Near-Infrared Spectroscopy of Ultracompact \ion{H}{2} regions in W51A with NIFS/ALTAIR

TL;DR

This study used near-infrared integral field spectroscopy to identify and classify massive stars in W51A, determining its distance and age spread, and providing insights into star formation in this active region.

Contribution

First application of NIFS/ALTAIR IFU observations to classify massive stars in W51A, refining distance and age estimates of the region.

Findings

Derived a spectrophotometric distance of 4.80 ± 1.27 kpc.

Spectral types ranged from O8 to O9.5 for five stars.

Age spread of 1.5-4 Myr among studied stars.

Abstract

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