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Controlling graphene’s doping will be critically important for its incorporation into future electronic and optoelectronic devices. Noncovalent functionalization through adsorption of organic molecules on graphene’s surface has proved to be a promising route for achieving p- or n-type doping. However, due to the poor adhesion of the molecules, these tend to desorb over time under standard environmental conditions or in the presence of certain solvents. The resulting reversal in the achieved chemical doping is a major obstacle to using organic molecules as noncovalent graphene dopants. In this work, we present a simple method for achieving long-term p- and n-doping of graphene devices through vapor phase evaporation of organic molecules, followed by encapsulation under an inert Al2O3film. This film, grown via an optimized atomic layer deposition process, ensures long-term doping stability, as confirmed by electrical transport and Raman spectroscopy measurements. The doping is maintained even after storing the devices for six weeks in ambient conditions and immersing them in a dopant removing solvent, demonstrating that the film is as an effective barrier against environmental degradation of the doped devices.

Aims. Recent studies have detected linear polarization in L dwarfs in the optical Iband. Theoretical models have been developed to explain this polarization. These models predict higher polarization at shorter wavelengths. We discuss the polarization in the Rand Iband of 4 ultra cool dwarfs.

Context. Free-floating substellar candidates with estimated theoretical masses of as low as ~5 Jupiter masses have been found in the ~3 Myr old σOrionisopen cluster. As the overlap with the planetary mass domain increases, the question of how these objects form becomes important. The determination of their number density and whether a mass cut-off limit exists is crucial to understanding their formation.

We present radial velocities of the M8V-type, very low-mass star vB 10obtained at four different epochs between 2001 and 2008. We use high-resolution (R~ 20 000) near-infrared (J-band) spectra taken with the nirspecinstrument on the Keck II telescope. Our data suggest that vB 10shows radial velocity variability with an amplitude of ~1 km s-1, a result that is consistent with the recent finding of a massive planet companion around the star. More velocity measurements and a better sampling of the orbital phase are required to precisely constrain the orbital parameters and the individual masses of the pair.

Context. Ultra-cool dwarfs of the L spectral type (Teff= 1400–2200 K) are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering.

Context. The knowledge of the binary properties of metal-poor and solar-metallicity stars can shed light on the potential differences between the formation processes responsible for both types of objects.

Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger.

Context. The young σOrionis cluster is an important location for studying the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although it is a fundamental parameter, has not been well determined yet.

Context. Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated.

Aims.The nature of S Ori 70 (S Ori J053810.1-023626), a faint mid-T type object found towards the direction of the young σOrionis cluster, is still under debate. We intend to find out whether it is a field brown dwarf or a 3-Myr old planetary-mass member of the cluster.

Context.Classical T Tauri stars are young solar-type stars accreting material from their circumstellar disks. Thanks to a favorable inclination of the system, the classical T Tauri star AA Tau exhibits periodic optical eclipses as the warped inner disk edge occults the stellar photosphere.

Aims.We searched for infrared flux excesses of planetary-mass candidates in the σOrionis cluster (~3 Myr, ~350 pc).

Aims.We investigate the mass function in the substellar domain down to a few Jupiter masses in the young σOrionis open cluster (3±2 Ma, d= $360^{\rm +70}_{\rm -60}$pc).

Context.We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys.

We have conducted deep photometric searches for substellar members of the Praesepe (0.5-1 Gyr) and σOrionis (3 Myr) star clusters using the Sloan i'and z'broad-band filters, with the 3.5-m and the 5-m Hale telescopes on the Calar Alto and Palomar Observatories. The total area surveyed was 1177 arcmin2and 1122 arcmin2towards the central regions of Praesepe and σOrionis, respectively. The 5-σdetection limit of our survey is measured at $i' = 24.5$and $z' = 24$mag, which according to state-of-the-art evolutionary models corresponds to masses of 50-55 MJup(Praesepe) and 6 MJup(σOrionis), i.e., well within the substellar regime. Besides recovering previously known cluster members reported in the literature, we have identified new photometric candidates in both clusters whose masses expand the full range covered by our study. In σOrionis, follow-up near-infrared photometry has allowed us to confirm the likely cluster membership of three newly discovered planetary-mass objects. The substellar mass function of σOrionis, which is complete from the star–brown dwarf borderline down to 7 MJup, rises smoothly with a slope of $\alpha = 0.6\,^{+0.5}_{-0.1}$(dN/d$M \sim M^{-\alpha}$). One of the faintest Praesepe candidates for which we have obtained follow-up near-infrared JHKsphotometry closely fits the expected optical and infrared photometric sequence of the cluster. From its colors, we have estimated its spectral type to be between L4 and L6. If confirmed as a true Praesepe member, it would become the first L-type brown dwarf (50-60 MJup) identified in an intermediate-age star cluster. Our derivation of the Praesepe mass function, which is based on state-of-the-art evolutionary models, depends strongly on the cluster age. For the youngest possible ages (500-700 Myr), our results suggest that there is a deficit of Praesepe brown dwarfs in the central regions of the cluster, while the similarity between the Praesepe and σOrionis mass functions increases qualitatively for models older than 800 Myr.

