Ultracool Atmospheres Research Articles

The Polar Vortex Hypothesis: Evolving, Spectrally Distinct Polar Regions Explain Short- and Long-term Light-curve Evolution and Color–Inclination Trends in Brown Dwarfs and Giant Exoplanets

Recent studies revealed viewing-angle-dependent color and spectral trends in brown dwarfs, as well as long-term photometric variability (∼100 hr). The origins of these trends are yet unexplained. Here, we propose that these seemingly unrelated sets of observations stem from the same phenomenon: the polar regions of brown dwarfs and directly imaged exoplanets are spectrally different from lower-latitude regions, and they evolve over longer timescales, possibly driven by polar vortices. We explore this hypothesis via a spatiotemporal atmosphere model capable of simulating time series and disk-integrated spectra of ultracool atmospheres. We study three scenarios with different spectral and temporal components: a null hypothesis without polar vortex, and two scenarios with polar vortices. We find that the scenarios with polar vortex can explain the observed infrared color–inclination trend and the variability amplitude–inclination trend. The presence of spectrally distinct, time-evolving polar regions in brown dwarfs and giant exoplanet atmospheres raises the possibility that one-dimensional static atmospheric models may be insufficient for reproducing ultracool atmospheres in detail.

Primeval very low-mass stars and brown dwarfs – VIII. The first age benchmark L subdwarf, a wide companion to a halo white dwarf

Abstract We report the discovery of five white dwarf + ultracool dwarf systems identified as common proper motion wide binaries in the Gaia Catalogue of Nearby Stars. The discoveries include a white dwarf + L subdwarf binary, VVV 1256−62AB, a gravitationally-bound system located 75.6$^{+1.9}_{-1.8}$ pc away with a projected separation of 1375$^{+35}_{-33}$ au. The primary is a cool DC white dwarf with a hydrogen dominated atmosphere, and has a total age of $10.5^{+3.3}_{-2.1}$ Gyr, based on white dwarf model fitting. The secondary is an L subdwarf with a metallicity of [M/H] = $-0.72^{+0.08}_{-0.10}$ (i.e. [Fe/H] = −0.81 ± 0.10) and Teff = 2298$^{+45}_{-43}$ K based on atmospheric model fitting of its optical to near infrared spectrum, and likely has a mass just above the stellar/substellar boundary. The subsolar metallicity of the L subdwarf and the system’s total space velocity of 406 km/s indicates membership in the Galactic halo, and it has a flat eccentric Galactic orbit passing within 1 kpc of the centre of the Milky Way every ∼0.4 Gyr and extending to 15-31 kpc at apogal. VVV 1256−62B is the first L subdwarf to have a well-constrained age, making it an ideal benchmark of metal-poor ultracool dwarf atmospheres and evolution.

Ultracool dwarfs observed with the Spitzer Infrared Spectrograph – III. Dust grains in young L dwarf atmospheres are heavier

ABSTRACT Analysis of all archival 5–14 micron spectra of field ultracool dwarfs from the Infrared Spectrograph on the Spitzer Space Telescope has shown that absorption by silicates in the 8–11 micron region is seen in most L-type (1300 to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the atmospheres of L dwarfs and is strongest at L4–L6 spectral types. Herein we compare averages of the mid-infrared silicate absorption signatures of L3–L7 dwarfs that have low (≲104.5 cm s−2) versus high (≳105 cm s−2) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity, with young atmospheres having a broader, redder, and more asymmetric absorption profile. This indicates a difference in grain size and composition between dust condensates in young and old mid-L dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs matches expectations for ∼1 micron-sized amorphous iron- and magnesium-bearing pyroxene (MgxFe1 − xSiO3) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller (≲0.1 μm) and more volatile amorphous enstatite (MgSiO3) or SiO grains. This is the first direct spectroscopic evidence for gravity-dependent sedimentation of dust condensates in ultracool atmospheres. It confirms theoretical expectations for lower sedimentation efficiencies in low-gravity atmospheres and independently confirms their increased dustiness.

