Atmospheric effects of stellar cosmic rays on Earth-like exoplanets orbiting M-dwarfs

M-dwarf stars are generally considered favourable for rocky planet detection. However, such planets may be subject to extreme conditions due to possible high stellar activity. The goal of this work is to determine the potential effect of stellar cosmic rays on key atmospheric species of Earth-like planets orbiting in the habitable zone of M-dwarf stars and show corresponding changes in the planetary spectra. We build upon the cosmic rays model scheme of Grenfell et al. (2012), who considered cosmic ray induced NOx production, by adding further cosmic ray induced production mechanisms (e.g. for HOx) and introducing primary protons of a wider energy range (16 MeV - 0.5 TeV). Previous studies suggested that planets in the habitable zone that are subject to strong flaring conditions have high atmospheric methane concentrations, while their ozone biosignature is completely destroyed. Our current study shows, however, that adding cosmic ray induced HOx production can cause a decrease in atmospheric methane abundance of up to 80\%. Furthermore, the cosmic ray induced HOx molecules react with NOx to produce HNO$_3$, which produces strong HNO$_3$ signals in the theoretical spectra and reduces NOx-induced catalytic destruction of ozone so that more than 25\% of the ozone column remains. Hence, an ozone signal remains visible in the theoretical spectrum (albeit with a weaker intensity) when incorporating the new cosmic ray induced NOx and HOx schemes, even for a constantly flaring M-star case. We also find that HNO$_3$ levels may be high enough to be potentially detectable. Since ozone concentrations, which act as the key shield against harmful UV radiation, are affected by cosmic rays via NOx-induced catalytic destruction of ozone, the impact of stellar cosmic rays on surface UV fluxes is also studied.Comment: 14 pages, 12 figure

Quantum read-out for cold atomic quantum simulators

Quantum simulators allow to explore static and dynamical properties of otherwise intractable quantum many-body systems. In many instances, however, the read-out limits such quantum simulations. In this work, we introduce an innovative experimental read-out exploiting coherent non-interacting dynamics. Specifically, we present a tomographic recovery method allowing to indirectly measure the second moments of the relative density fluctuations between two one-dimensional superfluids, which until now eluded direct measurements. Applying methods from signal processing, we show that we can reconstruct the relative density fluctuations from non-equilibrium data of the relative phase fluctuations. We employ the method to investigate equilibrium states, the dynamics of phonon occupation numbers and even to predict recurrences. The method opens a new window for quantum simulations with one-dimensional superfluids, enabling a deeper analysis of their equilibration and thermalization dynamics

Two Component Heat Diffusion Observed in CMR Manganites

We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434 v2: Abstract correcte

The effect of stellar limb darkening values on the accuracy of the planet radii derived from photometric transit observations

We study how the precision of the exoplanet radius determination is affected by our present knowledge of limb darkening in two cases: when we fix the limb darkening coefficients and when we adjust them. We also investigate the effects of spots in one-colour photometry. We study the effect of limb darkening on the planetary radius determination both via analytical expressions and by numerical experiments. We also compare some of the existing limb darkening tables. When stellar spots affect the fit, we replace the limb darkening coefficients, calculated for the unspotted cases, with effective limb darkening coefficients to describe the effect of the spots. There are two important cases. (1) When one fixes the limb darkening values according to some theoretical predictions, the inconsistencies of the tables do not allow us to reach accuracy in the planetary radius of better than 1-10% (depending on the impact parameter) if the host star's surface effective temperature is higher than 5000 K. Below 5000 K the radius ratio determination may contain even 20% error. (2) When one allows adjustment of the limb darkening coefficients, the a/Rs ratio, the planet-to-stellar radius ratio, and the impact parameter can be determined with sufficient accuracy (<1%), if the signal-to-noise ratio is high enough. However, the presence of stellar spots and faculae can destroy the agreement between the limb darkening tables and the fitted limb darkening coefficients, but this does not affect the precision of the planet radius determination. We also find that it is necessary to fit the contamination factor, too. We conclude that the present inconsistencies of theoretical stellar limb darkening tables suggests one should not fix the limb darkening coefficients. When one allows them to be adjusted, then the planet radius, impact parameter, and the a/Rs can be obtained with the required precision.Comment: Astronomy & Astrophysics Vol. 549, A9 (2013) - 11 page

New Insights into Cosmic Ray induced Biosignature Chemistry in Earth-like Atmospheres

