Balmer lines in cool dwarf stars II. Effective temperatures and calibration of colour indices

Effective temperatures obtained from synthesis of the extended profile wings of the first four Balmer lines are presented for more than 100 dwarfs and subgiants of different metal abundances and surface gravities in the temperature range from 5000 to 6500 K. Line formation is based on homogeneous ODF blanketed model atmospheres in LTE. The resulting temperatures of the more metal-rich stars differ systematically from those determined by reference to synthetic broad- or intermediate-band colours such as B−V, b−y, R−I or V−K. While the Balmer line temperatures give room to only very small individual errors and result in a convincingly small mean error for all four lines, the scatter against temperatures determined from broad-band colours is by far outside the internal errors claimed in recent applications