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A map of the surface of a brown dwarf reveals features that suggest patchy clouds, providing the mechanism for the dispersal of atmospheric dust as brown dwarfs cool with age. Casting a weather eye on a nearby brown dwarf The recently discovered system known as Luhman 16AB is a binary consisting of two brown dwarfs — objects much bigger than planets but not big enough to become stars — and is a mere six light years from us. Only Alpha Centauri and Barnard's star are closer. Ian Crossfield et al . have now mapped the surface of brown dwarf Luhman 16B in the infrared and find large-scale surface patterns indicative of patchy clouds. Monitoring suggests that the characteristic timescale for the evolution of global weather patterns is about one day. Further observations of the evolution of weather patterns on brown dwarfs could provide a new benchmark for understanding how global circulation conditions affect dusty atmospheres on brown dwarfs and giant extrasolar planets. Brown dwarfs—substellar bodies more massive than planets but not massive enough to initiate the sustained hydrogen fusion that powers self-luminous stars 1 , 2 —are born hot and slowly cool as they age. As they cool below about 2,300 kelvin, liquid or crystalline particles composed of calcium aluminates, silicates and iron condense into atmospheric ‘dust’ 3 , 4 , which disappears at still cooler temperatures (around 1,300 kelvin) 5 , 6 . Models to explain this dust dispersal include both an abrupt sinking of the entire cloud deck into the deep, unobservable atmosphere 5 , 7 and breakup of the cloud into scattered patches 6 , 8 (as seen on Jupiter and Saturn 9 ). However, hitherto observations of brown dwarfs have been limited to globally integrated measurements 10 , which can reveal surface inhomogeneities but cannot unambiguously resolve surface features 11 . Here we report a two-dimensional map of a brown dwarf’s surface that allows identification of large-scale bright and dark features, indicative of patchy clouds. Monitoring suggests that the characteristic timescale for the evolution of global weather patterns is approximately one day.

The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars 1 . Observations of broad emission lines could potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds 2 . The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk  continuum and the emission lines 3 —a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow 4 , whereas in others they arise from orbiting gas clouds 5 . Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10 −4 arcseconds 3 , 6 ). Here we report a spatial offset (with a spatial resolution of 10 −5 arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 10 8 solar masses. We infer a disk radius of 150 light days; a radius of 100–400 light days was found previously using reverberation mapping 7 – 9 . The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time. High-angular-resolution observations of the quasar 3C 273 reveal that it has a relatively small but thick disk, viewed nearly face-on, in which material is orbiting the central supermassive black hole.

Many galaxies are thought to have supermassive black holes at their centres 1 —more than a million times the mass of the Sun. Measurements of stellar velocities 2 , 3 , 4 , 5 , 6 , 7 and the discovery of variable X-ray emission 8 have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 ± 1.5) × 10 6 solar masses ( M ⊙ ). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

We use the TGAS proper motions and parallaxes as well as published and new radial velocities to study the dynamics of nearby moving groups. In particular we try to determine their age using backtracing of the individual members to a common origin. We find that the current data, probably the radial velocities, do not allow to reach a successful conclusion.

Hubble Space Telescope imaging observations of two nearby brown dwarfs, DENIS-P J1228.2-1547 and Kelu 1, made with the near-infrared camera and multiobject spectrometer (NICMOS), show that the DENIS object is resolved into two components of nearly equal brightness with a projected separation of 0.275 arc second (5 astronomical units for a distance of 18 parsecs). This binary system will be able to provide the first dynamical measurement of the masses of two brown dwarfs in only a few years. Upper limits to the mass of any unseen companion in Kelu 1 yield a planet of 7 Jupiter masses aged 0.5 × 10$^9$ years, which would have been detected at a separation larger than about 4 astronomical units. This example demonstrates that giant planets could be detected by direct imaging if they exist in Jupiter-like orbits around nearby young brown dwarfs.

AstraLux is a Lucky Imaging camera for the Calar Alto 2.2-m telescope, based on an electron-multiplying high speed CCD. By selecting only the best 1-10% of several thousand short exposure frames, AstraLux provides nearly diffraction limited imaging capabilities in the SDSS i' and z' filters over a field of view of 24×24 arcseconds. By choosing commercially available components wherever possible, the instrument could be built in short time and at comparably low cost. We briefly present the instrument design, the data reduction pipeline, and summarise the performance and characteristics.

