Results

Dynamic mineral clouds on HD 189733b II. Monte Carlo radiative transfer for 3D cloudy exoplanet atmospheres: combining scattering and emission spectra

G. Lee, K. Wood, I. Dobbs-Dixon, A. Rice, Ch. Helling, (accepted to A&A)

As the 3D spatial properties of exoplanet atmospheres are being observed in increasing detail by current and new generations of telescopes, the modelling of the 3D scattering effects of cloud forming atmospheres with inhomogeneous opacity structures becomes increasingly important to interpret observational data. We model the scattering and emission properties of a simulated cloud forming, inhomogeneous opacity, hot Jupiter atmosphere of HD 189733b. We compare our results to available HST and Spitzer data and quantify the effects of 3D multiple-scattering on observable properties of the atmosphere. We discuss potential observational properties of HD 189733b for the upcoming TESS and CHEOPS missions. We develop a Monte Carlo radiative transfer code and apply it to post-process output of our 3D radiative-hydrodynamic, cloud formation simulation of HD 189733b. We employ three variance reduction techniques; next event estimation, survival biasing and composite emission biasing to improve signal-to-noise of the output.For cloud particle scattering events, a log-normal area distribution is constructed from the 3D cloud formation RHD results and stochastically sampled in order to model the Rayleigh and Mie scattering behaviour of a mixture of grain sizes. Stellar photon packets incident on the eastern dayside hemisphere show predominantly Rayleigh, single-scattering behaviour, while multiple-scattering occurs on the western hemisphere. Combined scattered and thermal emitted light predictions are consistent with published HST and Spitzer secondary transit observations. Our model predictions are also consistent with geometric albedo constraints from optical wavelength ground based polarimetry and HST B-band measurements. We predict an apparent geometric albedo for HD 189733b of 0.205 and 0.229, in the TESS and CHEOPS photometric bands respectively.


Robust detection of quasi-periodic variability: A HAWKI mini survey of late T dwarfs

S. Littlefair, B. Burningham, Ch. Helling, (accepted for publication in MNRAS)

We present HAWK-I J-band light curves of five late-type T dwarfs (T6.5-T7.5) with a typical duration of four hours, and investigate the evidence for quasi-periodic photometric variability on intra-night timescales. Our photometry reaches precisions in the range 7-20 mmag, after removing instrumental systematics that correlate with sky background, seeing and airmass. Based upon a Lomb-Scargle periodogram analysis, the latest object in the sample – ULAS J2321 (T7.5) – appears to show quasi-periodic variability with a period of 1.64 hours and an amplitude of 3 mmag. Given the low amplitude of variability and presence of systematics in our lightcurves, we discuss a Bayesian approach to robustly determine if quasi-periodic variability is present in a lightcurve affected by red noise. Using this approach, we conclude that the evidence for quasi-periodic variability in ULAS J2321 is not significant. As a result, we suggest that studies which identify quasi-periodic variables using the false alarm probability from a Lomb-Scargle periodogram are likely to over-estimate the number of variable objects, even if field stars are used to set a higher false alarm probability threshold. Instead we argue that a hybrid approach combining a false alarm probability cut, followed by Bayesian model selection, is necessary for robust identification of quasi-periodic variability in lightcurves with red noise.


Dust in brown dwarfs and extra-solar planets V. Cloud formation in carbon- and oxygen-rich environments

Ch. Helling, D. Tootill, P. Woitke, and G. Lee, (accepted for publication in A&A)

Recent observations indicate potentially carbon-rich (C/O>1) exoplanet atmospheres. Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/O ratio as additional parameter but carbon-rich cloud formation modeling is a challenge for the models applied. The determination of the habitable zone for exoplanets requires the treatment of cloud formation in chemically different regimes. We aim to model cloud formation processes for carbon rich exoplanetary atmospheres. Disk models show that carbon-rich or near-carbon-rich niches may emerge and cool carbon planets may trace these particular stages of planetary evolution. We extend our kinetic cloud formation model by including carbon seed formation and the formation of C[s], TiC[s], SiC[s], KCl[s], and MgS[s] by gas-surface reactions. We solve a system of dust moment equations and element conservation for a pre-scribed Drift-Phoenix atmosphere structure to study how a cloud structure would change with changing initial C/O_0=0.43 … 10.0.

The material composition of cloud particles does change with height (here: represented by log pgas [bar], and is very different between the oxygen-rich case (C/O=0.43) and the carbon-rich cases (C/O=1.1, 1.5).

The material composition of cloud particles does change with height (here: represented by log pgas [bar], and is very different between the oxygen-rich case (C/O=0.43) and the carbon-rich cases (C/O=1.1, 1.5).

From our work we conclude: While it is almost impossible that pure carbon can condense in any oxygen-rich atmosphere based on an equilibrium gas-phase chemistry composition, carbon and carbonaceous material appears in abundance if C/O>1. It might therefore be thinkable that either i) the atmosphere of the giant exoplanet WASP-12b is indeed carbon-rich as the detection of HCN and C2H2 would suggest, or ii) the original atmosphere was nearly carbon-rich and an inefficient cloud formation tipped it over in the carbon-rich regime (like in Fig. 9 in Helling et al. 2014, or iii) external irradiation (e.g cosmic rays) drives a strong ion-neutral chemistry which leads to the emergence of larger carbo-hydrate molecules (Rimmer et al. 2014).
 

 


VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres

S.-M. Tsai, J.R. Lyon, L. Grosheintz, P.R. Rimmer, D. Kitzmann, K. Heng, 2017, ApJS 228, 20

An open-source and validated chemical kinetics code for studying hot exoplanetary atmospheres is presented (VULCAN). It is constructed for gaseous chemistry from 500 to 2500 K using a reduced C- H-O chemical network with about 300 reactions. It uses eddy diffusion to mimic atmospheric dynamics and excludes photochemistry. We have provided a full description of the rate coefficients and thermodynamic data used. VULCAN is validated by reproducing chemical equilibrium and by comparing its output versus the disequilibrium-chemistry calculations of Moses et al. and Rimmer & Helling.

Validating a new, gas-kinetic code (VULCAN) by comparing to results from ARGO which used STAND2015 from Rimmer & Helling (2016). STAND2015 was constructed for studying the chemistry of lightning and life, hence the potential emergence of complex key molecules.

Validating a new, gas-kinetic code (VULCAN) by comparing to results from ARGO which used STAND2015 from Rimmer & Helling (2016). STAND2015 was constructed for studying the chemistry of lightning and life, hence the potential emergence of complex key molecules.

