arXiv subjects

H. Lammer

Publications and source records attributed to H. Lammer.

At least 37 records · Page 2Linked to original sources

arXiv2020

Modeling the Ly$α$ transit absorption of the hot Jupiter HD 189733b

Hydrogen-dominated atmospheres of hot exoplanets expand and escape 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 Ly$α$ line during transits of several close-in exoplanets, indicating such extended atmospheres. For the hot Jupiter HD 189733b, this absorption shows temporal variability. Variations in stellar XUV emission and/or stellar wind conditions have been invoked to explain this effect. We apply a 1D hydrodynamic upper atmosphere model and a 3D MHD stellar wind flow model to study the effect of variations of the stellar XUV and wind conditions on the neutral hydrogen distribution, including the production of energetic neutral atoms (ENAs), and the related Ly$α$ transit signature. We obtain comparable, albeit slightly higher Ly$α$ absorption as observed in 2011 with a stellar XUV flux of $1.8\times10^4$ erg cm$^{-2}$ s$^{-1}$, rather typical activity conditions for this star. Flares similar to the one observed 8 h before the transit are unlikely to have caused a significant modulation of the transit signature. The resulting Ly$α$ 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 on the stellar wind parameters. Since the transit absorption can be modeled with typical stellar XUV and wind conditions, it is possible that the non-detection 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/spatial variations in Ly$α$ affecting the determination of the transit absorption.

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arXiv2020

Three-dimensional hydrodynamic simulations of the upper atmosphere of $π$ Men c: comparison with Ly$α$ transit observations

Aims: We aim at constraining the conditions of the wind and high-energy emission of the host star reproducing the non-detection of Ly$α$ planetary absorption. Methods: We model the escaping planetary atmosphere, the stellar wind, and their interaction employing a multi-fluid, three-dimensional hydrodynamic code. We assume a planetary atmosphere composed of hydrogen and helium. We run models varying the stellar high-energy emission and stellar mass-loss rate, further computing for each case the Ly$α$ synthetic planetary atmospheric absorption and comparing it with the observations. Results: We find that a non-detection of Ly$α$ in absorption employing the stellar high-energy emission estimated from far-ultraviolet and X-ray data requires a stellar wind with a stellar mass-loss rate about six times lower than solar. This result is a consequence of the fact that, for $π$ Men c, detectable Ly$α$ absorption can be caused exclusively by energetic neutral atoms, which become more abundant with increasing the velocity and/or the density of the stellar wind. By considering, instead, that the star has a solar-like wind, the non-detection requires a stellar ionising radiation about four times higher than estimated. This is because, despite the fact that a stronger stellar high-energy emission ionises hydrogen more rapidly, it also increases the upper atmosphere heating and expansion, pushing the interaction region with the stellar wind farther away from the planet, where the planet atmospheric density that remains neutral becomes smaller and the production of energetic neutral atoms less efficient. Conclusions: Comparing the results of our grid of models with what is expected and estimated for the stellar wind and high-energy emission, respectively, we support the idea that the atmosphere of $π$ Men c is likely not hydrogen-dominated.

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arXiv2020

Stellar Driven Evolution of Hydrogen-Dominated Atmospheres from Earth-Like to Super-Earth-Type Exoplanets

In the present chapter we present the results of evolutionary studies of exoplanetary atmospheres. We mostly focus on the sub- to super-Earth domain, although these methods are applicable to all types of exoplanets. We consider both thermal and nonthermal loss processes. The type of thermal loss mechanism depends on so-called escape parameter $β$, which is the ratio of the gravitational energy of a particle to its thermal energy. While $β$ is decreasing, an exoplanet switches from classical Jeans to modified Jeans and finally to blow-off escape mechanisms. During blow-off the majority of the atmospheric particles dispose of enough energy to escape the planet's gravity field. This leads to extreme gas losses. Although nonthermal losses never exceed blow-off escape, they are of significant importance for planets with relatively weak Jeans-type escape. From the diversity of nonthermal escape mechanisms, in the present chapter we focus on ion pickup and discuss the importance of other loss mechanisms. The general conclusion of the chapter is, that escape processes strongly shape the evolution of the exoplanets and determine, if the planet loses its atmosphere due to erosion processes or, on the contrary, stays as mini-Neptune type body, which can probably not be considered as a potential habitat as we know it.