Uniform Forward-modeling Analysis of Ultracool Dwarfs. II. Atmospheric Properties of 55 Late-T Dwarfs

Abstract We present a large uniform forward-modeling analysis for 55 late-T (T7–T9) dwarfs, using low-resolution (R ≈ 50–250) near-infrared (1.0–2.5 μm) spectra and cloudless Sonora–Bobcat model atmospheres. We derive the objects’ effective temperatures, surface gravities, metallicities, radii, masses, and bolometric luminosities using our newly developed Bayesian framework, and use the resulting population properties to test the model atmospheres. We find (1) our objects’ fitted metallicities are 0.3–0.4 dex lower than those of nearby stars; (2) their ages derived from spectroscopic parameters are implausibly young (10 Myr–0.4 Gyr); (3) their fitted effective temperatures show a similar spread to empirical temperature scales at a given spectral type but are ∼50–200 K hotter for ≥T8 dwarfs; and (4) their spectroscopically inferred masses are unphysically small (mostly 1–8 M Jup). These suggest the Sonora–Bobcat assumptions of cloudless and chemical-equilibrium atmospheres do not adequately reproduce late-T dwarf spectra. We also find a gravity and metallicity dependence of effective temperature as a function of spectral type. Combining the resulting parameter posteriors of our sample, we quantify the degeneracy between the fitted surface gravity and metallicity such that an increase in Z combined with a 3.4× increase in logg results in a spectrum that has similar fitted parameters. We note the systematic difference between the late-T dwarf spectra and Sonora–Bobcat models is on average ≈2%–4% of the objects’ peak J-band fluxes over the 1.0–2.5 μm range, implying modeling systematics will exceed measurement uncertainties when analyzing data with J-band S/N ≳ 50. Using our large, high-quality sample, we examine the spectral-fitting residuals as a function of wavelength and atmospheric properties to discern how to improve the model assumptions. Our work constitutes the largest analysis of brown dwarf spectra using multimetallicity models and the most systematic examination of ultracool model atmospheres to date.

The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc

Abstract We present the identification of the second discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79−763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594″ (6471au). The primary star COCONUTS-2A has strong stellar activity (Hα, X-ray, and ultraviolet emission) and is rapidly rotating (Prot = 2.83 days), from which we estimate an age of 150–800 Myr. Comparing equatorial rotational velocity derived from the Transiting Exoplanet Survey Satellite (TESS) light curve to spectroscopic vsini, we find that COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of T eff = 434 ± 9 K, a surface gravity of log g = 4.11 − 0.18 + 0.11 dex, and a mass of M = 6.3 − 1.9 + 1.5 M Jup based on hot-start evolutionary models, leading to a mass ratio of 0.016 − 0.005 + 0.004 for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806−661B), and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b’s infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity ( log ( L bol / L ⊙ ) = − 6.384 ± 0.028 dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means that such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.

Cloud Atlas: Unraveling the Vertical Cloud Structure with the Time-series Spectrophotometry of an Unusually Red Brown Dwarf

Abstract Rotational modulations of emission spectra in brown dwarf and exoplanet atmospheres show that clouds are often distributed non-uniformly in these ultracool atmospheres. The spatial heterogeneity in cloud distribution demonstrates the impact of atmospheric dynamics on cloud formation and evolution. In this study, we update the Hubble Space Telescope (HST) time-series data analysis of the previously reported rotational modulations of WISEP J004701+680352—an unusually red late-L brown dwarf with a spectrum similar to that of the directly imaged planet HR 8799e. We construct a self-consistent spatially heterogeneous cloud model to explain the HST and the Spitzer time-series observations, as well as the time-averaged spectra of WISE 0047. In the heterogeneous cloud model, a cloud thickness variation of around one pressure scale height explains the wavelength dependence in the HST near-IR spectral variability. By including disequilibrium CO/CH4 chemistry, our models also reproduce the redder color of WISE 0047 compared to that of field brown dwarfs. We discuss the impact of vertical cloud structure on atmospheric profile and estimate the minimum eddy diffusivity coefficient for other objects with redder colors. Our data analysis and forward modeling results demonstrate that time-series spectrophotometry with a broad wavelength coverage is a powerful tool for constraining heterogeneous atmospheric structure.

