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 
 ------------------
    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 
 -----------------------------
    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
 ------------------------
    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 
 ----------------------
    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
 -------------------------
    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'.