Christiane Helling

The shocking transit of WASP-12b: modelling the observed early ingress in the near-ultraviolet

Near-ultraviolet (near-UV) observations of WASP-12b have revealed an early ingress compared to the optical transit light curve. 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 or without bow shock. We find that we can reproduce the transit light curves 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.

CONSISTENT SIMULATIONS OF SUBSTELLAR ATMOSPHERES AND NONEQUILIBRIUM DUST CLOUD FORMATION

We aim to understand cloud formation in substellar objects. We combined the non-equilibrium, stationary cloud model of Helling, Woitke & Thi (2008; seed formation, growth, evaporation, gravitational settling, element conservation) with the general-purpose model atmosphere code Phoenix (radiative transfer, hydrostatic equilibrium, mixing length theory, chemical equilibrium) in order to consistently calculate cloud formation and radiative transfer with their feedback on convection and gas phase depletion. We calculate the complete 1D model atmosphere structure and the chemical details of the cloud layers. The DriftPhoenix models enable the first stellar atmosphere simulation that is based on the actual cloud formation process. The resulting (T, p) profiles differ considerably from the previous limiting Phoenix cases Dusty and Cond. A tentative comparison with observations demonstrates that the determination of effective temperatures based on simple cloud models has to be applied with care. Based on our new models, we suggest a mean Teff = 1800K for the L-dwarf twin-binary system DENIS J0205-1159 which is up to 500K hotter than suggested in the literature. We show transition spectra for gas-giant planets which form dust clouds in their atmospheres and evaluate photometric fluxes for a WASP-1 type system.

DISCOVERY OF AN ∼23 M Jup BROWN DWARF ORBITING ∼700 AU FROM THE MASSIVE STAR HIP 78530 IN UPPER SCORPIUS

We present the discovery of a substellar companion on a wide orbit around the ~ 2.5 M ☉ star HIP 78530, which is a member of the 5 Myr old Upper Scorpius association. We have obtained follow-up imaging over two years and show that the companion and primary share common proper motion. We have also obtained JHK spectroscopy of the companion and confirm its low surface gravity, in accordance with the young age of the system. A comparison with DRIFT-PHOENIX synthetic spectra indicates an effective temperature of 2800 ± 200 K and a comparison with template spectra of young and old dwarfs indicates a spectral type of M8 ± 1. The mass of the companion is estimated to be 19-26 M Jup based on its bolometric luminosity and the predictions of evolutionary models. The angular separation of the companion is 45, which at the distance of the primary star, 156.7 pc, corresponds to a projected separation of ~710 AU. This companion features one of the lowest mass ratios (~0.009) of any known companion at separations greater than 100 AU.

Ionization in atmospheres of brown dwarfs and extrasolar planets. I. The role of electron avalanche

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.

Physical and orbital properties of β Pictoris b

M.B., G.C., A.M.L., J.R., H.B., F.A., and D.H. acknowledge financial support from the French National Research Agency (ANR) through project grant, ANR10- BLANC0504-01, ANR-07-BLAN-0221, ANR-2010-JCJC-0504-01, and ANR- 2010-JCJC-0501-01. ChH and DH highlight EU financial support under FP7 by starting grant. J.L.B. Ph.D is funded by the LabEx Exploration Spatiale des Environnements Planetaires (ESEP) # 2011-LABX-030.

Optical and Near-Infrared Spectroscopy of the L Subdwarf SDSS J125637.13-022452.4

