Alert Notice 840: Anticipated primary eclipse of b Per centered on November 28 2023 UT

November 10, 2023

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Dr. Donald F. Collins (AAVSO member), Dr. Robert Zavala (US Naval Observatory, Flagstaff Station), Jason Sanborn (Lowell Observatory), and Dr. Anatoly Miroshnichenko (University of N. Carolina, Greensboro) have requested high time-resolved photometry of the bright star b Persei during the upcoming predicted primary eclipse of the AB inner pair by the third star. They provide the following material:

Call to Observe Primary Eclipse in the Triple Star b Persei in late November 2023

The mid-eclipse prediction of b Persei is centered on November 28.3 ± .2 UT, 2023 (JD 2460276.8  ± .2)  The observations in V (and B if available) or green and blue if DSLR are desired for several hours each night for about 2 weeks: November 20 through December 4.  Long time-series are requested during the eclipse November 25 through November 30.  Out-of-eclipse observations are very important on account of the ellipsoidal variation of b Persei.  The out-of-eclipse light curve serves as a variable base-line for all observers to aid the separation of light levels of eclipse from the base curve.

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 no eclipses, but ellipsoidal variation: ~0.06 peak-to-peak magnitude variation centered on magnitude 4.6 (Stebbins 1923). The brightest star of the pair is spectral class A2V. Radial velocity studies (Hill et al. 1976) determined the existence of the third star that orbits the A-B pair with a period of about 702 days in a highly eccentric orbit.  AAVSO observers detected the first ever eclipse/transit involving the third star of the system in a January-February 2013 campaign (AAVSO Alert Notice 476). (See AAVSO Alert Notice 610 for the light curves of the first few campaigns).  

Since the detection of the first eclipse of b Persei in 2013, 9 AAVSO campaigns have observed time-resolved eclipses (AAVSO Alert Notices: 507 (Jan 2015), 537 (Feb 2016), 563 (Dec 2016), 610 (Jan 2018), 655 (Oct 2018), 688 (Dec 2019), 721 (Oct 2020),  761 (Dec 2021), and  791 (Sep 2022)).  Figure 1 shows the light curves from the four previous primary eclipse light curves (LC's).  The LC's  for eclipses of b Persei are much more complicated than those of binary star eclipses.  The A-B close-orbiting pairs' orbital plane closely matches the plane of the sky, while the C star's orbital plane (90 deg inclination) causes the C star to alternately block the some of the light of each the A and B stars.   The LC's  all exhibit several dips in brightness during the eclipses.  All the LC's  are different because the period of the third star's orbit is not commensurate with that of the inner stars.  Figure 2 indicates the predicted LC for the November 2023 eclipse.

The simulated light curves in Figures 1 and 2 were produced by calculating the net light flux for the three stars as the third star transits the close-orbiting inner A-B pair both for primary and secondary eclipses.  Reasonable values for the light fluxes, sizes, and separations of the three stars were assumed during all transits.  The mid-eclipse times are defined as the time when the center of the transiting third star passes closest to the center-of-mass of the AB pair.  The mid-eclipse times for each eclipse are searched that best match the simulated light curves to the observed data as judged by eye for any single eclipse.  The photometric period (704.64 ±.06 d) for the C component is determined by linear regression of the mid-eclipse times for the last four primary eclipses observed by AAVSO.  A similar calculation for the last primary eclipse (December 2021), using only three previous primary eclipse times, predicted a mid-eclipse date that was 0.21 d earlier than the observed mid-eclipse indicated in Figure 1.  A similar calculation based on four previous secondary eclipses had predicted a date that was within 0.44 d of the observed date for the September 2022 secondary eclipse.   The accuracy of the 704 d period of the third star has steadily improved as more eclipses were observed.

Observers are asked to obtain high-resolution time-series observations (CCD, or PEP) of long duration in V  (and B if available) 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 November 2023 eclipse (November 20- December 4). Out-of-eclipse data are needed to calibrate the various offsets expected from different observing systems.  DSLR users should submit their data from the green channel (TG).

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/CMOS 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 Robert Buchheim’s permission). Bright star photometry has a number of problems since the brightness of well-focused images have to be short.  Atmospheric scintillations become very large with short exposures, and are best averaged out with many exposures covering 30-60 sec total integration times.  Slight de-focusing helps to gather more signal before saturating CCD detectors, but de-focusing should not distort the star profiles out of a Gaussian-like profile.  The processing tools after acquisition from AAVSO database allow post image time-binning so observers do not need to perform the time-binning.  Continuous observations may be conducted throughout the night or as bursts of at least 10-20 observations every 30-60 minutes. Please try to follow the same protocol while observing before, during, and after the eclipse.  Photoelectric (PEP) observations are extremely valuable and support a much larger dynamic range than CCD's.

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

Spectroscopy will also be extremely valuable, especially during this primary eclipse in which the C star will block some of the light from the A and B stars at different times throughout the eclipse. We hope to see small changes in some parts of the spectra if the A or B stars are totally or partially eclipsed. It will be especially valuable if spectra obtained an eclipse of the A star that will reveal any rotational RV shifts of the A star.

Please send any spectroscopy observations to Dr. Anatoly Miroshnichenko <>. All spectroscopy data should be displayed with the following guidelines:

The FITS header should include: coordinates of the observing place, the observer name, RA & DEC of the star, UT time of the observation (better mid-point), exposure time in seconds. The intensities need to be in ADU (not normalized to the continuum) and wavelength scale needs to be geocentric.

If the standard cannot be met, Dr. Miroshnichenko would need specific explanations of the data-taking circumstances.

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).

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]

Hill, G., et. al., 1976, The Radio-flaring Triple System b Persei, Astrophysical Journal, 208: 152-164.

Stebbins, J. 1923, The Ellipsoidal Variable Star, b Persei, ApJ. LVII, 1-6.


Figure 1.  The four primary eclipses of b Persei that have been observed previous AAVSO observing campaigns.  The symbols and colors represent individual observations for different observers.  The dates of the mid-eclipses are indicated by the vertical arrows.  The dates of mid-eclipse for all the simulated curves are obtained by the best visual fit to the observations by determining be best date when the third star passes closest to the center of mass of the closely rotating pair.  The solid lines are the simulations described in the text.

Figure 2.  The expected light curve for the November 2023 eclipse using the parameters for the previous eclipses and extrapolating the date of mid-eclipse from the ephemeris.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material supplied by Dr. Collins.


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