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arXiv2020

A census of Coronal Mass Ejections on solar-like stars

Coronal Mass Ejections (CMEs) may have major importance for planetary and stellar evolution. Stellar CME parameters, such as mass and velocity, have yet not been determined statistically. So far only a handful of stellar CMEs has been detected mainly on dMe stars using spectroscopic observations. We therefore aim for a statistical determination of CMEs of solar-like stars by using spectroscopic data from the ESO phase 3 and Polarbase archives. To identify stellar CMEs we use the Doppler signal in optical spectral lines being a signature of erupting filaments which are closely correlated to CMEs. We investigate more than 3700 hours of on-source time of in total 425 dF-dK stars. We find no signatures of CMEs and only few flares. To explain this low level of activity we derive upper limits for the non detections of CMEs and compare those with empirically modelled CME rates. To explain the low number of detected flares we adapt a flare power law derived from EUV data to the Hα regime, yielding more realistic results for Hα observations. In addition we examine the detectability of flares from the stars by extracting Sun-as-a-star Hα light curves. The extrapolated maximum numbers of observable CMEs are below the observationally determined upper limits, which indicates that the on-source times were mostly too short to detect stellar CMEs in Hα. We conclude that these non detections are related to observational biases in conjunction with a low level of activity of the investigated dF-dK stars.

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arXiv2019

Escape and evolution of Mars' CO2 atmosphere: Influence of suprathermal atoms

With a Monte-Carlo model we investigate the escape of hot oxygen and carbon from the martian atmosphere for four points in time in its history corresponding to 1, 3, 10, and 20 times the present solar EUV flux. We study and discuss different sources of hot oxygen and carbon atoms in the thermosphere and their changing importance with the EUV flux. The increase of the production rates due to higher densities resulting from the higher EUV flux competes against the expansion of the thermosphere and corresponding increase in collisions. We find that the escape due to photodissociation increases with increasing EUV level. However, for the escape via some other reactions, e.g.~dissociative recombination of O$_2^+$, this is only true until the EUV level reaches 10 times the present EUV flux, and then the rates start to decrease. Furthermore, our results show that Mars could not have had a dense atmosphere at the end of the Noachian epoch, since such an atmosphere would not have been able to escape until today. In the pre-Noachian era, most of a magma ocean and volcanic activity related outgassed CO$_2$ atmosphere could have been lost thermally until the Noachian epoch, when non-thermal loss processes such as suprathermal atom escape became dominant. Thus, early Mars could have been hot and wet during the pre-Noachian era with surface CO$_2$ pressures larger than 1 bar during the first 300 Myr after the planet's origin.

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arXiv2019

Solar XUV and ENA-driven water loss from early Venus' steam atmosphere

The influence of the hydrogen hydrodynamic upper atmosphere escape, driven by the solar soft X-ray and extreme ultraviolet radiation (XUV) flux, on an expected magma ocean outgassed steam atmosphere of early Venus is studied. By assuming that the young Sun was either a weak or moderate active young G star, we estimated the water loss from a hydrogen dominated thermosphere due to the absorption of the solar XUV flux and the precipitation of solar wind produced energetic hydrogen atoms (ENAs). The production of ENAs and their interaction with the hydrodynamic extended upper atmosphere, including collision-related feedback processes, have been calculated by means of Monte Carlo models. ENAs that collide in the upper atmosphere deposit their energy and heat the surrounding gas mainly above the main XUV energy deposition layer. It is shown that precipitating ENAs modify the thermal structure of the upper atmosphere, but the enhancement of the thermal escape rates caused by these energetic hydrogen atoms is negligible. Our results also indicate that the majority of oxygen arising from dissociated H$_2$O molecules is left behind during the first 100 Myr. It is thus suggested that the main part of the remaining oxygen has been absorbed by crustal oxidation.

