S. J. Warren, D. J. Mortlock, S. K. Leggett, D. J. Pinfield, D. Homeier, S. Dye, R. F. Jameson, N. Lodieu, P. W. Lucas, A. J. Adamson, F. Allard, D. Barrado y N… 展开作者
S. J. Warren, D. J. Mortlock, S. K. Leggett, D. J. Pinfield, D. Homeier, S. Dye, R. F. Jameson, N. Lodieu, P. W. Lucas, A. J. Adamson, F. Allard, D. Barrado y Navascues, M. Casali, K. Chiu, N. C. Hambly, P. C. Hewett, P. Hirst, M. J. Irwin, A. Lawrence, M. C. Liu, E. L. Martin, R. L. Smart, L. Valdivielso, B. P. Venemans
(abridged) We report the discovery of a very cool brown dwarf, ULAS J003402.77-005206.7 (ULAS J0034-00), identified in UKIDSS DR1. We provide optical, near-infrared, and mid-infrared photometry of the source, and two near-infrared spectra. Comparing the spectral energy distribution of ULAS J0034-00 to that of the T8 brown dwarf 2MASS J0415-09, the latest-type and coolest well-studied brown dwarf to date, with Teff~750 K, we find evidence that ULAS J0034-00 is significantly cooler. First, the measured values of the near-infrared absorption spectral indices imply a later classification, of T8.5. Second, the H-[4.49] colour provides an empirical estimate of the temperature of 540<Teff<660 K (+/-2sig range). Third, the J- and H-band peaks are somewhat narrower in ULAS J0034-00, and detailed comparison against spectral models calibrated to 2MASS J0415-09 yields an estimated temperature lower by 60-120 K relative to 2MASS J0415-09 i.e. 630<Teff<690 K (+/-2sig), and lower gravity or higher metallicity according to the degenerate combination -0.5<delta(log g-2[m/H])<-0.25 (+/-2sig). Combining these estimates, and considering systematics, it is likely the temperature lies in the range 600<Teff<700 K. Despite the low inferred Teff we find no evidence for strong absorption by NH3 over the wavelength range 1.51-1.56 um. Evolutionary models imply that the mass and age are in the ranges 15-36 M(Jup) and 0.5-8 Gyr, respectively. The measured proper motion, of (0.37+/-0.07)arcsec/yr, combined with the photometrically estimated distance of 14-22 pc, implies a tangential velocity of ~30 km/s. ULAS J0034-00 is significantly bluer than 2MASS J0415-09 in Y-J, so future searches should allow for the possibility that cooler T dwarfs are bluer still.
The UKIRT Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of infrared surveys. Here we combine the data from two UKIDSS components, the Large Area Survey (LAS) and the Galactic Cluster Survey (GCS), with 2MASS data to produce an infrared proper motion survey for low mass stars and brown dwarfs. In total we detect 267 low mass stars and brown dwarfs with significant proper motions. We recover all ten known single L dwarfs and the one known T dwarf above the 2MASS detection limit in our LAS survey area and identify eight additional new candidate L dwarfs. We also find one new …
We present the results of a deep wide-field near-infrared survey of 12 square degrees of the Pleiades conducted as part of the UKIDSS Deep Infrared Sky Survey (UKIDSS) Galactic Cluster Survey (GCS). We have extracted over 340 high probability proper motion members down to 0.03 solar masses using a combination of UKIDSS photometry and proper motion measurements obtained by cross-correlating the GCS with data from the Two Micron All Sky Survey (2MASS), the Isaac Newton (INT) and the Canada-France-Hawai'i (CFHT) telescopes. Additionally, we have unearthed 73 new candidate brown dwarf members on t…
We present the optical and near-infrared photometry and spectroscopy of four faint T dwarfs newly discovered from the UKIDSS first data release. The sample, drawn from an imaged area of ~136 square degrees to a depth of Y=19.9 (5-sigma, Vega), is located in the SDSS Southern Equatorial Stripe, a region of significant future deep imaging potential. We detail the selection and followup of these objects, three of which are spectroscopically confirmed brown dwarfs ranging from type T2.5 to T7.5, and one is photometrically identified as early T. Their magnitudes range from Y=19.01 to 19.88 with der…
