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Another cluster of red supergiants close to RSGC1

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Negueruela, I.; González-Fernández, C.; Marco, A.; Clark, J. S. and Martínez-Núñez, S. (2010). Another cluster of red supergiants close to RSGC1. Astronomy & Astrophysics, 513, article no. A74.

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c 2010 ESO

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Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.1051/0004-6361/200913373

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Another cluster of red supergiants close to RSGC1

I. Negueruela1, C. González-Fernández1 ,A.Marco1,J.S.Clark2, and S. Martínez-Núñez1

1 Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain e-mail: [email protected] 2 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Received 29 September 2009 / Accepted 5 January 2010

ABSTRACT

Context. Recent studies have revealed massive clusters in a region of the close to the tip of the Long Bar. These clusters are heavily obscured and are characterised by a population of red supergiants. Aims. We analyse a previously unreported concentration of bright red ∼16 away from the cluster RSGC1 Methods. We utilised near IR photometry to identify candidate red supergiants and then K-band spectroscopy of a sample to charac- terise their properties. Results. We find a compact clump of eight red supergiants and five other candidates at some distance, one of which is spectro- scopically confirmed as a red supergiant. These objects must form an , which we name . Because of the high reddening and strong field contamination, the cluster sequence is not clearly seen in 2MASS or UKIDSS near-IR photometry. From = . the analysis of the red supergiants, we infer an extinction AKS 1 9 and an age close to 20 Myr. Conclusions. Though this cluster is smaller than the three known previously, its properties still suggest a mass in excess of 10 000 M. Its discovery corroborates the hypothesis that star formation in this region has happened on a wide scale between ∼10 and ∼20 Myr ago. Key words. stars: evolution – supergiants – : structure – open clusters and associations: individual: Alicante 8

1. Introduction of objects thought to be the progenitors of type IIn supernovae (Smartt et al. 2009). > 4 Over the past few , the census of massive (Mcl ∼ 10 M) In this paper, we report the discovery of one more cluster clusters in the Milky Way has steadily increased, with the dis- of red supergiants in the immediate vicinity of RSGC1, which covery of three such clusters near the Galactic centre (Krabbe we designate as Alicante 8 = RSGC41. Identified visually in et al. 1995; Nagata et al. 1995; Cotera et al. 1996; Figer et al. 2MASS KS images as a concentration of bright stellar sources 1999) and the realisation that Westerlund 1 has a mass of the or- near  = 24d.60, b =+0d.39 (see Fig. 1), we utilised near-IR 5 derof10 M (Clark et al. 2005). Similar clusters are known in photometry to identify potential cluster members, nine of which many other and are typical of starburst environments, were subsequently observed