It reproduces the models of HD 189733b and HD 209458b by Moses et al., which employ a network with nearly 1600 reactions. Further validation of VULCAN is made by examining the theoretical trends produced when the temperature-pressure profile and carbon-to-oxygen ratio are varied. Assisted by a sensitivity test designed to identify the key reactions responsible for producing a specific molecule, we revisit the quenching approximation and find that it is accurate for methane but breaks down for acetylene, because the disequilibrium abundance of acetylene is not directly determined by transport-induced quenching, but is rather indirectly controlled by the disequilibrium abundance of methane. Therefore, we suggest that the quenching approximation should be used with caution and must always be checked against a chemical kinetics calculation. A one-dimensional model atmosphere with 100 layers, computed using VULCAN, typically takes several minutes to complete. VULCAN is written in Python and is publicly available as part of the Exoclimes Simulation Platform (ESP).


Dynamic mineral clouds on HD 189733b I. 3D RHD with kinetic, non-equilibrium cloud formation

G. Lee, I. Dobbs-Dixon, Ch. Helling, K. Bognar, P. Woitke, 2016, A&A 594, 48

3D modelling of cloud formation in atmospheres of extrasolar planets coupled to the atmospheric radiative, hydrodynamic and thermo-chemical properties has long been an open challenge. We present a 3D radiative-hydrodynamic (RHD) atmosphere model of HD 189733b fully coupled to a kinetic, microphysical mineral cloud formation model. We include the feedback effects of cloud advection and settling, gas phase element advection and depletion/replenishment and include the radiative effects of cloud and gas opacity. The 3D Navier-Stokes equations are solved consistently with a two-stream radiative transfer scheme coupled with the cloud moment conservation equations. We model the cloud particles as a mix of mineral materials which change in size and composition as they travel through atmospheric thermo-chemical environments.

 Meridional polar slices of atmospheric cloud properties at the equator (θ = 0◦).

3D Cloud properties for the giant gas planet HD189733b: Meridional polar slices of atmospheric cloud properties at the equator (θ = 0◦) show that the cloud particles are made of different fractions of materials (TiO2[s], Mg2SiO4[s], SiO[s] etc) depending on their location inside the atmosphere.

The local cloud properties such as number density, grain size and material composition are time-dependently calculated. Gas phase element depletion as a result of cloud formation are calculated and uncondensed elements are advected across the globe. Mean cloud particle sizes are typically sub-micron (0.01-0.5 mum) at pressures less than 1 bar with hotter equatorial regions containing the smallest grains. Denser cloud structures occur near terminator regions and deeper (~1 bar) atmospheric layers. A thick, greyer opaque cloud layer is found to be at 100 mbar – 10 bar which contains micron sized or larger cloud particles. The cloud material composition differs across the globe, with various minerals dominating dependent on local thermo-chemical conditions. Silicate materials such as MgSiO3[s] are found to be abundant at mid-high latitudes. Elements involved in the cloud formation can be depleted by several orders of magnitude, with the exception of Oxygen which is depleted by a maximum of 30%.


Lightning climatology of exoplanets and brown dwarfs guided by Solar System data

G. Hodosan, Ch. Helling, R. Assensio-Torres, I. Vorgul, P.R. Rimmer, 2016, MNRAS 461, 3927

Clouds form on extrasolar planets and brown dwarfs where lightning could occur. Lightning is a tracer of atmospheric convection, cloud formation and ionization processes, and may be significant for the formation of prebiotic molecules.

Radio energy distribution of lightning strokes from Earth (WWLLN, 2013) based on data from the whole year compared to a month (2013, December), which do not distinguish between intra-cloud and cloud-to-ground lightning, and optical energy distribution of Jovian and Saturnian flashes (bottom).

Radio energy distribution of lightning strokes from
Earth (WWLLN, Dec  2013; top), and optical energy distribution of Jovian and Saturnian flashes (bottom). No difference is made between intra-cloud and cloud-ground lightning.

We study lightning climatology for the different atmospheric environments of Earth, Venus, Jupiter and Saturn. We present lightning distribution maps for Earth, Jupiter and Saturn, and flash densities for these planets and Venus, based on optical and/or radio measurements from the WWLLN and STARNET radio networks, the LIS/OTD satellite instruments, the Galileo, Cassini, New Horizons and Venus Express spacecraft. We also present flash densities calculated for several phases of two volcano eruptions, Eyjafjallajokull’s (2010) and Mt Redoubt’s (2009). We estimate lightning rates for transiting and directly imaged extrasolar planets and brown dwarfs. Based on the large variety of exoplanets, six categories are suggested for which we use the lightning occurrence information from the Solar System. We examine lightning energy distributions for Earth, Jupiter and Saturn. We discuss how strong stellar activity may support lightning activity. We provide a lower limit of the total number of flashes that might occur on transiting planets during their full transit as input for future studies. We find that volcanically very active planets might show the largest lightning flash densities. When applying flash densities of the large Saturnian storm from 2010/11, we find that the exoplanet HD 189733b would produce high lightning occurrence even during its short transit.

see also LEAP Blog for the world 15 July 2016


Is lightning a possible source of the radio emission on HAT-P-11b?

G. Hodosan, P.B. Rimmer, Ch. Helling, 2016, MNRAS 461, 1222

Lightning induced radio emission has been observed on Solar system planets. There have been many attempts to observe exoplanets in the radio wavelength, however, no unequivocal detection has been reported. Lecavelier des Etangs et al. carried out radio transit observations of the exoplanet HAT-P-11b, and suggested that a small part of the radio flux can be attributed to the planet.

The amount of HCN produced by lightning at pressures 0.1 bar, in a hydrogen-rich atmosphere of an irradiated exoplanet with strong winds, may in some cases yield detectable quantities that linger in the atmosphere for 2 ... 3 yr after the advent of the lightning storm.

Chemical time scale of HCN: The amount of HCN produced by lightning at pressures 0.1 bar, in a hydrogen-rich atmosphere of an irradiated exoplanet with strong winds, may in some cases yield detectable quantities that linger in the atmosphere for 2 … 3 yr  after the advent of the lightning storm.

