T. Penz

The Astronomical Observatory of Brera, Merate, Lombardy, Italy

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Publications (87)174.88 Total impact

  • Article: Erratum to
    Planetary and Space Science 03/2012; 62(1):160-161. · 2.11 Impact Factor
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    ABSTRACT: We present thermal mass loss calculations over evolutionary time scales for the investigation if the smallest transiting rocky exoplanets CoRoT-7b (∼1.68REarth) and Kepler-10b (∼1.416REarth) could be remnants of an initially more massive hydrogen-rich gas giant or a hot Neptune-class exoplanet. We apply a thermal mass loss formula which yields results that are comparable to hydrodynamic loss models. Our approach considers the effect of the Roche lobe, realistic heating efficiencies and a radius scaling law derived from observations of hot Jupiters. We study the influence of the mean planetary density on the thermal mass loss by placing hypothetical exoplanets with the characteristics of Jupiter, Saturn, Neptune, and Uranus to the orbital location of CoRoT-7b at 0.017 AU and Kepler-10b at 0.01684 AU and assuming that these planets orbit a K- or G-type host star. Our findings indicate that hydrogen-rich gas giants within the mass domain of Saturn or Jupiter cannot thermally lose such an amount of mass that CoRoT-7b and Kepler-10b would result in a rocky residue. Moreover, our calculations show that the present time mass of both rocky exoplanets can be neither a result of evaporation of a hydrogen envelope of a "Hot Neptune" nor a "Hot Uranus"-class object. Depending on the initial density and mass, these planets most likely were always rocky planets which could lose a thin hydrogen envelope, but not cores of thermally evaporated initially much more massive and larger objects. (Cited by 18 in Google Scholar)
    Planetary and Space Science 10/2011; 59(13):1472-1481. · 2.11 Impact Factor
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    ABSTRACT: HD 209458b is an exoplanet found to transit the disk of its parent star. Observations have shown a broad absorption signature about the Lyα stellar line during transit, suggesting the presence of a thick cloud of atomic hydrogen around the "hot Jupiter" HD 209458b. This work expands on an earlier work studying the production of energetic neutral atoms (ENAs) as a result of the interaction between the stellar wind and the exosphere. We present an improved flow model of HD 209458b and use stellar wind values similar to those in our solar system. We find that the ENA production is high enough to explain the observations, and we show that—using expected values for the stellar wind and exosphere—the spatial and velocity distributions of ENAs would give absorption in good agreement with the observations. We also study how the production of ENAs depends on the exospheric parameters and establish an upper limit for the obstacle standoff distance at approximately 4-10 planetary radii. Finally, we compare the results obtained for the obstacle standoff distance with existing exomagnetospheric models and show how the magnetic moment of HD 209458b can be estimated from ENA observations.
    The Astrophysical Journal 01/2010; · 6.73 Impact Factor
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    09/2009;
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    ABSTRACT: Aims. We study the relative role of EUV and X-ray radiation in the heating of hydrogen-rich planet atmospheres with different composition and electron content.Methods. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere.Results. Heating rates and efficiencies have been computed, together with column density cut-offs at which photons of given energies stop their heating production inside the atmosphere. Assuming 100 eV as the energy borderline between the extreme ultraviolet spectral range and X-rays we find that when the absorbing hydrogen column density is higher than $10^{20}$ cm$^{-2}$ only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers.Conclusions. Using emission spectra from a sample of solar-type stars of different ages representative of the Sun's main sequence lifetime, we have derived the corresponding heating rates. We find that the existence of an energetic cross-over in atmospheric heating is present for all stars in the sample.
