The population of OB supergiants in the starburst cluster Westerlund 1

Abstract

Context. After leaving the main sequence, massive stars undergo complex evolution, which is still poorly understood. With a population of hundreds of OB stars, the starburst cluster Westerlund 1 offers an unparallelled environment to study their evolutionary tracks. Aims. We characterise a large sample of evolved OB stars in the cluster, with the aim of determining cluster parameters and place stars in an evolutionary sequence. Methods. We used the FORS2 instrument on the VLT to obtain intermediate-resolution spectroscopy over the range 5800-9000Å of about a hundred stars selected as likely members of the cluster based on their photometry. We developed criteria for their spectral classification using only spectral features in the range observed. We discuss these criteria, useful for spectral classification of earlytype stars in the GAIA spectral region, in the appendix. Using these criteria, we obtain spectral classifications, probably accurate to one subtype, for 57 objects, most of which had no previous classification or a generic classification. Results. We identify more than 50 objects as OB supergiants. We find almost 30 luminous early-B supergiants and a number of less luminous late-O supergiants. In addition, we find a few mid B supergiants with very high luminosity, some of them displaying signs of heavy mass loss. All these stars form a sequence compatible with theoretical evolutionary tracks. In addition, two early B supergiants also show indication of heavy mass loss and may represent the evolutionary phase immediately prior to the Wolf-Rayet stage. We investigate cluster properties using the spectral types and existing photometry. We find that the reddening law to the cluster does not deviate strongly from standard, even though extinction is quite variable, with an average value A V = 10.8. Though evolutionary tracks for high-mass stars are subject to large uncertainties, our data support an age of > ∼ 5 Myr and a distance d ≈ 5 kpc for Westerlund 1. Conclusions. The spectral types observed are compatible with a single burst of star formation (the age range is very unlikely to be > 1 Myr). Westerlund 1 shows its potentiality as a laboratory for massive star evolution, which can be fulfilled by detailed study of the population presented here.

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

“…3) and that are actually used in this work are marked by additional flags. We also provide the photometric mass determinations and additional information extracted from the 2MASS-PSC (Bonanos 2007;Negueruela et al 2010). An example of the information available in the catalog is provided in Table 1, the full version is available online.…”

“…We matched the catalogs of optical sources with the full list of near-infrared ones, obtaining an overlap of 63 stars in total within the SofI field of view. Among the whole set of optical sources, 10 do not have a measurement of their near-infrared magnitudes, 4 from Bonanos (2007) and 6 from Negueruela et al (2010). These stars are marked in Fig.…”

“…3. The position of the stars without near-infrared magnitude measurements but with optical magnitudes are displayed as magenta diamonds (stars from Bonanos 2007) and yellow triangles (stars from Negueruela et al 2010).…”

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…The longslit mode with a 0.3 slit was used for all observations, yielding a resolution of ∼7000. Data reduction was accomplished following the methodology described in Negueruela et al (2010). The resultant spectra from both this, our higher resolution VLT/FLAMES run (Ritchie et al 2009a) and published JHK-band observations ) are presented in Figs. 2−4; we note that low S/N as a result of interstellar reddening precludes meaningful discussion of the spectrum shortwards of ∼5000 Å.…”

Studying the YMC population of M83: how long clusters remain embedded, their interaction with the ISM and implications for GC formation theories

“…For example, the Arches cluster towards the centre of our Galaxy has an age of 2.5-4 Myr (Blum et al 2001) and is observed to be free of gas. Other clusters of similar masses, such as NGC 3603 (aged ∼1-2 Myr) or Westerlund 1 (aged ∼3-6 Myr; Brandner et al 2008;Negueruela, Clark & Ritchie 2010;Gennaro et al 2011) also have no dense gas bound to them. It has been found that even for lower mass systems than those already mentioned, the clusters can potentially remove gas in <1 Myr (Seale et al 2012).…”