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 rad io 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 a n infrared excess and so appears to be no longer accreting. The K magn itude and the bolometric luminosity allow us to place limits on the lo cation of the ionizing star in the H-R diagram. The 3 sigma upper limi t on the effective temperature of the ionizing star is 42,500 K. We fa vor a luminosity appropriate for star with a mass in excess of about 6 0 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 l ine for massive stars at twice solar metallicity must be significantly redder than the ZAMS.