    Astronomy and Astrophysics 03/2009; · 5.08 Impact Factor
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    ABSTRACT: Aims. We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. Methods. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws. Results. By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <M. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances ≤0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances ≤0.02 AU. (Cited by 62 in Google Scholar)
    Astronomy and Astrophysics 01/2009; 506:399-410. · 5.08 Impact Factor
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    ABSTRACT: Replying to: A. Lecavelier des Etangs, A. Vidal-Madjar & J.-M. Désert 456, 10.1038/nature07402 (2008)Lecavelier des Etangs et al. object to the conclusion by Holmström et al. that radiation pressure alone cannot explain the Lyman-alpha absorption observed during transits of HD 209458b. We agree that hydrogen atoms can be accelerated to large velocities by radiation pressure. However, with our model we cannot reproduce the observed spectrum, as shown in the Supplementary Information and Fig. 3 of ref. 2.
    Nature 12/2008; 456(7219):E1-2. · 38.60 Impact Factor
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    ABSTRACT: Cosmic strings are topological defects which were generated at a transition phase of the very early Universe and are probably responsible for large-scale structure forming. However, they may pull through all history and exist in the recent epoch. Thus, they can have influence for the recent Universe interacting with different objects. We consider the cosmic string behavior in the vicinity of a spinning black hole by means of a numerical simulation. Here we present preliminary results of this work via a comparison of cosmic string and magnetic flux tube behavior in the Kerr metric. Such an approach follows from the similarity of the equations which describe these objects. Therefore, many aspects of this behavior may be comparable.It turns out that the cosmic string behavior at an early stage copies the flux tube movement in some degree. Involved in differential rotation, the central part of the cosmic string starts to lose energy and angular momentum due to string braking. Stretching and twisting around the event horizon, the central part of the string gains negative energy in the ergosphere. To compensate these losses, positive energy is subsequently generated and apparently can be extracted from the ergosphere as in the flux tube case. Because of an increase of the numerical errors the code breaks down near the event horizon and only initial stages of the negative energy creation can be observed. In comparison with the cosmic string, further simulations of the flux tube behavior clearly demonstrate an energy extraction process which is attended by relativistic jet forming. Consequently, within the frame of direct analogy, we consider our result as the very beginning of cosmic string jet formation in Kerr geometry.
    Advances in Space Research 08/2008; · 1.18 Impact Factor
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    ABSTRACT: Since low mass M stars show a higher level of stellar activity compared to solar-like stars and because of the closer orbital distance of their habitable zones compared to that of the Solar System, terrestrial exoplanets within M star habitable zones are expected to be much more strongly influenced by stellar winds and dense plasma ejected from the host star by coronal mass ejections. The efficiency of atmospheric erosion of CO2-rich exoplanets, having the size and mass similar to that of the Earth, due to dense stellar plasma flows within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and EUV radiation (XUV) wavelengths over long time periods, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by the CO2 IR radiation in the 15μm band. Our study shows that intense XUV radiation of active M-stars, together with the photochemical production of excited atomic oxygen results in atmospheric expansion and extended exospheres which can interact with the stellar plasma flow. Using the calculated thermospheric neutral and ion densities, we applied a 3-D magnetohydrodynamic and a test particle model for calculating the non-thermal loss rates from the extended exospheres of magnetized and non-magnetized Earth-like exoplanets. The consequences of our preliminary results for the evolution of habitable planets within active M star environments are discussed.
    "14th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun", held on 5-10 November, 2006.; 04/2008
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    ABSTRACT: Since low mass M stars show a higher level of stellar activity compared to solar-like stars, and because of the closer orbital distance of their habitable zones compared to that of the Solar System, terrestrial exoplanets within M star habitable zones are expected to be much more strongly influenced by stellar winds and dense plasma ejected from the host star by coronal mass ejections. The efficiency of atmospheric erosion of CO_2-rich exoplanets, having the size and mass similar to that of the Earth, due to dense stellar plasma flows within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and EUV radiation (XUV) wavelengths over long time periods, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by the CO_2 IR radiation in the 15μm band. Our study shows that intense XUV radiation of active M-stars, together with the photochemical production of excited atomic oxygen results in atmospheric expansion and extended exospheres which can interact with the stellar plasma flow. Using the calculated thermospheric neutral and ion densities, we applied a 3-D magnetohydrodynamic and a test particle model for calculating the non-thermal loss rates from the extended exospheres of magnetized and non-magnetized Earth-like exoplanets. The consequences of our preliminary results for the evolution of habitable planets within active M star environments are discussed.