Aims.Properties of multiplicity, such as the multiplicity frequency, the distribution of mass ratios and periods, provide important informations on the formation processes at work. Studying these properties in different environments and at different ages allows us to constrain the models of formation and to understand the influence of the environment on the star formation.

We present a spectrophotometric analysis of S Ori J053825.4-024241, a candidate member close to the substellar boundary of the young (1–8 Myr), nearby (~360 pc) σOrionis star cluster. Our optical and near-infrared photometry and low-resolution spectroscopy indicate that S Ori J053825.4-024241 is a likely cluster member with a mass estimated from evolutionary models at 0.06$^{+0.07}_{-0.02}$$M_\odot$, which makes the object a probable brown dwarf. The radial velocity of S Ori J053825.4-024241 is similar to the cluster systemic velocity. This target, which we have classified as an M 6.$0\pm1$.0 low-gravity object, shows excess emission in the near-infrared and anomalously strong photometric variability for its type (from the blue to the Jband), suggesting the presence of a surrounding disc. The optical spectroscopic observations show a continuum excess at short wavelengths and a persistent and resolved Hαemission (pseudo-equivalent width of ~-250 Å) in addition to the presence of other forbidden and permitted emission lines, which we interpret as indicating accretion from the disc and possibly mass loss. We conclude that despite the low mass of S Ori J053825.4-024241, this object exhibits some of the properties typical of active classical T Tauri stars.

We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the south-east region of the young (~3 Myr), nearby (~350 pc) σOrionis open cluster. A field of ~1000 arcmin2was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ~0.4 mag. A correlation between the near-infrared excess in the Ksband, strong Hαemission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60 MJup, and the rotational velocity of 14 ±4 km s-1for one of them.

We explored the regions within a radius of 25´´ around 473 nearby, low-metallicity G- to M-type stars using $(VR)I$optical filters and small-aperture telescopes. About 10% of the sample was searched up to angular separations of 90´´. We applied photometric and astrometric techniques to detect true physical companions to the targets. The great majority of the sample stars was drawn from the Carney–Latham surveys; their metallicities range from roughly solar to [Fe/H] = -3.5 dex. Our I-band photometric survey detected objects that are between 0 and 5 mag fainter (completeness) than the target stars; the maximum dynamical range of our exploration is 9 mag. We also investigated the literature, and inspected images from the Digitized Sky Surveys to complete our search. By combining photometric and proper motion measurements, we retrieved 29 previously known companions, and identified 13 new proper motion companions. Near-infrared 2MASS photometry is provided for the great majority of them. Low-resolution optical spectroscopy (386–1000 nm) was obtained for eight of the new companion stars. These spectroscopic data confirm them as cool, late-type, metal-depleted dwarfs, with spectral classes from esdK7 to sdM3. After comparison with low-metallicity evolutionary models, we estimate the masses of the proper motion companion stars to be in the range 0.5–0.1 $M_{\odot}$. They are moving around their primary stars at projected separations between ~32 and ~57 000 AU. These orbital sizes are very similar to those of solar-metallicity stars of the same spectral types. Our results indicate that about 15% of the metal-poor stars have stellar companions in wide orbits, which is in agreement with the binary fraction observed among main sequence G- to M-type stars and T Tauri stars.

