Proposal #62

We propose to use the AAVSOnet telescopes to photometrically monitor one of the best examples of a colliding winds binary system in the sky, WR 140 (HD 193793). This massive system consists of a carbon-rich WC7 star and a hot O5.5 star companion in an eccentric (e=0.887), long-period (P=7.98 years) binary. The winds of the system move fast (~2500 km/s) and collide to form a dense bow-shock (Fahed et al. 2011, MNRASm 418, 2). After the periastron passage, the carbon-rich material from the Wolf-Rayet star's wind condenses with the hydrogen-rich material from the O star's wind in this shock region to produce hot dust (e.g., Williams et al. 2009, MNRAS, 395, 1749). One of the main qualities of this system that sets it apart from other colliding winds binaries is that the orbit has been visually resolved on the sky with interferometry (Monnier et al. 2011, ApJ, 742, L1), giving precise masses of the components of 14.9+/-0.5 M_sun for the Wolf-Rayet star and 35.9+/-1.3 M_sun for the O star.

Many studies of WR 140 have been done for studying the colliding winds with optical spectroscopy (e.g., Fahed et al. 2011, MNRASm 418, 2), the X-ray light curve (Corcoran et al. 2011 arXiv:1101.1422), and X-ray spectroscopy (e.g., Sugawara et al., 2015, PASJ, 67, 121). The dust production has been predominantly studied through analysis of the infrared brightening that occurs after the periastron passage (Williams et al. 2009, MNRAS, 395, 1749). Marchenko et al. (2003, ApJ, 596, 1295) have done the only photometric monitoring of the system over two periastron passages. During the 1993 passage, nothing unusual was found, but in the 2003 passage, large dips were observed in the optical. These were associated with changes in the geometry and attenuation towards the system as the dust formed. The changes in the B-V and U-B color indices gave indications to the sizes of the dust grains that were formed in the time after the periastron passage.

The next periastron passage will occur on December 18, 2016. We have issued AAVSO Alert #546 and Special Notice #419 for observations of this important system. We are requesting nightly snapshots starting as soon as possible through phase 0.1 (2017 October) of the system in BVRI to better constrain the dust formation process and grain size. The AAVSOnet Bright Star Monitor telescopes are ideally situated to study this system, as the observing sequence indicates that the nearest reliable comparison stars of similar brightness are typically 45-60 arcmin away on the sky. While the placement in Cygnus makes observing in January - March exceptionally difficult, the large field-of-view coupled with the relatively short exposure times for a S/N>100 (<5 sec per filter) should make this a relatively simple system to observe during the crucial months.

In addition to the dust occultations in early 2017, we will investigate the overall flux of the system close to periastron. The PI and his colleagues have found several other WR+O colliding-winds systems with eccentric orbits to show a flux excess that peaks at periastron following a relationship inversely proportional to the separation of the stars in the system. These systems include R144 and R145 in the LMC and gamma Vel in the Milky Way. For gamma Vel, we have a plethora of spectra taken by both professional and amateur facilities during a six-month campaign with BRITE photometry. We are currently testing if these variations are caused by changes in line flux, or if it is from changes in the continuum. With the high eccentricity and mass-loss rates for the two stars in WR 140, we may expect to see these changes in the months around periastron passage.