We present infrared photometry of the WC8 Wolf-Rayet system WR 48a observed with telescopes at ESO , the SAAO and the AAT between 1982 and 2011 which show a slow decline in dust emission from the previously reported outburst in 1978–79 until about 1997 , when significant dust emission was still evident .
This was followed by a slow rise , accelerating to reach and overtake the first ( 1978 ) photometry , demonstrating that the outburst observed in 1978–79 was not an isolated event , but that they recur at intervals of 32+ years .
This suggests that WR 48a is a long-period dust maker and colliding-wind binary ( CWB ) .
The locus of WR 48a in the ( H - L ) , K colour-magnitude diagram implies that the rate of dust formation fell between 1979 and about 1997 and then increased steadily until 2011 .
Superimposed on the long-term variation are secondary ( ‘ mini ’ ) eruptions in ( at least ) 1990 , 1994 , 1997 , 1999 and 2004 , characteristic of relatively brief episodes of additional dust formation .
Spectra show evidence for an Oe or Be companion to the WC8 star , supporting the suggestion that WR 48a is a binary system and indicating a system luminosity consistent with the association of WR 48a and the young star clusters Danks 1 and Danks 2 .
The range of dust formation suggests that these stars are in an elliptical orbit having e \sim 0.6 .
The size of the orbit implied by the minimum period , together with the WC wind velocity and likely mass-loss rate , implies that the post-shock WC wind is adiabatic throughout the orbit – at odds with the observed dust formation .
A similar conflict is observed in the ‘ pinwheel ’ dust-maker WR 112 .