Plaskett's Star (HD 47129): Still More Curious

The properties of the components of Plaskett's Star (HD 47129) have been the subject of a continuing controversy for over seven decades. In a recent paper, Bagnuolo, Gies, & Wiggs analyzed a set of 17 UV spectra of this object from IUE archives. A tomography algorithm was used to produce the individual spectra of the two components. The physical properties of the stars were estimated including the mass ratio (q = 1.18), the UV intensity ratio (r_UV = 0.53), the interpolated MK types (07.3 I and 06.2 I, respectively), and the rotational velocities (v sin i ≈ 88 km s^-1 and 310 km s^-1, respectively).

These properties of the primary and secondary of RD 47129 were questioned by Underhill based on visual spectra. We now present reconstructions of the component spectra in four visual wavebands with data obtained using the CHARA 1 m Multiple Telescope Telescope (MTT). Data were taken at nine epochs well distributed in orbital phase about the 14.4 day period and separated with a tomography algorithm. An analysis produces results consistent with those of Bagnuolo, Gies, & Wiggs. Specifically, we find that the secondary is roughly 1-1.5 subclasses earlier than the primary (O7 vs. O8.5 from He II λ4542: He I λ4471 ratio. The intensity ratio in the visual is about 0.5 (primary brighter). The mass ratio appears to be similar to that estimated from the UV lines (q = 1.18). The primary lines are sharp and are compatible with a relatively low rotational velocity from the primary: V sin i = 105 km s^-1. The secondary lines appear to be rotationally broadened, and we estimate that V sin i = 370 km s^-1 for this star.

We suggest that the main reason for the discrepancy between our interpretation and Underhill's is abundance differences in the stars. Specifically, the primary appears to be depleted in C and O and enriched in N. We briefly discuss the implications for the evolutionary history of this interesting binary.