    03/2008; 384:303.
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    ABSTRACT: Absorption in the stellar Lyman-alpha (Lyalpha) line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet. This has been interpreted as hydrogen atoms escaping from the planet's exosphere, possibly undergoing hydrodynamic blow-off, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Lyalpha absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations.
    Nature 03/2008; 451(7181):970-2. · 38.60 Impact Factor
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    T. Penz, G. Micela
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    ABSTRACT: Aims.We study the influence of the X-ray luminosity distribution of dM stars on mass loss from planets on close-in orbits.Methods.Using the X-ray luminosity of the Pleiades, the Hyades, and field dM stars, we construct a scaling law for the radiation environment of dM stars for ages between 0.1 and 10 Gyr. An energy-limited escape approach is used to calculate the influence of thermal mass loss on planetary distribution functions.Results.We show that the X-ray luminosity distribution of nearby dM stars can be described by using a scaling law derived from observations of open clusters with a given age. It is shown that the X-ray flux from dM stars is significantly less than the flux from dG stars for a given orbital distance. Therefore, loss processes have less of an impact on the mass evolution of planets orbiting dM stars. We found that the mass loss is negligible for hydrogen-rich Jupiter-mass planets at orbits $>$0.02 AU, while Neptune-mass planets are influenced up to 0.05 AU. At orbits of 0.02 AU, Roche lobe effects are also having a strong impact on the mass-loss evolution. Because of the low mass of dM stars, Roche lobe effects are less effective for loss processes at planets orbiting these stars. Finally, if we use only the X-ray luminosity of their host stars for the energy input to the atmosphere, we obtain a lower limit for the mass loss of GJ876d and GJ674b. This does not allow us to conclude whether they are remnants of eroded Jupiter-mass planets.
    http://dx.doi.org/10.1051/0004-6361:20078873. 01/2008;
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    ABSTRACT: We investigate the efficiency of the atmospheric mass loss due to hydrodynamic blow-off over the lifetime of the exoplanet HD209458b by studying numerically its hydrogen wind for host star X-ray and EUV (XUV) fluxes between 1 and 100 times that of the present Sun. We apply a time-dependent numerical algorithm which is able to solve the system of hydrodynamic equations straight through the transonic point of the flow including Roche lobe effects. The mass loss rates are calculated as functions of the absorbed energy in the thermosphere. Depending on the heating efficiency for a hydrogen-rich thermosphere the maximum temperature obtained in our study at 1.5Rpl by neglecting IR cooling is about 5000–10,000 K for heating efficiencies of 10% and 60%, respectively. We find that the upper atmosphere of HD209458b experiences hydrodynamic blow-off even at such low temperatures if one does not neglect gravitational effects caused by the proximity of the planet to its Roche lobe boundary. Depending on the heating efficiency, we find from the solution of the hydrodynamic equations of mass, momentum, and energy balance that energy-limited mass loss rate estimations overestimate the realistic mass loss rate at present time for HD209458b by several times. Using the maximum heating efficiency for hydrogen–rich atmospheres of 60% we find that HD209458b may experience an atmospheric mass loss rate at present time of about . The mass loss rate evolves to higher values for higher XUV fluxes expected during the early period of the planet's host star evolution, reaching values of several times . The integrated mass loss is found to be between 1.8% and 4.4% of the present mass of HD209458b. We found that the influence of the stellar tidal forces on atmospheric loss (the Roche lobe effect) is not significant at 0.045 AU. For a similar exoplanet, but at closer orbital distances , the combined effect of the Roche lobe and the high XUV radiation result in much higher thermal loss rates of about and even more for early stages. This leads to a total loss over 4 Gyr of 27.5% of the planetary mass. (Cited by 35 in Google Scholar)
    Planetary and Space Science 01/2008; · 2.11 Impact Factor