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arXiv2019

Global 3D hydrodynamic modeling of in-transit Lyα absorption of GJ436b

Using a global 3D, fully self-consistent, multi-fluid hydrodynamic model, we simulate the escaping upper atmosphere of the warm Neptune GJ436b, driven by the stellar XUV radiation impact and gravitational forces and interacting with the stellar wind. Under the typical parameters of XUV flux and stellar wind plasma expected for GJ436, we calculate in-transit absorption in Lyα and find that it is produced mostly by Energetic Neutral Atoms outside of the planetary Roche lobe, due to the resonant thermal line broadening. At the same time, the influence of radiation pressure has been shown to be insignificant. The modelled absorption is in good agreement with the observations and reveals such features as strong asymmetry between blue and red wings of the absorbed Lyα line profile, deep transit depth in the high velocity blue part of the line reaching more than 70%, and the timing of early ingress. On the other hand, the model produces significantly deeper and longer egress than in observations, indicating that there might be other processes and factors, still not accounted, that affect the interaction between the planetary escaping material and the stellar wind. At the same time, it is possible that the observational data, collected in different measurement campaigns, are affected by strong variations of the stellar wind parameters between the visits, and therefore, they cannot be reproduced altogether with the single set of model parameters.

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arXiv2019

The Kepler-11 system: evolution of the stellar high-energy emission and {initial planetary} atmospheric mass fractions

The atmospheres of close-in planets are strongly influenced by mass loss driven by the high-energy (X-ray and extreme ultraviolet, EUV) irradiation of the host star, particularly during the early stages of evolution. We recently developed a framework to exploit this connection and enable us to recover the past evolution of the stellar high-energy emission from the present-day properties of its planets, if the latter retains some remnants of their primordial hydrogen-dominated atmospheres. Furthermore, the framework can also provide constraints on planetary initial atmospheric mass fractions. The constraints on the output parameters improve when more planets can be simultaneously analysed. This makes the Kepler-11 system, which hosts six planets with bulk densities between 0.66 and 2.45g cm^{-3}, an ideal target. Our results indicate that the star has likely evolved as a slow rotator (slower than 85\% of the stars with similar masses), corresponding to a high-energy emission at 150 Myr of between 1-10 times that of the current Sun. We also constrain the initial atmospheric mass fractions for the planets, obtaining a lower limit of 4.1% for planet c, a range of 3.7-5.3% for planet d, a range of 11.1-14% for planet e, a range of 1-15.6% for planet f, and a range of 4.7-8.7% for planet g assuming a disc dispersal time of 1 Myr. For planet b, the range remains poorly constrained. Our framework also suggests slightly higher masses for planets b, c, and f than have been suggested based on transit timing variation measurements. We coupled our results with published planet atmosphere accretion models to obtain a temperature (at 0.25 AU, the location of planet f) and dispersal time of the protoplanetary disc of 550 K and 1 Myr, although these results may be affected by inconsistencies in the adopted system parameters.

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arXiv2019

Modelling atmospheric escape and MgII near-ultraviolet absorption of the highly irradiated hot Jupiter WASP-12b

