Near-infrared Spectroscopy of Ultracompact H ii Regions in W51A with NIFS/ALTAIR*

W51A is the most active star formation region of Giant \ion{H}{2} W51. It harbors two massive proto-clusters W51e and W51\,IRS2, which are very rare in Galaxy. We aim to identify new born stars UCHII regions derive its distance age. performed IFU observations with NIFS+ALTAIR nine targets sub-region. The modulus was obtained using spectral classification $K$-band a reddening law appropriate inner Galactic plane. derived types for five them, ranging from O8 O9.5, similar those radio continuum data, except sources that we assigned somewhat later type. included another seven objects precise other works, allowed us better constrain estimate. Our spectrophotometric d=4.80\,$\pm$\,1.27\,kpc good agreement rotation model trigonometric parallaxes, placing near tangent point Sagittarius arm. conclude studied this work have an age spread 1.5-4 Myr, substantially older than thought date.