Astronomy: Say hello to Algol's new companion star candidates

Date:
November 4, 2021
Source:
University of Helsinki
Summary:
Astronomers have analyzed observations of Algol. They argue that
Algol has many companion stars which have not been detected from
earlier observations.



FULL STORY ========================================================================== Algol is an eclipsing binary, where the two stars Algol A and Algol B
orbit around their common centre of mass. Their orbital period is 2.867
days. The abbreviation for this binary system is Algol AB.


========================================================================== Algol's primary eclipses occur when the dimmer Algol B partially covers
the brighter Algol A. These primary eclipses last ten hours, and they can
be observed with naked eye. Goodricke (1783) determined Algol's orbital
period from naked eye observations of these events. The primary eclipses
would be repeated regularly exactly after 2.867 days, if nothing disturbed
the motions of Algol AB binary system. All these future eclipses could
be computed from the multiples of the constant period 2.867 days.

The presence of a third member Algol C in this multiple star system was confirmed in late 1950s. Algol C and Algol AB orbit around their common
centre of mass. One round takes 1.86 years. The orbital motions of Algol
C and Algol AB cause a light travel time effect. We observe the primary eclipses earlier when Algol AB is closer to us, and later when Algol AB
is further away from us.

During every 1.86 years round, Algol C causes the same regular positive
and negative time shifts in the observed eclipse epochs of Algol AB. The
range of these time shifts is only about nine minutes. Due to these
time shifts, the observed eclipse epochs (O=Observed) differ from the
computed constant period eclipse epochs (C=Computed). These differences
are called the O-C data.

There may be even four or five new companions Lauri Jetsu analyses the
O-C data of Algol between November 1782 and October 2018. He applies his recently formulated Discrete Chi-square Method to these data. This method
is designed for detecting regular periodic signals. These detections
succeed even if the signals are superimposed on an irregular aperiodic
trend. From O-C data of Algol AB, the Discrete Chi-square Method can
detect the light travel time effect signals of five or six companion
star candidates. The O-C data alone can not be used to establish the
exact number of these candidates. One of these candidates is the known
"old" companion Algol C.

The orbital periods of the other four or five "new" companion star
candidates are between 20 and 219 years.

- These stars are candidates until new observations confirm their
existence, says Jetsu. He also shows that the periodic signals of these candidates can predict the observed Algol's O-C changes.

Why have these candidates not been detected earlier? Algol is so close
to us that we can observe its eclipses with naked eye. These new Algol's companion candidate stars would be literally on our backyard.

"The paradox is that Algol is 'too bright,' " says Jetsu. "Algol can hide
these new companion candidate stars even from our most powerful modern
space telescopes, just like our Sun can hide all other stars during
daytime," says Jetsu. He points out that for example the cutting-edge
equipment onboard the Gaia satellite could not detect these Algol's
new companion candidates. Jetsu argues that future interferometric
observations may be used to directly confirm the existence of at least
some of these new Algol's companion candidates.

========================================================================== Story Source: Materials provided by University_of_Helsinki. Original
written by Riitta-Leena Inki. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Lauri Jetsu. Say Hello to Algol's New Companion Candidates. The
Astrophysical Journal, 2021; 920 (2): 137 DOI:
10.3847/1538-4357/ac1351 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211104115259.htm

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