|
|
|
|
Variable Star Of The Month
July, 2002: CI Cygni
CI Cygni's Days of Discovery
 | | The Henry Draper (HD) Catalogue, named for the first photographer of stellar
spectra, is a nine volume publication (released from 1918 through 1924) that contains the positions, though
not accurate, and stellar spectra for most stars down to about 8th magnitude. This catalog is most often
used at present for its valuable spectral information, which Annie Jump Cannon classified for an impressive
225,300 stars. |
Like many variable stars, CI Cygni was discovered in the early
1900s, in an era of a virtual variable star awakening when the differing classes of such stars were yet to
be established. Evidence of CI Cyg's variable behavior was first recognized by Annie Jump Cannon in 1922
when examining Harvard spectrum plates during her work with the Henry Draper Catalogue. The star, which
revealed a peculiar spectrum combining two distinct types of spectra, could not be classified with any of
the known types of the time. The unusual object was announced to the astronomical world by Harlow Shapley in
1922, whereby he noted a similarity to the spectral characteristics of recurrent nova RS Ophiuchi (Shapley 1922). Further investigation into the nature
of the star ensued when Cannon examined more than 300 Harvard photographs taken between 1890 and 1922, where
it was then categorized as an irregular variable. Still intrigued by the unique characteristics, Naomi
Greenstein later analyzed Harvard photographic plates of the variable for the interval of 1890-1936. During
her study she noticed that while CI Cyg varied irregularly between photographic magnitudes 12 and 13, there
was a periodicity of about 900 days between successive minima. Also of interest, was a remarkable rise in
photographic magnitude 12.1 to 10.7 that occurred in 1911 and lasted for about 200 days. Features of the
light curves and oddities in spectra led Greenstein to suggest similarities between CI Cyg and another
unusual star, Z And (Greenstein 1937). Z And was later
classified to be the prototype of the symbiotic class of stars -- a term coined by Paul Merrill in the 1940s
to describe the class of stars with cool M-type photospheric absorption lines and very high temperature
emission lines -- with CI Cyg as part of the symbiotic family.  | | Maria Mitchell (1818-1889) was one of the most famous American scientists of
the 19th century. Among the many awards and honors bestowed upon her, she was the first female professor of
astronomy in the United States. Mitchell is pictured here, just left of center, with a few of the many
female students she fostered in astronomy. Image: AAVSO archives. |
Interest in CI
Cyg arose again in the 1960s when prompted by CI Cyg's close proximity to Cygnus X-1 and the curiosity
enshrouded with its possible association, which has now been refuted. By studying the Maria Mitchell
Observatory photographic plates from 1916 to 1967, Dorrit Hoffleit compiled a composite light curve, fitting
an 855 day period to its variations, thereby confirming the report by Balfour Whitney of the University of
Oklahoma that the light curve of CI Cyg resembled that of an eclipsing binary star with a period of 855 days
(Hoffleit 1968), following the ephemeris: JD MIN = 2411902 + 855d.25 E (see, for instance, Mattei 1981 and
Mikolajewska 1985). Furthermore, she also noted yet another departure from the mean curve, similar to that
found by Greenstein, when a nova-like outburst occurred in May of 1937 and the star reached photographic
magnitude 10.2 (Hoffleit 1968). This interesting variable was introduced to the AAVSO by well-known
astronomer, AAVSO past president, and good friend to the association, Dorrit Hoffleit. CI Cyg joined the
ranks of the AAVSO during its 1971 outburst, when alerted of its activity by long-time member, observer, and
past AAVSO president Wayne Lowder who had been observing this object a few years prior to its inclusion in
the AAVSO observing program. Since that time, over 250 observers have adopted CI Cyg into their observing
regiment and have contributed over 17,000 observations of this variable to the AAVSO International Database! Symbiotics and CI Cygni
Symbiotic stars are a group of relatively rare and interesting variables, with about 150 known members in
the class. The objects are believed to be interactive close binary system composed of a late-type giant and
a hot star, imbedded in a web of nebulosity. The "hot" component of the system may be a main sequence or