Cloud Atlas: High-contrast Time-resolved Observations of Planetary-mass Companions

Abstract Directly imaged planetary-mass companions offer unique opportunities in atmospheric studies of exoplanets. They share characteristics of both brown dwarfs and transiting exoplanets, and therefore are critical for connecting atmospheric characterizations for these objects. Rotational phase mapping is a powerful technique to constrain the condensate cloud properties in ultra-cool atmospheres. Applying this technique to directly imaged planetary-mass companions will be extremely valuable for constraining cloud models in low mass and surface-gravity atmospheres and for determining the rotation rate and angular momentum of substellar companions. Here, we present Hubble Space Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll differential imaging and hybrid point-spread function modeling, we achieve sub-percent photometric precision for all three observations. We find tentative modulations (<2σ) for AB Pic B and 2M0122B, but cannot reach conclusive results on 2M1207b due to strong systematics. The relatively low significance of the modulation measurements cannot rule out the hypothesis that these planetary-mass companions have the same vertical cloud structures as brown dwarfs. Our rotation rate measurements, combined with archival period measurements of planetary-mass companions and brown dwarfs, do not support a universal mass-rotation relation. The high precision of our observations and the high occurrence rates of variable low-surface-gravity objects encourage high-contrast time-resolved observations with the James Webb Space Telescope.

Cloud Atlas: Hubble Space Telescope Near-infrared Spectral Library of Brown Dwarfs, Planetary-mass Companions, and Hot Jupiters

Abstract Bayesian atmospheric retrieval tools can place constraints on the properties of brown dwarfs' and hot Jupiters' atmospheres. To fully exploit these methods, high signal-to-noise spectral libraries with well-understood uncertainties are essential. We present a high signal-to-noise spectral library (1.10–1.69 μm) of the thermal emission of 76 brown dwarfs and hot Jupiters. All our spectra have been acquired with the Hubble Space Telescope’s Wide Field Camera 3 instrument and its G141 grism. The near-infrared spectral types of these objects range from L4 to Y1. Eight of our targets have estimated masses below the deuterium-burning limit. We analyze the database to identify peculiar objects and/or multiple systems, concluding that this sample includes two very-low-surface-gravity objects and five intermediate-surface-gravity objects. In addition, spectral indices designed to search for composite-atmosphere brown dwarfs indicate that eight objects in our sample are strong candidates to have such atmospheres. None of these objects are overluminous, so their composite atmospheres are unlikely to be companion-induced artifacts. Five of the eight confirmed candidates have been reported as photometrically variable, suggesting that composite atmospheric indices are useful in identifying brown dwarfs with strongly heterogeneous cloud covers. We compare hot Jupiters and brown dwarfs in a near-infrared color–magnitude diagram. We confirm that the coldest hot Jupiters in our sample have spectra similar to mid-L dwarfs, and the hottest hot Jupiters have spectra similar to those of M-dwarfs. Our sample provides a uniform data set of a broad range of ultracool atmospheres, allowing large-scale comparative studies and providing an HST legacy spectral library.

Environmental effects on the ionisation of brown dwarf atmospheres

Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient. Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets. Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects. Results. The external radiation environment plays an important role for the atmospheric ionisation of very low-mass, ultra-cool objects. Lyman-continuum irradiation greatly increases the level of ionisation in the uppermost atmospheric regions. Our results suggest that a shell of an almost fully ionised atmospheric gas emerges for brown dwarfs in star-forming regions and brown dwarfs in white dwarf binary systems. As a consequence, brown dwarf atmospheres can be magnetically coupled, which is the presumption for chromospheric heating to occur and for aurorae to emerge. First tests for assumed chromosphere-like temperature values suggest that the resulting free-free X-ray luminosities are comparable with those observed from non-accreting brown dwarfs in star-forming regions.

Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

Abstract Time-resolved observations of brown dwarfs’ rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the 1.1–1.7 μm broadband light curve has an amplitude of 1.206% ± 0.025% and period of 15.4 ± 0.5 hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 μm water absorption band, indicating that the characteristic condensate particle sizes are large (>1 μm). We detect significantly (4.4σ) lower modulation amplitude in the 1.4 μm water absorption band and find that HN Peg B’s spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.

