We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey ( UKIDSS ) Third Data Release : ULAS J101721.40+011817.9 ( ULAS1017 ) , ULAS J123828.51+095351.3 ( ULAS1238 ) and ULAS J133553.45+113005.2 ( ULAS1335 ) .
We detail optical and near-infrared photometry for all three sources , and mid-infrared photometry for ULAS1335 .
We use near-infrared spectra of each source to assign spectral types T8p ( ULAS1017 ) , T8.5 ( ULAS1238 ) and T9 ( ULAS1335 ) to these objects .
ULAS1017 is classed as a peculiar T8 ( T8p ) due to appearing as a T8 dwarf in the J -band , whilst exhibiting H and K -band flux ratios consistent with a T6 classification .
Through comparison to BT-Settl model spectra we estimate that ULAS1017 has 750 { K } \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $ \textstyle < $ } } { % \sim } $ } T _ { eff } \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $% \textstyle < $ } } { \sim } $ } 850 { K } , and 5.0 \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $ \textstyle < $ } } { \sim } $% } \log { \it g } ( cms ^ { -2 } ) \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $% \textstyle < $ } } { \sim } $ } 5.5 , assuming solar metallicity .
This estimate for gravity is degenerate with varying metallicity .
We estimate that ULAS1017 has an age of 1.6–15 Gyr , a mass of 33–70 M _ { J } and lies at a distance of 31–54 pc .
We do not estimate atmospheric parameters for ULAS1238 due to a lack of K -band photometry .
We extend the unified scheme of to the type T9 and suggest the inclusion of the W _ { J } index to replace the now saturated J -band indices .
We propose ULAS1335 as the T9 spectral type standard .
ULAS1335 is the same spectral type as ULAS J003402.77-005206.7 and CFBDS J005910.90-011401.3 .
We argue that given the similarity of the currently known > T8 dwarfs to the rest of the T dwarf sequence , the suggestion of the Y0 spectral class for these objects is premature .
Comparison of model spectra with that of ULAS1335 suggest a temperature below 600K , possibly combined with low-gravity and/or high-metallicity .
We find ULAS1335 to be extremely red in near to mid-infrared colours , with H - [ 4.49 ] ~ { } = ~ { } 4.34 \pm 0.04 .
This is the reddest near to mid-infrared colour yet observed for a T dwarf .
The near to mid-infrared spectral energy distribution of ULAS1335 further supports T _ { eff } < 600 { K } , and we estimate T _ { eff } \sim 550 - 600 { K } for ULAS1335 .
We estimate that ULAS1335 has an age of 0.6–5.3 Gyr , a mass of 15–31 M _ { J } and lies at a distance of 8–12 pc .