Up until around 1980, the Stingray was an ordinary B1 post-AGB star, but then
it suddenly sprouted bright emission lines like in a planetary nebula (PN), and
soon after this the Hubble Space Telescope (HST) discovered a small PN around
the star, so apparently we have caught a star in the act of ionizing a PN. We
report here on a well-sampled light curve from 1889 to 2015, with unique
coverage of the prior century plus the entire duration of the PN formation plus
three decades of its aftermath. Surprisingly, the star anticipated the 1980's
ionization event by declining from B=10.30 in 1889 to B=10.76 in 1980. Starting
in 1980, the central star faded fast, at a rate of 0.20 mag/year, reaching
B=14.64 in 1996. This fast fading is apparently caused by the central star
shrinking in size. From 1994-2015, the V-band light curve is almost entirely
from the flux of two bright [OIII] emission lines from the unresolved nebula,
and it shows a consistent decline at a rate of 0.090 mag/year. This steady
fading (also seen in the radio and infrared) has a time scale equal to that
expected for ordinary recombination within the nebula, immediately after a
short-duration ionizing event in the 1980s. We are providing the first direct
measure of the rapidly changing luminosity of the central star on both sides of
a presumed thermal pulse in 1980, with this providing a strong and critical set
of constraints, and these are found to sharply disagree with theoretical models
of PN evolution.Comment: ApJ accepted, 54 pages, 4 figures, one long data tabl
