The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to the NextGen dwarf atmosphere models, however spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We re-visit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of NLTE calculations for a few selected models.Comment: ApJ, in press (November 1999), 13 pages, also available at http://dilbert.physast.uga.edu/~yeti/PAPERS and at ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.g

Infrared Colors at the Stellar/Substellar Boundary

We present new infrared photometry for 61 halo and disk stars around the stellar/substellar boundary. These data are combined with available optical photometry and astrometric data to produce color--color and absolute magnitude--color diagrams. The disk and halo sequences are compared to the predictions of the latest model atmospheres and structural models. We find good agreement between observation and theory except for known problems in the V and H passbands probably due to incomplete molecular data for TiO, metal hydrides and H$_2$O. The metal--poor M subdwarfs are well matched by the models as oxide opacity sources are less important in this case. The known extreme M subdwarfs have metallicities about one--hundredth solar, and the coolest subdwarfs have T$_{eff}\sim 3000$ K with masses $\sim$0.09M/M$_{\odot}$. The grainless models are not able to reproduce the flux distributions of disk objects with T$_{eff} <$ 2500 K, however a preliminary version of the NextGen--Dusty models which includes homogeneous formation and extinction by dust grains {\it is} able to match the colors of these very cool objects. The least luminous objects in this sample are GD165B, three DENIS objects --- DBD0205, DBD1058 and DBD1228 --- and Kelu-1. These have T$_{eff}\sim$ 2000 K and are at or below the stellar limit with masses $\leq$0.075M/M$_{\odot}$. Photometry alone cannot constrain these parameters further as the age is unknown, but published lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass $\leq$0.06M/M$_{\odot}$).Comment: ApJ, in press. 18 pages. Also available at ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dM_preprint

A deep, wide-field search for substellar members in NGC 2264

We report the first results of our ongoing campaign to discover the first brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region for which our optical studies have revealed a very high density of potential candidates - 236 in $<$ 1 deg$^2$ - from the substellar limit down to at least $\sim$ 20 M$_{\rm Jup}$ for zero reddening. Candidate BD were first selected using wide field ($I,z$) band imaging with CFH12K, by reference to current theoretical isochrones. Subsequently, 79 (33%) of the $I,z$ sample were found to have near-infrared 2MASS photometry ($JHK_s$ $\pm$ 0.3 mag. or better), yielding dereddened magnitudes and allowing further investigation by comparison with the location of NextGen and DUSTY isochrones in colour-colour and colour-magnitude diagrams involving various combinations of $I$,$J$,$H$ and $K_s$. We discuss the status and potential substellarity of a number of relatively unreddened (A$_{\rm v}$ $<$ 5) likely low-mass members in our sample, but in spite of the depth of our observations in $I,z$, we are as yet unable to unambiguously identify substellar candidates using only 2MASS data. Nevertheless, there are excellent arguments for considering two faint (observed $I$ $\sim$ 18.4 and 21.2) objects as cluster candidates with masses respectively at or rather below the hydrogen burning limit. More current candidates could be proven to be cluster members with masses around 0.1 M$_{\odot}$ {\it via} gravity-sensitive spectroscopy, and deeper near-infrared imaging will surely reveal a hitherto unknown population of young brown dwarfs in this region, accessible to the next generation of deep near-infrared surveys.Comment: 10 pages, 12 figures, accepted by A&

Gravitational microlensing as a test of stellar model atmospheres

We present calculations illustrating the potential of gravitational microlensing to discriminate between classical models of stellar surface brightness profiles and the recently computed ``Next Generation'' models of Hauschildt et al. These spherically-symmetric models include a much improved treatment of molecular lines in the outer atmospheres of cool giants -- stars which are very typical sources in Galactic bulge microlensing events. We show that the microlensing signatures of intensively monitored point and fold caustic crossing events are readily able to distinguish between NextGen and the classical models, provided a photometric accuracy of 0.01 magnitudes is reached. This accuracy is now routinely achieved by alert networks, and hence current observations can discriminate between such model atmospheres, providing a unique insight on stellar photospheres.Comment: 4 pages, 4 figures, Astronomy & Astrophysics (Letters), vol. 388, L1 (2002

Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster

We present near-infrared photometry and optical spectroscopy of very low-mass stars and brown dwarf candidates in the Pleiades open cluster. The membership status of these objects is assessed. Eight objects out of 45 appear to be non-members. A search for companions among 34 very low-mass Pleiades members (M$\le$0.09 M$_\odot$) in high-spatial resolution images obtained with the Hubble Space Telescope and the adaptive optics system of the Canada-France-Hawaii telescope produced no resolved binaries with separations larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find evidence for a binary sequence in the color-magnitude diagrams, in agreement with the results of Steele & Jameson (1995) for higher mass stars. We compare the multiplicity statistics of the Pleiades very low-mass stars and brown dwarfs with that of G and K-type main sequence stars in the solar neighborhood (Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency of wide binary systems (separation >27 AU) among the Pleiades very low-mass members. We briefly discuss how this result can fit with current scenarios of brown dwarf formation. We correct the Pleiades substellar mass function for the contamination of cluster non-members found in this work. We find a contamination level of 33% among the brown dwarf candidates identified by Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary, we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass bin is still rising but the contribution to the total mass of the cluster is declining in the brown dwarf regime.Comment: to be published in The Astrophysical Journa

Evolutionary models for very-low-mass stars and brown dwarfs with dusty atmospheres

We present evolutionary calculations for very-low-mass stars and brown dwarfs based on synthetic spectra and non-grey atmosphere models which include dust formation and opacity, i.e. objects with \te\simle 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after $\sim 2$ Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L-dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L-dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below \te\approx 1300-1400 K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility ofa brown dwarf dearth in $J,H,K$ color-magnitude diagrams around this temperature.Comment: 38 pages, Latex file, uses aasms4.sty, accepted for publication in Ap

Air Traffic Management Safety Challenges

The primary goal of the Air Traffic Management (ATM) system is to control accident risk. ATM safety has improved over the decades for many reasons, from better equipment to additional safety defences. But ATM safety targets, improving on current performance, are now extremely demanding. Safety analysts and aviation decision-makers have to make safety assessments based on statistically incomplete evidence. If future risks cannot be estimated with precision, then how is safety to be assured with traffic growth and operational/technical changes? What are the design implications for the USA’s ‘Next Generation Air Transportation System’ (NextGen) and Europe’s Single European Sky ATM Research Programme (SESAR)? ATM accident precursors arise from (eg) pilot/controller workload, miscommunication, and lack of upto- date information. Can these accident precursors confidently be ‘designed out’ by (eg) better system knowledge across ATM participants, automatic safety checks, and machine rather than voice communication? Future potentially hazardous situations could be as ‘messy’ in system terms as the Überlingen mid-air collision. Are ATM safety regulation policies fit for purpose: is it more and more difficult to innovate, to introduce new technologies and novel operational concepts? Must regulators be more active, eg more inspections and monitoring of real operational and organisational practices