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    ABSTRACT: We revisit an example of “quasi-steady” magnetic reconnection at the dayside magnetopause on February 11, 1998, observed by Equator-S and Geotail at the dawnside magnetopause. Phan et al. [Phan, T.D. et al., 2000. Extended magnetic reconnection at the Earth’s magnetopause from detection of bi-directional jets. Nature 404, 848–850.] reported oppositely directed jets at these spacecrafts and inferred a length of the reconnection line of about 38RE. Pinnock et al. [Pinnock, M., Chisham, G., Coleman, I.J., Freeman, M.P., Hairston, M., Villain, J.-P., 2003. The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field. Ann. Geophys. 21, 1467–1482.] used measurements from SuperDARN radars to show that the reconnection electric field was variable. Here we complement this work by obtaining snapshots of the reconnection electric field from the in situ observations. To do this, we apply a reconstruction method based on a model of compressible Petschek-type magnetic reconnection. This independent method uses magnetic field observations as input data to calculate the reconnection electric field. We obtain average values of Erec in the range of 0.4–2.4 mV/m. Further we infer a distance perpendicular to the reconnection line of 0.4–0.6RE. The model results are compared with the two studies mentioned above. It thus appears that while the transfer of momentum for this event is indeed large-scale, the actual rate depends on the time it is measured.
    Advances in Space Research 01/2008; 41(10):1551-1555. · 1.18 Impact Factor
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    ABSTRACT: A global magnetohydrodynamic numerical simulation is used to study the large-scale structure and formation location of flux transfer events (FTEs) in synergy with in situ spacecraft and ground-based observations. During the main period of interest on the 14 February 2001 from 0930 to 1100 UT the Cluster spacecraft were approaching the Northern Hemisphere high-latitude magnetopause in the postnoon sector on an outbound trajectory. Throughout this period the magnetic field, electron, and ion sensors on board Cluster observed characteristic signatures of FTEs. A few minutes delayed to these observations the Super Dual Auroral Radar Network (SuperDARN) system indicated flow disturbances in the conjugate ionospheres. These ``two-point'' observations on the ground and in space were closely correlated and were caused by ongoing unsteady reconnection in the vicinity of the spacecraft. The three-dimensional structures and dynamics of the observed FTEs and the associated reconnection sites are studied by using the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) MHD code in combination with a simple open flux tube motion model (Cooling). Using these two models the spatial and temporal evolution of the FTEs is estimated. The models fill the gaps left by measurements and allow a ``point-to-point'' mapping between the instruments in order to investigate the global structure of the phenomenon. The modeled results presented are in good correlation with previous theoretical and observational studies addressing individual features of FTEs.
    Journal of Geophysical Research, v.113 (2008). 01/2008;
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    T. Penz, G. Micela, H. Lammer
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    ABSTRACT: Aims.We investigate the influence of high-energy stellar radiation at close-in orbits on atmospheric mass loss during the stellar evolution of a G-type star.Methods.High-energy stellar luminosity varies over a wide range for G field stars. The X-ray luminosity distributions from the Pleiades, the Hyades, and the field are used to derive a scaling law for the evolution of the stellar X-ray luminosity distribution. A modified energy-limited escape approach is taken for calculating atmospheric mass loss for a broad range of planetary parameters.Results.We show that the evolution of close-in exoplanets strongly depends on the detailed X-ray luminosity history of their host stars, which varies over several orders-of-magnitude for G stars. Stars located in the high-energy tail of the luminosity distribution can evaporate most of its planets within 0.5 AU, while a significant fraction of planets can survive if exposed to a moderate X-ray luminosity. We show the change on an initial planetary mass distribution caused by atmospheric escape.