Self-consistent atmosphere modeling with cloud formation for low-mass stars and exoplanets

Context: Low-mass stars and extrasolar planets have ultra-cool atmospheres where a rich chemistry occurs and clouds form. The increasing amount of spectroscopic observations for extrasolar planets requires self-consistent model atmosphere simulations to consistently include the formation processes that determine cloud formation and their feedback onto the atmosphere. Aims: Complement the MARCS model atmosphere suit with simulations applicable to low-mass stars and exoplanets in preparation of E-ELT, JWST, PLATO and other upcoming facilities. Methods: The MARCS code calculates stellar atmosphere models, providing self-consistent solutions of the radiative transfer and the atmospheric structure and chemistry. We combine MARCS with DRIFT, a kinetic model that describes cloud formation in ultra-cool atmospheres (seed formation, growth/ evaporation, gravitational settling, convective mixing, element depletion). Results: We present a small grid of self-consistently calculated atmosphere models for $T_ \text{eff} = 2000 - 3000$ K with solar initial abundances and $\log(g) = 4.5$. Cloud formation in stellar and sub-stellar atmospheres appears for $T_\text{eff} < 2700$ K and has a significant effect on the structure and the spectrum of the atmosphere for $T_\text{eff} < 2400$ K. We have compared the synthetic spectra of our models with observed spectra and found that they fit the spectra of mid to late type M-dwarfs and early type L-dwarfs well. We also test DRIFT-MARCS for an example exoplanet and demonstrate that our simulations reproduce the Spitzer observations for WASP-19b rather well for $T_{\rm eff}=2600$ K, $\log(g)=3.2$ and solar abundances. Our model points at an exoplanet with a deep cloud-free atmosphere with a substantial day-night energy transport and no temperature inversion.

Modification of “Pressed” Atmospheres in Active Regions of Ultracool Stars

Ultracool stars usually have active regions, which is confirmed by their high-power radiofrequency emission modulated by the star axial rotation. The interpretation of this emission is commonly based on the electron cyclotron maser mechanism realized in the active regions. A plasma mechanism of radiofrequency emission is not considered, because ultracool star atmospheres are tightly “pressed” against the star surface, and the plasma frequency is much lower than the electron gyrofrequency (fL ≪ fB) at the coronal levels. This paper explores active regions of ultracool stars for the possible existence of a system of coronal magnetic loops carrying electric current generated by photospheric convection. It is shown that current dissipation induces a temperature increase inside the loops to about 107 K, which causes an increase in the scale of height of the inhomogeneous atmosphere and, at the coronal levels, effectuates condition fL ≫ fB, at which the plasma mechanism of radiofrequency emission prevails over the electron cyclotron maser mechanism. The magnetic loop parameters, intensity of electric currents generated by the photospheric convection, and efficiency of plasma heating inside the magnetic loops are evaluated on the example of the brown dwarf TVLM513-46546. The scale of the height of the modified atmosphere, which appears to be comparable to the star radius, is calculated; it is shown that the soft X-ray flow created by the hot modified atmosphere inside a coronal magnetic loop is about equal to that observed for brown dwarf TVLM513-46546.

MAPS OF EVOLVING CLOUD STRUCTURES IN LUHMAN 16AB FROM HST TIME-RESOLVED SPECTROSCOPY

ABSTRACT WISE J104915.57-531906.1 is the nearest brown dwarf binary to our solar system, consisting of two brown dwarfs in the L/T transition: Luhman 16A and B. In this paper, we present the first map of Luhman 16A, and maps of Luhman 16B for two epochs. Our maps were created by applying Aeolus, a Markov-Chain Monte Carlo code that maps the top-of-the-atmosphere (TOA) structure of brown dwarf and other ultracool atmospheres, to light curves of Luhman 16A and B using the Hubble Space Telescope’s G141 and G102 grisms. Aeolus retrieved three or four spots in the TOA of Luhman 16A and B, with a surface coverage of 19%–32% (depending on an assumed rotational period of 5 hr or 8 hr) or 21%–38.5% (depending on the observational epoch), respectively. The brightness temperature of the spots of the best-fit models was ∼200 K hotter than the background TOA. We compared our Luhman 16B map with the only previously published map. Interestingly, our map contained a large TOA spot that was cooler (ΔT ∼ 51 K) than the background, which lay at low latitudes, in agreement with the previous Luhman 16B map. Finally, we report the detection of a feature reappearing in Luhman 16B light curves that are separated by tens of hundreds of rotations from each other. We speculate that this feature is related to TOA structures of Luhman 16B.