Here, we assume that this signal is real, and study if this radio emission could be caused by lightning with similar energetic properties like in the Solar system. We find that a lightning storm with 3.8 x 106 times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. The optical emission of such thunderstorm would be comparable to that of the host star. We show that HCN produced by lightning chemistry is observable 2-3 yr after the storm, which produces signatures in the L (3.0-4.0 \mu m) and N (7.5-14.5 \mu m) infrared bands. We conclude that it is unlikely that the observed radio signal was produced by lightning, however, future, combined radio and infrared observations may lead to lightning detection on planets outside the Solar system.

press release


Direct Imaging discovery of a second planet candidate around the possibly transiting planet host CVSO 30

T. Schmidt, R. Neuhäuser, C. Briceño et al., 2016, A&A 593, 75

Direct imaging has developed into a very successful technique for the detection of exoplanets in wide orbits, especially around young stars. Directly imaged planets can be both followed astrometrically on their orbits and observed spectroscopically and thus provide an essential tool for our understanding of the early solar system.
We surveyed the 25 Ori association for direct-imaging companions. This association has an age of only few million years. Among other targets, we observed CVSO 30, which has recently been identified as the first T Tauri star found to host a transiting planet candidate.
We report on photometric and spectroscopic high-contrast observations with the Very Large Telescope, the Keck telescopes, and the Calar Alto observatory. They reveal a directly imaged planet candidate close to the young M3 star CVSO 30.
The JHK-band photometry of the newly identified candidate is at better than 1σ consistent with late-type giants, early-T and early-M dwarfs, and free-floating planets. Other hypotheses such as galaxies can be excluded at more than 3.5σ. A lucky imaging z’ photometric detection limit z’ = 20.5 mag excludes early-M dwarfs and results in less than 10 MJup for CVSO 30 c if bound. We present spectroscopic observations of the wide companion that imply that the only remaining explanation for the object is that it is the first very young (<10 Myr) L – T-type planet bound to a star, meaning that it appears bluer than expected as a result of a decreasing cloud opacity at low effective temperatures. Only a planetary spectral model is consistent with the spectroscopy, and we deduce a best-fit mass of 4-5 Jupiter masses (total range 0.6-10.2 Jupiter masses).
This means that CVSO 30 is the first system in which both a close-in and a wide planet candidate are found to have a common host star. The orbits of the two possible planets could not be more different: they have orbital periods of 10.76 h and about 27 000 yr. The two orbits may have formed during a mutual catastrophic event of planet-planet scattering.


Ionisation and discharge in cloud-forming atmospheres of brown dwarfs and extrasolar planets

Ch. Helling, P.B. Rimmer, I.M. Rodriguez-Barrera, K. Wood, G.B. Robertson, C.R. Stark; 2016, Special Issue Plasma Physics and Controlled Fusion (PPCF) 58, 4003

Brown dwarfs and giant gas extrasolar planets have cold atmospheres with a rich chemical compositions from which mineral cloud particles form. Their particle sizes and material composition vary with height, and the mineral cloud particles are charged due to triboelectric processes in such dynamic atmospheres. The dynamics of the atmospheric gas is driven by the irradiating host star and/or by the rotation of the objects that changes during its lifetime. Thermal gas ionisation in these ultra-cool but dense atmospheres allows electrostatic interactions and magnetic coupling of a substantial atmosphere volume. Combined with a strong magnetic field >> B_Earth, a chromosphere and aurorae might form as suggested by radio and X-ray observations of brown dwarfs. Non-equilibrium processes like cosmic ray ionisation and discharge processes in clouds will increase the local pool of free electrons in the gas. Cosmic rays and lighting discharges also alter the composition of the local atmospheric gas such that tracer molecules might be identified. Cosmic rays affect the atmosphere through air showers which was modelled with a 3D Monte Carlo radiative transfer code to be able to visualise their spacial extent (see figure).

3D Monte Carlo simulation of a Cosmic Ray triggered air shower in a Jupiter-like atmosphere. The proton trajectoriers (blue) and the positive pions (white) are triggered by a primary CR event of 10^20eV.

3D Monte Carlo simulation of a Cosmic Ray triggered air shower in a Jupiter-like atmosphere. The proton trajectoriers (blue) and the positive pions (white) are triggered by a primary CR event of 10^20eV.

Given a certain degree of thermal ionisation of the atmospheric gas, we suggest that electron attachment to charge mineral cloud particles is too inefficient to cause an electrostatic disruption of the cloud particles. Cloud particles will therefore not be destroyed by Coulomb explosion for the local temperature in the collisional dominated brown dwarf and giant gas planet atmospheres. However, the cloud particles are destroyed electrostatically in regions with strong gas ionisation. The potential size of such cloud holes would, however, be too small and might occur too far inside the cloud to mimic the effect of, e.g., magnetic field induced star spots.


The mineral clouds on HD 209458b and HD189733b

Ch. Helling, G. Lee, I. Dobbs-Dixon, I., N. Mayne, D.S. Amundsen, J. Khaimova, A.A. Unger, J. Manners, D. Acreman, C. Smith; 2016, MNRAS 460, 855

classical_substellar_point

Vertical Cloud structure of the giant gas planets HD189733b (red) and HD209458b (blue). Note, e.g., the differences in cloud particles sizes (panel 3).

3D atmosphere model results are used to comparatively study the kinetic, non-equilibrium cloud formation in the atmospheres of two example planets guided by the giant gas planets HD209458b and HD189733b. Rather independently of hydrodynamic model differences, our cloud modelling suggests that both planets are covered in mineral clouds throughout the entire modelling domain. Both planets harbour chemically complex clouds that are made of mineral particles that have a height-dependent material composition and size. The remaining gas-phase element abundances strongly effects the molecular abundances of the atmosphere in the cloud forming regions. Hydrocarbon and cyanopolyyne molecules can be rather abundant in the inner, dense part of the atmospheres of HD189733b and HD209458b. No one value for metallicity and the C/O ratio can be used to describe an extrasolar planet. Our results concerning the presence and location of water in relation to the clouds explain some of the observed differences between the two planets. In HD189733b, strong water features have been reported while such features appear less strong for HD209458b. By considering the location of the clouds in the two atmospheres, we see that obscuring clouds exist high in the atmosphere of HD209458b, but much deeper in HD189733b. We further conclude that the (self-imposed) degeneracy of cloud parameters in retrieval methods can only be lifted if the cloud formation processes are accurately modelled in contrast to prescribing them by independent parameters.

see also LEAP Blog for the world 4 May 2016


Flash ionisation signature in coherent cyclotron emission from Brown Dwarfs

I. Vorgul & Ch. Helling; 2016, MNRAS 458, 1041

Brown dwarfs form mineral clouds in their atmospheres, where charged particles can produce large-scale discharges in form of lightning resulting in substantial sudden increase of local ionisation.

