171,142 research outputs found
Atmospheres of brown dwarfs
No full textChH highlights financial support of the European Community under the FP7 by an ERC starting grant. SLC acknowledges financial support of University of LeicesterBrown Dwarfs are the coolest class of stellar objects known to date. Our present perception is that Brown Dwarfs follow the principles of star formation, and that Brown Dwarfs share many characteristics with planets. Being the darkest and lowest mass stars known makes Brown Dwarfs also the coolest stars known. This has profound implication for their spectral fingerprints. Brown Dwarfs cover a range of effective temperatures which cause brown dwarfs atmospheres to be a sequence that gradually changes from a M-dwarf-like spectrum into a planet-like spectrum. This further implies that below an effective temperature of ≲ 2800K, clouds form already in atmospheres of objects marking the boundary between M-Dwarfs and brown dwarfs. Recent developments have sparked the interest in plasma processes in such very cool atmospheres: sporadic and quiescent radio emission has been observed in combination with decaying Xray-activity indicators across the fully convective boundary.Peer reviewe
The shocking transit of WASP-12b : modelling the observed early ingress in the near ultraviolet
No full text4 pages, 2 figuresNear ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.Peer reviewe
Ionization in atmospheres of brown dwarfs and extrasolar planets V. Alfvén ionization
No full textObservations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbor localized plasmas in their atmospheric environments. For low-mass objects, the degree of thermal ionization is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma that is the source of the radiation? We propose Alfvén ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (?10<sup>�7</sup>) that they constitute plasmas. We outline the criteria required for Alfvén ionization and demonstrate its applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs, and M dwarfs with both solar and sub-solar metallicities. We find that Alfvén ionization is most efficient at mid to low atmospheric pressures where a seed plasma is easier to magnetize and the pressure gradients needed to drive the required neutral flows are the smallest. For the model atmospheres considered, our results show that degrees of ionization of 10<sup>�6</sup>-1 can be obtained as a result of Alfvén ionization. Observable consequences include continuum bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g., He, Mg, H<sub>2</sub>, or CO lines). Forbidden lines are also expected from the metastable population. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models. The occurrence of Alfvén ionization may also be applicable to other astrophysical environments such as protoplanetary disks
Theory of Precipitating Dust Formation in Substellar Atmospheres
No full textA kinetic description of dust formation including gravitational settling has been presented in Woitke & Helling (2003a). An application to a prescribed brown dwarf atmosphere structure shows, that a stratified large-scale distribution has to be expected for the dust as well as for the molecular content of the atmosphere
Expect the unexpected : non-equilibrium processes in brown dwarf atmospheres
No full textChH highlights financial support of the European Community under the FP7 by an ERC starting grant 257341Brown Dwarf atmosphere are a chemically extremely rich, one example being the formation of clouds driven by the phase-non-equilibrium of the atmospheric gas. Cloud formation modelling is an integral part of any atmosphere simulation used to interpret spectral observations of ultra-cool objects and to determine fundamental parameters like log(g) and Teff. This proceeding to the workshop 'GAIA and the Unseen: The Brown Dwarf Question' first summarizes what a model atmosphere simulation is, and then advocates two ideas: A) The use of a multitude of model families to determine fundamental parameters with realistic confidence interval. B) To keep an eye on the unexpected, like for example, ionisation signatures resulting plasma processesPeer reviewe
Entwicklung eines formalen Beschreibungsmodells für das geisteswissenschaftliche Forschungsdatenmanagement - eine qualitative Untersuchung von Beratungsprotokollen zur bedarfsorientierten Beschreibung, Strukturierung und Modellierung des Managements von digitalen Forschungsdaten
No full textIm Rahmen der Dissertation "Entwicklung eines formalen Beschreibungsmodells für das geisteswissenschaftliche Forschungsdatenmanagement - eine qualitative Untersuchung von Beratungsprotokollen zur bedarfsorientierten Beschreibung, Strukturierung und Modellierung des Managements von digitalen Forschungsdaten" von Patrick Helling wurde ein formales Beschreibungsmodell zur Definition des Forschungsdatenmanagements (FDM) mit einem Fokus auf die Geistes- und Sozialwissenschaften entwickelt. Das Modell wurde mit Hilfe einer qualitativen Inhaltsanalyse, die auf ein anonymisiertes Korpus bestehend aus 146 FDM-Beratungsprotokollen angewendet wurde, entwickelt. Entsprechende qualitative und quantitative Analysen des kodierten Korpus ermöglichen die datenbasierte Beschreibung, Definition und Konkretisierung von FDM-Bedarfsmustern sowie notwendiger Kompetenzprofile zur Bedienung ebenjener FDM-Bedarfe. Das formale Beschreibungsmodell fungiert dabei als gemeinsame Sprache für ein neues Tiefenverständnis über das Management von Forschungsdaten, das in seiner Form sowohl methodisch als auch inhaltlich Pionierstatus hat. Eine vorgelagerte quantitative Beforschung der FDM-Versorgungslandschaft mit Fokus auf die organisatorische, strukturelle und inhaltliche Ausgestaltung von FDM-Beratungsservices im deutschsprachigen Raum situiert das Beschreibungsmodell grundsätzlich