We present first results from a deep (J = 18.7), wide-field (6.5 square degrees) infrared (ZYJHK) survey in the Upper Sco association conducted within the science verification phase of the UKIRT Infrared Deep Sky Survey Galactic Cluster Survey (GCS). Cluster members define a sequence well separated from field stars in the (Z-J,Z) colour-magnitude diagram. We have selected a total of 164 candidates with J = 10.5-18.7 mag from the (Z-J,Z) and (Y-J,Y) diagrams. We further investigated the location of those candidates in the other colour-magnitude and colour-colour diagrams to weed out contaminant…
We present the discovery of two new late-T dwarfs identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 2 (DR2). These T dwarfs are nearby old T dwarfs along the line of sight to star-forming regions and open clusters targeted by the UKIDSS GCS. They are found towards the Alpha Per cluster and Orion complex, respectively, from a search in 54 square degrees surveyed in five filters. Photometric candidates were picked up in two-colour diagrams, in a very similar manner to candidates extracted from the UKIDSS Large Area Survey (LAS) but taking advan…
We describe the goals, design, implementation, and initial progress of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven year sky survey which began in May 2005, using the UKIRT Wide Field Camera. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H_2. The Large Area Survey, the Galactic Clusters Survey, and the Galactic Plane Survey cover approximately 7000 square degrees to a depth of K~18; the Deep Extragalactic Survey covers 35 square degrees to K~21, and the Ultra Deep Survey covers 0.77 square degrees to K~23. Summed together UKIDSS i…
We review the current picture of disks around cool stars and brown dwarfs, including disk fractions, mass estimates, disk structure and dispersal, accretion, dust composition, and the debris disk phase. We discuss these in the framework of recent planet formation models.
We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIDSS and 2MASS data. This survey covers ~275 square degrees to depth of K~15 mag, equivalent to a mass of 0.05 solar masses assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between 0.05 < M < 0.075 solar masses. A high proportion of these L dwarfs appear to be photometric binaries.
Observations of brown dwarfs provide important feedback on theories of atmospheres and inner structure of substellar objects. Brown dwarf binary systems furthermore offer the unique opportunity to determine the mass of individual brown dwarfs, which is one of the fundamental astrophysical quantities.
Brown dwarfs, which straddle the mass range between stars and planets, appear to be common both in the solar neighborhood and in star-forming regions. Their ubiquity makes the question of their origin an important one both for our understanding of brown dwarfs themselves as well as for theories on the formation of stars and planets. Studies of young sub-stellar objects could provide valuable insight into their formation and early evolution. Here I report on the latest results from our observational programs at Keck, VLT and Magellan on the disk and accretion properties of young brown dwarfs. W…
Using the Subaru Telescope, we have obtained multiple near-infrared spectra of the cool brown dwarf, SDSS 1624+00, in search of spectral variability in an 80 minute time span. We have found the suspected variability of water vapor absorption throughout the observations, which requires confirmation by a longer time baseline. After coadding the spectra, we have obtained a high-quality spectrum covering 1.05 to 1.8 um. Three kinds of spectral indicators, the water vapor bands, methane band, and KI lines in J band, suggest that SDSS 1624+00 is warmer and dustier than Gl 229B.
