AAVSO CONFERENCE, OCTOBER 2005 - PART 2/4
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John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2005 November 27
Introduction
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The American Association of Variable Star Observers held its fall
convention in Newton, Massachusetts, on 13-15 October 2005. The
meeting was far too complex and lengthy to summarize in a single
article. I'm issuing a series of four articles to adequately treat the
material generated by the meeting. This is the second in the series, covering
talks and discussions about individual stars.
Polaris - discussion
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The North Star, alpha Ursae Minoris, is a delta Cephei star that
caused a sensation in the late 1980s. Its pulsations petered out! From
a modest 2/10 magnitude amplitude thruout the 20th century, it quickly
'turned off'. To the visual observer, the star became constant, with
no discernible change in brightness.
Detailed study showed it was still varying but only by about 2/100
magnitude, far too small to detect by visual techniques. It also
continued to pulsate weakly as revealed by radial velocity features in
its spectrum.
By 2002 it began to revive, very slowly increasing its amplitude
to a few hundredths magnitude.
On top of this there is confirmed a longterm increase in Polaris's
period by ~3-1/2 seconds/year, or about 6 minutes since 1900.
Polaris may have swelled over a couple millennia by a full
magnitude, from 3rd to 2nd. This is suggested by assessments of its
average brightness from Ptolemaeus thru the 19th and 20th century.
Polaris's amplitude may have always been small, so its variations were
not recognized until the early 20th century.
The distance to Polaris, as a delta Cephei star, was always of
critical importance. It is too far away for good parallax by
traditional methods. The HIPPARCOS spaceprobe put it no closer than
132 parsecs, 430 lightyears.
Polaris may be shifting on the HR diagram toward the red side of
the instability strip to eventually become a semiregular variable
star. Now it is at ab magn -3.4, spectral type F7, lum class I-II.
V725 Sagittarii - John Percy &a
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This star, in southern Sagittarius. was noticed by Henrietta Swope
in 1928. It was classed as a Cepheid of ~14d period. She studied it
until 1935 and found that it steadily increased its period from 14d to
22d. This was at the time an incredible behavior for a variable star.
The star was neglected in the late 1930s thru the 1960s, with only
scattered observations. Fitting these to extrapolated cycles showed
that the period continued to lengthen. Measurements back to 1889, on
Harvard plates, suggest the period was shorter in the prior decades.
By the mid 1960s it was 40d.
It was studied continually since the late 1960s. By the 1990s the
period grew to about 70d and is about 90d in the early 2000s.
The period also became erratic with time. By the 1970s it was no
longer possible to treat V725 Sgr as a Cepheid but more like a
semiregular star similar to R Scuti.
Spectra were first obtained in the 1960s, due to the low
declination and proximity of interfering brighter stars before them.
The spectrum drifted from F in the 1960s, to G in the 1970s, to M now.
The star is now considered a Pop II supergiant.
The star's amplitude increased gradually from 0.3 magnitude in the
1930s to 0.5 magnitude today. There were a couple instances of sudden
momentary fading, as if by veiling dust, as faint as 16th magnitude.
Its average magnitude increased from 13-1/2 to 12-1/2 magn over the
1930-1990 span.
This star is under rapid evolution out of the instability strip
back to the red giant region after one of its 'blue loop' excursions
as a giant star. The time scale of the changes agrees with theory but
this is the first time a star was observed in realtime to undergo such
radical alteration of behavior in the red giant phase of its life.
eta Aquilae - Doug West &a
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eta Aquilae, in southern Aquila, is a delta Cephei star observable
by eye or binoculars. In the past, it was studied by home astronomers
only in the optical band by visual techniques. Recently, modern
CCDgraphs allow home astronomers to collect infrared data about
variable stars.
The two bands easiest to employ are the J band at 1.25um and H
band at 1.65un. While these bands are passed thru by the atmosphere, a
dry climate is required for consistently good results. The width of
the bands passed to the telescope is sensitive to the moisture in the
air. It narrows with increasing moisture, constricting the influx of
infrared radiation and distorting the star's perceived brightness.
The tactic is to insert a narrow-band filter to exclude the outer
parts of the bandwidth. The ensures that the same range of wavelengths
is collected regardless of air moisture.
eta Aquilae is used as a calibration star because of its regular
and predictable behavior as a Cepheid and its proximity to the
celestial equator. The phase of the infrared cycle lags behind by
about 0.2 from that of the optical range.
