Large-Scale Periodic Variability of the Wind of the Wolf-Rayet Star WR1 (HD 4004)

TL;DR

This study reveals large-scale, periodic variability in the wind of Wolf-Rayet star WR1, likely caused by co-rotating interaction regions, with implications for understanding stellar wind structures and rotation rates.

Contribution

It provides the first detailed analysis linking wind variability in WR1 to co-rotating interaction regions, suggesting a common mechanism in similar Wolf-Rayet stars.

Findings

Detected a 16.9-day periodicity in photometry and spectroscopy.

Observed non-sinusoidal light-curve with a coherence timescale of about 60 days.

Estimated stellar rotation speeds based on variability analysis.

Abstract

We present the results of an intensive photometric and spectroscopic monitoring campaign of the WN4 Wolf-Rayet (WR) star WR1=HD4004. Our broadband V photometry covering a timespan of 91 days shows variability with a period of P=16.9$^{+0.6}_{-0.3}$ days. The same period is also found in our spectral data. The light-curve is non-sinusoidal with hints of a gradual change in its shape as a function of time. The photometric variations nevertheless remain coherent over several cycles and we estimate that the coherence timescale of the light-curve is of the order of 60 days. The spectroscopy shows large-scale line-profile variability which can be interpreted as excess emission peaks moving from one side of the profile to the other on a timescale of several days. Although we cannot unequivocally exclude the unlikely possibility that WR1 is a binary, we propose that the nature of the variability we have found strongly suggests that it is due to the presence in the wind of the WR star of large-scale structures, most likely Co-rotating Interaction Regions (CIRs), which are predicted to arise in inherently unstable radiatively driven winds when they are perturbed at their base. We also suggest that variability observed in WR6, WR134 and WR137 is of the same nature. Finally, assuming that the period of CIRs is related to the rotational period, we estimate the rotation rate of the four stars for which sufficient monitoring has been carried out; i.e. v$_{rot}$=6.5, 40, 70 and 275 km/s for WR1, WR6, WR134 and WR137, respectively.