Red opticaland near-infrared spectroscopy are presented for SDSS J125637.13 022452.4,one of only fourL subdwarfsreported to date.Thesedata conrm thelow-tem perature,m etal-poornature ofthissource,asindicated by prom inentm etal-hydride bands,alkalilines,and collision-induced H2 absorption. The opticaland near-infrared spectra ofSDSS J1256 0224 are sim ilar to those ofthe sdL4 2M ASS J16262034+3925190,and we derive a classication ofsdL3.5 based on the prelim inary schem eofBurgasser,Cruz,& Kirkpatrick.The kinem aticsofSDSS J1256 0224 areconsistentwith m em bership in the Galactic inner halo,with estim ated U V W space velocities indicating a slightly prograde,eccentricand inclined Galacticorbit(3.5 . R. 11 kpc; jZm axj = 7.5 kpc).Com parison to synthetic spectra com puted with the Phoenix code,including the recentim plem entation ofkinetic condensate form ation (D rift-Phoenix),indicate Teff � 2100{2500 K and (M /H) � 1.5 to 1.0 forlogg � 5.0{5.5 (cgs),although therearecleardiscrepanciesbetween m odeland observed spectra particularly in thered opticalregion.Thestrongerm etal-oxidebandspresentin the D rift-Phoenix m odelspectra,a resultofphase-non-equilibrium abundancesofgrain species,appearsto contradict priorsuggestionsthatgrain form ation isinhibited in m etal-pooratm ospheres;conclusivestatem ents on the m etallicity dependence ofgrain form ation eciency are as yet prem ature. In addition,an apparentshiftin thetem peraturescaleofL subdwarfsrelativeto L dwarfsm ay obviatetheneed for m odied grain chem istry to explain som eoftheform ersuniquespectralcharacteristics. Subjectheadings: stars: chem ically peculiar | stars: individual( SDSS J125637.13 022452.4)| stars:low m ass,brown dwarfs| subdwarfs

Disk Evolution, Element Abundances and Cloud Properties of Young Gas Giant Planets

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, in particular discussing the effects of unusual, non-solar carbon and oxygen abundances. 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. These deviations stem from the separate evolution of gas and dust in the disk, where the dust forms the planet cores, followed by the final run-away accretion of the left-over gas. This gas will contain only traces of elements like C, N and O, because those elements have frozen out as ices. 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. In this regions, C/O is found to approach unity after about 5 Myrs, scaling with the cosmic ray ionization rate assumed. 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 premordial 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, premordial element abundances are considered as suggested by disk models.

Characterization of the gaseous companion κ Andromedae b - New Keck and LBTI high-contrast observations

Context. We previously reported the direct detection of a low mass companion at a projected separation of 55 2 AU around the B9 type star Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. Aims. We present new angular di erential imaging (ADI) images of the system at 2.146 (Ks), 3.776 (L’), 4.052 (NB 4:05) and 4.78 m (M’) obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We aim to determine the near-infrared spectral energy distribution (SED) of the companion and use it to characterize the object. Methods. We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young/old objects and to a set of seven PHOENIX-based atmospheric models of cool objects accounting for the formation of dust. We used evolutionary models to derive mass estimates considering a wide range of plausible initial conditions. Finally, we used dedicated formation models to discuss the possible origin of the companion. Results. We derive a more accurate J = 15:86 0:21, H = 14:95 0:13, Ks = 14:32 0:09 mag for And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at Ks and L’ band. We derive NB 4:05 = 13:0 0:2 and M 0 = 13:3 0:3 mag and estimate Log10(L=L ) = 3:76 0:06. Atmospheric models yield Te = 1900 +100 K. They do not set constrains on the surface gravity. “Hot-start” evolutionary models predict masses of 14 +25 MJup based on the luminosity and temperature estimates, and considering a conservative age range for the system (30 +120 Myr). “warm-start” evolutionary tracks constrain the mass to M 11MJup. Conclusions. The mass of Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of And b.

Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets. III. Breakdown Conditions for Mineral Clouds

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. Cloud particles can be expected to carry considerable charges which may trigger discharge events via small-scale processes between individual cloud particles (intra-cloud discharges) or large-scale processes between clouds (inter-cloud discharges). 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. 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 ionization and the number density of cloud particles allows us to suggest the efficiency with which discharges will occur in planetary atmospheres.

Modelling the local and global cloud formation on HD 189733b

G.L. and Ch.H. highlight the financial support of the European community under the FP7 ERC starting grant 257431.