The Next Generation Sky Survey (NGSS) is a proposed NASA MIDEX mission to map the entire sky in four infrared bandpasses - 3.5, 4.7, 12, and 23 um. The seven-month mission will use a 50-cm telescope and four-channel imager to survey the sky from a circular orbit above the Earth. Expected sensitivities will be half a million times that of COBE/DIRBE at 3.5 and 4.7 um and a thousand times that of IRAS at 12 and 23 um. NGSS will be particularly sensitive to brown dwarfs cooler than those presently known. Deep absorption in the methane fundamental band at 3.3 um and a predicted 5-um overluminosity…
We present near-infrared (1.15-2.50 microns) medium-resolution (R = 1700) spectroscopy of a sample of 23 brown dwarf candidates in the young Upper Sco association. We confirm membership of 21 brown dwarfs based on their spectral shape, comparison with field dwarfs, and presence of weak gravity-sensitive features. Their spectral types range from M8 to L2 with an uncertainty of a subclass, suggesting effective temperatures between 2700 and 1800 K with an uncertainty up to 300 K and masses in the 30-8 Mjup range. Among the non-members, we have uncovered a field L2 dwarf at a distance of 120-140 p…
We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradia…
We have imaged $ε$ Indi B, the closest brown dwarf binary known, with VISIR at the VLT in three narrow-band mid-infrared bandpasses located around 8.6$μ$m, 10.5$μ$m and 11.3$μ$m. We are able to spatially resolve both components, and determine accurate mid-infrared photometry for both components independently. In particular, our VISIR observations probe the NH$_3$ feature in the atmospheres of the cooler and warmer brown dwarfs. For the first time, we can disentangle the contributions of the two components, and find that % our photometry of $ε$ IndiBb is in good agreement with recent ``cloud-fr…
We explore the spectral and atmospheric properties of brown dwarfs cooler than the latest known T dwarfs. Our focus is on the yet-to-be-discovered free-floating brown dwarfs in the \teff range from $\sim$800 K to $\sim$130 K and with masses from 25 to 1 \mj. This study is in anticipation of the new characterization capabilities enabled by the launch of SIRTF and the eventual launch of JWST. We provide spectra from $\sim$0.4 \mic to 30 \mic, highlight the evolution and mass dependence of the dominant H$_2$O, CH$_4$, and NH$_3$ molecular bands, consider the formation and effects of water-ice clo…
After a discussion of the physical processes in brown dwarfs, we present a complete, precise definition of brown dwarfs and of planets inspired by the internal physics of objects between 0.1 and 0.001 M_sun. We discuss observational techniques for characterizing low-luminosity objects as brown dwarfs, including the use of the lithium test and cooling curves. A brief history of the search for brown dwarfs leads to a detailed review of known isolated brown dwarfs with emphasis on those in the Pleiades star cluster. We also discuss brown dwarf companions to nearby stars, paying particular attenti…
We report the discovery of a widely separated (258$\farcs3\pm0\farcs$4) T dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was initially identified from the Two Micron All Sky Survey (2MASS). Its near-infrared spectrum shows the 1.6 and 2.2 $\micron$ CH$_4$ absorption bands characteristic of T dwarfs, while its common proper motion with the Gl 570ABC system confirms companionship. Gl 570D (M$_J$ = 16.47$\pm$0.07) is nearly a full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and estimates of L = (2.8$\pm$0.3)x10$^{-6}$ L$_{\sun}$ and T$_{eff}$ = 75…
The formation and evolution of brown dwarfs are currently "hot topics" in cool star research. Latest observations and modeling efforts on disks, accretion, outflows, spatial distribution, and binarity in the context of the formation by ejection model and alternative scenarios were presented and discussed vigorously in this splinter session. A second major topic was rotation and activity of these objects. This part included observations of rotation periods and amplitudes, variability, and X-ray activity, and their consequences for the generation of magnetic fields and the cool atmospheres.
In this paper, we review the current theory of very low mass stars model atmospheres including the coolest known M~dwarfs, M~subdwarfs, and brown dwarfs, i.e. T$_{eff} \leq 5,000 $K and $-2.0 \le [M/H] \le +0.0$. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in deriving the effective temperature scale of M~dwarfs. We also present the latest results of the models related to the search for brown dwarfs.