We present two-dimensional multi-fluid numerical modelling of the upper atmosphere of the hot Jupiter WASP-12b. The model includes hydrogen chemistry, and self-consistently describes the expansion of the planetary upper atmosphere and mass loss due to intensive stellar irradiation, assuming a weakly magnetized planet. We simulate the planetary upper atmosphere and its interaction with the stellar wind (SW) with and without the inclusion of tidal force and consider different XUV irradiation conditions and SW parameters. With the inclusion of tidal force, even for a fast SW, the escaping planetary material forms two streams, propagating towards and away from the star. The atmospheric escape and related mass loss rate reaching the value of 10^12 gs^-1 appear to be mostly controlled by the stellar gravitational pull. We computed the column density and dynamics of MgII ions considering three different sets of SW parameters and XUV fluxes. The simulations enable to compute the absorption at the position of the Mg h line and to reproduce the times of ingress and egress. In case of a slow SW and without accounting for tidal force, the high orbital velocity leads to the formation of a shock approximately in the direction of the planetary orbital motion. In this case, mass loss is proportional to the stellar XUV flux. At the same time, ignoring of tidal effects for WASP-12b is a strong simplification, so the scenario with a shock, altogether is an unrealistic one.

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arXiv2019

Extreme hydrodynamic losses of Earth-like atmospheres in the habitable zones of very active stars

Aims. In this letter, we calculate for the first time the full transonic hydrodynamic escape of an Earth-like atmosphere. We consider the case of an Earth-mass planet with an atmospheric composition identical to that of the current Earth orbiting at 1 AU around a young and very active solar mass star. Methods. To model the upper atmosphere, we used the Kompot Code, which is a first-principles model that calculates the physical structures of the upper atmospheres of planets, taking into account hydrodynamics and the main chemical and thermal processes taking place in the upper atmosphere of a planet. This model enabled us to calculate the 1D vertical structure of the atmosphere using as input the high-energy spectrum of a young and active Sun. Results. The atmosphere has the form of a transonic hydrodynamic Parker wind, which has an outflow velocity at the upper boundary of our computational domain that exceeds the escape velocity. The outflowing gas is dominated by atomic nitrogen and oxygen and their ion equivalents and has a maximum ionization fraction of 20%. The mass outflow rate is found to be 1.8x10^9 g s^-1, which would erode the modern Earth's atmosphere in less than 0.1 Myr. Conclusions. This extreme mass loss rate suggests that an Earth-like atmosphere cannot form when the planet is orbiting within the habitable zone of a very active star. Instead, such an atmosphere can only form after the activity of the star has decreased to a much lower level. This happened in the early atmosphere of the Earth, which was likely dominated by other gases such as CO2. Since the time it takes for the activity of a star to decay is highly dependent on its mass, this is important for understanding possible formation timescales for planets orbiting low-mass stars.

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arXiv2019

Transit Ly-$α$ signatures of terrestrial planets in the habitable zones of M dwarfs

We modeled the transit signatures in the Lya line of a putative Earth-sized planet orbiting in the HZ of the M dwarf GJ436. We estimated the transit depth in the Lya line for an exo-Earth with three types of atmospheres: a hydrogen-dominated atmosphere, a nitrogen-dominated atmosphere, and a nitrogen-dominated atmosphere with an amount of hydrogen equal to that of the Earth. We calculated the in-transit absorption they would produce in the Lya line. We applied it to the out-of-transit Lya observations of GJ 436 obtained by the HST and compared the calculated in-transit absorption with observational uncertainties to determine if it would be detectable. To validate the model, we also used our method to simulate the deep absorption signature observed during the transit of GJ 436b and showed that our model is capable of reproducing the observations. We used a DSMC code to model the planetary exospheres. The code includes several species and traces neutral particles and ions. At the lower boundary of the DSMC model we assumed an atmosphere density, temperature, and velocity obtained with a hydrodynamic model for the lower atmosphere. We showed that for a small rocky Earth-like planet orbiting in the HZ of GJ436 only the hydrogen-dominated atmosphere is marginally detectable with the STIS/HST. Neither a pure nitrogen atmosphere nor a nitrogen-dominated atmosphere with an Earth-like hydrogen concentration in the upper atmosphere are detectable. We also showed that the Lya observations of GJ436b can be reproduced reasonably well assuming a hydrogen-dominated atmosphere, both in the blue and red wings of the Lya line, which indicates that warm Neptune-like planets are a suitable target for Lya observations. Terrestrial planets can be observed in the Lya line if they orbit very nearby stars, or if several observational visits are available.

