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 Brγ line. By comparison with radio observations, we determine that the extinction to the nebula is A_K = 2.14 with a 3 σ 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 σ 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 10⁶ yr of the ZAMS. Since the age of the UC H II region is estimated to be only about 10⁵ 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.