arXiv subjects

H. Lammer

Publications and source records attributed to H. Lammer.

At least 55 records · Page 3Linked to original sources

arXiv2017

$\texttt{PyTranSpot}$ - A tool for multiband light curve modeling of planetary transits and stellar spots

Several studies have shown that stellar activity features, such as occulted and non-occulted starspots, can affect the measurement of transit parameters biasing studies of transit timing variations and transmission spectra. We present $\texttt{PyTranSpot}$, which we designed to model multiband transit light curves showing starspot anomalies, inferring both transit and spot parameters. The code follows a pixellation approach to model the star with its corresponding limb darkening, spots, and transiting planet on a two dimensional Cartesian coordinate grid. We combine $\texttt{PyTranSpot}$ with an MCMC framework to study and derive exoplanet transmission spectra, which provides statistically robust values for the physical properties and uncertainties of a transiting star-planet system. We validate $\texttt{PyTranSpot}$'s performance by analyzing eleven synthetic light curves of four different star-planet systems and 20 transit light curves of the well-studied WASP-41b system. We also investigate the impact of starspots on transit parameters and derive wavelength dependent transit depth values for WASP-41b covering a range of 6200-9200 $Å$, indicating a flat transmission spectrum.

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arXiv2017

Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets

Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, large amounts of the mantles' volatile contents may be outgassed. The resulting H$_2$O/CO$_2$ dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity and the high stellar EUV luminosities. Protoplanets forming later from such degassed building blocks could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species (O, CO$_2$, noble gases). The full range of possible EUV evolution tracks of a solar-mass star is taken into account to investigate the escape from Mars-sized embryos at different orbital distances. The envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. We investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with observations. Starting from solar values, consistent isotope ratios (Ne, Ar) can be found for different solar EUV histories, as well as assumptions about the initial atmosphere (either pure steam or a mixture with accreted H). Our results generally favor an early accretion scenario with a small amount of accreted H and a low-activity Sun, because in other cases too much CO$_2$ is lost during evolution, which is inconsistent with Venus' present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO$_2$ which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then very massive steam atmospheres would be required.

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arXiv2017

Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating

Low-mass M stars are plentiful in the Universe and often host small, rocky planets detectable with the current instrumentation. Recently, seven small planets have been discovered orbiting the ultracool dwarf TRAPPIST-1\cite{Gillon16,Gillon17}. We examine the role of electromagnetic induction heating of these planets, caused by the star's rotation and the planet's orbital motion. If the stellar rotation and magnetic dipole axes are inclined with respect to each other, induction heating can melt the upper mantle and enormously increase volcanic activity, sometimes producing a magma ocean below the planetary surface. We show that induction heating leads the three innermost planets, one of which is in the habitable zone, to either evolve towards a molten mantle planet, or to experience increased outgassing and volcanic activity, while the four outermost planets remain mostly unaffected.

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arXiv2017

Effect of stellar wind induced magnetic fields on planetary obstacles of non-magnetized hot Jupiters

We investigate the interaction between the magnetized stellar wind plasma and the partially ionized hydrodynamic hydrogen outflow from the escaping upper atmosphere of non- or weakly magnetized hot Jupiters. We use the well-studied hot Jupiter HD 209458b as an example for similar exoplanets, assuming a negligible intrinsic magnetic moment. For this planet, the stellar wind plasma interaction forms an obstacle in the planet's upper atmosphere, in which the position of the magnetopause is determined by the condition of pressure balance between the stellar wind and the expanded atmosphere, heated by the stellar extreme ultraviolet (EUV) radiation. We show that the neutral atmospheric atoms penetrate into the region dominated by the stellar wind, where they are ionized by photo-ionization and charge exchange, and then mixed with the stellar wind flow. Using a 3D magnetohydrodynamic (MHD) model, we show that an induced magnetic field forms in front of the planetary obstacle, which appears to be much stronger compared to those produced by the solar wind interaction with Venus and Mars. Depending on the stellar wind parameters, because of the induced magnetic field, the planetary obstacle can move up to ~0.5-1 planetary radii closer to the planet. Finally, we discuss how estimations of the intrinsic magnetic moment of hot Jupiters can be inferred by coupling hydrodynamic upper planetary atmosphere and MHD stellar wind interaction models together with UV observations. In particular, we find that HD 209458b should likely have an intrinsic magnetic moment of 10-20% that of Jupiter.