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

Brown dwarfs and giant gas extrasolar planets have cold atmospheres with rich chemical compositions from which mineral cloud particles form. Their properties, like 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 , 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 in a certain volume which was modelled with a 3D Monte Carlo radiative transfer code to be able to visualise their spacial extent. 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.

DISCOVERY OF ROTATIONAL MODULATIONS IN THE PLANETARY-MASS COMPANION 2M1207b: INTERMEDIATE ROTATION PERIOD AND HETEROGENEOUS CLOUDS IN A LOW GRAVITY ATMOSPHERE

ABSTRACT Rotational modulations of brown dwarfs have recently provided powerful constraints on the properties of ultra-cool atmospheres, including longitudinal and vertical cloud structures and cloud evolution. Furthermore, periodic light curves directly probe the rotational periods of ultra-cool objects. We present here, for the first time, time-resolved high-precision photometric measurements of a planetary-mass companion, 2M1207b. We observed the binary system with Hubble Space Telescope/Wide Field Camera 3 in two bands and with two spacecraft roll angles. Using point-spread function-based photometry, we reach a nearly photon-noise limited accuracy for both the primary and the secondary. While the primary is consistent with a flat light curve, the secondary shows modulations that are clearly detected in the combined light curve as well as in different subsets of the data. The amplitudes are 1.36% in the F125W and 0.78% in the F160W filters, respectively. By fitting sine waves to the light curves, we find a consistent period of 10.7 &minus; 0.6 &plus; 1.2 ?> hr and similar phases in both bands. The J- and H-band amplitude ratio of 2M1207b is very similar to a field brown dwarf that has identical spectral type but different J–H color. Importantly, our study also measures, for the first time, the rotation period for a directly imaged extra-solar planetary-mass companion.

AEOLUS: A MARKOV CHAIN MONTE CARLO CODE FOR MAPPING ULTRACOOL ATMOSPHERES. AN APPLICATION ON JUPITER AND BROWN DWARFHSTLIGHT CURVES

Deducing the cloud cover and its temporal evolution from the observed planetary spectra and phase curves can give us major insight into the atmospheric dynamics. In this paper, we present Aeolus, a Markov chain Monte Carlo code that maps the structure of brown dwarf and other ultracool atmospheres. We validated Aeolus on a set of unique Jupiter Hubble Space Telescope (HST) light curves. Aeolus accurately retrieves the properties of the major features of the Jovian atmosphere, such as the Great Red Spot and a major 5 μm hot spot. Aeolus is the first mapping code validated on actual observations of a giant planet over a full rotational period. For this study, we applied Aeolus to J- and H-band HST light curves of 2MASS J21392676+0220226 and 2MASS J0136565+093347. Aeolus retrieves three spots at the top of the atmosphere (per observational wavelength) of these two brown dwarfs, with a surface coverage of 21% ± 3% and 20.3% ± 1.5%, respectively. The Jupiter HST light curves will be publicly available via ADS/VIZIR.

Reference study to characterize plasma and magnetic properties of ultracool atmospheres

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. We utilise the {\sc Drift-Phoenix} model grid where the local atmospheric structure is determined by the global parameters T$_{\rm eff}$, $\log(g)$ and [M/H]. Our results show that it is not unreasonable to expect H$_{\alpha}$ 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 T$_{\rm eff}$. 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.