Cyclotron emission beams are refracted by the atmosphere material and may therefore not only interact with flash-ionisation lightning-events but also emerge with a wide opening-angle (4).

Cyclotron emission beams (1, 2, 3, 4) are refracted by the atmosphere material and may therefore not only interact with flash-ionisation lightning-events but also emerge with a wide opening-angle, e.g. (4).

Brown dwarfs are observed to emit cyclotron radio emission. We show that signatures of strong transient atmospheric ionisation events (flash ionisation) can be imprinted on a pre-existing radiation. Detection of such flash ionisation events will open investigations into the ionisation state and atmospheric dynamics. Such events can also result from explosion shock waves, material outbursts or (volcanic) eruptions. We present an analytical model that describes the modulation of a pre-existing electromagnetic radiation by a time-dependent (flash) conductivity that is characteristic for flash ionisation events like lightning. Our conductivity model reproduces the conductivity function derived from observations of Terrestrial Gamma Ray Flashes, and is applicable to astrophysical objects with strong temporal variations in the local ionization, as in planetary atmospheres and protoplanetary disks. We show that the field responds with a characteristic flash-shaped pulse to a conductivity flash of intermediate intensity. More powerful ionisation events result in smaller variations of the initial radiation, or in its damping. We show that the characteristic damping of the response field for high-power initial radiation carries information about the ionisation flash magnitude and duration. The duration of the pulse amplification or the damping is consistently shorter for larger conductivity variations and can be used to evaluate the intensity of the flash ionisation. Our work suggests that cyclotron emission could be probe signals for electrification processes inside BD atmosphere.


Atmospheric electrification in dusty, reactive gases in the solar system and beyond

Ch. Helling, R.G. Harrison, F. Honary, D.A. Diver, K. Aplin, I. Dobbs-Dixon, U. Ebert, S. Inutsuka, F.J. Gordillo-Vazquez, S. Littlefair; 2016, Surveys of Geophysics (SGeo) 37, 405

Figure5_upperpart

Comparing Earth, Saturn and Brown Dwarfs.

Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) Brown Dwarfs which are amongst the oldest objects in the universe. Despite of this diversity, solar system planets, extrasolar planets and Brown Dwarfs have broadly similar global temperatures between 300K and 2500K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emission. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emission that potentially originate from accelerated electrons on Brown Dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation.

see also LEAP Blog for the world 15 Feb 2016


The First Millimeter Detection of a Non-Accreting Ultracool Dwarf

P.K.G. Williams, S.L. Casewell, C.R. Stark, S.P. Littlefair, Ch. Helling, E. Berger, 2015, ApJ 815, 64

The well-studied M9 dwarf TVLM 513-46546 is a rapid rotator (P_rot ~ 2 hr) hosting a stable, dipolar magnetic field of ~3 kG surface strength. Here we report its detection with ALMA at 95 GHz at a mean flux density of $56 \pm 12$ uJy, making it the first ultracool dwarf detected in the millimeter band, excluding young, disk-bearing objects. We also report flux density measurements from unpublished archival VLA data and new optical monitoring data from the Liverpool Telescope.

Light curves of TVLM 513 in millimeter (ALMA LSB; upper panel) and optical (lower panel) bands.

Light curves of TVLM 513 in millimeter (ALMA LSB; upper panel) and optical (lower panel) bands.

The ALMA data are consistent with a power-law radio spectrum that extends continuously between centimeter and millimeter wavelengths. We argue that the emission is due to the synchrotron process, excluding thermal, free-free, and electron cyclotron maser emission as possible sources. During the interval of the ALMA observation that phases with the maximum of the object’s optical variability, the flux density is higher at a ~1.8 sigma significance level. These early results show how ALMA opens a new window for studying the magnetic activity of ultracool dwarfs, particularly shedding light on the particle acceleration mechanism operating in their immediate surroundings.

press release


A Chemical Kinetics Network for Lightning and Life in Planetary Atmospheres

P.R. Rimmer & Ch. Helling, 2016, ApJS  224, 9

There are many open questions about prebiotic chemistry in both planetary and exoplanetary environments. The increasing number of known exoplanets and other ultra-cool, substellar objects has propelled the desire to detect life and prebiotic chemistry outside the solar system. We present an ion-neutral chemical network constructed from scratch, Stand2015, that treats hydrogen, nitrogen, carbon and oxygen chemistry accurately within a temperature range between 100 K and 30000 K. Formation pathways for glycine and other organic molecules are included. The network is complete up to H6C2N2O3. Stand2015 is successfully tested against atmospheric chemistry models for HD209458b, Jupiter and the present-day Earth using a simple 1D photochemistry/diffusion code. Fig_IonChemOur results for the early Earth agree with those of Kasting (1993) for CO2, H2, CO and O2, but do not agree for water and atomic oxygen. We use the network to simulate an experiment where varied chemical initial conditions are irradiated by UV light. The result from our simulation is that more glycine is produced when more ammonia and methane is present. Very little glycine is produced in the absence of any molecular nitrogen and oxygen. This suggests that production of glycine is inhibited if a gas is too strongly reducing. Possible applications and limitations of the chemical kinetics network are also discussed.

see also LEAP Blog for the world 19 Jan 2016


Extended Baseline Photometry of Rapidly Changing Weather Patterns on the Brown Dwarf Binary Luhman-16

R.A. Street, B.J. Fulton, A. Scholz, K. Horne, Ch. Helling, D. Juncher, G. Lee, S. Valenti, S.; 2015, ApJ, 812, 161

Luhman-16 (WISE J1049-5319) was recently discovered to be a nearby (∼2 pc) brown dwarf binary that exhibits a high degree of photometric variability (Δm ∼ 0.1 mag). This is thought to be due to the evolution of “cloud” features on the photosphere, but Luhman-16 has been found to show unusually rapid changes, possibly resulting from fast-evolving “weather.” This target is of particular interest because it consists of a co-evolutionary pair of brown dwarfs spanning the transition between L and T spectral types (L7.5 and T0.5), which are expected to be associated with changes in cloud surface coverage. Being comparatively bright (I ∼ 15.5 mag), the target is well suited for observation with the new Las Cumbres Observatory Global Telescope Network (LCOGT) of 1 m telescopes. We present long-time baseline photometric observations from two of LCOGT’s southern hemisphere sites, which were used in tandem to monitor Luhman-16 for up to 13.25 hr at a time (more than twice the rotation period), for a total of 41.2 days in the SDSS-i‧ and Pan-STARRS-Z filters. We use this data set to characterize the changing rotational modulation, which may be explained by the evolution of cloud features at varying latitudes on the surfaces of the two brown dwarfs.


