Sky & Telescope - March 2024 - 38

Variable Stars
1946
For decades after the 1866 eruption, T CrB was
observed to glow at around 10th magnitude, yet with
chaotic variability of around 0.5 magnitude or so. The
most persistent of the star's observers was Leslie
Peltier. He started monitoring T CrB in 1920 from his
home in Delphos, Ohio, convinced that the star would
erupt again. This was insightful, as the concept of a
recurrent nova was unknown at the time. In his book
Starlight Nights, Peltier says, " For more than twentyfive
years I looked in on it from night to night as it
tossed and turned in fitful slumber. " In 1945, Peltier
observed that the star was suddenly fading fast (what
we now call the pre-eruption dip) and had the insight
that this activity marked an imminent outburst. " Then,
one night in February 1946 it stirred, slowly opened
its eyes, then quickly threw aside the draperies of its
couch and rose! And where was I, its self-appointed
guardian on that once-in-a-lifetime night when it
awoke? I was asleep! Self-pity comes easy at 2:30 on
a cold February morning so I went back to my warm
bed with the comforting thought that I owed it to my
family, at least, to take care of my health. " The pride
of discovery instead went to N. F. H. Knight, a veteran
English variable-star observer. Peltier continued, " I
still have the feeling that T could have shown me more
consideration. We had been friends for many years;
on thousands of nights I had watched over it as it
slept and then, it arose in my hour of weakness as I
nodded at my post. I am still watching it [up to 1973]
but now it is with a wary eye. There is no warmth
between us anymore. "
the same script it did last time. And if it does, we should also
see a pre-eruption brightness dip - something astronomers
are anxiously watching for. Amateurs around the world are
reporting magnitude estimates on the website of the American
Association of Variable Star Observers (AAVSO), aavso.org.
Sure enough, the star finally began to fade in March 2023.
For the 1945-46 event, the pre-eruption dip started 1.1±0.3
years before the nova. If the current cycle follows the same
pattern, the timing of the 2023 dip implies an eruption date
of 2024.4±0.3! That means the star should brighten to prominence
again some time between January and September 2024.
In any case, we can be confident that T CrB will go into outburst
soon and that it's likely to happen sometime in 2024.
Mysteries and Big Questions
The upcoming eruption will provide a wonderful opportunity
to solve some of T CrB's deep mysteries, including the
causes of the high-state, pre-eruption dip, and the secondary
eruption. What powers those events, and where in the system
do they happen? Why is the timing the same each eruption
cycle? Why are these phenomena apparently unique to T CrB?
Also mysterious is the evolution of the T CrB system. A
fundamental problem is that we don't even know what powers
its very high accretion rate. Initially I expected that the
red giant was expanding normally and pushing more and
more gas in its upper atmosphere " over the edge " to fall onto
the white dwarf. But this mechanism is orders-of-magnitude
too small. So, what drives the T CrB system?
Another set of questions relates to whether T CrB will
eventually explode as a Type Ia supernova. This is a prominent
part of the supernova progenitor problem, a long-running
conundrum regarding the nature of the systems that
produce Type Ia supernovae - the variety used as standard
candles to measure cosmological distances. The underlying
binary pair consists of at least one white dwarf that's pushed
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1945
38 MARCH 2024 * SKY & TELESCOPE
Secondary
eruption
Fading at end
of high state
Pre-eruption dip
1955
1965
1975
Year
B or V magnitude
http://www.aavso.org
Sky & Telescope - March 2024

Table of Contents for the Digital Edition of Sky & Telescope - March 2024
