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

TL;DR

This study investigates spectroscopic and photometric variability in massive stars of Westerlund 1, revealing pulsations and wind inhomogeneities that inform stellar evolution models and challenge existing theories for high-mass stars.

Contribution

It provides the first comprehensive variability characterization of Westerlund 1's massive stars using data from the 1960s onward, highlighting pulsations and wind structures across spectral types.

Findings

Spectroscopic variability due to wind asphericity and pulsations observed in both hot and cool stars.

Detection of photospheric pulsations across a range of spectral types, including hypergiants.

Constraints on stellar evolution pathways based on the presence of luminous hypergiants and red supergiants.

Abstract

Massive stars exhibit 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 utilising the large body of data that has accumulated since the 1960s. Spectroscopic variability attributable to both wind asphericity and photospheric pulsations was present amongst both the hot and cool stellar populations. Given the limitations imposed by the data, we were unable to determine the physical origin of the wind structure inferred for the OB supergiants, although the inhomogineities in the winds of the Wolf Rayets are likely 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 OB stars. The spectroscopically variable red supergiants (M2-5 Ia) are also potential pulsators but require further observations to confirm this hypothesis. Determination of the amplitude and periodicity of these pulsations as a function of temperature, luminosity and evolutionary state holds open the prospect of identifying the physical mechanisms driving the instabilities that constrain and define stellar evolution in the upper reaches of the HR diagram, while the presence of highly luminous yellow hypergiants and red supergiants within Wd1 place strong constraints on post-main sequence evolutionary pathways, apparently contradicting current theoretical predictions for >25Msun stars at solar metallicites. [ABRIDGED]