Methane band and Spitzer mid-IR imaging of L and T dwarf candidates in the Pleiades

We present Spitzer observations at [3.6] and [4.5] μm together with the methane short (1.58 μm) − methane long (1.69 μm) colour for three cool dwarfs in the Pleiades, PLZJ23, PLZJ93 and PLZJ100. We determine the effective temperatures of PLZJ23 and PLZJ93 to be ≈1200 and ≈1100 K. From the broad-band photometry we place an upper limit of 1100K on the effective temperature of PLZJ100 but lack the data required to determine the value more precisely. These temperatures are in the T dwarf regime, yet the methane colours indicate no methane is present.We attribute this to youth/low gravity in line with theoretical expectations. However, we find even less methane is present than predicted by the models. PLZJ23 and PLZJ93 are also very bright in the [3.6] μm waveband (PLZJ100 is not measured) compared to field brown dwarfs which can also be explained by this lack of methane. The definition of the T spectral class is the appearance of methane absorption, so strictly, via this definition, PLZJ93 and PLZJ100 cannot be described as T dwarfs. The colours of these two objects are, however, not compatible with those of L dwarfs. Thus we have a classification problem and cannot assign these objects a (photometric) spectral type.