With the recent discoveries of terrestrial planets around active M-dwarfs, destruction processes masking the possible presence of life are receiving increased attention in the exoplanet community. We investigate potential biosignatures of planets having Earth-like (N$_2$-O$_2$) atmospheres orbiting in the habitable zone of the M-dwarf star AD Leo. These are bombarded by high energetic particles which can create showers of secondary particles at the surface. We apply our cloud-free 1D climate-chemistry model to study the influence of key particle shower parameters and chemical efficiencies of NOx and HOx production from cosmic rays. We determine the effect of stellar radiation and cosmic rays upon atmospheric composition, temperature, and spectral appearance. Despite strong stratospheric O$_3$ destruction by cosmic rays, smog O$_3$ can significantly build up in the lower atmosphere of our modeled planet around AD Leo related to low stellar UVB. N$_2$O abundances decrease with increasing flaring energies but a sink reaction for N$_2$O with excited oxygen becomes weaker, stabilizing its abundance. CH$_4$ is removed mainly by Cl in the upper atmosphere for strong flaring cases and not via hydroxyl as is otherwise usually the case. Cosmic rays weaken the role of CH$_4$ in heating the middle atmosphere so that H$_2$O absorption becomes more important. We additionally underline the importance of HNO$_3$ as a possible marker for strong stellar particle showers. In a nutshell, uncertainty in NOx and HOx production from cosmic rays significantly influences biosignature abundances and spectral appearance.Comment: Manuscript version after addressing all referee comments. Published in Ap

The habitability of stagnant-lid Earths around dwarf stars

The habitability of a planet depends on various factors, such as delivery of water during the formation, the co-evolution of the interior and the atmosphere, as well as the stellar irradiation which changes in time. Since an unknown number of rocky exoplanets may operate in a one-plate convective regime, i.e., without plate tectonics, we aim at understanding under which conditions planets in such a stagnant-lid regime may support habitable surface conditions. Understanding the interaction of the planetary interior and outgassing of volatiles with the atmosphere in combination with the evolution of the host star is crucial to determine the potential habitability. M-dwarf stars in particular possess a high-luminosity pre-main sequence phase which endangers the habitability of planets around them via water loss. We therefore explore the potential of secondary outgassing from the planetary interior to rebuild a water reservoir allowing for habitability at a later stage. We compute the boundaries of the habitable zone around M, K, G, and F-dwarf stars using a 1D cloud-free radiative-convective climate model accounting for the outgassing history of CO2 and H2O from an interior evolution and outgassing model for different interior compositions and stellar luminosity evolutions. The outer edge of the habitable zone strongly depends on the amount of CO2 outgassed from the interior, while the inner edge is mainly determined via the stellar irradiation, as soon as a sufficiently large water reservoir has been outgassed. A build-up of a secondary water reservoir for planets around M-dwarf stars is possible even after severe water loss during the high luminosity pre-main sequence phase as long as some water has been retained within the mantle. Earth-like stagnant-lid planets allow for habitable surface conditions within a continuous habitable zone that is dependent on interior composition.Comment: 15 pages, accepted by A&A, abstract shortene

Probing the atmosphere of a sub-Jovian planet orbiting a cool dwarf

We derive the 0.01 $\mu$m binned transmission spectrum, between 0.74 and 1.0 $\mu$m, of WASP-80b from low resolution spectra obtained with the FORS2 instrument attached to ESO's Very Large Telescope. The combination of the fact that WASP-80 is an active star, together with instrumental and telluric factors, introduces correlated noise in the observed transit light curves, which we treat quantitatively using Gaussian Processes. Comparison of our results together with those from previous studies, to theoretically calculated models reveals an equilibrium temperature in agreement with the previously measured value of 825K, and a sub-solar metallicity, as well as an atmosphere depleted of molecular species with absorption bands in the IR ($\gg 5\sigma$). Our transmission spectrum alone shows evidence for additional absorption from the potassium core and wing, whereby its presence is detected from analysis of narrow 0.003 $\mu$m bin light curves ($\gg 5\sigma$). Further observations with visible and near-UV filters will be required to expand this spectrum and provide more in-depth knowledge of the atmosphere. These detections are only made possible through an instrument-dependent baseline model and a careful analysis of systematics in the data.Comment: 13 pages, 11 figures, 3 tables. Accepted for publication in MNRA