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arXiv2017

Stellar Coronal Mass Ejections I. Estimating occurrence frequencies and mass-loss rates

Stellar coronal mass ejections (CMEs) may play an important role in mass- and angular momentum loss of young Sun-like stars. If occurring frequently, they may also have a strong effect on planetary evolution by increasing atmospheric erosion. So far it has not been possible to infer the occurrence frequency of stellar CMEs from observations. Based on their close relation with flares on the Sun, we develop an empirical model combining solar flare-CME relationships with stellar flare rates to estimate the CME activity of young Sun-like and late-type main-sequence stars. By comparison of the obtained CME mass-loss rates with observations of total mass-loss rates, we find that our modeled rates may exceed those from observations by orders of magnitude for the most active stars. This reveals a possible limit to the extrapolation of such models to the youngest stars. We find that the most uncertain component in the model is the flare-CME association rate adopted from the Sun, which does not properly account for the likely stronger coronal confinement in active stars. Simple estimates of this effect reveal a possible suppression of CME rates by several orders of magnitude for young stars, indicating that this issue should be addressed in more detail in the future.

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arXiv2017

Two regimes of interaction of a Hot Jupiter's escaping atmosphere with the stellar wind and generation of energized atomic hydrogen corona

The interaction of escaping upper atmosphere of a hydrogen rich non-magnetized analog of HD209458b with a stellar wind of its host G-type star at different orbital distances is simulated with a 2D axisymmetric multi-fluid hydrodynamic model. A realistic sun-like spectrum of XUV radiation which ionizes and heats the planetary atmosphere, hydrogen photo-chemistry, as well as stellar-planetary tidal interaction are taken into account to generate self-consistently an atmospheric hydrodynamic outflow. Two different regimes of the planetary and stellar winds interaction have been modelled. These are: 1) the "captured by the star" regime, when the tidal force and pressure gradient drive the planetary material beyond the Roche lobe towards the star, and 2) the "blown by the wind" regime, when sufficiently strong stellar wind confines the escaping planetary atmosphere and channels it into the tail. The model simulates in details the hydrodynamic interaction between the planetary atoms, protons and the stellar wind, as well as the production of energetic neutral atoms (ENAs) around the planet due to charge-exchange between planetary atoms and stellar protons. The revealed location and shape of the ENA cloud either as a paraboloid shell between ionopause and bowshock (for the "blown by the wind" regime), or a turbulent layer at the contact boundary between the planetary stream and stellar wind (for the "captured by the star" regime) are of importance for the interpretation of Lyα absorption features in exoplanetary transit spectra and characterization of the plasma environments.

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arXiv2016

Aeronomical constraints to the minimum mass and maximum radius of hot low-mass planets

Stimulated by the discovery of a number of close-in low-density planets, we generalise the Jeans escape parameter taking hydrodynamic and Roche lobe effects into account. We furthermore define $Λ$ as the value of the Jeans escape parameter calculated at the observed planetary radius and mass for the planet's equilibrium temperature and considering atomic hydrogen, independently of the atmospheric temperature profile. We consider 5 and 10 $M_{\oplus}$ planets with an equilibrium temperature of 500 and 1000 K, orbiting early G-, K-, and M-type stars. Assuming a clear atmosphere and by comparing escape rates obtained from the energy-limited formula, which only accounts for the heating induced by the absorption of the high-energy stellar radiation, and from a hydrodynamic atmosphere code, which also accounts for the bolometric heating, we find that planets whose $Λ$ is smaller than 15-35 lie in the "boil-off" regime, where the escape is driven by the atmospheric thermal energy and low planetary gravity. We find that the atmosphere of hot (i.e. $T_{\rm eq}\gtrapprox$ 1000 K) low-mass ($M_{\rm pl}\lessapprox$ 5 $M_{\oplus}$) planets with $Λ$ < 15-35 shrinks to smaller radii so that their $Λ$ evolves to values higher than 15-35, hence out of the boil-off regime, in less than $\approx$500 Myr. Because of their small Roche lobe radius, we find the same result also for hot (i.e. $T_{\rm eq}\gtrapprox$ 1000 K) higher mass ($M_{\rm pl}\lessapprox$ 10 $M_{\oplus}$) planets with $Λ$ < 15-35, when they orbit M-dwarfs. For old, hydrogen-dominated planets in this range of parameters, $Λ$ should therefore be $\geq$15-35, which provides a strong constraint on the planetary minimum mass and maximum radius and can be used to predict the presence of aerosols and/or constrain planetary masses, for example.

