DRAMATIC EVOLUTION OF THE DISK-SHAPED SECONDARY IN THE ORION TRAPEZIUM STAR θ¹Ori B₁(BM Ori):MOSTSATELLITE OBSERVATIONS

Abstract

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.

Variability of OB stars from TESS southern Sectors 1–13 and high-resolution IACOB and OWN spectroscopy

“…HD 37022 (θ 1 Ori C, O7 Vp, Sota et al 2011) is a magnetic star (Donati et al 2002;Wade et al 2006). The TESS pixels are contaminated by the nearby eclipsing binary θ 1 Ori B (Windemuth et al 2013), which dominates the light contribution. We do note variability outside the eclipses which might come from any of the components of θ 1 Ori.…”

Stable and unstable accretion in the classical T Tauri stars IM Lup and RU Lup as observed byMOST

“…A similar effect was also found in the classical T Tau star 2MASS 06401258+1005404 observed twice by CoRoT; in 2008 the star showed a light-curve modulated with a stable spot, while in 2011 the light curve was irregular and dominated by accretion bursts . Prompted by the most recent results obtained for TW Hya (Siwak et al 2014), both the probable discovery of the accretion-regime alternation and the discovery of short-term occultations, we used MOST in a continuation of our previous, already cited work, but within a broader context of investigation of young stars Siwak et al 2011bSiwak et al , 2013Cody et al 2013;Windemuth et al 2013;Zwintz et al 2013). For the present analysis, we have chosen two well-known southern stars RU Lup and IM Lup, the brightest among the six known CTTSs in the Lupus 2 Star Forming Region (Comerón 2008).…”

Space-Based Photometry of Binary Stars: From Voyager to TESS

“…Windemuth et al [97] used MOST to observe the perplexing EB θ 1 Ori B 1 , also called BM Ori. This consists of a B3 star in orbit with a larger but less massive component which is surmised to be a self-luminous disc-shaped object.…”