GASEOUS MEAN OPACITIES FOR GIANT PLANET AND ULTRACOOL DWARF ATMOSPHERES OVER A RANGE OF METALLICITIES AND TEMPERATURES

We present new calculations of Rosseland and Planck gaseous mean opacities relevant to the atmospheres of giant planets and ultracool dwarfs. Such calculations are used in modeling the atmospheres, interiors, formation, and evolution of these objects. Our calculations are an expansion of those presented in Freedman et al. (2008) to include lower pressures, finer temperature resolution, and also the higher metallicities most relevant for giant planet atmospheres. Calculations span 1 microbar to 300 bar, and 75 K to 4000 K, in a nearly square grid. Opacities at metallicities from solar to 50 times solar abundances are calculated. We also provide an analytic fit to the Rosseland mean opacities over the grid in pressure, temperature, and metallicity. In addition to computing mean opacities at these local temperatures, we also calculate them with weighting functions up to 7000 K, to simulate the mean opacities for incident stellar intensities, rather than locally thermally emitted intensities. The chemical equilibrium calculations account for the settling of condensates in a gravitational field and are applicable to cloud-free giant planet and ultracool dwarf atmospheres, but not circumstellar disks. We provide our extensive opacity tables for public use.

Colour–magnitude diagrams of transiting Exoplanets – II. A larger sample from photometric distances

Colour-magnitude diagrams form a traditional way of presenting luminous objects in the Universe and compare them to each others. Here, we estimate the photometric distance of 44 transiting exoplanetary systems. Parallaxes for seven systems confirm our methodology. Combining those measurements with fluxes obtained while planets were occulted by their host stars, we compose colour-magnitude diagrams in the near and mid-infrared. When possible, planets are plotted alongside very low-mass stars and field brown dwarfs, who often share similar sizes and equilibrium temperatures. They offer a natural, empirical, comparison sample. We also include directly imaged exoplanets and the expected loci of pure blackbodies. Irradiated planets do not match blackbodies; their emission spectra are not featureless. For a given luminosity, hot Jupiters' daysides show a larger variety in colour than brown dwarfs do and display an increasing diversity in colour with decreasing intrinsic luminosity. The presence of an extra absorbent within the 4.5 $\mu$m band would reconcile outlying hot Jupiters with ultra-cool dwarfs' atmospheres. Measuring the emission of gas giants cooler than 1000 K would disentangle whether planets' atmospheres behave more similarly to brown dwarfs' atmospheres than to blackbodies, whether they are akin to the young directly imaged planets, or if irradiated gas giants form their own sequence.

BROWN DWARF PHOTOSPHERES ARE PATCHY: AHUBBLE SPACE TELESCOPENEAR-INFRARED SPECTROSCOPIC SURVEY FINDS FREQUENT LOW-LEVEL VARIABILITY

Condensate clouds strongly impact the spectra of brown dwarfs and exoplanets. Recent discoveries of variable L/T transition dwarfs argued for patchy clouds in at least some ultracool atmospheres. This study aims to measure the frequency and level of spectral variability in brown dwarfs and to search for correlations with spectral type. We used HST/WFC3 to obtain spectroscopic time series for 22 brown dwarfs of spectral types ranging from L5 to T6 at 1.1-1.7 $\mu$m for $\approx$40 min per object. Using Bayesian analysis, we find 6 brown dwarfs with confident $(p>95\%)$ variability in the relative flux in at least one wavelength region at sub-percent precision, and 5 brown dwarfs with tentative $(p>68\%)$ variability. We derive a minimum variability fraction $f_{min}=27^{+11}_{-7}\%$ over all covered spectral types. The fraction of variables is equal within errors for mid L, late L and mid T spectral types; for early T dwarfs we do not find any confident variable but the sample is too small to derive meaningful limits. For some objects, the variability occurs primarily in the flux peak in the J or H band, others are variable throughout the spectrum or only in specific absorption regions. Four sources may have broad-band peak-to-peak amplitudes exceeding 1%. Our measurements are not sensitive to very long periods, inclinations near pole-on and rotationally symmetric heterogeneity. The detection statistics are consistent with most brown dwarf photospheres being patchy. While multiple-percent near-infrared variability may be rare and confined to the L/T transition, low-level heterogeneities are a frequent characteristic of brown dwarf atmospheres.