A VLT/FLAMES survey for massive binaries in Westerlund 1

Abstract

Context. The first soft gamma-ray repeater was discovered over three decades ago, and was subsequently identified as a magnetar, a class of highly magnetised neutron star. It has been hypothesised that these stars power some of the brightest supernovae known, and that they may form the central engines of some long duration gamma-ray bursts. However there is currently no consenus on the formation channel(s) of these objects. Aims. The presence of a magnetar in the starburst cluster Westerlund 1 implies a progenitor with a mass ≥40 M , which favours its formation in a binary that was disrupted at supernova. To test this hypothesis we conducted a search for the putative pre-SN companion.Methods. This was accomplished via a radial velocity survey to identify high-velocity runaways, with subsequent non-LTE model atmosphere analysis of the resultant candidate, Wd1-5. Results. Wd1-5 closely resembles the primaries in the short-period binaries, Wd1-13 and 44, suggesting a similar evolutionary history, although it currently appears single. It is overluminous for its spectroscopic mass and we find evidence of He-and N-enrichement, O-depletion, and critically C-enrichment, a combination of properties that is difficult to explain under single star evolutionary paradigms. We infer a pre-SN history for Wd1-5 which supposes an initial close binary comprising two stars of comparable (∼41 M + 35 M ) masses. Efficient mass transfer from the initially more massive component leads to the mass-gainer evolving more rapidly, initiating luminous blue variable/common envelope evolution. Reverse, wind-driven mass transfer during its subsequent WC Wolf-Rayet phase leads to the carbon pollution of Wd1-5, before a type Ibc supernova disrupts the binary system. Under the assumption of a physical association between Wd1-5 and J1647-45, the secondary is identified as the magnetar progenitor; its common envelope evolutionary phase prevents spin-down of its core prior to SN and the seed magnetic field for the magnetar forms either in this phase or during the earlier episode of mass transfer in which it was spun-up. Conclusions. Our results suggest that binarity is a key ingredient in the formation of at least a subset of magnetars by preventing spin-down via core-coupling and potentially generating a seed magnetic field. The apparent formation of a magnetar in a Type Ibc supernova is consistent with recent suggestions that superluminous Type Ibc supernovae are powered by the rapid spin-down of these objects.

ALMA observations of the supergiant B[e] star Wd1-9

“…3 and shows a broad single peak with a gaussian-fitted FWHM of 163±5 km s −1 . The peak velocity -59±2 km s −1 is consistent with the cluster velocity for Westerlund 1 (Clark et al 2014). Also plotted in Fig.…”

Expansion of Kes 73, A Shell Supernova Remnant Containing a Magnetar

“…Binarity may be necessary to remove the progenitor envelope and allow ms spin periods (Turolla et al 2015), though evidence of any surviving companion is scarce. (Clark et al (2014) (Arnaud 1996), SPEX (Kaastra & Mewe 2000), Astropy (The Astropy Collaboration 2013), matplotlib (Hunter 2007), Numpy (Walt et al 2011), Scipy (Jones et al 2001), APLpy (Robitaille & Bressert 2012) Facilities: CXO APPENDIX Here we describe arguments similar to those in the Discussion above, to show that somewhat lower-mass progenitors than those discussed there ( 30M ) should also produce wind bubbles inconsistent with our observations of Kes 73: an ambient density of 2 cm −3 and a radius of about 6 pc. For a 25M rotating Geneva model, the brief RSG stage ends long before its explosion because of extensive mass loss, although its wind-blown bubble might be much smaller in size than for the more massive stars.…”

Population synthesis of isolated neutron stars with magneto-rotational evolution – II. From radio-pulsars to magnetars

“…40% of the total pulsar population being born with a binary massive progenitors, possibly leading to the formation of more magnetized NSs (Spruit 2008;Langer 2012;Clark et al 2014).…”