Three brown dwarfs in different evolutionary stages have been observed with the Chandra X-ray Observatory. Combining the new observations with previous studies presented in the literature yields a brown dwarf sample that reaches well down into the cooling phase of brown dwarfs, allowing to investigate the possible influence of effective temperature on X-ray activity. Combining our results with published data allows us to consider a subsample of high-mass brown dwarfs (with 0.05-0.07 Msun), and another one characterized by similar effective temperature (with 2400-2800 K). Our findings support t…
We present the first comprehensive, comparative Spitzer/IRS study of the dust and gas properties of disks around young sun-like stars (K1-M5) and cool stars/brown dwarfs (M5-M9). The comparison of these two large samples of over 60 sources reveal major differences in the evolution of both the dust and gas components.We report the first detections of organic molecules in disks around brown dwarfs. The detection rate statistics and the line flux ratios of HCN and C2H2 show a striking diference between the two samples, demonstrating a significant under-abundance of HCN relative to C2H2 in the dis…
We discuss color selection of rare objects in a wide-field, multiband survey spanning from the optical to the mid-infrared. Simple color criteria simultaneously identify and distinguish two of the most sought after astrophysical sources: the coolest brown dwarfs and the most distant quasars. We present spectroscopically-confirmed examples of each class identified in the IRAC Shallow Survey of the Bootes field of the NOAO Deep Wide-Field Survey. ISS J142950.9+333012 is a T4.5 brown dwarf at a distance of approximately 42 pc, and ISS J142738.5+331242 is a radio-loud quasar at redshift z=6.12. Ou…
Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the yet cooler Y-type spectral class (Teff <~ 500K?) has not been seen. Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5 Mj) are rare. Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic o…
We report the discovery of two L dwarfs (the new spectral class defined for dwarfs cooler than the M type) in a two-epoch CCD proper motion survey of 413 square degrees, complemented by infrared photometry from DENIS. One of them has a strong lithium line and is therefore a brown dwarf. The other is a common proper motion companion to the mid-M dwarf LHS 102 (GJ 1001), which has a well determined trigonometric parallax. LHS 102B is thus the coolest L dwarf of known distance and luminosity. Its infrared absolute photometry are very well reproduced by the Allard et al DUSTY models.
We present preliminary non-grey model atmospheres and interiors for cool brown dwarfs. The resulting synthetic spectra are compared to available spectroscopic and photometric observations of the coolest brown dwarf yet discovered, Gl229B (Nakajima \etal, 1995). Despite recognized shortcomings of current methane opacities, we find that the model spectra provide an excellent fit to most of the spectral features of the brown dwarf. We confirm the presence of methane absorption and the substellar nature of Gl229B. These preliminary models set an upper limit for the effective temperature of 1000~K.…
We examine the recent results of the MACHO collaboration towards the Large Magellanic Cloud (Alcock et al. 1996) in terms of a halo brown dwarf or white dwarf population. The possibility for most of the microlensing events to be due to brown dwarfs is totally excluded by large-scale kinematic properties. The white dwarf scenario is examined in details in the context of the most recent white dwarf cooling theory (Segretain et al. 1994) which includes explicitely the extra source of energy due to carbon-oxygen differentiation at crystallization, and the subsequent Debye cooling. We show that the…
Over the past decade considerable effort, both observational and theoretical, has been directed towards a more accurate determination of the stellar lower main sequence and of the sub-stellar domain covered by Brown Dwarfs and Planets. A review of the recent improvement in the theory of cool model atmospheres is presented.
AA Dor is an eclipsing, close, post common-envelope binary (PCEB) consisting of a sdOB primary star and an unseen cool companion with an extraordinary low mass. The evolution of this system appears enigmatic - results of a recent spectral analysis of the primary has shown that the secondary is formally a brown dwarf. We present results of a spectral analysis which is based on phase-resolved high-resolution and high-SN spectroscopy.
