Identifying two groups of massive stars aligned in the l similar to 38 degrees Galactic direction

Ramírez Alegría, S.;Herrero, A.;Rübke, K.;Marín-Franch, A.;García, M.;Borissova, J. 2018. Identifying two groups of massive stars aligned in the l similar to 38 degrees Galactic direction. Astronomy and Astrophysics 614, DOI: 10.1051/0004-6361/201731720

Recent near-infrared data have contributed to unveiling massive and obscured stellar populations in both new and previously known clusters in our Galaxy. These discoveries have lead us to view the Milky Way as an active star-forming machine.

Aims. We look for young massive cluster candidates as over-densities of OB-type stars. The first search, focused on the Galactic direction l = 38 degrees, resulted in the detection of two objects with a remarkable population of OB-type star candidates.

Methods. With a modified version of the friends-of-friends algorithm AUTOPOP and using 2MASS and UKIDSS-GPS near-infrared (J, H, and K) photometry for one of our cluster candidates (named Masgomas-6) we selected 30 stars for multi-object and long-slit H and K band spectroscopy. With the spectral classification and the near-infrared photometric data, we derive individual distance, extinction, and radial velocity.

Results. Of the 30 spectroscopically observed stars, 20 are classified as massive stars, including OB-types (dwarfs, giants and supergiants), two red supergiants, two Wolf Rayets (WR122-11 and the new WR122-16), and one transitional object (the LBV candidate IRAS 18576+0341). The individual distances and radial velocities do not agree with a single cluster, indicating that we are observing two populations of massive stars in the same line of sight: Masgomas-6a and Masgomas-6b. The first group of massive stars, located at 3.9(-0.3)(+0.4) kpc, contains bothWolf Rayets and most of the OB-dwarfs; the second group, located at 9.6 +/- 0.4 kpc, hosts the LBV candidate and an evolved population of supergiants. We are able to identify massive stars at two Galactic arms, but we cannot clearly identify whether these massive stars form clusters or associations.

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