Red supergiants around the obscured open cluster Stephenson 2

Abstract

Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the ScutumCrux 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 ∼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 Å 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 ∼35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we found ∼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 ∼6 kpc). In addition, we found evidence of several populations of massive stars at different distances along this line of sight.

Identification of red supergiants in nearby galaxies with mid-IR photometry

“…Different criteria have been proposed depending on the available resolution. The spectral classification of low-resolution spectra is described in Ginestet et al (1994), Negueruela et al (2011Negueruela et al ( , 2012, and Massey (1998), who presented a classification of RSGs in M 31 and M 33. However, the resolution of our spectra is not high enough to identify luminosity-sensitive features (Fe, Ti) apart from the Ca II triplet, which we used for identifying RSGs in our sample.…”

A multi-wavelength analysis of the diffuse H ii region G25.8700+0.1350

“…These clusters are separated from the center of G25.8700+0.1350 by 39 ′ 8 and 22 ′ 6 respectively. Moreover, the largest concentration of red supergiants (RSGs) known in our Galaxy (Negueruela et al 2012;Dorda et al 2016) can be found around these clusters. According to the high density of RSGs, this region probably is one of the most intense star-forming places in the Galaxy.…”

Characterisation of red supergiants in theGaiaspectral range

“…Due to their high luminosity and low temperature, RSGs appear very bright in the infrared, and thus are easily observable at very large distances, even if they are affected by high extinction. Thanks to this, in the past few years, several massive and highly reddened clusters have been discovered in the inner Galaxy (for example Davies et al 2007;Negueruela et al 2012), in the region where the tip of the Galactic bar is believed to touch the base of the Scutum arm, revealing recent widespread massive star formation in this part of the Milky Way.…”

“…Even the CaT becomes unable to separate clearly LC I from LC II and LC III (Negueruela et al 2011). However, the classification is still roughly possible if the spectral type (SpT) is known, as this will predict the TiO bandhead depths, and therefore warn about the erosion suffered by other spectral features (Negueruela et al 2011(Negueruela et al , 2012.…”

“…1 As a last step, we obtained velocities along the line of sight for these objects. This calculation follows that outlined in Negueruela et al (2012), with only one particularity: to correct the wavelength calibration from instrumental flexures, we use the subtracted sky spectrum to cross-correlate the sky emission lines between objects. By doing this, we guarantee that all our measurements will have the same instrumental signature that can then be corrected by comparing with velocity standards.…”