[Iaude] CBET 5374: T CORONAE BOREALIS = NOVA CrB 1787

[quai at eps.harvard.edu]

                                                  Electronic Telegram No. 5374
Central Bureau for Astronomical Telegrams
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Prepared using the Tamkin Foundation Computer Network


T CORONAE BOREALIS = NOVA CORONAE BOREALIS 1787
     B. E. Schaefer, Louisiana State University, Baton Rouge, reports his
discovery of a long-lost nova eruption of the recurrent nova T CrB at visual
magnitude somewhat brighter than 7.8, based on unpublished letters and
catalogues of the English astronomer Rev. Francis Wollaston (F.R.S.,
Chislehurst, England).  Wollaston first mentioned the star in a letter to
Sir William Herschel, dated 1787 Dec. 28, complaining that the star position
that he measured shortly before writing the letter was not that of Herschel's
double star "V 75" (HD 143707).  Herschel's double star is 0.39 degree away
from Wollaston's measured position.  The new star position is R.A. = 239.95
+/- 0.11 deg, Decl. = +25.912 +/- 0.008 deg (equinox J2000.0), with T CrB
being inside this error box, and no other star is within or near this region
to faint magnitude levels.  Wollaston gives these coordinates in his catalogue
"A Specimen of a General Astronomical Catalogue Arranged in Zones of North
Polar Distance, and Adapted to Jan. 1, 1790" (sent to the printers in Oct.
1788).  Wollaston wrote that his speciality was astrometry of stars in CrB,
and that all his astrometric measures were based on at least four separate
measures with both large and small telescopes.  The limiting magnitude for
Wollaston's astrometry is near V = 7.8.  The strong connection from
Wollaston's star to T CrB comes from his accurate astrometry, where no other
star or event can confuse.  That T CrB appeared somewhat brighter than V = 7.8
demonstrates that T CrB must have been in eruption at Christmastime in 1787.
     Wollaston did not recognize his star as being a nova or any sort of a
transient.  To get confidence in the nova identification, we must consider
a variety of possible confusing artifacts:  (1) Wollaston's star cannot be
any minor planet or comet, because he made astrometric measures on several
nights and he would have seen any celestial motion.  Further, Wollaston was
an experienced comet-astrometry observer, and any comet would be recognized
by its coma in his telescopic views.  (2) Wollaston's star is unlikely to be
any sort of variable star other than T CrB itself, because the error region
and all around is empty of stars to deep magnitudes, and the many surveys
for variability, including Gaia, would have picked up any dwarf nova or
Mira-type variable in the area.  (3) Wollaston's star cannot be a supernova
because such should have left a bright, 237-year-old remnant.  Further, any
supernova peaking brighter than V = 7.8 would be roughly 2.7 Mpc distant and
impossibly far outside any host galaxy.  (4) The coordinates of Wollaston's
star cannot be some sort of observational error, bookkeeping mistake, or typo
because Wollaston performed accurate astrometry on at least four occasions.
The nearest star brighter than V = 7.8 is 0.39 degree away, and this is too
large for any observational error.  Further, the probability of any error
landing on the position of T CrB is too small to be acceptable.
     T CrB has its famous nova eruptions peaking in 1866.4 and 1946.1,
reaching visual magnitude 1.7 (Kamenchuk 1946, IAUC 1038), and there is strong
expectation that T CrB will again erupt within a few months, in 2024.4 +/-
0.3 yr.  With the newly-discovered eruption in 1787.9, the average recurrence
time is 78.8 years with an RMS scatter of 0.7 years, or 1 per century, with
the average being 78.8 +/- 0.4 years.  Full details are given in Schaefer
(2023, JHA 54, 436-455; see also URL https://arxic.org/abs/2308.13668).


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2024 CBAT
2024 March 28                    (CBET 5374)              Daniel W. E. Green