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arXiv2016

Indications of stellar prominence oscillations on fast rotating stars: the cases of HK Aqr and PZ Tel

We present the analysis of six nights of spectroscopic monitoring of two young and fast rotating late-type stars, namely the dMe star HK Aqr and the dG/dK star PZ Tel. On both stars we detect absorption features reminiscent of signatures of co-rotating cool clouds or prominences visible in H$α$. Several prominences on HK Aqr show periodic variability in the prominence tracks which follow a sinusoidal motion (indication of prominence oscillations). On PZ Tel we could not find any periodic variability in the prominence tracks. By fitting sinusoidal functions to the prominence tracks we derive amplitudes and periods which are similar to those of large amplitude oscillations seen in solar prominences. In one specific event we also derive a periodic variation of the prominence track in the H$β$ spectral line which shows an anti-phase variation with the one derived for the H$α$ spectral line. Using these parameters and estimated mass density of a prominence on HK Aqr we derive a minimum magnetic field strength of $\sim$2G. The relatively low strength of the magnetic field is explained by the large height of this stellar prominence ($\ge$ 0.67 stellar radii above the surface).

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arXiv2016

On the ultraviolet anomalies of the WASP-12 and HD 189733 systems: Trojan satellites as a plasma source

We suggest an additional possible plasma source to explain part of the phenomena observed for the transiting hot Jupiters WASP-12b and HD 189733b in their ultraviolet (UV) light curves. In the proposed scenario, material outgasses from the molten surface of Trojan satellites on tadpole orbits near the Lagrange points L$_4$ and L$_5$. We show that the temperature at the orbital location of WASP-12b is high enough to melt the surface of rocky bodies and to form shallow lava oceans on them. In case of WASP-12b, this leads to the release of elements such as Mg and Ca, which are expected to surround the system. The predicted Mg and Ca outgassing rates from two Io-sized WASP-12b Trojans are $\approx 2.2 \times 10^{27}$ s$^{-1}$ and $\approx 2.2 \times 10^{26}$ s$^{-1}$, respectively. Trojan outgassing can lead to the apparent lack of emission in Mg{\sc ii}\,h\&k and Ca{\sc ii}\,H\&K line cores of WASP-12. For HD 189733b, the mechanism is only marginally possible due to the lower temperature. This may be one of the reasons that couldn't explain the early ingress of HD 189733b observed in the far-UV (FUV) C{\sc ii} doublet due to absence of carbon within elements outgassed by molten lava. We investigate the long-term stability region of WASP-12b and HD 189733b in case of planar and inclined motion of these satellites and show that unlike the classical exomoons orbiting the planet, Io-sized Trojans can be stable for the whole systems life time.

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arXiv2016

Identifying the "true" radius of the hot sub-Neptune CoRoT-24b by mass loss modelling

For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii R$_{\rm T}$ of 3.7$\pm$0.4 R$_{\oplus}$ and 4.9$\pm$0.5 R$_{\oplus}$, and masses of $\le$5.7 M$_{\oplus}$ and 28$\pm$11 M$_{\oplus}$, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming R$_{\rm T} \approx$ R$_{\rm PL}$, where R$_{\rm PL}$ is the planetary radius at the pressure of 100 mbar, we obtained for CoRoT-24b unrealistically high thermally-driven hydrodynamic escape rates. This is due to the planet's high temperature and low gravity, independent of the stellar EUV flux. Such high escape rates could last only for $<$100 Myr, while R$_{\rm PL}$ shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, R$_{\rm PL}$ must be therefore located at $\approx 1.9-2.2$ R$_{\oplus}$ and high altitude hazes/clouds possibly extinct the light at R$_{\rm T}$. Our analysis constraints also the planet's mass to be 5$-$5.7 M$_{\oplus}$. For CoRoT-24c, R$_{\rm PL}$ and R$_{\rm T}$ lie too close together to be distinguished in the same way. Similar differences between R$_{\rm PL}$ and R$_{\rm T}$ may be present also for other hot, low-density sub-Neptunes.

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arXiv2016

Thermal mass loss of protoplanetary cores with hydrogen-dominated atmospheres: The influences of ionization and orbital distance

We investigate the loss rates of the hydrogen atmospheres of terrestrial planets with a range of masses and orbital distances by assuming a stellar extreme ultraviolet (EUV) luminosity that is 100 times stronger than that of the current Sun. We apply a 1D upper atmosphere radiation absorption and hydrodynamic escape model that takes into account ionization, dissociation and recombination to calculate hydrogen mass loss rates. We study the effects of the ionization, dissociation and recombination on the thermal mass loss rates of hydrogen-dominated super-Earths and compare the results to those obtained by the energy-limited escape formula which is widely used for mass loss evolution studies. Our results indicate that the energy-limited formula can to a great extent over- or underestimate the hydrogen mass loss rates by amounts that depend on the stellar EUV flux and planetary parameters such as mass, size, effective temperature, and EUV absorption radius.