CY Aquarii - Ron Zissell
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This star, of 11th magnitude in northeast Aquarius, is often
termed a 'dwarf Cepheid'. It is only about one solar mass, A1 to A2
spectrum, +2.5 absolute magnitude. It sits in the instability strip
near where it intersects the Main Sequence.
When discovered it had the shortest period of any known variable
star, 88 minutes. It is a favorite star for astronomy exercises in
photometry because a large fraction of its cycle can be observed
within one lab session.
During Zissell's 40-year study of this star, from the mid 1960s
thru mid 2000s, its period has slowly lengthened. Superimposed on this
trend is an oscillation which thwart fitting to a simple monotonic
curve.
One possible cause for the anomaly is an unseen companion star.
Such a star would exert Doppler motion in the visible star and add or
subtract light travel time to the variations. The estimated period of
the binary system is about 63 years, so the drift in light travel
approximates in timescale the 40-year record for CY Aquarii.
To analyze longterm trends in variable stars, the definition of
'time' is crucial. The plots showed a discontinuity at at 1972, when
atomic time was substituted for solar time. The newer measures were
clocked in UTC, which is based on a smaller second than the one for
old UT until 1972. The graph has an artificial break. Worse, the
second of solar time varied over he years as the time signals were
tampered with to keep the UT clock in step with the rotation of the
Earth. That is, in place of today's leapsecond, to accomplish this
end, the extra bit of time was added in slices to the second at
assorted instants thruout the year.
The other point in time definition is that the Julian Day Number
is not at all a system of timekeeping. JDN is the time scale for
variable star work to simplify calendar arithmetic. JDN merely is a
translation of the prevailing instant of UT and not and independent
scheme of clocking off time units.
Julian Day Number is broken at the UT-UTC boundary, too. As one
flaw in JDN, it can not deal with leapsecond. It maps the leapsecond,
as a fraction greater than unity, into the next Julian day. This
instant duplicates the first second of the next UTC day.
delta Cephei - Grant Foster
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This talk covered the general properties of delta Cephei stars as
demonstrated by 65 stars in the AAVSO database of visual observations.
delta Cephei stars on the whole over many decades, or a human
lifespan, have secular trend of increasing or decreasing period. The
accumulated shift is many minutes to an hour.
At first this seems to upset the period-luminosity relation but
the shift is only a minuscule fraction of the period while the
magnitude assessments are taken only to 1/10 magnitude. So there is a
leeway for the period within a given 1/10 magn zone.
Stars can suddenly switch to a shorter or longer period for
timescale of 500 to 1000 days, then they return to the former longterm
trend as if nothing happened. The switch could be large enough, if not
recognized, to cause error in applying the period-luminosity curve.
The profile of a delta Cephei star can be surprisingly closely
modeled by a three-line curve. The first line runs from a minimum to
the next maximum. Line #2 runs from maximum to about 2/3 maximum at
about 1/2 cycle. Line #3 completes the cycle to the next minimum and
is of slightly lesser slope than line #2. For some specimina of star,
lines #2 and #3 are of so similar slope that they become a single
line, making the model a two-line curve. This 3, or 2, line model fits
the Fourier description of the real profile.
delta Cephei stars typicly have a bump on the declining slope of
the cycle. Stars with periods less than 5 days have no bump. The bump
grows on the fall side of the cycle for periods between 5d and 25d.
At about 25d period the bump merges with the peak and broadens it.
Greater than 25d, the bump moves to the rise part of the cycle.
delta Cephei stars can have two simultaneous periods or overtones
of the main period. These beat against each other to strengthen or
weaken the profile, much like sound or water waves add or subtract. In
the case of Polaris, the beat nulled out the cycle to make the star
turn off its variations for about a decade.
One ongoing need for visual observations is for precise timings,
to the minute, of Cepheid star maxima or minima. This can be done with
uncalibrated CCD images taken across the peak or valley of the cycle.
Continuation
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This is the second of four articles about the AAVSO 2005 October
convention. The articles are named 'aavso05a.htm', '...b.htm',
'...c.htm', '...d.htm'.