摘要：Context. Massive stars form on different scales that range from large,
dispersed OB associations to compact, dense starburst clusters. The complex structure of
regions of massive star formation and the involved short timescales provide a challenge
for our understanding of their birth and early evolution. As one of the most massive and
luminous star-forming region in our Galaxy, W49 is the ideal place to study the formation
of the most massive stars.
Aims. By classifying the massive young stars that are deeply embedded in
the molecular cloud of W49, we aim to investigate and trace the star formation history of
this region.
Methods. We analyse near-infrared K-band spectroscopic
observations of W49 from LBT/LUCI combined with JHK images obtained with NTT/SOFI and LBT/LUCI.
Based on JHK-band photometry and K-band spectroscopy, the
massive stars are placed in a Hertzsprung Russell diagram. By comparison with evolutionary
models, their age and hence the star formation history of W49 can be investigated.
Results. Fourteen O-type stars, as well as two young stellar objects
(YSOs), are identified by our spectroscopic survey. Eleven O stars are main sequence stars
with subtypes ranging from O3 to O9.5 and masses ranging from ~20 M⊙ to
~120 M⊙. Three of the
O stars show strong wind features and are considered to be Of-type supergiants with masses
beyond 100 M⊙. The two YSOs show CO emission, which
is indicative of the presence of circumstellar disks in the central region of the massive
cluster. The age of the cluster is estimated as ~1.5 Myr, with star formation continuing in different parts of
the region. The ionising photons from the central massive stars have not yet cleared the
molecular cocoon surrounding the cluster. W49 is comparable to extragalactic star-forming
regions, and it provides us with a unique chance to study a starburst in detail.
关键词：stars: formation;stars: massive;supergiants;infrared:
stars;techniques: spectroscopic;open clusters and associations:
individual: W49
