  The star formation rates (SFRs) and metallicities of a sample of 294 galaxies
with visually classified, strong, large-scale bars are compared to a control
sample of unbarred disk galaxies selected from the Sloan Digital Sky Survey
Data Release 4. The fibre (inner few kpc) metallicities of barred galaxies are
uniformly higher (at a given mass) than the unbarred sample by ~0.06 dex.
However, the fibre SFRs of the visually classified barred galaxies are higher
by about 60% only in the galaxies with total stellar mass log M > 10. The metal
enhancement at log M<10 without an accompanying increase in the SFR may be due
to a short-lived phase of early bar-triggered star formation in the past,
compared to on-going star formation rate enhancements in higher mass barred
galaxies. There is no correlation between bar length or bar axial ratio with
the enhancement of the star formation rate. In order to assess the relative
importance of star formation triggered by bars and galaxy-galaxy interactions,
SFRs are also determined for a sample of close galaxy pairs. Both mechanisms
appear to be similarly effective at triggering central star formation for
galaxies with log M>10. However, due to the much lower fraction of pairs than
bars, bars account for ~3.5 times more triggered central star formation than
interactions.
