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AAVSO Alert Notice 563: Observations requested for b Per December 2016 eclipse

December 2, 2016: An AAVSO campaign on the bright star b Per has been launched at the request of Dr. Donald F. Collins, who writes:

"Dr. Donald F. Collins (AAVSO member, Swannanoa, NC), Dr. Robert Zavala (US Naval Observatory, Flagstaff Station), Dr. Anatoly Miroshnichenko (University of North Carolina at Greensboro), and Jason Sanborn (Lowell Observatory) have requested high time-resolution observations of the bright star b Persei [HIP 20070, SAO 29531, HR 1324, HD 26961, V ~ 4.57] during the next predicted eclipse, or transit, of the third star across the system as it is predicted to transit the inner pair of the triple system **DURING THE WEEK CENTERED ON DECEMBER 15, 2016**. Well-sampled light curves can provide timing data for the orbits with much greater precision than astrometry or spectroscopy alone.

"Three campaigns by the AAVSO (AAVSO Alert Notice 476, January 4, 2013; AAVSO Alert Notice 507, January 7, 2015; and AAVSO Alert Notice 537, February 26, 2016) have successfully observed three transits between the inner A-B pairs each with the third star. The second transit (January 2015) was observed 704 days after the first observed transit. Long time-series observations of the January 2015 transit detected multiple dips in the light curve as the third star alternately transited the A and B components and the inner rotating system. A third transit was observed successfully by AAVSO observers in March 2016. During March 2016 transit the third star transited on the opposite sides of the inner A-B pair from the first two observed transits. We are trying to sort out whether the third star is blocked by the inner A-B pair or the A-B pair blocks the light of the third star in each. Results of the previous campaigns may be seen at the links below:

January 2013:  http://inside.warren-wilson.edu/~dcollins/bPersei/

January 2015:  https://www.aavso.org/campaign-highlight-b-persei

and http://inside.warren-wilson.edu/~dcollins/bPersei/bPersei2015Eclipse/b_Persei2015eclipse.html

The last link above includes an animation attempting to simulate the eclipse of the orbiting stars in the system.

The plot showing the result of the March 2016 campaign may be seen below.

"The inner pair of stars [in b Per] orbit in a low-inclination (non-eclipsing) orbit with a 1.523 day period that shows a small ellipsoidal variation (0.06 V p-p) due to the gravitational distortion of the two stars. This is shown as the small out-of-eclipse eclipse oscillations in the [figure showing the March 2016 eclipse that may be seen on the AAVSO webpage for this Alert Notice]. The third star of the system transits the inner pair every 702 - 705 days, no eclipse had been detected before the 2013 AAVSO campaign.  

"Observers are asked to obtain high-resolution time-series observations in V during the eclipses as well as time-series observations of the system out of eclipse **BEGINNING ABOUT DECEMBER 8, 2016, [AND CONTINUING FOR ABOUT A WEEK AFTER THE ECLIPSE]** in order to calibrate the varying offsets from different observing systems. It is recommended to use the star HIP 20156 = SAO 39457 = HR 1330 = HD 27084 (the star labeled '55') in the AAVSO finder chart at 5.456 V as the comparison star. For a check star HIP 20370 (J2000 RA, dec = 04 19 13.24, +50 02 55.30) may be used if the observer's field of view is about one degree. This star is not in the AAVSO sequence. Any other AAVSO sequence stars may be used as check stars if available in the observer's field of view.

"CCD and DSLR observers should follow some simple guidelines to observe bright stars. Integration times should be kept longer than about 15 seconds by stopping down the aperture of the optics if necessary. Otherwise atmospheric scintillations add significantly to the noise. With short focal-length telescopes or lenses - needed for large fields of view for bright stars and the sequences - de-focusing (FWHM ~ 4 pixels) is also necessary to avoid under-sampling among pixels on the detector array. Co-adding the photometry data (between 5 and 20 observations averaged) or stacking the same number of images will reduce the point-to-point noise even further. It is also important to obtain out-of-eclipse observations both before and after the event in order to match the varying zero point offsets from different observers and to match the out-of-eclipse light curves with the ellipsoidal light curve of b Persei. Observers should also concentrate on obtaining time-series of long duration each night - especially in mid-eclipse as the transit time duration between any two stars is about six hours.

"We also need a wider longitudinal spread in observers in order to insure that observations cover the entire transit event that experiences several eclipses and re-emergences during the 2-4 day event. Observers from western N. America, Pacific islands, and Asia are especially needed in addition to continued good coverage from Europe and eastern North America.

"Photoelectric observers are especially encouraged to contribute to the campaign.

"Dr. Anatoly Miroshnichenko and his students at the University of North Carolina at Greensboro have been obtaining extensive high-resolution spectroscopy of the b Persei system this observing season to help with the campaign."

A thread has been created on the Campaigns and Observation Reports forum at (https://www.aavso.org/content/december-2016-b-per-campaign) to share rapid developments, observing techniques, and notices concerning the eclipse progress.

Coordinates for b Per:  R.A. 04 18 14.62  Dec. +50 17 43.8  (J2000)

Charts with a comparison star sequence for b Per may be created using the AAVSO Variable Star Plotter (VSP). Please choose comp/check stars as requested above, if possible.

Please submit observations to the AAVSO International Database using the name b PER. Do NOT also send observations to Dr. Collins.

This campaign is being followed on the AAVSO Observing Campaigns webpage.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen primarily utilizing text provided by Dr. Donald Collins.

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