Wide Field Imaging Surveys

Sensitive wide-field imaging surveys in the future will detect the largest numbers of new sub-stellar objects. Indeed, the vast majority of known brown dwarfs were discovered using the present generation of wide-area imaging surveys: DENIS, 2MASS, and SDSS (Sect. 5.3.1), where they are seen as isolated free-floating objects. However, those surveys have revealed only a small fraction of the expected sub-stellar objects in the solar neighborhood, and barely hinted at their diversity. The census of sub-stellar objects in the solar neighborhood remains woefully incomplete, especially at the cool end of the brown dwarf sequence (spectral type T8).

A second generation of sensitive wide-field surveys is now under way, with the UK Infrared Digital Sky Survey (UKIDSS) in operation on the 3.8 meter UK Infrared Telescope (UKIRT) since 2005, and the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) survey expected to commence on the first of three 1.8 meter telescopes in 2007. UKIDSS is 3 magnitudes more sensitive than 2MASS, and will therefore be correspondingly more effective at detecting very cool brown

Wavelength fj.m

Fig. 5.13. The expected spectroscopic characteristics of cool brown dwarfs in the wavelength range covered by JWST's Near InfraRed Spectrograph (NIRSPEC) (from the model calculations by Burrows, Sudarsky & Lunine, 2003). The spectra are labeled by temperature and the main features identified. Note the appearance of ammonia at — 600K and the weakening of the alkaline lines (e.g. Na I) at Teff < 450K.

Wavelength fj.m

Fig. 5.13. The expected spectroscopic characteristics of cool brown dwarfs in the wavelength range covered by JWST's Near InfraRed Spectrograph (NIRSPEC) (from the model calculations by Burrows, Sudarsky & Lunine, 2003). The spectra are labeled by temperature and the main features identified. Note the appearance of ammonia at — 600K and the weakening of the alkaline lines (e.g. Na I) at Teff < 450K.

dwarfs, with effective temperatures 500-700 K. Kendall et al (2007) recently announced the first T dwarf discoveries from UKIDSS data.

Even cooler brown dwarfs should be detectable with the planned 8.4 meter Large Synoptic Survey Telescope (LSST; commissioning expected circa 2012), which will scan the entire sky every 3 days. LSST will be able to detect very faint, fast-moving, nearby brown dwarfs, which may have evaded previous shallower surveys performed on smaller telescopes. The search for cool brown dwarfs is also a primary scientific goal of the upcoming Wide-Field Infrared Survey Explorer (WISE; mid-infrared; 0.4 m) space mission, an all-sky 3^m survey due for launch in 2009. All of these programs aim to push detections to temperatures below 600K, where, as discussed in Sect. 5.2.1, the presence of significant absorption by ammonia will lead to sufficient changes in the spectroscopic appearance to warrant their classification in a new spectral type, Y. Figure 5.13 shows the expected appearance of late-T and Y dwarfs at near- and mid-infrared wavelengths.

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