124 results found
Moore L, Stallard T, Galand MIF, 2016, Upper atmospheres of the giant planets, Heliophysics: Active Stars, their Astrospheres, and Impacts on Planetary Environments, Editors: Schrijver, Bagenal, Sojka, Publisher: Cambridge University Press, Pages: 175-200, ISBN: 9781107090477
Chadney JM, Galand M, Koskinen TT, et al., 2016, EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars, Astronomy & Astrophysics, Vol: 587, ISSN: 1432-0746
The composition and structure of the upper atmospheres of Extrasolar GiantPlanets (EGPs) are affected by the high-energy spectrum of their host starsfrom soft X-rays to EUV. This emission depends on the activity level of thestar, which is primarily determined by its age. We focus upon EGPs orbiting K-and M-dwarf stars of different ages. XUV spectra for these stars areconstructed using a coronal model. These spectra are used to drive both athermospheric model and an ionospheric model, providing densities of neutraland ion species. Ionisation is included through photo-ionisation andelectron-impact processes. We find that EGP ionospheres at all orbitaldistances considered and around all stars selected are dominated by thelong-lived H$^+$ ion. In addition, planets with upper atmospheres where H$_2$is not substantially dissociated have a layer in which H$_3^+$ is the major ionat the base of the ionosphere. For fast-rotating planets, densities ofshort-lived H$_3^+$ undergo significant diurnal variations, with the maximumvalue being driven by the stellar X-ray flux. In contrast, densities oflonger-lived H$^+$ show very little day/night variability and the magnitude isdriven by the level of stellar EUV flux. The H$_3^+$ peak in EGPs with upperatmospheres where H$_2$ is dissociated under strong stellar illumination ispushed to altitudes below the homopause, where this ion is likely to bedestroyed through reactions with heavy species. The inclusion of secondaryionisation processes produces significantly enhanced ion and electron densitiesat altitudes below the main EUV ionisation peak, as compared to models that donot include electron-impact ionisation. We estimate infrared emissions fromH$_3^+$, and while, in an H/H$_2$/He atmosphere, these are larger from planetsorbiting close to more active stars, they still appear too low to be detectedwith current observatories.
Raghuram S, Bhardwaj A, Galand M, 2016, Prediction of forbidden ultraviolet and visible emissions in comet 67P/Churyumov-Gerasimenko, Astrophysical Journal, Vol: 818, ISSN: 1538-4357
Remote observation of spectroscopic emissions is a potential tool for theidentification and quantification of various species in comets. CO Cameron band(to trace \cod) and atomic oxygen emissions (to trace H$_2$O and/or CO$_2$, CO)have been used to probe neutral composition in the cometary coma. Using acoupled-chemistry emission model, various excitation processes controlling COCameron band and different atomic oxygen and atomic carbon have been modelledin comet 67P-Churyumov-Gerasimenko at 1.29~AU (perihelion) and at 3~AUheliocentric distances, which is being explored by ESA's Rosetta mission. Theintensities of CO Cameron band, atomic oxygen and atomic carbon emission linesas a function of projected distance are calculated for different CO and CO$_2$volume mixing ratios relative to water. Contributions of different excitationprocesses controlling these emissions are quantified. We assess how CO$_2$and/or CO volume mixing ratios with respect to H$_2$O can be derived based onthe observed intensities of CO Cameron band, atomic oxygen, and atomic carbonemission lines.The results presented in this work serve as base linecalculations to understand the behaviour of low out-gassing cometary coma andcompare them with the higher gas production rate cases (e.g. comet Halley).Quantitative analysis of different excitation processes governing thespectroscopic emissions is essential to study the chemistry of inner coma andto derive neutral gas composition.
The discovery of almost two thousand exoplanets has revealed an unexpectedlydiverse planet population. We see gas giants in few-day orbits, whole multi-planet systemswithin the orbit of Mercury, and new populations of planets with masses between that of theEarth and Neptune—all unknown in the Solar System. Observations to date have shown thatour Solar System is certainly not representative of the general population of planets in ourMilky Way. The key science questions that urgently need addressing are therefore: What areexoplanets made of? Why are planets as they are? How do planetary systems work and whatcauses the exceptional diversity observed as compared to the Solar System? The EChO(Exoplanet Characterisation Observatory) space mission was conceived to take up thechallenge to explain this diversity in terms of formation, evolution, internal structure andplanet and atmospheric composition. This requires in-depth spectroscopic knowledge of theatmospheres of a large and well-defined planet sample for which precise physical, chemicaland dynamical information can be obtained. In order to fulfil this ambitious scientificprogram, EChO was designed as a dedicated survey mission for transit and eclipsespectroscopy capable of observing a large, diverse and well-defined planet sample withinits 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signalfrom the star and planet are differentiated using knowledge of the planetary ephemerides,allows us to measure atmospheric signals from the planet at levels of at least 10−4 relative tothe star. This can only be achieved in conjunction with a carefully designed stable payloadand satellite platform. It is also necessary to provide broad instantaneous wavelengthcoverage to detect as many molecular species as possible, to probe the thermal structureof the planetary atmospheres and to correct for the contaminating effects of the stellarphotosphere. This requires wavelength coverage of at l
Fuselier SA, Altwegg K, Balsiger H, et al., 2015, ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet, Astronomy & Astrophysics, Vol: 583, ISSN: 1432-0746
Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-closeinvestigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within thethin coma.Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km fromthe comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access tothe nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured thelow-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19),CO+, and CO+2(masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions includeH3O+and HCO+(masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relativelysimple model is constructed to account for the low H3O+/H2O+and HCO+/CO+ratios observed in a water dominated coma. Resultsfrom this simple model are compared with results from models that include a more detailed chemical reaction network.Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex modelsthat include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ratiosare predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ratios and the observed ratiosare often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densitiescan exceed that of water.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.