No abstract

As part of a program to determine the dynamical masses of Cepheids, we have imaged the nearest and brightest Cepheid, Polaris, with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and Wide Field Camera 3. Observations were obtained at three epochs between 2007 and 2014. In these images, as in HST frames obtained in 2005 and 2006, which we discussed in a 2008 paper, we resolve the close companion Polaris Ab from the Cepheid Polaris Aa. Because of the small separation and large magnitude difference between Polaris Aa and Ab, we used point-spread function deconvolution techniques to carry out astrometry of the binary. Based on these new measurements, we have updated the elements for the 29.59 year orbit. Adopting the distance to the system from the recent Gaia Data Release 2, we find a dynamical mass of 3.45 ± 0.75 M ⊙ for the Cepheid, although this is preliminary and will be improved by CHARA measurements covering periastron. As is the case for the recently determined dynamical mass for the Cepheid V1334 Cyg, the mass of Polaris is significantly lower than the “evolutionary mass” predicted by fitting to evolutionary tracks in the Hertzsprung–Russell diagram. We discuss several questions and implications raised by these measurements, including the pulsation mode, which instability-strip crossing the stars are in, and possible complications such as rotation, mass loss, and binary mergers. The distant third star in the system, Polaris B, appears to be older than the Cepheid, based on isochrone fitting. This may indicate that the Cepheid Polaris is relatively old and is the result of a binary merger, rather than being a young single star.

The eclipsing binary θ 1 Orionis B 1 , variable star designation BM Ori, is the faintest of the four well-known Trapezium stars at the heart of the Orion Nebula. The primary is a B3 star (∼6 M ⊙ ) but the nature of the secondary (∼2 M ⊙ ) has long been mysterious, since the duration and shape of primary eclipse are inappropriate for any sort of ordinary star. Here we report nearly continuous photometric observations obtained with the MOST satellite over ∼4 cycles of the 6.47 d binary period. The light curve is of unprecedented quality, revealing a deep, symmetric primary eclipse as well as a clear reflection effect and secondary eclipse. In addition, there are other small disturbances, some of which repeat at the same phase over the four cycles monitored. The shape of the primary light curve has clearly evolved significantly over the past 40 years. While its overall duration and depth have remained roughly constant, the slopes of the descent and ascent phases are significantly shallower now than in the past and its distinctive flat-bottomed "pseudo-totality" is much less obvious or even absent in the most recent data. We further demonstrate that the primary eclipse was detected at X-ray wavelengths during the Chandra Orion Ultradeep Project (COUP) study. The light curve continues to be well modeled by a self-luminous and reflective disk-shaped object seen nearly edge-on orbiting the B3 primary. The dramatic change in shape over four decades is modeled as an opacity variation in a tenuous outer envelope or disk of the -2secondary object. We presume that the secondary is an extremely young protostar at an earlier evolutionary phase than can be commonly observed elsewhere in the Galaxy and that the opacity variations observed are related to its digestion of some accreted matter over the last 50 -100 years. Indeed, this object deserves continued observational and theoretical attention as the youngest known eclipsing binary system.

No abstract