Proposal #157

Proposal for bright star monitor – Texas or New Mexico to observe time-series of the October 2020 secondary eclipse of b Persei as the third star in the system passes behind the AB close pair.

By Donald F. Collins
(dcollins@warren-wilson.edu>

Refs:
1) AAVSO campaign alert 721
2) VSX: b Per (000-BBG-774)

It is desired to request one of the Bright Star Monitors (BSMs) in the N. American southwest to observe the October 2020 secondary eclipse of b Persei. The recent upgrades to the BSMs will permit accurate bright star photometry with controllable de-focusing and remote operation to vastly improve the observations of b Persei eclipses. More usable nights – especially from the N. American southwest will expand the longitude coverage of b Persei time-series observations. AAVSO observations, since 2013 when these eclipses were first observed, have determined the the period of the third star is 704.5 ± 0.5 d. This differs from Hill's (1976 – see alert notice or VSX for ref) determination of 702 d. Previous observations have been contributed mostly by European and eastern N. American observers. Continued eclipse observations are needed to resolve many of the other physical parameters of the three stars of the system: sizes, separations, spectral classes and temperatures, luminosities, separations, and eventually evolution of the orbits.

Observations are requested nightly between the dates: October 20, 2020 and Oct. 30, 2020). This includes both out-of-eclipse and in-eclipse observations during the three days of eclipse duration. The out-of-eclipse observations are especially important because the A-B pair undergo ellipsoidal brightness variations (about 0.06 mag peak-to-peak) of about 1 cycle in 0.77 d. This is the dominant 2nd harmonic of the ellipsoidal variations (orbit period: about 1.53 d). The ellipsoidal variation is used as the “baseline” for the various offsets of non-transformed AAVSO observations.

Duration of the BSM request: 11 days (October 20, 2020 – October 30, 2020 incl.) (from 30 deg elevation until dawn). Operators of the BSM will determination the optimal exposure times, binning, and defocusing and the seeing conditions.

Maximum cadence time: 0.04d (1 hr). (based on rate of change expected during eclipse and the error of observations (0.02 mag). Smaller cadence cycle time if possible.

Defocus to approx double the FWHM from sharp focus – permits longer exposure times. This will also need to be determined.

Binning: determined by the site.

Filters and exposures: V (1-5 sec) B (2-10 sec). Exposures will be determined by operators monitoring the image quality.

Number of images for each observation: 30 for each filter. This and the cadence time should permit multiplexing other targets throughout the night. Many exposures are needed to average-out the scintillation fluctuations of short exposures. Number to be optimized at the site.

BSM station requested: Texas - 1st choice; New Mexico - 2nd choice.