A serendipitous survey for variability amongst the massive stellar population of Westerlund 1

Abstract

Aims. Massive stars are known to demonstrate significant spectroscopic and photometric variability over a wide range of timescales. However the physical mechanisms driving this behaviour remain poorly understood. Westerlund 1 presents an ideal laboratory for studying these processes in a rich, coeval population of post-main sequence stars and we present a pathfinding study aimed at characterising their variability. Methods. To do this we utilised the large body of spectroscopic and photometric data that has accumulated for Wd1 during the past decade of intensive studies, supplemented with the sparser historical observations extending back to the early 1960s. Results. Despite the heterogeneous nature of this dataset, we were able to identify both spectroscopic and photometric variability amongst every class of evolved massive star present within Wd 1. Spectroscopic variability attributable to both wind asphericity and photospheric pulsations was present amongst both the hot and cool hypergiants and the former, also with the Wolf Rayets. Given the limitations imposed by the data, we were unable to determine the physical origin of the wind structure inferred for the OB supergiants, noting that it was present in both single pulsating and binary stars. In contrast we suspect that the inhomogineities in the winds of the Wolf Rayets are driven by binary interactions and, conversely, by pulsations in at least one of the cool hypergiants. Photospheric pulsations were found for stars ranging from spectral types as early as O9 I through to the mid F Ia + yellow hypergiants -with a possible dependence on the luminosity class amongst the hot supergiants. The spectroscopically variable red supergiants (M2-5 Ia) are also potential pulsators but require further observations to confirm this hypothesis. Given these findings it was therefore rather surprising that, with the exception of W243, no evidence of the characteristic excursions of both luminous blue variables and yellow hypergiants was found. Nevertheless, future determination of the amplitude and periodicity of these pulsations as a function of temperature, luminosity and evolutionary state holds out the tantalising possibility of constraining the nature of the physical mechanisms driving the instabilities that constrain and define stellar evolution in the upper reaches of the HR diagram. Relating to this, the lack of secular evolution amongst the cool hypergiants and the presence of both high-luminosity yellow hypergiants and red supergiants within Wd1 potentially place strong constraints on post-main sequence evolutionary pathways, with the latter result apparently contradicting current theoretical predictions for >25 M stars at solar metallicites.

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…However, to date no direct mass determination exists for a member of Wd1; as well as providing confirmation of the current understanding of the cluster derived from spectroscopic studies, this is of importance for confirming the high progenitor mass for the magnetar CXOU J164710.2-455216 that has been inferred from its membership of Wd1 (Muno et al 2006). In addition, the distribution of evolved stars in Wd1 offers the prospect of demanding tests for evolutionary models, with both the distribution of WR subtypes and the large number of cool hypergiants in both YHG and RSG phases at odds with current predictions (Clark et al 2010a).…”

“…therein; see also Clark et al 2010a;Negueruela et al 2010 population support a single burst of star formation at an age ∼5 Myr and a distance ∼5 kpc Negueruela et al 2010), with the identification of ∼O8V stars in the cluster (Clark et al, in prep. ) and a population of lower-luminosity late-O II−III stars just evolving off the MS both fully consistent with this derived age.…”

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…However, both studies are hampered by small sample sizes, which comprise stars which are known pulsators and hence radial velocity (RV) variables (cf. Clark et al 2010), potentially leading to significantly biased RV determinations. Finally Koumpia & Bonanos (2007) make multiple observations of four eclipsing binaries within Wd1 to determine their orbital parameters, from which mean values of v sys ∼ −40 ± 6 km s −1 are found (or v sys ∼ −45 ± 14 km s −1 depending on the assumptions made regarding the twin components of .…”

“…Of these, object D has been classified as an OB supergiant, and hence potentially the massive pre-SN companion we predict; classification spectroscopy of the remaining stars in combination with an RV survey of the full cluster population would be of considerable interest to determine if any of the three are indeed overluminous chemically peculiar runaway analogues to Wd1-5. Alternatively, if disruption is avoided, binaries containing magnetars should 7 By analogy to the optical properties of Of?p stars, Clark et al (2010) cited Wd1-24 as a possible highly magnetic star, based on the variable C iii+Pa 16 ∼8500 Å blend. However, further observations have revealed that this behaviour is instead due to a variable contribution from a late-O binary companion (Ritchie et al, in prep.).…”

Detection of a 1.59 h period in the B supergiant star HD 202850

“…Many B-type supergiants are reported to show spectroscopic and photometric variability (e.g., Fullerton et al 1996;Kaufer et al 1997Kaufer et al , 2006Lefever et al 2007;Clark et al 2010). In addition, spectral lines of B supergiants are found to be much wider than expected from pure stellar rotation.…”