Transcript The Probability and Effects of an Asteroid Impact with Earth

Visual Observations
of Delta Cephei: Time
to Update the Finder
Chart
David Turner
Saint Mary’s University
d Cephei has a long history as a
variable star, its variability being
discovered by John Goodricke in
October 1784. His friend Edward
Piggot had discovered the
variability of the Cepheid  Aquilae
a month earlier in September 1784.
d Cephei
 Aquilae
Polaris — the
brightest of 40
classical Cepheids
visible without
optical aids.
The ready accessibility of δ Cephei for
amateur or student observations has
been pointed out in the literature from
time to time, e.g. van de Kamp, Sky &
Telescope, 12, 208, 1953.
Recent article on δ Cephei on the AAVSO website.
AAVSO finder chart with ζ Cep and ε Cep as
reference stars.
A more detailed comparison chart for δ
Cephei from the AAVSO. Problem: ζ is
clearly brighter than ι to the eye, and closer
to β in brightness.
Specifically: 10 years of regular observation by
two AAVSO observers (AAP, SSW) relative to the
photoelectric light curve. Visual observations do
not match the amplitude or extrema of δ Cep.
Alternate method (Turner JRASC, 93, 228, 1999).
Alternate comparison chart for δ Cephei tied to
photoelectric V-magnitudes from the Bright Star
Catalogue. At faint limits the eye can distinguish
differences of 0m.1 or better (Turner 2000).
See Turner (JAAVSO, 28, 116, 2000).
Cassiopeia
Delta Cephei
comparison
chart
3.52
d Cephei
Cepheus
4.29
3.35
4.19
3.77
Lacerta
An accurate ephemeris for a Cepheid is
essential for proper phasing of the
observations. For δ Cephei:
HJDmax = 2442756.49 + 5d.36627 E
First observations of δ Cephei tied to
the alternate comparison chart.
Phased photoelectric observations of δ
Cephei from the literature.
A comparison of eye V magnitudes
with the photoelectric V light curve.
Note the 0m.15 offset.
A similar comparison with eye
estimates by the author for η Aquilae.
There is no offset here.
Ditto for ζ Geminorum.
A combination of observations from 1999-2000
with every five data points averaged in phase.
The results are comparable to photoelectric data.
Additional observations of δ Cephei
from 2009.
... and from 2010.
David
Turner
John
Goodricke
A study of the evolutionary period decrease in δ
Cephei from 230 years of observation.
The problem lies in the adopted reference
magnitudes for the two main comparison
stars: ζ Cep and ε Cep. van de Kamp (1953)
lists values of 3.60 and 4.36 for the stars from
Potsdam Durchmusterung photovisual pv
magnitudes. Their photoelectric V magnitudes
are 3.35 and 4.19, respectively, while AAVSO
chart values are 3.6 and 4.2. Since all good eye
estimates of δ Cephei are made under normal
light conditions, the use of AAVSO charts
artificially depresses the observed light
amplitude and skews light maximum from its
true brightness.
Star
ζ Cep
ε Cep
ι Cep
α Cep
υ Cep
α Lac
V
3.35
4.19
3.52
2.44
4.29
3.77
BV
SpT
1.57 K1.5 Ib
0.28 F0 IV
1.05 K0 III
0.22 A7 V
0.52 A2 Ia
0.01 A1 V
Vtrans
3.60
4.21
3.68
2.45
4.35
3.74
Vchart
3.6
4.2
3.5
2.4
...
...
The study by Richard Stanton (JAAVSO, 10, 1,
1981) relating pe V magnitudes to visual
magnitudes on AAVSO charts. He derived the
relationship: mvis = V + 0.182 (BV)  0.15
Ron Zissel’s (1998) study of the visual magnitude
system adopted the same formula… for
observations with the dark-adapted eye.
The wavelength dependence of the photoelectric
V filter relative to that for the dark-adapted eye.
But the eye’s spectral response for normal
lighting conditions is much closer to the
wavelength sensitivity of the Johnson V filter.
Since most observations of d Cephei are
done in normal lighting conditions, use
of a comparison chart with star
magnitudes adjusted for dark adaption
is inappropriate. Observations near the
visual limit are also more accurate than
those made of bright stars using
telescopes, so the ideal observations of δ
Cephei are made by eye or binoculars
before the eye becomes dark adapted.