We report the results of a synoptic study of the photometric and spectroscopic variability of the classical T Tauri star AA Tau on timescales ranging from a few hours to several weeks. The AA Tau light curve had been previously shown to vary with a 8.2 d period, exhibiting a roughly constant brightness level, interrupted by quasi-cyclic fading episodes, which we interpreted as recurrent eclipses of the central star by the warped inner edge of its accretion disk (Bouvier et al. [CITE]). Our observations show the system is dynamic and presents non-stationary variability both in the photometry and spectroscopy. The star exhibits strong emission lines that show substantial variety and variability in their profile shapes and fluxes. Emission lines such as Hαand Hβshow both infall and outflow signatures and are well reproduced by magnetospheric accretion models with moderate mass accretion rates ($10^{-8}{-}10^{-9}~ {M}_\odot~{\rm yr}^{-1}$) and high inclinations ($i \ge 60\degr$). The veiling shows variations that indicate the presence of 2 rotationally modulated hot spots corresponding to the two  magnetosphere poles. It correlates well with the $\ion{He}{i}$line flux, with $B-V$and the Vexcess flux. We have indications of a time delay between the main emission lines (Hα, Hβand $\ion{He}{i}$) and veiling, the lines formed farther away preceding the veiling changes. The time delay we measure is consistent with accreted material propagating downwards the accretion columns at free fall velocity from a distance of about 8 $R_\star$. In addition, we report periodic radial velocity variations of the photospheric spectrum which might point to the existence of a 0.02 $M_\odot$object orbiting the star at a distance of 0.08 AU. During a few days, the eclipses disappeared, the variability of the system was strongly reduced and the line fluxes and veiling severely depressed. We argue that this episode of quiescence corresponds to the temporary disruption of the magnetic configuration at the disk inner edge. The smooth radial velocity variations of inflow and outflow diagnostics in the Hαprofile yield further evidence for large scale variations of the magnetic configuration on a timescale of a month. These results may provide the first clear evidence for large scale instabilities developping in T Tauri magnetospheres as the magnetic field lines are twisted by differential rotation between the star and the inner disk.The interaction between the inner accretion disk and the stellar magnetosphere thus appears to be a highly dynamical and time dependent process.

VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the σOrionis cluster with masses spanning from 1 $M_{\odot}$down to about 0.013 $M_{\odot}$(i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hαemission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li iin absorption at 6708 Å, have allowed us to tentatively classify σOrionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hαline has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hαemission, we estimate that the minimum disk frequency of the σOrionis low-mass population is in the range 5–12%.

We report on the finding of the strongest Hαemission – pseudoequivalent width of 705 Å – known so far in a young, late type dwarf. This object, named as S Ori 71, is a substellar candidate member of the 1–8 Myr star cluster σOrionis. Due to its overluminous location in color-magnitude diagrams, S Ori 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021–0.012 $M_\odot$, depending on evolutionary models and possible binarity. The broad Hαline of S Ori 71 appears asymmetric, indicative of high velocity mass motions in the Hαforming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.

We present intermediate- and low-resolution optical spectra around Hαand Li iλ6708 Å for a sample of 25 low mass stars and 2 brown dwarfs with confirmed membership in the pre-main sequence stellar σOrionis cluster. Our observations are intended to investigate the age of the cluster. The spectral types derived for our target sample are found to be in the range K6–M8.5, which corresponds to a mass interval of roughly 1.2–0.02 $M_{\odot}$on the basis of state-of-the-art evolutionary models. Radial velocities (except for one object) are found to be consistent with membership in the Orion complex. All cluster members show considerable Hαemission and the Li iresonance doublet in absorption, which is typical of very young ages. We find that our pseudo-equivalent widths of Hαand Li i(measured relative to the observed local pseudo-continuum formed by molecular absorptions) appear rather dispersed (and intense in the case of Hα) for objects cooler than M3.5 spectral class, occurring at the approximate mass where low mass stars are expected to become fully convective. The least massive brown dwarf in our sample, S Ori 45 (M8.5, ~0.02 $M_{\odot}$), displays variable Hαemission and a radial velocity that differs from the cluster mean velocity. Tentative detection of forbidden lines in emission indicates that this brown dwarf may be accreting mass from a surrounding disk. We also present recent computations of Li iλ6708 Å curves of growth for low gravities and for the temperature interval (about 4000–2600 K) of our sample. The comparison of our observations to these computations allows us to infer that no lithium depletion has yet taken place in σOrionis, and that the observed pseudo-equivalent widths are consistent with a cluster initial lithium abundance close to the cosmic value. Hence, the upper limit to the σOrionis cluster age can be set at 8 Myr, with a most likely value around 2–4 Myr.

