Bibcode
Burningham, Ben; Pinfield, D. J.; Leggett, S. K.; Tamura, M.; Lucas, P. W.; Homeier, D.; Day-Jones, A.; Jones, H. R. A.; Clarke, J. R. A.; Ishii, M.; Kuzuhara, M.; Lodieu, N.; Zapatero Osorio, M. R.; Venemans, B. P.; Mortlock, D. J.; Barrado Y Navascués, D.; Martin, E. L.; Magazzù, A.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 391, Issue 1, pp. 320-333.
Advertised on:
11
2008
Citations
104
Refereed citations
85
Description
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
(NIR) photometry for all three sources, and mid-IR photometry for
ULAS1335. We use NIR 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 750K <~ Teff <~ 850K, and 5.0 <~
logg(cms-2) <~ 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 33-70MJ 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 Burgasser et al. to the type T9 and suggest the
inclusion of the WJ 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 600 K,
possibly combined with low gravity and/or high metallicity. We find
ULAS1335 to be extremely red in NIR to mid-IR colours, with H - [4.49] =
4.34 +/- 0.04. This is the reddest NIR to mid-IR colour yet observed for
a T dwarf. The NIR to mid-IR spectral energy distribution of ULAS1335
further supports Teff < 600 K, and we estimate
Teff ~ 550-600K for ULAS1335. We estimate that ULAS1335 has
an age of 0.6-5.3 Gyr, a mass of 15-31MJ and lies at a
distance of 8-12 pc.