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arXiv2015

The Evolution of Stellar Rotation and the hydrogen atmospheres of habitable-zone Terrestrial Planets

Terrestrial planets formed within gaseous protoplanetary disks can accumulate significant hydrogen envelopes. The evolution of such an atmosphere due to XUV driven evaporation depends on the activity evolution of the host star, which itself depends sensitively on its rotational evolution, and therefore on its initial rotation rate. In this letter, we derive an easily applicable method for calculating planetary atmosphere evaporation that combines models for a hydrostatic lower atmosphere and a hydrodynamic upper atmosphere. We show that the initial rotation rate of the central star is of critical importance for the evolution of planetary atmospheres and can determine if a planet keeps or loses its primordial hydrogen envelope. Our results highlight the need for a detailed treatment of stellar activity evolution when studying the evolution of planetary atmospheres.

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arXiv2015

Thermosphere and geomagnetic response to interplanetary coronal mass ejections observed by ACE and GRACE: Statistical results

For the period July 2003 to August 2010, the interplanetary coronal mass ejection (ICME) catalogue maintained by Richardson and Cane lists 106 Earth-directed events, which have been measured in-situ by plasma and field instruments onboard the ACE satellite. We present a statistical investigation of the Earth's thermospheric neutral density response by means of accelerometer measurements collected by the GRACE satellites, which are available for 104 ICMEs in the data set, and its relation to various geomagnetic indices and characteristic ICME parameters such as the impact speed, southward magnetic field strength (Bz). The majority of ICMEs causes a distinct density enhancement in the thermosphere, with up to a factor of eight compared to the pre-event level. We find high correlations between ICME Bz and thermospheric density enhancements (~0.9), while the correlation with the ICME impact speed is somewhat smaller (~0.7). The geomagnetic indices revealing the highest correlations are Dst and SYM-H (~0.9), the lowest correlations are obtained for kp and AE (~0.7), which show a nonlinear relation with the thermospheric density enhancements. Separating the response for the shock sheath region and the magnetic structure of the ICME, we find that the Dst and SYM-H reveal a tighter relation to the Bz minimum in the magnetic structure of the ICME, whereas the polar cap indices show higher correlations with the Bz minimum in the shock sheath region. Since the strength of the Bz component - either in the sheath or the magnetic structure of the ICME - is highly correlated (~0.9) with the neutral density enhancement, we discuss the possibility of satellite orbital decay estimates based on magnetic field measurements at L1, i.e. before the ICME hits the Earth's magnetosphere. This will further stimulate progress in space weather understanding and applications regarding satellite operations.

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arXiv2015

Transiting exoplanets from the CoRoT space mission XXVII. CoRoT-28b, a planet orbiting an evolved star, and CoRoT-29b, a planet showing an asymmetric transit

Context. We present the discovery of two transiting extrasolar planets by the satellite CoRoT. Aims. We aim at a characterization of the planetary bulk parameters, which allow us to further investigate the formation and evolution of the planetary systems and the main properties of the host stars. Methods. We used the transit light curve to characterize the planetary parameters relative to the stellar parameters. The analysis of HARPS spectra established the planetary nature of the detections, providing their masses. Further photometric and spectroscopic ground-based observations provided stellar parameters (log g,Teff,v sin i) to characterize the host stars. Our model takes the geometry of the transit to constrain the stellar density into account, which when linked to stellar evolutionary models, determines the bulk parameters of the star. Because of the asymmetric shape of the light curve of one of the planets, we had to include the possibility in our model that the stellar surface was not strictly spherical. Results. We present the planetary parameters of CoRoT-28b, a Jupiter-sized planet (mass 0.484+/-0.087MJup; radius 0.955+/-0.066RJup) orbiting an evolved star with an orbital period of 5.208 51 +/- 0.000 38 days, and CoRoT-29b, another Jupiter-sized planet (mass 0.85 +/- 0.20MJup; radius 0.90 +/- 0.16RJup) orbiting an oblate star with an orbital period of 2.850 570 +/- 0.000 006 days. The reason behind the asymmetry of the transit shape is not understood at this point. Conclusions. These two new planetary systems have very interesting properties and deserve further study, particularly in the case of the star CoRoT-29.