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arXiv2018

3D Aeronomy Modeling of Close-in Exoplanets

We present a 3D fully selfconsistent multi-fluid hydrodynamic aeronomy model to study the structure of a hydrogen dominated expanding upper atmosphere around the hot Jupiter HD 209458b and the warm Neptune GJ 436b. In comparison to previous studies with 1D and 2D models, the present work finds such 3D features as zonal flows in upper atmosphere reaching up to 1 km/s, the tilting of the planetary outflow by Coriolis force by up to 45 degrees and its compression around equatorial plane by tidal forces. We also investigated in details the influence of Helium (He) on the structure of the thermosphere. It is found that by decrease of the barometric scale-height, the He presence in the atmosphere strongly affects the H2 dissociation front and the temperature maximum.

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arXiv2018

Supermassive Hot Jupiters Provide More Favourable Conditions for the Generation of Radio Emission via the Cyclotron Maser Instability - A Case Study Based on Tau Bootis b

We investigate under which conditions supermassive hot Jupiters can sustain source regions for radio emission, and whether this emission could propagate to an observer outside the system. We study Tau Bootis b-like planets (a supermassive hot Jupiter with 5.84 Jupiter masses and 1.06 Jupiter radii), but located at different orbital distances (between its actual orbit of 0.046 AU and 0.2 AU). Due to the strong gravity of such planets and efficient radiative cooling, the upper atmosphere is (almost) hydrostatic and the exobase remains very close to the planet, which makes it a good candidate for radio observations. We expect similar conditions as for Jupiter, i.e. a region between the exobase and the magnetopause that is filled with a depleted plasma density compared with cases where the whole magnetosphere cavity is filled with hydrodynamically outward flowing ionospheric plasma. Thus, unlike classical hot Jupiters like the previously studied planets HD 209458b and HD 189733b, supermassive hot Jupiters should be in general better targets for radio observations.

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arXiv2018

Effective induction heating around strongly magnetized stars

Planets that are embedded in the changing magnetic fields of their host stars can experience significant induction heating in their interiors caused by the planet's orbital motion. For induction heating to be substantial, the planetary orbit has to be inclined with respect to the stellar rotation and dipole axes. Using WX~UMa, for which the rotation and magnetic axes are aligned, as an example, we show that for close-in planets on inclined orbits, induction heating can be stronger than the tidal heating occurring inside Jupiter's satellite Io; namely, it can generate a surface heat flux exceeding 2\,W\,m$^{-2}$. An internal heating source of such magnitude can lead to extreme volcanic activity on the planet's surface, possibly also to internal local magma oceans, and to the formation of a plasma torus around the star aligned with the planetary orbit. A strongly volcanically active planet would eject into space mostly SO$_2$, which would then dissociate into oxygen and sulphur atoms. Young planets would also eject CO$_2$. Oxygen would therefore be the major component of the torus. If the O{\sc i} column density of the torus exceeds $\approx$10$^{12}$\,cm$^{-2}$, the torus could be revealed by detecting absorption signatures at the position of the strong far-ultraviolet O{\sc i} triplet at about 1304\,Å. We estimate that this condition is satisfied if the O{\sc i} atoms in the torus escape the system at a velocity smaller than 1--10\,km\,s$^{-1}$. These estimates are valid also for a tidally heated planet.

