The determination of for metal-poor A-type stars using and 2MASS , and magnitudes ^*,**

¹ Kitt Peak National Observatory, National Optical Astronomy Observatories, Box 26732, Tucson, AZ 85726-6732, USA
² Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, via del Fosso del Cavaliere, 00133 Roma, Italy
³ Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: castelli@ts.astro.it

Corresponding author: T. Kinman, kinman@noao.edu

Received: 5 April 2002
Accepted: 27 May 2002

Abstract

Effective temperatures (T_eff) can be determined from ,   and  colours that are derived from 2MASS magnitudes. This gives another way to estimate the T_eff of faint blue halo stars (V   15) whose temperatures are now usually deduced from . Transformations (adapted from Carpenter [CITE]) 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 ,  and  for a variety of metallicities and  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]) whose T_eff  are significantly cooler than those we derive by an amount that increases with increasing temperature.