We present the status of an ongoing study to built a a high resolution near infrared Echelle spectrograph (NAHUAL) for the 10.4-m-Gran Telescopio Canarias (GTC) which will be especially optimised for planet searches by means of high precision radial velocity measurements. We show that infrared radial velocity programs are particularly suitable to search for planets very low mass stars and brown dwarfs, as well as active stars. The goal of NAHUAL is to reach an accuracy of the radial velocity measurement of a few m/s, which would allow the detection of planets with a few earth-masses orbiting l…
We report the discovery of CFBDS J005910.90-011401.3 (hereafter CFBDS0059), the coolest brown dwarf identified to date. We found CFBDS0059 using i' and z' images from the Canada-France-Hawaii Telescope (CFHT), and present optical and near-infrared photometry, Keck laser guide star adaptive optics imaging, and a complete near-infrared spectrum, from 1.0 to 2.2 $μ$m. A side to side comparison of the near-infrared spectra of CFBDS0059 and ULAS J003402.77-005206.7 (hereafter ULAS0034), previously the coolest known brown dwarf, indicates that CFBDS0059 is ~50+/-15K cooler. We estimate a temperature…
We present predictions for the numbers of ultra--cool dwarfs in the Galactic disk population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction vs. effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables s…
Monte Carlo simulations of the field substellar mass function (MF) are presented. Starting from various representations of the MF below 0.1 M$_{\sun}$ and the stellar birth rate, luminosity functions (LFs) and T$_{eff}$ distributions are produced for comparison with observed samples. These distributions exhibit distinct minima in the mid-type L dwarf regime followed by a rise in number density for fainter/cooler brown dwarfs, predicting many more T-type and cooler brown dwarfs in the field even for relatively shallow mass functions. Deuterium-burning brown dwarfs dominate field objects with 40…
We present a near-infrared spectrum of the close, detached white dwarf + brown dwarf binary WD0137-349 (Maxted et al 2006), that directly reveals the substellar companion through an excess of flux longwards of 1.95 microns. We best match the data with a white dwarf + L8 composite model. For ages ~1 Gyr, the spectral type of the cool secondary is in agreement with the mass determined by Maxted et al. (2006) from radial velocity measurements (0.053 +/- 0.006Msun), and supports an evolutionary scenario in which the brown dwarf survived a previous phase of common envelope evolution which resulted …
Electron-degenerate, pressure-ionized hydrogen (usually referred to as metallic hydrogen) is the principal constituent of brown dwarfs, the long-sought objects which lie in the mass range between the lowest-mass stars (about eighty times the mass of Jupiter) and the giant planets. The thermodynamics and transport properties of metallic hydrogen are important for understanding the properties of these objects, which, unlike stars, continually and slowly cool from initial nondegenerate (gaseous) states. Within the last year, a brown dwarf (Gliese 229 B) has been detected and its spectrum observed…
Using a simulated disk brown dwarf (BD) population, we find that new large area infrared surveys are expected to identify enough BDs covering wide enough mass--age ranges to potentially measure the mass function down to ~0.03Mo, and the BD formation history out to 10 Gyr, at a level capable of establishing if BD formation follows star formation. We suggest these capabilities are best realised by spectroscopic calibration of BD properties (Teff, g and [M/H]) which, when combined with a measured luminosity and an evolutionary model can give BD mass and age relatively independent of BD atmosphere…
Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for …
We present intermediate and low resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a CCD-based IZ survey covering an area of 1 sq. deg in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs, having spectral types in the range M6-M9. Five out of the seven objects can be considered as Pleiades members on the basis of their radial velocities, Halpha emission and other gravity sensitive atomic features like the NaI doublet at 818.3 and 819.5 nm. According to current …
We suggest that the low viscosity in close binary accretion disks during quiescence is due to magnetic fields from the companion star. In very late evolutionary phase the companion stars become brown dwarfs and have cooled down to such low a temperature that this process cannot work anymore. The extremely low viscosity in WZ Sge stars supports this connection between companion stars and viscosity. We further suggest that magnetic activity in such very cool stars is cut off by their poor electric conductivity.