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arXiv2015

Origin and Stability of Exomoon Atmospheres - Implications for Habitability

We study the origin and escape of catastrophically outgassed volatiles (H$_2$O, CO$_2$) from exomoons with Earth-like densities and masses of $0.1M_{\oplus}$, $0.5M_{\oplus}$ and $1M_{\oplus}$ orbiting an extra-solar gas giant inside the habitable zone of a young active solar-like star. We apply a radiation absorption and hydrodynamic upper atmosphere model to the three studied exomoon cases. We model the escape of hydrogen and dragged dissociation products O and C during the activity saturation phase of the young host star. Because the soft X-ray and EUV radiation of the young host star may be up to $\sim$100 times higher compared to today's solar value during the first 100 Myr after the system's origin, an exomoon with a mass $ < 0.25M_{\oplus}$ located in the HZ may not be able to keep an atmosphere because of its low gravity. Depending on the spectral type and XUV activity evolution of the host star, exomoons with masses between $\sim0.25-0.5M_{\oplus}$ may evolve to Mars-like habitats. More massive bodies with masses $ > 0.5M_{\oplus}$, however, may evolve to habitats that are a mixture of Mars-like and Earth-analogue habitats, so that life may originate and evolve at the exomoon's surface.

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arXiv2015

Outgassing History and Escape of the Martian Atmosphere and Water Inventory

The evolution and escape of the martian atmosphere and the planet's water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet's origin and lasted $\sim$500 Myr. Because of the high EUV flux of the young Sun and Mars' low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO$_2$ atmosphere of a few tens to a few hundred mbar around $\sim$4--4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure.

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arXiv2015

Extreme hydrodynamic atmospheric loss near the critical thermal escape regime

By considering martian-like planetary embryos inside the habitable zone of solar-like stars we study the behavior of the hydrodynamic atmospheric escape of hydrogen for small values of the Jeans escape parameter $β< 3$, near the base of the thermosphere, that is defined as a ratio of the gravitational and thermal energy. Our study is based on a 1-D hydrodynamic upper atmosphere model that calculates the volume heating rate in a hydrogen dominated thermosphere due to the absorption of the stellar soft X-ray and extreme ultraviolet (XUV) flux. We find that when the $β$ value near the mesopause/homopause level exceeds a critical value of $\sim$2.5, there exists a steady hydrodynamic solution with a smooth transition from subsonic to supersonic flow. For a fixed XUV flux, the escape rate of the upper atmosphere is an increasing function of the temperature at the lower boundary. Our model results indicate a crucial enhancement of the atmospheric escape rate, when the Jeans escape parameter $β$ decreases to this critical value. When $β$ becomes $\leq$2.5, there is no stationary hydrodynamic transition from subsonic to supersonic flow. This is the case of a fast non-stationary atmospheric expansion that results in extreme thermal atmospheric escape rates.

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arXiv2015

Atmosphere expansion and mass loss of close-orbit giant exoplanets heated by stellar XUV: I. Modeling of hydrodynamic escape of upper atmospheric material

In the present series of papers we propose a consistent description of the mass loss process. To study the effects of intrinsic magnetic field of a close-orbit giant exoplanet (so-called Hot Jupiter) on the atmospheric material escape and formation of planetary inner magnetosphere in a comprehensive way, we start with a hydrodynamic model of an upper atmosphere expansion presented in this paper. While considering a simple hydrogen atmosphere model, we focus on selfconsistent inclusion of the effects of radiative heating and ionization of the atmospheric gas with its consequent expansion in the outer space. Primary attention is paid to investigation of the role of specific conditions at the inner and outer boundaries of the simulation domain, under which different regimes of material escape (free- and restricted- flow) are formed. Comparative study of different processes, such as XUV heating, material ionization and recombination, H3+ cooling, adiabatic and Lyman-alpha cooling, Lyman-alpha reabsorption is performed. We confirm basic consistence of the outcomes of our modeling with the results of other hydrodynamic models of expanding planetary atmospheres. In particular, we obtain that under the typical conditions of an orbital distance 0.05 AU around a Sun-type star a Hot Jupiter plasma envelope may reach maximum temperatures up to ~9000K with a hydrodynamic escape speed ~9 km/s resulting in the mass loss rates ~(4-7)*10^10 g*s . In the range of considered stellar-planetary parameters and XUV fluxes that is close to mass loss in the energy limited case. The inclusion of planetary intrinsic magnetic fields in the model is a subject of the following up paper (Paper II).

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