We present empirical metallicity-dependent calibrations of effective temperature against colours for dwarfs of luminosity classes IV and V and for giants of luminosity classes II and III , based on a collection from the literature of about two hundred nearby stars with direct effective temperature measurements of better than 2.5 per cent .
The calibrations are valid for an effective temperature range 3,100 – 10,000 K for dwarfs of spectral types M5 to A0 and 3,100 – 5,700 K for giants of spectral types K5 to G5 .
A total of twenty-one colours for dwarfs and eighteen colours for giants of bands of four photometric systems , i.e .
the Johnson ( UBVR _ { J } I _ { J } JHK ) , the Cousins ( R _ { C } I _ { C } ) , the Sloan Digital Sky Survey ( SDSS , gr ) and the Two Micron All Sky Survey ( 2MASS , JHK _ { s } ) , have been calibrated .
Restricted by the metallicity range of the current sample , the calibrations are mainly applicable for disk stars ( [ Fe/H ] \gtrsim -1.0 ) .
The normalized percentage residuals of the calibrations are typically 2.0 and 1.5 per cent for dwarfs and giants , respectively .
Some systematic discrepancies at various levels are found between the current scales and those available in the literature ( e.g .
those based on the infrared flux method IRFM or spectroscopy ) .
Based on the current calibrations , we have re-determined the colours of the Sun .
We have also investigated the systematic errors in effective temperatures yielded by the current on-going large scale low- to intermediate-resolution stellar spectroscopic surveys .
We show that the calibration of colour ( g - K _ { s } ) presented in the current work provides an invaluable tool for the estimation of stellar effective temperature for those on-going or upcoming surveys .