This work aims to constrain the physical nature of umbral dots ( UDs ) using high-resolution spectropolarimetry .
Full Stokes spectra recorded by the spectropolarimeter on Hinode of 51 UDs in a sunspot close to the disk center are analyzed .
The height dependence of the temperature , magnetic field vector , and line-of-sight velocity across each UD is obtained from an inversion of the Stokes vectors of the two Fe I lines at 630 nm .
No difference is found at higher altitudes ( -3 \leq \log ( \tau _ { 500 } ) \leq - 2 ) between the UDs and the diffuse umbral background .
Below that level the difference rapidly increases , so that at the continuum formation level ( \log ( \tau _ { 500 } ) = 0 ) we find on average a temperature enhancement of 570 K , a magnetic field weakening of 510 G , and upflows of 800 m s ^ { -1 } for peripheral UDs , whereas central UDs display an excess temperature of on average 550 K , a field weakening of 480 G , and no significant upflows .
The results for , in particular , the peripheral UDs , including cuts of magnetic vector and velocity through them , look remarkably similar to the output of recent radiation MHD simulations .
They strongly suggest that UDs are produced by convective upwellings .