SDSS J010448.46+153501.8 has previously been classified as an sdM9.5 subdwarf .
However , its very blue J - K colour ( -0.15 \pm 0.17 ) suggests a much lower metallicity compared to normal sdM9.5 subdwarfs .
Here , we re-classify this object as a usdL1.5 subdwarf based on a new optical and near-infrared spectrum obtained with X-shooter on the Very Large Telescope .
Spectral fitting with BT-Settl models leads to T _ { eff } = 2450 \pm 150 K , [ Fe/H ] = - 2.4 \pm 0.2 and log g = 5.5 \pm 0.25 .
We estimate a mass for SDSS J010448.46+153501.8 of 0.086 \pm 0.0015 M _ { \sun } which is just below the hydrogen-burning minimum mass at [ Fe/H ] = - 2.4 ( \sim 0.088 M _ { \sun } ) according to evolutionary models .
Our analysis thus shows SDSS J0104+15 to be the most metal-poor and highest mass substellar object known to-date .
We found that SDSS J010448.46+153501.8 is joined by another five known L subdwarfs ( 2MASS J05325346+8246465 , 2MASS J06164006 - 6407194 , SDSS J125637.16 - 022452.2 , ULAS J151913.03 - 000030.0 and 2MASS J16262034+3925190 ) in a ‘ halo brown dwarf transition zone ’ in the T _ { eff } – [ Fe/H ] plane , which represents a narrow mass range in which unsteady nuclear fusion occurs .
This halo brown dwarf transition zone forms a ‘ substellar subdwarf gap ’ for mid L to early T types .