Reference study to characterise plasma and magnetic properties of ultra-cool atmospheres

M.I. Rodríguez-Barrera, Ch. Helling, C.R. Stark, A.M. Rice, 2015, MNRAS 454, 3977

Radio and X-ray emission from brown dwarfs suggest that an ionised gas and a magnetic field with a sufficient flux density must be present. We perform a reference study for late M-dwarfs, brown dwarfs and giant gas planet to identify which ultra-cool objects are most susceptible to plasma and magnetic processes. Only thermal ionisation is considered. V-ionized-f_e We utilise the Drift-Phoenix model grid where the local atmospheric structure is determined by the global parameters Teff, log(g) and [M/H].
Our results show that it is not unreasonable to expect Hα or radio emission to origin from Brown Dwarf atmospheres as in particular the rarefied upper parts of the atmospheres can be magnetically coupled despite having low degrees of thermal gas ionisation. Such ultra-cool atmospheres could therefore drive auroral emission without the need for a companion’s wind or an outgassing moon. The minimum threshold for the magnetic flux density required for electrons and ions to be magnetised is well above typical values of the global magnetic field of a brown dwarf and a giant gas planet. Na+, K+ and Ca+ are the dominating electron donors in low-density atmospheres (low log(g), solar metallicity) independent of Teff. Mg+ and Fe+ dominate the thermal ionisation in the inner parts of M-dwarf atmospheres. Molecules remain unimportant for thermal ionisation. Chemical processes (e.g. cloud formation) affecting the most abundant electron donors, Mg and Fe, will have a direct impact on the state of ionisation in ultra-cool atmospheres.

see also LEAP Blog for the world 10 Nov 2015


Modelling the local and global cloud formation on HD 189733b

G. Lee, Ch. Helling, I. Dobbs-Dixon, D. Juncher, 2015, A&A 580

Context. Observations suggest that exoplanets such as HD 189733b form clouds in their atmospheres which have a strong feedback onto their thermodynamical and chemical structure, and overall appearance. cloud map Aims. Inspired by mineral cloud modelling efforts for Brown Dwarf atmospheres, we present the first spatially varying kinetic cloud model structures for HD 189733b. Methods. We apply a 2-model approach using results from a 3D global radiation-hydrodynamic simulation of the atmosphere as input for a detailed, kinetic cloud formation model. Sampling the 3D global atmosphere structure with 1D trajectories allows us to model the spatially varying cloud structure on HD 189733b. The resulting cloud properties enable the calculation of the scattering and absorption properties of the clouds. Results. We present local and global cloud structure and property maps for HD 189733b. The calculated cloud properties show variations in composition, size and number density of cloud particles which are strongest between the dayside and nightside. Cloud particles are mainly composed of a mix of materials with silicates being the main component. Cloud properties, and hence the local gas composition, change dramatically where temperature inversions occur locally. The cloud opacity is dominated by absorption in the upper atmosphere and scattering at higher pressures in the model. The calculated 8 μm single scattering Albedo of the cloud particles are consistent with Spitzer bright regions. The cloud particles scattering properties suggest that they would sparkle/reflect a midnight blue colour at optical wavelengths.

see also LEAP Blog for the world 29 May 2015


Inhomogeneous cloud coverage through the Coulomb explosion of dust in substellar atmospheres

C.R. Stark, Ch. Helling, D.A. Diver, 2015, A&A 579

Recent observations of brown dwarf spectroscopic variability in the infrared infer the presence of patchy cloud cover. 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. As a result, the dust population is depleted, and the cloud opacity become optically thin in the wavelength range 0.1-10 μm.


Multiwaveband photometry of the irradiated brown dwarf WD0137-349B

S.L. Casewell, K.A. Lawrie, P.F.L. Maxted, M.S. Marley, J.J. Fortney, R.B. Rimmer, S.P. Littlefair, G. Wynn, M.R. Burleigh, Ch. Helling, 2015, MNRAS 447

WD0137-349 is a white dwarf-brown dwarf binary system in a 116 minute orbit. We present radial velocity observations and multiwaveband photometry from V, R and I in the optical, to J, H and Ks in the near-IR and 3.6, 4.5, 5.8 and 8.0 μm in the mid-IR. The photometry and lightcurves show variability in all wavebands, with the amplitude peaking at 4.5 μm, where the system is also brightest. Fluxes and brightness temperatures were computed for the heated and unheated atmosphere of the brown dwarf (WD0137-349B) using synthetic spectra of the white dwarf using model atmosphere simulations. We show that the flux from the brown dwarf dayside is brighter than expected in the Ks and 4.5 μm bands when compared to models of irradiated brown dwarfs with full energy circulation and suggest this over-luminosity may be attributed to H2 fluorescence or H3+ being generated in the atmosphere by the UV irradiation.

see also LEAP Blog for the world 16 Feb 2015


Dust in brown dwarfs and extra-solar planets IV. Assessing TiO2 and SiO nucleation for cloud formation modeling

G. Lee, Ch. Helling, H. Giles, S. T. Bromley, 2015, A&A 575

Clouds form in atmospheres of brown dwarfs and planets. TiO2 and SiO nucleation The cloud particle
formation processes are similar to the dust formation process studied in circumstellar shells of AGB stars and in Supernovae. We re-address the question of the primary nucleation species in view of new (TiO2)N-cluster data and new SiO vapour pressure data. We apply the density functional theory using the computational chemistry package Gaussian 09 to derive updated thermodynamical data for (TiO2)N-clusters as input for our TiO2 seed formation model. We test different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation or a prescribed Drift-Phoenix atmosphere structure. Updated Gibbs free energies for the (TiO2)N-clusters are presented, and a slightly temperature dependent surface tension for T=500 … 2000K with an average value of σ = 480.6 erg cm-2. The TiO2-seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO-nucleation. Despite the higher abundance of SiO over TiO2 in the gas phase, TiO2 remains considerably more efficient in forming condensation seeds by homogeneous nucleation followed by heterogeneous grain growth. The paper discusses also the effect on the cloud structure.

see also LEAP Blog for the world 20 Jan 2015


Atmospheres of Brown Dwarfs

Christiane Helling & Sarah Casewell
Review in The Astronomy & Astrophysics Review 2015, Vol. 22 (1) (Springer)

Brown 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. StratIonAtmosphere 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.

