (170–979 km/s). These results are used to derive velocity-dependent triplet-to-singlet ratios and emission cross-sections of the Cv K-series following single-electron capture. Comparison with existing measurements of integral charge-changing cross-sections indicates that auto-ionizing multi-electron capture is a significant reactions channel. This reaction may indirectly populate the n = 2 states and thus add strength to the K α emission of Cv ions thereby co-determining the hardness ratio of K-series emission of Cv.

scholarly journals Stellar-wind theory for O and B stars

1970 ◽  
Vol 36 ◽  
pp. 236-237
Author(s):  
Philip M. Solomon
Keyword(s):  
Mass Loss ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Conclusive Evidence ◽  
Driving Mechanism ◽  
Gravitational Acceleration ◽  
Hot Stars ◽  
B Stars ◽  
Wind Theory ◽  
Ultraviolet Observations

The rocket-ultraviolet observations of strong Doppler-shifted absorption lines of Siiv, Civ, Nv and other ions in the spectrum of O and B supergiants clearly indicate a high velocity outflow of matter from these stars. The presence of moderate ionisation stages in the stellar wind is conclusive evidence that the flow cannot be due to a high temperature corona as is the case for the solar wind. It is shown that the driving mechanism for the hot-star mass loss is radiation pressure exerted on the gas through absorption in resonance lines occurring at wavelengths near the maximum of the star's continuum flux. In the upper layers of these stars the outward force per gram of matter due to the radiation pressure can greatly exceed the gravitational acceleration making a static atmosphere impossible.The problem of a steady-state moving reversing layer is formulated and the solution leads to predictions of mass-loss rates as a function of effective temperature and gravity for all hot stars. These results are in substantial agreement with the observations.

Spectroscopic Investigations of Herbig-Ae-Be-Stars

1982 ◽  
Vol 98 ◽  
pp. 501-507
Author(s):  
Ulrich Finkenzeller
Keyword(s):  
Mass Loss ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Main Sequence ◽  
Physical Structure ◽  
Pressure Effects ◽  
Theoretical Knowledge ◽  
Be Stars ◽  
Be Star ◽  
Moderate Mass

“Herbig-Ae-Be-Stars” are assumed to be pre-main sequence objects of moderate mass with line emitting envelopes of an unknown nature. From our present theoretical knowledge it is not clear whether the physical structure of these envelopes is dominated by mass accretion or mass loss induced by a stellar wind or radiation pressure effects. Radial velocities and remarks on peculiarities are given for the star HD 200 775, which seems to represent a typical Herbig-Ae-Be-star fairly well. A catalogue of about 60 supposed Herbig-Ae-Be-stars is presented and comments, in particular on the brighter members, are invited.

scholarly journals The warm-hot circumgalactic medium around EAGLE-simulation galaxies and its detection prospects with X-ray and UV line absorption

2020 ◽  
Vol 498 (1) ◽  
pp. 574-598 ◽  
Author(s):  
Nastasha A Wijers ◽  
Joop Schaye ◽  
Benjamin D Oppenheimer
Keyword(s):  
Column Density ◽  
Extreme Ultraviolet ◽  
Gas Well ◽  
X Ray ◽  
Filling Phase ◽  
Circumgalactic Medium ◽  
Halo Masses ◽  
Collisional Ionization ◽  
Far Ultraviolet ◽  
Halo Gas

ABSTRACT We use the EAGLE (Evolution and Assembly of GaLaxies and their Environments) cosmological simulation to study the distribution of baryons, and far-ultraviolet (O vi), extreme-ultraviolet (Ne viii), and X-ray (O vii, O viii, Ne ix, and Fe xvii) line absorbers, around galaxies and haloes of mass $\,{M}_{\rm {200c}}= 10^{11}$–$10^{14.5} \, \rm {M}_{\odot}$ at redshift 0.1. EAGLE predicts that the circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards temperatures corresponding to the collisional ionization peak for each ion, and towards high metallicities. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O vi, photoionization becomes significant. When presenting observables, we work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Virial-temperature collisional ionization equilibrium ion fractions are good predictors of column density trends with halo mass, but underestimate the diversity of ions in haloes. Halo gas dominates the highest column density absorption for X-ray lines, but lower density gas contributes to strong UV absorption lines from O vi and Ne viii. Of the O vii (O viii) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $\,{M}_{\rm {200c}}= 10^{12.0}{-}10^{13.5} \, \rm {M}_{\odot}$. We predict that the X-IFU will detect O vii (O viii) in 77 (46) per cent of the sightlines passing $\,{M}_{\star }= 10^{10.5}{-}10^{11.0} \, \rm {M}_{\odot}$ galaxies within $100 \, \rm {pkpc}$ (59 (82) per cent for $\,{M}_{\star }\gt 10^{11.0} \, \rm {M}_{\odot}$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O vi.

scholarly journals Modeling the Lyα transit absorption of the hot Jupiter HD 189733b

2020 ◽  
Vol 638 ◽  
pp. A49 ◽  
Author(s):  
P. Odert ◽  
N. V. Erkaev ◽  
K. G. Kislyakova ◽  
H. Lammer ◽  
A. V. Mezentsev ◽  
...  
Keyword(s):  
Stellar Wind ◽  
Extreme Ultraviolet ◽  
Neutral Hydrogen ◽  
Hydrogen Distribution ◽  
Modeling Framework ◽  
X Ray ◽  
Host Stars ◽  
Excess Absorption ◽  
Hot Jupiter

