The enigmatic WR46: A binary or a pulsator in disguise

Abstract

We discuss the observational history of the Wolf-Rayet object WR 46 (WN3p), including a re-investigation of the original discovery plates from early this century. We find that the reported presence of lines is a mis-interpretation of lines and conclude that the object did not change its spectral type since the first recording one century ago. We performed photometric monitoring in the period 1986–1999, and confirm that the object shows cyclical variability on a time scale of hours. The shape of the light curves varies from purely sinusoidal to irregular, and from an amplitude of nearly 01 to constancy. In addition, night-to-night variability of the mean brightness causes folded light curves to display a large scatter. We investigate the frequency behaviour of the photometric data. From the periodograms of our two large data sets, in 1989 and in 1991, we identify frequencies of significantly different values 7.08 cd-1 and 7.34 cd-1, respectively. Moreover, the 1989 data show strong evidence for an additional frequency cd-1. The periodograms of our eight smaller data sets show more ambiguous behaviour. We discuss whether these latter data show evidence for multi-frequency behaviour, or whether they can be reconciled with a single clock with a changing clock-rate. As pointed out by van Genderen et al. (1991), if the data are folded using twice the single-wave period, the light curves appear ellipsoidal with unequal minima. Although the difference in depth of the minima is hardly significant, it does occur in both large data sets. Moreover, the simultaneously obtained radial velocity measurements are in better agreement with the double-wave than the single-wave period (Paper II). Finally, Marchenko et al. (2000) observed WR 46 to have a single-wave period of the same order as the double-wave period identified here. The periodograms of the (V–W) colour index show that the colour changes are controlled by single-wave frequencies, or their sub-harmonics (double-wave periods), but not by fx. The colour variation of WR 46 is peculiar in the sense that the object is red when bright and blue when faint. Although the spectrum of WR 46 has been suggested to originate from a stellar disc, this peculiar colour behaviour is in line with its WR nature, which is also confirmed by its spectral variability (Marchenko et al. 2000; Paper II). In addition, our seasonal photometric averages of WR 46 show a rise from 1989 to 1991 of 012, confirming the brightening detected by the Hipparcos-satellite (Marchenko et al. 1998). Eventually, WR 46 brightened by about 025 and subsequently declined on a time scale of years. Such a rise is unique among the WR stars in the Hipparcos-survey, and has not been found anywhere else. We investigate the changes to the double-wave behaviour and mean colour-index coinciding with the period change and brightening. Interpretation of the object as either a multi-frequency non-radial WR pulsator, or a WR binary with possible large orbital decay is deferred to Paper III.