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 1100 K 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.