期刊
ASTRONOMY & ASTROPHYSICS
卷 547, 期 -, 页码 -出版社
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201219540
关键词
stars: evolution; supergiants; open clusters and associations: individual: Stephenson 2; Galaxy: structure; virtual observatory tools
资金
- Spanish Ministerio de Ciencia e Innovacion (MICINN) [AYA2010-21697-C05-05, AYA2011-24052, CSD2006-70]
- Generalitat Valenciana [ACOMP/2012/134]
- National Aeronautics and Space Administration
- National Science Foundation
- European Commission [CT920791, CT940627]
- STFC [ST/G002533/1, PP/D000963/1] Funding Source: UKRI
- Science and Technology Facilities Council [PP/D000963/1, ST/G002533/1] Funding Source: researchfish
Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Amongst these clusters, Stephenson 2, with a core grouping of 26 red supergiants, is a strong candidate to be the most massive young cluster in the Galaxy. Aims. Stephenson 2 is located close to a region where a strong over-density of red supergiants had been found. We explore the actual cluster size and its possible connection to this over-density. Methods. Taking advantage of Virtual Observatory tools, we have performed a cross-match between the DENIS, USNO-B1 and 2MASS catalogues to identify candidate obscured luminous red stars around Stephenson 2, and in a control nearby region. More than 600 infrared bright stars fulfill our colour criteria, with the vast majority having a counterpart in the I band and >400 being sufficiently bright in I to allow observation with a 4-m class telescope. We observed a subsample of similar to 250 stars, using the multi-object, wide-field, fibre spectrograph AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500-9000 angstrom range. We derived spectral types and luminosity classes for all these objects and measured their radial velocities. Results. Our targets turned out to be G and K supergiants, late (>= M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We found similar to 35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we found similar to 40 red supergiants with radial velocities incompatible in principle with a physical association. Conclusions. Our results show that Stephenson 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of red supergiants, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance similar to 6 kpc). In addition, we found evidence of several populations of massive stars at different distances along this line of sight.
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