Star formation: evidence of mass The rapidly spinning young star AB Doradus (AB Dor) is thought to have a low-mass companion star, detected as an astrometric ‘wobble’. It has proved elusive — even to the Hubble Space Telescope — but now a new instrument built to image extrasolar planets shows what it can do by observing the faint companion. The high-contrast NACO SDI adaptive optics camera at the European Southern Observatory reveals the object, dubbed AB Dor C, to be of very low mass for a star (90 times that of Jupiter). It is 400 °C cooler and 2.5 times fainter than predicted by stellar models. This suggests that most known brown dwarfs and extrasolar planets are heavier than was thought, and the new findings will be important for the design of future cameras intended to find extrasolar planets. See the cover story for more on the search for new planets. Mass is the most fundamental parameter of a star, yet it is also one of the most difficult to measure directly. In general, astronomers estimate stellar masses by determining the luminosity and using the ‘mass–luminosity’ relationship 1 , 2 , but this relationship has never been accurately calibrated for young, low-mass stars and brown dwarfs 3 . Masses for these low-mass objects are therefore constrained only by theoretical models 1 , 2 . A new high-contrast adaptive optics camera 4 , 5 , 6 enabled the discovery of a young (50 million years) companion only 0.156 arcseconds (2.3  au ) from the more luminous (> 120 times brighter) star AB Doradus A. Here we report a dynamical determination of the mass of the newly resolved low-mass companion AB Dor C, whose mass is 0.090 ± 0.005 solar masses. Given its measured 1–2-micrometre luminosity, we have found that the standard mass–luminosity relations 1 , 2 overestimate the near-infrared luminosity of such objects by about a factor of ∼2.5 at young ages. The young, cool objects hitherto thought to be substellar in mass are therefore about twice as massive, which means that the frequency of brown dwarfs and planetary mass objects in young stellar clusters has been overestimated.

Astrometric observations of binary brown dwarfs yield dynamical masses of the components independently of theoretical models. We give an update on our long-term high-resolution spectroscopic and photometric monitoring programme of spatially resolved binary brown dwarfs using ground-based adaptive optics and the Hubble Space Telescope. We present current orbital fits, including refined dynamical mass estimate of the Kelu-1 AB system. The results seem to support the previously reported trend that evolutionary and atmospheric models might underestimate the mass of very-low-mass stars and brown dwarfs.

A Mid-IR instrumentation study for OWL has been performed by the Max-Planck-Institut für Astronomie in Heidelberg (Germany), and a Dutch consortium led by the Leiden Observatory (The Netherlands). MIR imaging and spectroscopic observational capabilities are compared to contemporary IR to sub-millimeter facilities, especially concentrating on the MIR-capabilities of JWST(MIRI). Our best effort calculation of the sensitivity for both MIR imager and spectrograph indicate a huge discovery potential in numerous areas from our planetary system to the high redshift Universe. Here we focus on the field of exo-planets and nearby star formation. Starting with the science cases, top level requirements are deduced and summarized including MIR instrumental constrains for the telescope itself.

For a comprehensive understanding of planetary formation and evolution, we need to investigate the environment in which planets form: circumstellar disks. Here we present high-contrast imaging observations of V4046 Sagittarii, a 20-Myr-old close binary known to host a circumbinary disk. We have discovered the presence of rotating shadows in the disk, caused by mutual occultations of the central binary. Shadow-like features are often observed in disks1,2, but those found thus far have not been due to eclipsing phenomena. We have used the phase difference due to light travel time to measure the flaring of the disk and the geometrical distance of the system. We calculate a distance that is in very good agreement with the value obtained from the Gaia mission’s Data Release 2 (DR2), and flaring angles of α = (6.2 ± 0.6)° and α = (8.5 ± 1.0)° for the inner and outer disk rings, respectively. Our technique opens up a path to explore other binary systems, providing an independent estimate of distance and the flaring angle, a crucial parameter for disk modelling.Moving shadows have been seen on the circumbinary disk around V4046 Sgr, cast by eclipses of the central binary system. Using geometrical arguments, the degree of flaring of the disk and the distance to the system have been calculated.