  The derivation of the effective temperature of a star is a critical first
step in a detailed spectroscopic analysis. Spectroscopic methods suffer from
systematic errors related to model simplifications. Photometric methods may be
more robust, but are exposed to the distortions caused by interstellar
reddening. Direct methods are difficult to apply, since fundamental data of
high accuracy are hard to obtain. We explore a new approach in which the
spectrum is used to characterize a star's effective temperature based on a
calibration established by a small set of standard stars. We perform principal
component analysis on homogeneous libraries of stellar spectra, then calibrate
a relationship between the principal components and the effective temperature
using a set of stars with reliable effective temperatures. We find that our
procedure gives excellent consistency when spectra from a homogeneous set of
observations are used. Systematic offsets may appear when combining
observations from different sources. Using as reference the spectra of stars
with high-quality spectroscopic temperatures in the Elodie library, we define a
temperature scale for FG-type disk dwarfs with an internal consistency of about
50 K, in excellent agreement with temperatures from direct determinations, but
distinct from widely used scales based on the infrared flux method.