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the σOrionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M -mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show Hαin emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the σOrionis cluster, in the form dN/dM, keeps rising in the planetary domain.

Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ~2.1 days after the event and continued until 4 Apr. 2000. The flux decay constant of the optical afterglow in the R-band is -2.30 ±0.07 up to ~5 days, which is very likely due to the jet e ffect, and it is followed by a much steeper decay with constant -3.2 ±0.2, the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously with the event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ~2 days after the occurrence of the GRB (as expected from the jet effect) or a maximum, as observed in GRB 970508. The existence of a se cond break might indicate a steepening in the electron spectrum or the superposition of two events, resembling GRB 000301C. Once the afterglow emission vanished, contribution of a bright underlying supernova was found on the basis of the late-time R-band measurements, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our redshift determination of $z = 0.706$indicates that GRB 991208 is at 3.7 Gpc (for $H_{0}= 60$km s-1Mpc-1, $\Omega_{0}= 1$and $\Lambda_{0}= 0$), implying an isotropic energy release of 1.15 1053erg which may be relaxed by beaming by a factor >102. Precise astrometry indicates that the GRB coincides within 0.2″ with the host galaxy, thus supporting a massive star origin. The absolute magnitude of the galaxy is MB= -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of ($11.5 \pm 7.1$) $M_{\odot}$yr-1, which is much larger than the present-day rate in our Galaxy. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow was determined ~3.5 day after the burst (Dec. 12.0) implying a cooling frequency $\nu_{\rm c}$below the optical band, i.e. supporting a jet model with $p = -$2.30 as the index of the power-law electron distribution.

BVRICCD-based and near-IR (J) imaging, together with unfiltered photometry of the hot subdwarf B star PG 0856+121, are reported. Two close, faint, red, point-like sources are resolved. They account for the previously reported IR excess observed in this hot subdwarf. In addition, the new unfiltered differential photometry of PG 0856+121 confirms its previously reported pulsational nature. A comparison with the oscillation modes of stellar models suggests the presence of g modes.

We present the results of a deep CCD-based IZphotometric survey of a ~ 1 deg2area in the central region of the Pleiades Galactic open cluster. The magnitude coverage of our survey (from $I\,\sim\,17.5$down to 22) allows us to detect substellar candidates with masses between 0.075 and 0.03 $M_{\odot}$. Details of the photometric reduction and selection criteria are given. Finder charts prepared from the I-band images are provided.

Inductive fault current limiters operating with stacks of various small superconducting elements acting as secondaries were studied. The stacks consist of Y1Ba2Cu3O7??thin film washers or Bi1.8Pb0.26Sr2Ca2Cu3O10+xbulk rings. A central result of our work is an experimental demonstration that the limiting capability of the device is strongly reduced when several bulk rings are stacked, whereas it remains almost unchanged for thin film washers. The use of thin films should therefore allow us to build more efficient high power inductive limiters based on stacks of small washers.

GJ 569Bab is the first brown dwarf binary for which the mass of each component has been derived by solving the astrometric and spectroscopic orbit of the pair, i.e., independently of any theoretical assumption. This allows us to test the predictions of the various evolutionary models available in the literature. Particularly interesting are the predictions of lithium depletion for the mass (0.08–0.05M⊙) and likely age (300–800 Myr) of the substellar components. High-resolution optical spectra of GJ 569B (the pair is not resolved) obtained with HIRES at the Keck telescope show that there has been significant lithium depletion in both components. We will compare these results to state-of-the-art theoretical calculations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study here the limitation performance and the thermal recovery of inductive superconducting fault current limiters (SFCL) with special emphasis on the case in which the superconducting element (i.e. the secondary) has weak zones. Our experimental results indicate that the conduction of heat between a hot weak zone and its cold surroundings plays an important role in its refrigeration. This suggests that it could be interesting to design superconducting elements based on an appropriate distribution of artificial weak zones. In this way, the heat would be removed more efficiently than in the case of homogeneous samples, where the heating would be uniform. By using a numerical simulation it is found that, with a proper distribution of artificially created weak zones, it could be feasible to simultaneously get a good current limitation (as big as with homogeneous secondaries) and a shorter recovery time. These results suggest an unexpected way to improve the performance of any SFCL.

