We put the biological implications of observed increases in stellar UV during
flares in perspective by pointing out that the quiescent UV outputs of dMe stars
are very low compared with the Sun. Using data from Worden et al. (1984), YZ
Canis Minoris (Teff = 3097 K) has a total luminosity around 0.01 L⊙ , but in the
U-bandpass it is not quite 2.0 × 10−4 as luminous as the Sun. For a planet receiving
Ie (0.10 AU), the U-bandpass output of YZ Cmi would have to rise by a factor of
50 (4.2 stellar magnitudes) for incident UV-A to match that arriving continuously
at the top of the Earth’s atmosphere from the quiescent Sun. UV Ceti with less
than 3.0 × 10−3 L⊙ (Leggett et al., 1997) has a quiescent U-bandpass output some
1.1 × 10−5 that of out Sun (Lacy et al., 1976). A U-bandpass upsurge of over 260
times (>6.0 magnitudes) would be necessary before incident U-bandpass radiation
matched continuous Earth values at the Ie radius of 0.05 AU). Moreover, even
when flare UV exceeds the latter, it may be maintained for only a short time. UV
Ceti flare stars, as a typical example (described by Mochnacki and Zirin, 1980),
generally rise to a peak within 10 s and declined substantially within about 20 sec).
The quiescent U-bandpass component of insolation from AD Leonis (Teff = 3400
K), is just 0.03 solar at the Ie radius (0.15 AU). The ‘Great Flare’ of April 12,
1985 exceeded 4.6 magnitudes (an increase by a factor of >60) in the U-bandpass,
so UV-A at the Ie radius would have climbed to the continuous Earth level within
about 8 min, peaked sharply at 1.8 times the Earth level at 10 min, and fallen
below Earth level after 18 min.
