We build an accurate data base of 5200 HCN and HNC rotation–vibration energy levels, determined from existing laboratory data. 20 000 energy levels in the Harris et al. linelist are assigned approximate quantum numbers. These assignments, lab‐determined energy levels and Harris et al. energy levels are incorporated in to a new energy level list. A new linelist is presented, in which frequencies are computed using the lab‐determined energy levels where available, and the ab initio energy levels otherwise. The new linelist is then used to compute new model atmospheres and synthetic spectra for the carbon star WZ Cas. This results in better fit to the spectrum of WZ Cas in which the absorption feature at 3.56 μm is reproduced to a higher degree of accuracy than has previously been possible. We improve the reproduction of HCN absorption features by reducing the abundance of Si to [Si/H]=−0.5 dex, however, the strengths of the Δv= 2 CS band heads are overpredicted.

Using a simulated disc 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 present day mass function down to ∼0.03 M , and the BD formation history out to 10 Gyr, at a level that will be capable of establishing if BD formation follows star formation. We suggest these capabilities are best realized by spectroscopic calibration of BD properties (T eff , 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 models. Such calibration requires an empirical understanding of how BD spectra are affected by variations in these properties, and thus the identification and study of 'benchmark BDs' whose age and composition can be established independently.We identify the best sources of benchmark BDs as young open cluster members, moving group members, and wide (>1000 au) BD companions to both subgiant stars and high-mass white dwarfs (WDs). To accurately asses the likely number of wide companion BDs available, we have constrained the wide L dwarf companion fraction using the 2-Micron All Sky Survey (2MASS), and find a companion fraction of 2.7 +0.7 −0.5 per cent for separations of ∼1000-5000 au. This equates to a BD companion fraction of 34 +9 −6 per cent if one assumes an α ∼ 1 companion mass function. Using this BD companion fraction, we simulate populations of wide BD binaries, and estimate that 80 +21 −14 subgiant-BD binaries, and 50 +13 −10 benchmark WD-BD binaries could be identified using current and new facilities. The WD-BD binaries should all be identifiable using the Large Area Survey component of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey, combined with the Sloan Digital Sky Survey. Discovery of the subgiant-BD binaries will require a near-infrared imaging campaign around a large (∼900) sample of Hipparcos subgiants. If identified, spectral studies of these benchmark BD populations could reveal the spectral sensitivities across the T eff , g and [M/H] space probed by new surveys.

The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright The Royal Astronomical SocietyWe present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints (<= 1 Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has T-eff = 695 +/- 60 K, log g = 4.0-4.7 and an inferred mass of 5-20M(J). We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5-mu m region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of the dust opacity in the spectra of Ross 458C that was raised by Burgasser et al. We have found that the BT Settl models which also incorporate a condensate cloud model provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with f(sed) = 2 and we briefly discuss the influence of condensate clouds on T dwarf spectra

We present eight new T4.5–T7.5 dwarfs identified in the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) Data Release 1 (DR1). In addition we have recovered the T4.5 dwarf SDSS J020742.91+000056.2 and the T8.5 dwarf ULAS J003402.77−005206.7. Photometric candidates were picked up in two‐colour diagrams over 190 deg2 (DR1) and selected in at least two filters. All candidates exhibit near‐infrared spectra with strong methane and water absorption bands characteristic of T dwarfs and the derived spectral types follow the unified scheme of Burgasser et al.. We have found six new T4.5–T5.5 dwarfs, one T7 dwarf, one T7.5 dwarf and recovered a T4.5 dwarf and a T8.5 dwarf. We provide distance estimates which lie in the 15–85 pc range; the T7.5 and T8.5 dwarfs are probably within 25 pc of the Sun. We conclude with a discussion of the number of T dwarfs expected after completion of the LAS, comparing these initial results to theoretical simulations.

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