Space instrumentation

Abstract:

Thin-film transmission spectra of ammonium hydrosulfide (NH4SH) ice and ammonia (NH3) ice between 1300 and 12,000 cm-1 were used to determine the ice's optical constants. The films were grown on a sapphire substrate, and a Fourier-transform spectrometer and a grating spectrometer were used together to record the spectra. Lambert's law was used to directly determine the imaginary component of the complex refractive indices; from this, the real component was derived using the Kramers-Kronig algorithm. It is shown that, contrary to what is expected, the optical constants determined for NH3 ice at 80 K are in good agreement with those in the cubic phase, rather than the metastable one. The phase of the NH4SH ice was observed to change from amorphous to polycrystalline as the film was annealed to 160 K. © 2006 Optical Society of America.

Abstract:

Contexf. We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys. Aims. We aim to show that our methodologies for searching the ∼27 deg² of the LAS EDR are successful for finding both L and T dwarfs via cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs. Methods. Ultracool dwarfs are selected by combinations of their YJH(K) UKIDSS colours and SDSS DR4 z - J and i - z colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4 m and 8 m-class facilities. Results. Our main result is the discovery following CH4 imaging and spectroscopy of a T4.5 dwarf, ULAS J 1452+0655, lying ∼80 pc distant. A further T dwarf candidate, ULAS J 1301+0023, has very similar CH4 colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter LO dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey J-band limit, Conclusions. Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky. © ESO 2007.