Link to Springer online paper


Jupiter as a Giant Cosmic Ray Detector

P.B. Rimmer, C.R. Stark, Ch. Helling, 2014, ApJ 787, 25

We explore the feasibility of using the atmosphere of Jupiter to detect ultra-high-energy cosmic rays (UHECRs). The large surface area of Jupiter allows us to probe cosmic rays of higher energies than previously accessible. Cosmic ray extensive air showers in Jupiter’s atmosphere could in principle be detected by the Large Area Telescope (LAT) on the Fermi observatory. In order to be observed, these air showers would need to be oriented toward the Earth, and would need to occur sufficiently high in the atmosphere that the gamma rays can penetrate. We demonstrate that, under these assumptions, Jupiter provides an effective cosmic ray “detector” area of 3.3 × 107 km2.

cr_jupiter_fig3

Cosmic ray frequency as a function of the energy of the extensive airshower, E0 [eV], and the gamma ray fluence [erg cm-2] as observed by Fermi-LAT. The solid line indicates the maximum fluence from an extensive air shower of energy E0.

 

We predict that Fermi-LAT should be able to detect events of energy >1021 eV with fluence 10-7 erg cm-2 at a rate of about one per month. The observed number of air showers may provide an indirect measure of the flux of cosmic rays >~ 1020 eV. Extensive air showers also produce a synchrotron signature that may be measurable by Atacama Large Millimeter/submillimeter Array (ALMA). Simultaneous observations of Jupiter with ALMA and Fermi-LAT could be used to provide broad constraints on the energies of the initiating cosmic rays.

Disk evolution, element abundances and cloud properties of young gas giant planets

Ch. Helling, P. Woitke, P.B. Rimmer, I. Kamp, W.-F. Thi, R. Meijerink, 2014, Life 4, 142

We discuss the chemical pre-conditions for planet formation, in terms of gas and ice abundances in a protoplanetary disk, as function of time and position, and the resulting chemical composition and cloud properties in the atmosphere when young gas giant planets form. Large deviations between the abundances of the host star and its gas giants seem likely to occur if the planet formation follows the core-accretion scenario. ProDiMo protoplanetary disk models are used to predict the chemical evolution of gas and ice in the midplane. We find that cosmic rays play a crucial role in slowly un-blocking the CO, where the liberated oxygen forms water, which then freezes out quickly. Therefore, the C/O ratio in the gas phase is found to gradually increase with time, in a region bracketed by the water and CO ice-lines.

13003_099_czuo_mitdust

Carbon-to-oxygen ratio after cloud formation from an initial disk element abundance with C/Oinit=0.99.

 

In this regions, C/O is found to approach unity after about 5 Myrs, scaling with the cosmic ray ionisation rate. We then explore how the atmospheric chemistry and cloud properties in young gas giants are affected when the non-solar C/O ratios predicted by the disk models are assumed. The DRIFT cloud formation model is applied to study the formation of atmospheric clouds under the influence of varying primordial element abundances and its feedback onto the local gas. We demonstrate that element depletion by cloud formation plays a crucial role in converting an oxygen-rich atmosphere gas into carbon-rich gas when non-solar, primordial element abundances are considered as suggested by disk model.

Ionisation in atmospheres of brown dwarfs and extrasolar planets VI: Properties of large-scale discharge events

R. Bailey, Ch. Helling, G. Hodosan, C. Bilger, C. R. Stark, 2013, ApJ, 784, 43

Mineral clouds in substellar atmospheres play a special role as a catalyst for a variety of charge processes. If clouds are charged, the surrounding environment becomes electrically activated, and ensembles of charged grains are electrically discharging (e.g. by lightning), which significantly influences the local chemistry creating conditions similar to those thought responsible for life in early planetary atmospheres.

properties

Laboratory streamer properties applied to substellar atmospheres. (Briels et al. 2008; IOP Publishing. Reproduced with permission. All rights reserved.)

 

We note that such lightning discharges contribute also to the ionisation state of the atmosphere. We apply scaling laws for electrical discharge processes from laboratory measurements and numerical experiments to Drift-Phoenix model atmosphere results to model the discharge’s propagation downwards (as lightning) and upwards (as sprites) through the atmospheric clouds. We evaluate the spatial extent and energetics of lightning discharges.
The atmospheric volume affected (e.g. by increase of temperature or electron number) is larger in a brown dwarf atmosphere than in a giant gas planet’s. Our results suggest that the total dissipated energy in one event is <10^12 J for all models of initial solar metallicity. First attempts to show the influence of lightning on the local gas phase indicate an increase of small carbohydrate molecules like CH and CH2 at the expense of CO and CH4. Dust forming molecules are destroyed and the cloud particle properties are frozen-in unless enough time is available for complete evaporation. We summerise instruments potentially suitable to observe lightning on extrasolar objects.

see also LEAP Blog for the world 30 April 2014


The Influence of Galactic Cosmic Rays on Ion-Neutral Hydrocarbon Chemistry in the Upper Atmospheres of Free-Floating Exoplanets

Paul B. Rimmer, Christiane Helling, Camille Bilger 2013, JAsB 13carbon-aurora, 173

Volume fraction of various species as a function of the gas pressure for the model atmosphere of a free-floating giant gas planet.

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 10^6 eV < E < 10^12 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.

see also LEAP Blog for the world 14 March 2014


Electrostatic activation of prebiotic chemistry in substellar atmospheres

Craig R. Stark, Christiane Helling, Declan A. Diver, Paul Rimmer 2013, IJAsB 13, 165

Most likely series of ion-neutral reactions that lead to the formation of glycine. chemreac_to_glycine

Charged 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 ~ 1eV (~10000 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.

see also University press release , astrobits , universe today

see also LEAP Blog for the world 28 Jan 2014


Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets V: Alfven Ionization

Craig R. Stark, Christiane Helling, Declan A. Diver, Paul Rimmer 2013, ApJ 776, 11

Evolution of an electron overdensity due to its self-electrostatic field,in the e_avalanche_26082013presence of vertically oriented magnetic field. Similar effects can be expected for the Alfvén ionization process. Credit:MacLachlan et al. New J Phys.1, 063001, 2009

Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbour localized plasmas in their atmospheric environments. We propose Alfven ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (>10^-7) that they constitute plasmas. We outline the criteria required for Alfven ionization to occur and demonstrate it’s applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs and M-dwarfs. Our results show that degrees of ionization ranging from 10^-6 … 1 can be obtained. Furthermore, Alfven ionization alters the atmospheric chemistry via the creation of new ionic species not normally available in contemporary, thermally-driven atmospheric models. Observable consequences include continuum Bremsstrahlung emission superimposed with spectral lines from the plasma ion species.