Brown dwarfs are natural clocks, cooling and dimming over time due to insufficient core fusion. They are also numerous and present in nearly all Galactic environments, making them potentially useful chronometers for a variety of Galactic studies. For this potential to be realized, however, precise and accurate ages for individual sources are required, a prospect made difficult by the complex atmospheres and spectra of low-temperature brown dwarfs; degeneracy between mass, age and luminosity; and the lack of useful age trends in magnetic activity and rotation. In this contribution, I review fiv…
Radio activity has been observed in a large variety of stellar objects, including in the last few years, ultra-cool dwarfs. To explore the extent of long-term radio activity in ultra-cool dwarfs, we use data taken over an extended period of 9 hr from the Very Large Array of the source 2MASS J05233822-1403022 in September 2006, plus data taken in 2004. The observation taken in September 2006 failed to detect any radio activity at 8.46 GHz. A closer inspection of earlier data reveals that the source varied from a null detection on 3 May 2004, to $\approx$95 $μ$Jy on 17 May 2004, to 230 $μ$Jy on …
We present the discovery of a brown-dwarf companion to the 130-400 Myr-old G8 V star HD 203030. Separated by 11.9" (487 AU in projection) from its host star, HD 203030B has an estimated mass of 0.023 (+0.008;-0.011) solar masses. The K-band spectral type of L7.5+/-0.5 places HD 203030B near the critical L/T transition in brown dwarfs, which is characterized by the rapid disappearance of dust in sub-stellar photospheres. From a comparative analysis with well-characterized field L/T transition dwarfs, we find that, despite its young age, HD 203030B has a bolometric luminosity similar to the >1 G…
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project (COUP) to study the X-ray properties of 34 spectroscopically-identified brown dwarfs with near-infrared spectral types between M6 and M9 in the core of the Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray sources. The apparently low detection rate is in many cases related to the substantial extinction of these brown dwarfs; considering only the BDs with $A_V \leq 5$ mag, nearly half of the objects (7 out of 16) are detected in X-rays. Our 10-day long X-ray lightcurves of these objects exhibit str…
We have searched, so far in vain, for brown dwarfs or giant planets in the vicinity of several nearby stars using cameras built at Universite de Montreal attached to the CFHT Adaptive Optics system. Here, we show how pairs of images taken simultaneously through separate filters on, and off, the 1.6 micron methane absorption band, can be used to completely defeat speckles, the dominant noise source, while revealing cool brown dwarfs and giant planets in high contrast. We achieve the photon shot-noise limit for data taken without a focal plane mask but, so far, have had less success with data wh…
The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub-micron-sized dust grains accompanied by dust settling toward the disk mid-plane. Here we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circu…
We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf system, spectral classes at which most of the objects are brown dwarfs. Our atlas covers the optical region from H$α$ up to the near infrared at 1 $μ$m. We present spectral details of ultra-cool atmospheres at very high resolution ($R \sim 33 000$) and compare the spectra to model calculations. Our comparison shows that molecular features from VO and CaH, and atomic features from Cs and Rb are reasonably well fit by current models. On the other hand, features due to TiO, CrH, and water, and atomic Na and K reveal lar…
The formation of low-mass protostars and especially of brown dwarfs currently are ``hot topics'' in cool star research. The talks contributed to this splinter session discussed how low in mass and how low in luminosity objects might exist, if these substellar objects show evidence for circum(sub)stellar disks, and how the bottom of the mass function in young clusters after the formation process looks like. In a lively open discussion, a vast majority of the speakers and the audience expressed why, given the available data, a stellar-like formation mechanism down to the lowest masses should be …
We report on searches for radio emission from three of the nearest known young brown dwarfs using the Very Large Array. We have obtained sensitive upper limits on 3.6cm emission from 2MASSW J1207334-393254, TWA~5B and SSSPM J1102-3431, all of which are likely members of the $\sim$8-Myr-old TW Hydrae association. We derive constraints on the magnetic field strength and the number density of accelerated electrons, under the assumption that young brown dwarf atmospheres are able to produce gyrosynchrotron emission, as seems to be indicated in older brown dwarfs. For the young brown dwarf TWA~5B, …
Optical linear polarimetry of 8 ultra cool field dwarfs, with spectral types ranging from M9 to L8, is presented. The linear polarisation, P, of each dwarf we measured is P < 0.2%. Three dwarfs have polarisations compatible with zero, two are marginal detections, and three have significant polarisation. Due to their small distance, an insterstellar origin for the detected polarisation can be safely ruled out. Our detections confirm that dust is present in the atmosphere of these brown dwarfs and that the scattering geometry is not symmetric. Possibilities for asymmetry include the dwarfs rotat…
