Direct observations of the ionizing star in the ultracompact H II region G29.96-0.02: A strong constraint on the stellar birth line for massive stars

We have observed the ultracompact H II region G29.96-0.02 in the near-infrared J, H, and K bands and in the Bry line. By comparison with radio observations, we determine that the extinction to the nebula is A(K) = 2.14 with a 3 sigma uncertainty of 0.25. We identify the ionizing star and determine its intrinsic K magnitude. The star does not have an infrared excess and so appears to be no longer accreting. The K magnitude and the bolometric luminosity allow us to place limits on the location of the ionizing star in the H-R diagram. The 3 sigma upper limit on the effective temperature of the ionizing star is 42,500 K. We favor a luminosity appropriate for star with a mass in excess of about 60 M.. The limit on the temperature and luminosity exclude stars on the zero-age main sequence (ZAMS) and stars within 10(6) yr of the ZAMS. Since the age of the UC H II region is estimated to be only about 10(5) yr, we suggest that this is direct evidence that the stellar birth line for massive stars at twice solar metallicity must be significantly redder than the ZAMS.