Small hydrocarbon molecules in cloud-forming Brown Dwarf and giant gas planet atmospheres

Camille Bilger, Paul Rimmer, Christiane Helling 2013, MNRAS 435, 1888

We study the abundances of complex carbon-bearing molecules in the oxygen-rich
dust- forming atmospheres of Brown Dwarfs and giant gas planets. Results from Drift-phoenix atmosphere simulations, which include the feedback of phase-non-equilibrium dust cloud formation on the atmospheric structure and the gas-phase abundances, are utilised. The resulting element depletion leads to a shift in the carbon-to-oxygen ratio such that several hydrocarbon molecules and cyanopolyynene molecules can be present.
We determine chemical relaxation time-scales to evaluate if hydrocarbon molecules can be affected by transport-induced quenching. Our results suggest that a considerable amount of C2H6 and C2H2 could be expected in the upper atmospheres not only of giant gas planets, but also of Brown Dwarfs. However, the exact quenching height strongly depends on the data source used. These results will have an impact on future thermo-kinetic studies, as they change the inner boundary condition for those simulations.


Ionization in atmospheres of Brown Dwarfs and extrasolar planets IV. The Effect of Cosmic Rays

Paul Rimmer and Christiane Helling 2013, ApJ 774, 108

Cosmic rays provide an important source for free electrons in the Earth’s atmosphere and also in dense interstellar regions where they produce a prevailing background ionization. We utilize a Monte Carlo cosmic ray transport model for particle energies of 1 MeV < E < 1 GeV, and an analytic cosmic ray transport model for particle energies of 1 GeV < E < 1 TeV in order to investigate the cosmic ray enhancement of free electrons in substellar atmospheres of free-floating objects. For the model brown dwarf atmosphere, the electron fraction is enhanced significantly by cosmic rays when the pressure pgas < 10^-2 bar. Our example giant gas planet atmosphere suggests that the cosmic ray enhancement extends to 10^-4 – 10^-2 bar, depending on the effective temperature. For the model atmosphere of the example giant gas planet considered here (Teff = 1000 K), cosmic rays bring the degree of ionization to fe ~ 10^-8 when pgas < 10^-8 bar, suggesting that this part of the atmosphere may behave as a weakly ionized plasma. Although cosmic rays enhance the degree of ionization by over three orders of magnitude in the upper atmosphere, the effect is not likely to be significant enough for sustained coupling of the magnetic field to the gas.


Ionisation in atmospheres of Brown Dwarfs and extrasolar planets III. Breakdown conditions for mineral clouds

Christiane Helling, Moira Jardine, Craig Stark, Declan Diver 2013, Asigmacritpgrain_16003_5_3pJ 767, 136

Critical number of charges per dust grain [e/grain] needed for electric breakdown in mineral clouds inside atmospheres for objects with Teff =1600K and log(g)=3.0 (red) and log(g)=5.0 (brown).

Electric discharges were detected directly in the cloudy atmospheres of Earth, Jupiter and Saturn, are debatable for Venus, and indirectly inferred for Neptune and Uranus in our solar system. Sprites (and other types of transient luminous events) have been detected only on Earth, and are theoretically predicted for Jupiter, Saturn and Venus.
Cloud formation is a common phenomenon in ultra-cool atmospheres such as in Brown Dwarf and extrasolar planetary atmospheres. We investigate electrostatic breakdown characteristics, like critical field strengths and critical charge densities per surface, to demonstrate under which conditions mineral clouds undergo electric discharge events which may trigger or be responsible for sporadic X-ray emission.frac_ion_el_dust

Top : Degree of thermal gas ionisation (solid lines) and the dust number density per hydrogen (dashed lines). Bottom: The The efficiency with which a streamer may be initiated in a mineral cloud.

We apply results from our kinetic dust cloud formation model that is part of the Drift-Phoenix model atmosphere simulations. We present a first investigation of the dependence of the breakdown conditions in Brown Dwarf and giant gas exoplanets on the local gas-phase chemistry, the effective temperature and primordial gas-phase metallicity. Our results suggest that different intra-cloud discharge processes dominate at different heights inside mineral clouds: local coronal (point discharges) and small-scale sparks at the bottom region of the cloud where the gas density is high, and flow discharges and large-scale sparks near, and maybe above, the cloud top. The comparison of the thermal degree of ionisation and the number density of cloud particles allows us to suggest the efficiency with which discharges will occur in planetary atmospheres.


Energetic Charged Particles Above Thunderclouds

Martin Fuellekrug, Declan Diver, Jean-Louis Pincon, Alan D. R. Phelps, Anne Bourdon, Christiane Helling, Elisabeth Blanc, Farideh Honary, R. Giles Harrison, Jean-Andre Sauvaud, Jean-Baptiste Renard, Mark Lester, Michael Rycroft, Mike Kosch, Richard B. Horne, Serge Soula, Stephane Gaffet 2012, Surveys in Geophysics

satellit-taranis

The French government has committed to launch the satellite TARANIS to study transient coupling processes between the Earth’s atmosphere and near-Earth space. The prime objective of TARANIS is to detect energetic charged particles and hard radiation emanating from thunderclouds. The British Nobel prize winner C.T.R. Wilson predicted lightning discharges from the top of thunderclouds into space almost a century ago. However, new experiments have only recently confirmed energetic discharge processes which transfer energy from the top of thunderclouds into the upper atmosphere and near-Earth space; they are now denoted as transient luminous events, terrestrial gamma-ray flashes and relativistic electron beams. This meeting report builds on the current state of scientific knowledge on the physics of plasmas in the laboratory and naturally occurring plasmas in the Earth’s atmosphere to propose areas of future research. The report specifically reflects presentations delivered by the members of a novel Franco-British collaboration during a meeting at the French Embassy in London held in November 2011. The scientific subjects of the report tackle ionization processes leading to electrical discharge processes, observations of transient luminous events, electromagnetic emissions, energetic charged particles and their impact on the Earth’s atmosphere. The importance of future research in this area for science and society, and towards spacecraft protection, is emphasized.