The influence of galactic cosmic rays on ion-neutral hydrocarbon chemistry in the upper atmospheres of free-floating exoplanets
No full textThe authors highlight financial support of the European Community under the FP7 by an ERC starting grant.Cosmic rays may be linked to the formation of volatiles necessary for prebiotic chemistry. We explore the effect of cosmic rays in a hydrogen-dominated atmosphere, as a proof-of-concept that ion–neutral chemistry may be important for modelling hydrogen-dominated atmospheres. In order to accomplish this, we utilize Monte Carlo cosmic ray transport models with particle energies of 106 eV<E<1012 eV in order to investigate the cosmic-ray enhancement of free electrons in substellar atmospheres. Ion–neutral chemistry is then applied to a Drift–Phoenix model of a free-floating giant gas planet. Our results suggest that the activation of ion–neutral chemistry in the upper atmosphere significantly enhances formation rates for various species, and we find that C2H2, C2H4, NH3, C6H6 and possibly C10H are enhanced in the upper atmospheres because of cosmic rays. Our results suggest a potential connection between cosmic-ray chemistry and the hazes observed in the upper atmospheres of various extrasolar planets. Chemi-ionization reactions are briefly discussed, as they may enhance the degree of ionization in the cloud layer.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Inhomogeneous cloud coverage through the Coulomb explosion of dust in substellar atmospheres
No full textContext. Recent observations of brown dwarf spectroscopic variability in the infrared infer the presence of patchy cloud cover. Aims. This paper proposes a mechanism for producing inhomogeneous cloud coverage due to the depletion of cloud particles through the Coulomb explosion of dust in atmospheric plasma regions. Charged dust grains Coulomb-explode when the electrostatic stress of the grain exceeds its mechanical tensile stress, which results in grains below a critical radius a < aCoulcrit being broken up. Methods. This work outlines the criteria required for the Coulomb explosion of dust clouds in substellar atmospheres, the effect on the dust particle size distribution function, and the resulting radiative properties of the atmospheric regions. Results. Our results show that for an atmospheric plasma region with an electron temperature of Te = 10 eV (≈ 105 K), the critical grain radius varies from 10-7 to 10-4 cm, depending on the grains’ tensile strength. Higher critical radii up to 10-3 cm are attainable for higher electron temperatures. We find that the process produces a bimodal particle size distribution composed of stable nanoscale seed particles and dust particles with a ≥ aCoulcrit , with the intervening particle sizes defining a region devoid of dust. As a result, the dust population is depleted, and the clouds become optically thin in the wavelength range 0:1 - 10 μm, with a characteristic peak that shifts to higher wavelengths as more sub-micrometer particles are destroyed. Conclusions. In an atmosphere populated with a distribution of plasma volumes, this will yield regions of contrasting radiative properties, thereby giving a source of inhomogeneous cloud coverage. The results presented here may also be relevant for dust in supernova remnants and protoplanetary disks.Peer reviewe
Electrostatic activation of prebiotic chemistry in substellar atmospheres
No full textCharged dust grains in the atmospheres of exoplanets may play a key role in the formation of prebiotic molecules, necessary to the origin of life. Dust grains submerged in an atmospheric plasma become negatively charged and attract a flux of ions that are accelerated from the plasma. The energy of the ions upon reaching the grain surface may be sufficient to overcome the activation energy of particular chemical reactions that would be unattainable via ion and neutral bombardment from classical, thermal excitation. As a result, prebiotic molecules or their precursors could be synthesised on the surface of dust grains that form clouds in exoplanetary atmospheres. This paper investigates the energization of the plasma ions, and the dependence on the plasma electron temperature, in the atmospheres of substellar objects such as gas giant planets. Calculations show that modest electron temperatures of eV ( K) are enough to accelerate ions to sufficient energies that exceed the activation energies required for the formation of formaldehyde, ammonia, hydrogen cyanide and the amino acid glycine.Peer reviewe
- …