Context. Hydrogen-dominated atmospheres of hot exoplanets expand and escape hydrodynamically due to the intense heating by the X-ray and extreme ultraviolet (XUV) irradiation of their host stars. Excess absorption of neutral hydrogen has been observed in the Lyα line during transits of several close-in gaseous exoplanets, indicating such extended atmospheres. Aims. For the hot Jupiter HD 189733b, this absorption shows temporal variability. We aim to study if variations in stellar XUV emission and/or variable stellar wind conditions may explain this effect. Methods. We applied a 1D hydrodynamic planetary upper atmosphere model and a 3D magnetohydrodynamic stellar wind flow model to study the effect of variations of the stellar XUV irradiation and wind conditions at the planet’s orbit on the neutral hydrogen distribution. This includes the production of energetic neutral atoms (ENAs) and the related Lyα transit signature. Results. We obtain comparable, albeit slightly higher Lyα absorption than that observed in 2011 with a stellar XUV flux of 1.8 × 104 erg cm−2 s−1, rather typical activity conditions for this star. Flares with parameters similar to that observed eight hours before the transit are unlikely to have caused a significant modulation of the transit signature. We find that the resulting Lyα absorption is dominated by atmospheric broadening, whereas the contribution of ENAs is negligible, as they are formed inside the bow shock from decelerated wind ions that are heated to high temperatures. Thus, within our modeling framework and assumptions, we find an insignificant dependence of the absorption on the stellar wind parameters. Conclusions. Since the transit absorption can be modeled with typical stellar XUV and wind conditions, it is possible that the nondetection of the absorption in 2010 was affected by less typical stellar activity conditions, such as a very different magnitude and/or shape of the star’s spectral XUV emission, or temporal and/or spatial variations in Lyα affecting the determination of the transit absorption.

Panspermina Revisited: Astrophysical and Biological Constraints

1997 ◽  
Vol 161 ◽  
pp. 539-544
Author(s):  
Paul S. Wesson ◽  
Jeff Secker ◽  
James R. Lepock
Keyword(s):  
Solar System ◽  
Radiation Pressure ◽  
Biological Evolution ◽  
Star System ◽  
Initial Information ◽  
The Galaxy ◽  
Absorbing Material ◽  
Significant Probability ◽  
Red Giant ◽  
Micro Organisms

AbstractWe have carried out a series of calculations involving bacteria and viruses embedded in dust grains that are ejected from our solar system by radiation pressure, and travel through space to other star systems. Under many conditions, this kind of panspermia is impractical, mainly because the ultraviolet radiation of the present Sun kills the micro-organisms. However, if the organisms are shielded by an absorbing material like carbon and if ejection takes place in the late-Sun (red giant) phase, there is a significant probability that these micro-organisms can reach another star system alive. In addition to panspermia with living micro-organisms, we note that it is possible to seed the Galaxy with dead ones, whose DNA fragments might provide the initial information necessary to start biological evolution in favorable environments.

scholarly journals The domain of radiatively driven mass loss in the H-R diagram

1979 ◽  
Vol 83 ◽  
pp. 235-240 ◽  
Author(s):  
David C. Abbott
Keyword(s):  
Steady State ◽  
Mass Loss ◽  
Radiation Pressure ◽  
Effective Temperature ◽  
Stellar Wind ◽  
Radiation Force ◽  
Theoretical Uncertainty ◽  
Line Radiation ◽  
Consistent Model ◽  
Self Consistent

Previous work by Castor, Abbott, and Klein (1975) presented a self-consistent model of a steady-state stellar wind. They also showed qualitatively that for O stars at least a static atmosphere could not exist. This paper extends that result by calculating in detail the minimum luminosity as a function of effective temperature required for the line radiation force to exceed gravity. Within the observational and theoretical uncertainty there is a one-to-one correspondence between a star's calculated ability to self-initiate a stellar wind by radiation pressure alone and the observed presence of outflowing material in the UV resonance lines.

scholarly journals The dichotomy of atmospheric escape in AU Mic b

2020 ◽  
Vol 498 (1) ◽  
pp. L53-L57
Author(s):  
S Carolan ◽  
A A Vidotto ◽  
P Plavchan ◽  
C Villarreal D’Angelo ◽  
G Hazra
Keyword(s):  
Mass Loss ◽  
Stellar Wind ◽  
Evaporation Rate ◽  
Loss Rate ◽  
Extreme Ultraviolet ◽  
Mass Loss Rate ◽  
Young Star ◽  
Planetary Atmosphere ◽  
The Other ◽  
Atmospheric Escape

ABSTRACT Here, we study the dichotomy of the escaping atmosphere of the newly discovered close-in exoplanet AU Microscopii (AU Mic) b. On one hand, the high extreme-ultraviolet stellar flux is expected to cause a strong atmospheric escape in AU Mic b. On the other hand, the wind of this young star is believed to be very strong, which could reduce or even inhibit the planet’s atmospheric escape. AU Mic is thought to have a wind mass-loss rate that is up to 1000 times larger than the solar wind mass-loss rate ($\dot{\mathrm{ M}}_\odot$). To investigate this dichotomy, we perform 3D hydrodynamics simulations of the stellar wind–planetary atmosphere interactions in the AU Mic system and predict the synthetic Ly α transits of AU Mic b. We systematically vary the stellar wind mass-loss rate from a ‘no wind’ scenario to up to a stellar wind with a mass-loss rate of $1000~\dot{\mathrm{ M}}_\odot$. We find that, as the stellar wind becomes stronger, the planetary evaporation rate decreases from 6.5 × 1010  g s−1 to half this value. With a stronger stellar wind, the atmosphere is forced to occupy a smaller volume, affecting transit signatures. Our predicted Ly α absorption drops from $\sim 20{{\ \rm per\ cent}}$ in the case of ‘no wind’ to barely any Ly α absorption in the extreme stellar wind scenario. Future Ly α transits could therefore place constraints not only on the evaporation rate of AU Mic b, but also on the mass-loss rate of its host star.

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