Alert Notice 721: Anticipated secondary eclipse of b Persei centered on October 26 2020 UT

October 1, 2020

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Dr. Donald F. Collins and colleagues provide the following material:

“The bright (4.6 V) triple-star, eclipsing system b Persei has been the subject of several AAVSO campaigns to observe time-series of six eclipses (AAVSO Alert Notices 507 (Jan 2013), 537 (Feb 2016), 563 (Dec 2016), 610 (Jan 2018), 655 (Oct 2018), 688 (Dec 2019)).

“The first ever observed eclipse was not observed until an AAVSO campaign in 2013 (AAVSO Alert Notice 476 (Jan 2013)).

“Dr. Donald F. Collins (AAVSO member), Dr. Robert Zavala (US Naval Observatory, Flagstaff Station), Jason Sanborn (Lowell Observatory), and Dr. Anatoly Miroshnichenko (University N. Carolina, Greensboro) have requested high time-resolved observations of the bright star b Persei during the upcoming predicted secondary eclipse of the third star as it passes behind the AB inner pair.  The observation window should last about 2 weeks beginning October 18, 2020, which is about one week before the predicted date of mid-eclipse (October 26.4 ± 1 UT).   It is important that observers observe several long time-series observations both before the eclipse and after the eclipse as well as all parts of the eclipse.  

“The b Persei system (HD 26961, HIP 20070, HR 1324, SAO 24531) consists of three stars in a hierarchical system.  The A and B components are close-orbiting (1.5237 d period) in a low-inclined orbit that exhibits ellipsoidal variation (~0.06 magnitude variation) with no eclipses. The brightest star of the pair is spectral class A2V.  The other two stars in the system are believed to be spectral class F (Hill et al., 1976, ApJ, 208,152).   The recent campaigns by AAVSO observers have revealed the following:

1.    Both the primary and secondary transits show significant light curve dips in total brightness on the order of 0.1 mag to 0.5 mag.

2.    The orbital period for the C star is 704.9 ± 0.1 d based on the time difference between successive transits of the same primary/secondary phase of the C orbit. The previously published orbital period is 702 d (Hill 1976).

3.    Radial velocity observations by Anatoly Miroshnichenko, which separate the short period RV oscillations from the long period RV oscillation, have indicated that the eclipses observed in January 2015, December 2016, and November 2018 (Figure 1) were secondary eclipses in which the  C star transits behind the A-B pair.  We have also observed three primary eclipses: March 2016, February 2018, and January 2020 (Figure 2).  In the primary eclipses the C star temporarily blocks some of the light from the close-orbiting A-B pair.

4.    Each transit (or eclipse) is different – due to the lack of a simple commensurate relationship between the short period A-B stars and the long period of the of the AB-C triple system.

5.    The deepest relative minimum observed occurred during the primary eclipse of January 2020 at mJD 1867.4 (Figure 2).   The solid curve in Figure 2 (the simple model) calculates the result if the C star (spectral class F, luminosity 2 x Lsolar) is fully ingressed over the “disk” of the primary star A (spectral class A-V, luminosity 10 x Lsolar).  

Figure 1. The three previously-observed secondary eclipses of b Persei.  The solid black curves are a simple model fit to the observations.  The different colors and symbols in the observations represent the individual observations contributed by the various observers.  The dashed sinusoidal curve in the middle panel represents the graphical radial velocity of the component star A based on data from A. Miroshnichenko.  The red solid sinusoidal curves in each panel represent the fit to ellipsoidal data.  The heavy black curves are a simple model fit to the observations.

Figure 2. The three previously-observed by AAVSO primary eclipses of b Persei.  The different colors and symbols for the observations represent the different observers who contributed to the data.  The solid black curves are a simple model fit to the observations.  The lighter sinusoidal curves represent the fit to ellipsoidal data.

“Observers are asked to obtain high-resolution time-series observations of long duration in V (or the green channel from DSLR cameras) during the eclipses as well as as many as possible out-of-eclipse time-series observations of the system during the two week window centered on the October 26 eclipse (October 18-November 2).  Out-of-eclipse data are needed to calibrate the various offsets expected from different observing systems.

“It is recommended to use the star  labeled '55' (AUID 000-BLL-386 = HIP 20156 = SAO 39457 = HR 1330 = HD 270840) in the AAVSO finder chart at 5.456 V for the comparison star.  For a check star HIP 20370 (J2000 RA, Dec = 04 21 45.47 +50 02 06.64) 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 a check star if available in the observer's field of view. New observers are welcome - especially Asian and Pacific observers to help fill the gaps in the transit light curves.

“Guidelines for observing bright stars for both CCD and  DSLR cameras may be found in the Alert Notices for the earlier campaigns.  DSLR guidelines may also found in the AAVSO DSLR Observing Manual.  Another resource for DSLR photometry may be found by Buchheim (2018, used with Buchheim’s permission) ."

Coordinates (2000): R.A. 04 18 14.62   Dec. +50 17 43.8  (from VSX entry for b Per)

Finder charts with comparison stars for b Per may be created using the AAVSO Variable Star Plotter (VSP).

Please submit all photometry data to the AAVSO International Database via WebObs at the AAVSO site using the name “B PER”.

Be sure to subscribe to the forum threads given above to be advised of latest developments, eclipse onsets, and observing ideas.

Buchheim, R. K. 2018,  “Lessons from DSLR Photometry of b Per “Third Star” Eclipse (February 2018)”, SAS-2018 The Symposium on Telescope Science and ALPO Annual 2018 Meeting, Proceedings for the 37th Annual Conference of the Society for Astronomical Sciences [] pp 71-77.
[Note: Bob Buchheim has given AAVSO permission to make available a stand-alone copy of his presentation on the AAVSO web site for observers' easy access - dfc]

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Donald Collins and colleagues.


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