Clouds in brown dwarfs and giant planets

S. Metchev, D. Apai, J. Radigan, É. Artigau, A. Heinze, Ch. Helling, D. Homeier, S. Littlefair, C. Morley, A. Skemer, C. Stark

A growing body of observational and theoretical evidence points toward the importance of clouds in the atmospheres of ultra-cool brown dwarfs and giant planets. Empirically, the presence of clouds is inferred from the red, likely dusty atmospheres of young substellar objects, and from detections of periodic variability in a fraction of brown dwarfs – as expected from rotation and a patchy cloud cover. Theoretical models have progressed alongside by including ever more comprehensive atomic and molecular opacity tables, incorporating the treatment of non-equilibrium chemistry and clouds through vertical mixing and grain size/sedimentation parameters, and employing 3-D hydrodynamical simulations. In this proceeding we summarize the key issues raised during the first gathering of observers and theorists to discuss clouds and atmospheric circulation in non-irradiated ultra-cool dwarfs and giant planets.


M dwarf stars in the light of (future) exoplanet searches

B. Rojas-Ayala, E.J. Hilton, A.W. Mann, S. Lépine, E. Gaidos, X. Bonfils, Ch. Helling, T.J. Henry, L.A. Rogers, K. von Braun, A. Youdin

We present a brief overview of a splinter session on M dwarf stars as planet hosts that was organized as part of the Cool Stars 17 conference. The session was devoted to reviewing our current knowledge of M dwarf stars and exoplanets in order to prepare for current and future exoplanet searches focusing in low mass stars. We review the observational and theoretical challenges to characterize M dwarf stars and the importance of accurate fundamental parameters for the proper characterization of their exoplanets and our understanding on planet formation.


Dust cloud lightning in extraterrestrial atmospheres

Helling Ch., Jardine M., Diver D., Witte S, (2013, P&SS, 77,cosmic-ray-attenuation 152)

Cosmic ray attenuation (in %; solid lines, right axis) and gas-phase column density ρcol (in g/cm2; dotted lines, left axis)

Lightning is present in all solar system planets which form clouds in their atmospheres. Cloud formation outside our solar system is possible in objects with much higher temperatures than on Earth or on Jupiter: Brown dwarfs and giant extrasolar gas planets form clouds made of mixed materials and a large spectrum of grain sizes. These clouds are globally neutral but we argue that mineral clouds in brown dwarfs and extrasolar planets are susceptible to local discharge events and that the upper cloud layers are most suitable for powerful lightning-like discharges. We discuss various sources of atmospheric ionisation, including thermal ionisation and a first estimate of ionisation by cosmic rays, and argue that we should expect thunderstorms also in the atmospheres of brown dwarfs and giant gas planets which contain mineral clouds.


Ionisation in atmospheres of Brown Dwarfs and extrasolar planets II Dust-induced collisional ionization

Helling Ch., Jardine M., Mokler F. (2011, ApJ 532)ecol_16005_mitekine

Turbulent dust-dust collisional energies compared to the ionisation energy of mixed grain materials

Observations have shown that continuous radio emission and also sporadic H-alpha and X-ray emission are prominent in singular, low-mass objects later than spectral class M. These activity signatures are interpreted as being caused by coupling of an ionised atmosphere to the stellar magnetic field. In Brown Dwarfs, thermal gas processes are insufficient, but the formation of clouds sets in. Cloud particles can act as seeds for electron avalanches in streamers that ionise the ambient gas, and can lead to lightning and indirectly to magnetic field coupling, a combination of processes also expected for protoplanetary disks.neutralisationtimeetc5

Time-scale comparison suggest lightning regime in upper part of brown dwarf atmospheric clouds

However, the precondition is that the cloud particles are charged. We use results from Drift-Phoenix model atmospheres to investigate collisional processes. We show that ionisation by turbulence-induced dust-dust collisions is the most efficient kinetic process. Dust-dust collisions alone are not sufficient to improve the magnetic coupling of the atmosphere inside the cloud layers, but the charges supplied either on grains or within the gas phase as separated electrons can trigger secondary non-linear processes. We suggest that although thermal gas ionisation declines in objects across the fully-convective boundary, dust charging by collisional processes can play an important role in the lowest mass objects. The onset of atmospheric dust may therefore correlate with the anomalous X-ray and radio emission in atmospheres that are cool, but charged more than expected by pure thermal ionisation.


The influence of non-isotropic scattering of thermal emission on spectra of brown dwarfs and hot exoplanets.

de Kok, Helling, Stam, Woitke and Witte (2011, A&A 531, 67)moltenplanet

We calculate near-infrared thermal emission spectra, which includes scattering by clouds and haze. Initial temperature profiles and cloud optical depths are taken from the DRIFT-PHOENIX Brown Dwarf model.

We show that cloud particles tend to reduce fluxes in the near-infrared spectrum and make it more red compared to the clear sky case. This is especially apparent for particles that are large compared to the wavelength and have only little iron in them. Scattering particles will show deeper absorption features than absorbing (e.g. iron) particles and scattering and particle size will also affect the calculated infrared colours. Large particles tend to be strongly forward-scattering and we show that assuming isotropic scattering in this case also leads to very large errors in the spectrum. The fact that the thermal radiation can be scattered also means that it can be polarised.

It is interesting to note that the small dark particles predicted by DRIFT-PHOENIX will also result in a lower geometric albedo of the planet at visible wavelengths compared to Rayleigh scattering or relatively large enstatite particles. Because the small iron-rich particles are dark, this haze may contribute to the low albedos of some hot Jupiters. The darkening effect of small iron particles has already been noted in earlier works, but the DRIFT-PHOENIX model might provide a physical basis for this argument.


The role of electron avalanche processes in atmospheres of Brown Dwarfs.

Helling, Jardine, Witte and Diver (2011, ApJ 727)

iceland-volcano-lightning

Brown dwarf and extrasolar planet atmospheres form clouds which strongly influence the local chemistry and physics. These clouds are globally neutral obeying dust-gas charge equilibrium which is, on short timescales, inconsistent with the observation of stochastic ionization events of the solar system planets. We argue that a significant volume of the clouds in brown dwarfs and extrasolar planets is susceptible to local discharge events. These are electron avalanches triggered by charged dust grains. Such intra-cloud discharges occur on timescales shorter than the time needed to neutralize the dust grains by collisional processes. An ensemble of discharges is likely to produce enough free charges to suggest a partial and stochastic coupling of the atmosphere to a large-scale magnetic field.