We present an RI photometric survey covering an area of 430 arcmin2 around the multiple star σ Orionis. The observations were conducted with the 0.8 m IAC-80 Telescope at the Teide Observatory. The survey limiting R and I magnitudes are 22.5 and 21, and completeness magnitudes 21 and 20, respectively. We have selected 53 candidates from the I vs. R–I colour-magnitude diagram (I = 14–20) that follow the previously known photometric sequence of the cluster. Adopting an age of 2–4 Myr for the cluster, we find that these objects span a mass range from 0.35 M⊙ to 0.015 M⊙. We have performed J-band photometry of 52 candidates and Ks photometry for 12 of them, with the result that 50 follow the expected infrared sequence for the cluster, thus confirming with great confidence that the majority of the candidates are bona fide members. JHKs photometry from the Two Micron All Sky Survey (2MASS) is available for 50 of the candidates and are in good agreement with our data. Out of 48 candidates, which have photometric accuracies better than 0.1 mag in all bands, only three appear to show near-infrared excesses. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Observations taken at Calar Alto and Loiano led to the possible identification of the GRB 971227 optical counterpart. Details on the observations are presented.

Aims.We report the detection of transit timing variations (TTVs) well in excess of one hour in the Kepler multi-planet candidate system KOI 806. This system exhibits transits consistent with three separate planets – a Super-Earth, a Jupiter, and a Saturn – lying very nearly in a 1:2:5 resonance, respectively.

Context.The proper characterization of the least massive population of the young σOrionis star cluster is required to understand the form of the cluster mass function and its impact on our comprehension of the substellar formation processes. S Ori 70(T5.5  ±  1) and 73, two T-type cluster member candidates, are likely to have masses between 3 and 7 MJupif their age is 3 Myr. It awaits confirmation whether S Ori 73has a methane atmosphere.

Context. The fundamental properties of brown dwarfs evolve with age. Models describing the evolution of luminosities and effective temperatures, among other physical parameters, can be empirically constrained using brown dwarfs of various masses in star clusters of well-determined age and metallicity.Aims. We aim to carry out a spectroscopic and photometric characterization of low-mass brown dwarfs in the ~120 Myr old Pleiades open cluster.Methods. We obtained low-resolution, near-infrared spectra of the J= 17.4–18.8 mag candidate L-type brown dwarfs PLIZ 28 and 35, BRB 17, 21, 23, and 29, which are Pleiades members by photometry and proper motion. We also obtained spectra of the well-known J= 15.4–16.1 mag late M-type cluster members PPl 1, Teide 1, and Calar 3.Results. We find that the first six objects have early- to mid-L spectral types and confirm previously reported M-types for the three other objects. The spectra of the L0-type BRB 17 and PLIZ 28 present a triangular H-band continuum shape, indicating that this peculiar spectral feature persists until at least the age of the Pleiades. We add to our sample 36 reported M5–L0-type cluster members and collect their IC- and UKIDSS ZYJHK-band photometry. We confirm a possible interleaving of the Pleiades and field L-type sequences in the JHKabsolute magnitude versus spectral type diagrams, and quantify marginally redder Pleiades J–Kcolours, by 0.11 ±0.20 mag, possibly related to both reddening and youth. Using field dwarf bolometric correction – and effective temperature – spectral type relations, we obtain the Hertzsprung-Russell diagram of the Pleiades sample. Theoretical models reproduce the spectral sequence at M5.5–9, but appear to overestimate the luminosity or underestimate the effective temperature at L0–5.Conclusions. We classify six faint Pleiades brown dwarfs as early to mid L-type objects using low-resolution near-infrared spectra. We compare their properties to field dwarfs and theoretical models and estimate their masses to be in the range 0.025–0.035 $M_{\odot}$.

Context. The fundamental properties of brown dwarfs evolve with age. Models describing the evolution of luminosities and effective temperatures, among other physical parameters, can be empirically constrained using brown dwarfs of various masses in star clusters of well-determined age and metallicity.