In order to determine the relative frequency of occurence of sgB[e] stars in comparison to other post-MS sub-types we constructed a representative census of well-studied young (≤20 Myr), massive Galactic clusters. To accomplish this we employed the source lists of Massey et al. (1995, 2001), Eggenberger et al. (2002), Crowther et al. (2006b), Evans et al. (2011), Davies et al. (2012b) and Chené et al. (2013) to undertake a literature survey11, supplemented with the 3 Galactic Centre clusters and a number of aggregates associated with Giant H ii regions12; as expected the latter possessed no unambiguous post-MS stars and so are not considered further. This resulted in a total of ~68 stellar clusters and/or associations and where possible we present both cluster mass and age in Table A.1, along with associated uncertainties if quoted in the literature.
For simplicity we group stars into eight broad spectral classifications in Table A.1, based on our current understanding of the evolutionary sequence of post-MS stars; a more detailed breakdown is superfluous, given our simple aim of qualitatively determining the rarity, or otherwise, of sgB[e] stars. This results in a total of ≳600 stars.
Inevitably both the cluster census and the populations of individual clusters summarised in Table A.1 are likely to be incomplete and subject to observational biases that are difficult to quantify given the diverse detection strategies employed by the various authors; hence we consider the values quoted to be lower limits. Specifically:
The location of RSGC1-6 within a wider star forming regionsimilarly dominated by RSGs complicates assessment of clusterextent/membership. Moreover, interstellar extinction mandatestheir study in the far-red/near-IR, biasing any surveys against thedetection of blue supergiants and WRs, noting that populationsynthesis arguements would suggest broadly comparablynumbers of blue and red supergiantsin 10–20 Myr-old clusters (e.g.Davies et al. 2009).
More generally, the lack of systematic narrow-band imaging surveys for obscured clusters will hamper identification of intrinsically faint WRs, while the weak winds of lower-luminosity OB supergiants will not yield pronounced emission lines and hence will prevent their detection via such an observational strategy. We suspect that the second limitation is particularly problematic for the Galactic Centre proper and Quintuplet clusters, and is further compounded by the low S/N of available spectra for the latter aggregate, to the extent that in many cases we are unable to confidently determine accurate spectral types and/or luminosity classes for individual members (Figer et al. 1999; Liermann et al. 2009).
Any single-epoch observational survey will be biased against identifying LBVs; one might anticipate that both late-B supergiants and YHGs could be cool-phase LBVs.
The qualitative classification criteria used to distinguish between yellow super- and hyper-giants.
Nevertheless we suspect that sgB[e] stars will be amongst those least affected by incompleteness; their strong emission lines, intrinsic brightness and characteristic near- to mid-IR excess (e.g. Bonanos et al. 2009) render them easily identifiable. While we recognise the overlap in spectral morphologies of sgB[e] stars and cool-phase LBVs in the near-IR window (cf. S18 and GG Car versus AG Car; Morris et al. 1996) comparison of both near- and mid-IR colours allows for their separation (Zickgraf et al. 1986; Bonanos et al. 2009). Moreover, they may be distinguished from the spectroscopically similar but less evolved Herbig AeBe stars on the basis of their intrinsic luminosities.
Bearing the above caveats in mind, the results of this compilation appear broadly as expected (cf. Davies et al. 2009), with younger clusters being dominated by the apparently massive core H-burning early-mid O supergiants and WNLha stars and older clusters hosting increasing numbers of RSGs. Both WN and WC subtypes are present in young (<10 Myr) clusters; it is curently not possible to determine whether their absence in older clusters is a result of observational bias due to interstellar reddening and/or stellar evolution. Both LBVs/BHGs/WN9-11h stars and YHGs are present in small numbers- as anticipated due to the apparently short durations of these phases – in clusters spanning a comparatively wide range of ages (~2–12 Myr). This appears to support the predictions of e.g. Groh et al. (2013) that both high and low mass stars pass through such evolutionary phases (the former always remaining in the blue region of the HR diagram and the latter on the bluewards track of a red loop).
Only two instances of clusters hosting confirmed sgB[e] stars are found – Wd1 and the Cyg OB2 association (MWC349A). Two further stars are of interest. Firstly Martayan et al. (2008) identify one member of NGC 6611 – the emission line binary W503 – as demonstrating an IR excess, which the authors attribute to the presence of a mass-transfer accretion disc. Further observations of this object to better constrain its nature would be of considerable value. Secondly, a detatched equatorial ring is associated with the ~4 Myr old B1.5 Ia star Sher25, located in the outskirts of the young (~1 Myr) cluster NGC 3603 (Brandner et al. 1997). Although the current stellar spectrum lacks the forbidden lines characteristic of sgB[e] stars (Smartt et al. 2002), Smith et al. (2007) suggest that Sher 25 might represent the evolved descendant of such a star.
Finally we strongly caution against employing these data in order to quantitatively determine the progenitor masses of evolved stars. In several cases the ages of clusters have been determined solely from the presence of particular spectral types; therefore deriving progenitor masses for such stars from the age of the cluster introduces a circularity into the argument. Moreover in many of the remaining cases ages have been inferred from isochrone fitting to sparse datasets, while in the near-IR window favoured for the study of obscured clusters isochrones are essentially vertical, presenting an additional difficulty. Finally, it is increasingly clear that binarity plays a major role in post-MS evolution (e.g. Clark et al. 2011); given that systematic RV surveys have only been attempted for a handful of clusters any detailed analysis of the data presented here would appear premature.
Stellar content of Galactic young massive clusters.
© ESO, 2013