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arXiv2017

Young planets under extreme UV irradiation. I. Upper atmosphere modelling of the young exoplanet K2-33b

The K2-33 planetary system hosts one transiting ~5 R_E planet orbiting the young M-type host star. The planet's mass is still unknown, with an estimated upper limit of 5.4 M_J. The extreme youth of the system (<20 Myr) gives the unprecedented opportunity to study the earliest phases of planetary evolution, at a stage when the planet is exposed to an extremely high level of high-energy radiation emitted by the host star. We perform a series of 1D hydrodynamic simulations of the planet's upper atmosphere considering a range of possible planetary masses, from 2 to 40 M_E, and equilibrium temperatures, from 850 to 1300 K, to account for internal heating as a result of contraction. We obtain temperature profiles mostly controlled by the planet's mass, while the equilibrium temperature has a secondary effect. For planetary masses below 7-10 M_E, the atmosphere is subject to extremely high escape rates, driven by the planet's weak gravity and high thermal energy, which increase with decreasing mass and/or increasing temperature. For higher masses, the escape is instead driven by the absorption of the high-energy stellar radiation. A rough comparison of the timescales for complete atmospheric escape and age of the system indicates that the planet is more massive than 10 M_E.

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arXiv2017

Lyα Absorption at Transits of HD 209458b: A Comparative Study of Various Mechanisms Under Different Conditions

To shed more light on the nature of the observed Lyα absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6-8 percent can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10-15 percent absorption at the blue wing of the Lya line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.

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arXiv2017

Atmosphere Expansion and Mass Loss of Close-Orbit Giant Exoplanets heated by Stellar XUV. II. Effects of Planetary Magnetic Field, Structuring of inner Magnetosphere

This is the second paper in a series where we build a self-consistent model to simulate the mass-loss process of a close-orbit magnetized giant exoplanet, so-called hot Jupiter (HJ). In this paper we generalize the hydrodynamic (HD) model of an HJ expanding hydrogen atmosphere, proposed in the first paper, to include the effects of intrinsic planetary magnetic field. The proposed self-consistent axisymmetric 2D magnetohydrodynamics model incorporates radiative heating and ionization of the atmospheric gas, basic hydrogen chemistry for the appropriate account of major species composing HJ's upper atmosphere and related radiative energy deposition, and H3+ and Lyα cooling processes. The model also takes into account a realistic solar-type X-ray/EUV spectrum for calculation of intensity and column density distribution of the radiative energy input, as well as gravitational and rotational forces acting in a tidally locked planet-star system. An interaction between the expanding atmospheric plasma and an intrinsic planetary magnetic dipole field leads to the formation of a current-carrying magnetodisk that plays an important role for topology and scaling of the planetary magnetosphere. A cyclic character of the magnetodisk behavior, composed of consequent phases of the disk formation followed by the magnetic reconnection with the ejection of a ring-type plasmoid, has been discovered and investigated. We found that the mass-loss rate of an HD 209458b analog planet is weakly affected by the equatorial surface field <0.3 G, but is suppressed by an order of magnitude at the field of 1 G.

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arXiv2017

An Impacting Descent Probe for Europa and the other Galilean Moons of Jupiter

We present a study of an impacting descent probe that increases the science return of spacecraft orbiting or passing an atmosphere-less planetary body of the solar system, such as the Galilean moons of Jupiter. The descent probe is a carry-on small spacecraft (< 100 kg), to be deployed by the mother spacecraft, that brings itself onto a collisional trajectory with the targeted planetary body in a simple manner. A possible science payload includes instruments for surface imaging, characterisation of the neutral exosphere, and magnetic field and plasma measurement near the target body down to very low-altitudes (~1 km), during the probe's fast (~km/s) descent to the surface until impact. The science goals and the concept of operation are discussed with particular reference to Europa, including options for flying through water plumes and after-impact retrieval of very-low altitude science data. All in all, it is demonstrated how the descent probe has the potential to provide a high science return to a mission at a low extra level of complexity, engineering effort, and risk. This study builds upon earlier studies for a Callisto Descent Probe (CDP) for the former Europa-Jupiter System Mission (EJSM) of ESA and NASA, and extends them with a detailed assessment of a descent probe designed to be an additional science payload for the NASA Europa Mission.

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