The determination of Teff for metal-poor A-type stars using V and 2MASS J, H and K magnitudes

Effective temperatures (T_eff) can be determined from (V-J)_0, (V-H)_0 and (V-K)_0 colours that are derived from 2MASS magnitudes. This gives another way to estimate the T_eff of faint blue halo stars (V la 15) whose temperatures are now usually deduced from ^bo. Transformations (adapted from Carpenter \cite{carp01b}) are used to change colours derived from the 2MASS data to the Johnson system. T_eff is then derived from these colours using an updated Kurucz model. Tables are given to derive T_eff as a function of (V-J)_0, (V-H)_0 and (V-K)_0 for a variety of metallicities and log g suitable for blue horizontal branch and main sequence stars. The temperatures obtained in this way are compared with those in the recent literature for various stars with 5 <= V <= 15 and T_eff in the range 6500 to 9500 K; systematic differences are ~ 100 K. An exception is the sample of BHB stars observed by Wilhelm et al. (\cite{wbsl99}) whose T_eff are significantly cooler than those we derive by an amount that increases with increasing temperature. NOAO is operated by the Association of Universities for research in Astronomy, Inc., under contract with the National Science Foundation. Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: / /cdsweb.u-strasbg.fr/cgi-bin/ qcat?J/ A+A/391/1039