    Astronomy and Astrophysics 01/2008; · 5.08 Impact Factor
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    ABSTRACT: Magnetic reconnection is one of the most fundamental processes in the magnetosphere. We present here a simple method to determine the essential parameters of reconnection such as reconnected flux and location of the reconnection site out of single spacecraft data via remote sensing. On the basis of a time-dependent reconnection model, the dependence of the reconnected flux on the magnetic field z-component Bz is shown. The integral of Bz over time is proportional to the reconnected flux and depends on the distance between the reconnection site and the actual position where Bz is measured. This distance can be estimated from analysis of magnetic field Bz data. We apply our method to Cluster measurements in the Earth’s magnetotail.
    Advances in Space Research 01/2008; · 1.18 Impact Factor
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    ABSTRACT: The problem of steady-state magnetic reconnection in an infinite current layer in collisionless, incompressible, nonresistive plasma, except of the electron diffusion region, is examined analytically using the electron Hall magnetohydrodynamics approach. It is found that this approach allows reducing the problem to the magnetic field potential finding, while last one has to satisfy the Grad–Shafranov equation. The obtained solution demonstrates all essential Hall reconnection features, namely proton acceleration up to Alfvén velocities, the forming of Hall current systems and the magnetic field structure expected. It turns out that the necessary condition of steady-state reconnection to exist is an electric field potential jump across the electron diffusion region and the separatrices. Besides, the powerful mechanism of electron acceleration in X-line direction is required. It must accelerate electrons up to the electron Alfvén velocity inside the diffusion region and on the separatrixes. This is a necessary condition for steady-state reconnection as well.
    Advances in Space Research 01/2008; 41(10):1556-1561. · 1.18 Impact Factor
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    ABSTRACT: Observed atomic hydrogen absorption in the stellar Lyman-alpha line by the planet HD 209458b shows hydrogen with high velocity at great distances from the planet. This has been interpreted as hydrogen atoms undergoing hydrodynamic escape. We estimate the production of energetic neutral atoms from the interaction of the stellar wind with the extrasolar planet HD 209458b, and the implications for observations of atomic hydrogen. The estimated production of hydrogen-energetic neutral atoms is compared with observations.
    AGU Fall Meeting Abstracts. 11/2007; -1:0857.
  • T. Penz, G. Micela, H. Lammer
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    ABSTRACT: Aims: We investigate the influence of high-energy stellar radiation at close-in orbits on atmospheric mass loss during stellar evolution of a G-type star. Methods: High-energy stellar luminosity varies over a wide range for G field stars. The temporal evolution of the distribution of stellar X-ray luminosity and its influence on the evolution of close-in exoplanets is investigated. X-ray luminosity distributions from the Pleiades, the Hyades and the field are used to derive a scaling law for the evolution of the stellar X-ray luminosity distribution. A modified energy-limited escape approach is used to calculate atmospheric mass loss for a broad range of planetary parameters. Results: We show that the evolution of close-in exoplanets strongly depends on the detailed X-ray luminosity history of their host stars, which varies over several orders-of-magnitude for G stars. Stars located at the high-energy tail of the luminosity distribution can evaporate most of its planets within 0.5 AU, while for a moderate luminosity a significant fraction of planets can survive. We show the change on an initial planetary mass distribution caused by atmospheric escape.
    10/2007;

Publication Stats

659 Citations
174.88 Total Impact Points

Institutions

  • 2008–2009
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
    • Swedish Institute of Space Physics
      Kiruna, Norrbotten, Sweden
  • 2003–2004
    • Austrian Academy of Sciences
      • Institut für Weltraumforschung
      Vienna, Vienna, Austria
  • 1970
    • Karl-Franzens-Universität Graz
      • Fachbereich Theoretische Physik
      Graz, Styria, Austria