white dwarf star surrounded by an accretion disk, or it could be a compact star similar in nature to the
central component of a planetary nebula (Mikolajewska 1985). Spectroscopically, in the visual part of the
electromagnetic spectrum, symbiotics show a hybrid spectrum punctuated with low temperature absorption lines
contributed by the cool star and high-temperature emission lines contributed by the excited nebula. In
addition to the spectral features, symbiotic stars characterized by semi-periodic, nova-like rises in
brightness of up to 3 magnitudes. Additionally, they generally have orbital periods of about 2-3 years. (For
more detailed information about the symbiotic class of variables, see past VSOTM features, CH Cygni and Z
Andromedae, and references therein.) The symbiotic star CI Cyg is believed to consist of a normal M5
II star as the cool member, while the hot component is thought to be a main sequence star surrounded by an
accretion disk. Symbiotic variables like CI Cyg can serve as an astronomical laboratory for studying several
different physical processes that arise in these systems. Following, Mikolajewska (1992) answers the
question as to why it is important to study such symbiotic systems: There are
many reasons. First as in all binary systems, they provide direct information about the fundamental
parameters involved, such as masses and radii, that are essential to understanding stellar evolution.
Second, they interact. Interactions between stars in a binary system are of great interest in astrophysics
today, and symbiotics give us an excellent opportunity to study these processes under the very extreme
physical conditions found in these systems. Thus, symbiotics tell us about such basic physical processes as:
mass loss from red giants, accretion onto compact stars (main sequence stars, white dwarfs, neutron stars)
and the evolution of nova-like eruptions in very wide binary systems, and radiative processes in gaseous
nebulae. Third, the symbiotics are among the binaries with the longest known periods. Thus, each component
has enough time to fulfill its evolutionary destiny before their interaction begins and a symbiotic star is
born. The variety of possible combinations giving rise to the symbiotic phenomenon, combined with knowledge
of masses and radii of the stars involved, make symbiotics very important for understanding the last stages
of stellar evolution. Finally, last but not least, symbiotic stars simply do exist and it is natural that we
try to learn as much as possible about them. For instance, according to Kenyon et al.
(1991) and references therein, models of the evolution of binary stars suggest that systems like CI Cyg
evolve into a common envelope system, such that the red giant core and main sequence star share an extended
atmosphere that is ejected by the binary. The next phase is poorly understood, but it is theorized that
cataclysmic binaries may be the result. "CI Cyg lies at the brink of a luminous common envelope phase and
may provide important information concerning the formation of short period interacting binary systems in the
next few decades" (Kenyon et al. 1991). Thus, it is of great importance that observers continue to monitor
CI Cyg, other symbiotics, and even other types of variables so that we may have continuous detailed records
of various light curves that can be correlated with other types of observational data and theories.
Seeing the Light  |
| The light curve of CI Cygni from 1916-1967 as compiled by
Dorrit Hoffleit using Maria Mitchell Observatory plates. Click imgae to enlarge. |
The light curve of CI Cyg reveals two distinct types of variable behavior: that of an eclipsing binary and
that of an eruptive star. Eclipses: According to Whitney, as mentioned above, the eclipses are
seen to take place every 855 days. The duration of the events typically last 100 to 200 days, while the
amplitude has been seen to vary as little as 0.3 magnitudes to as much as 2.5 magnitudes. During the
eclipse, B-V and U-B become redder, suggesting that a late-type star is eclipsing a hotter component (Kenyon
1983). The period and duration of eclipse totality allow astronomers to estimate the radius of the eclipsed
object. In this case, it is about 5x1012 cm (Kenyon 1983).  |
| The 35-year AAVSO light curve of CI Cygni from 1967 to
2002. Click image to enlarge. |
Eruptions: There have been 5 recorded
outbursts in the recorded history of CI Cyg, which were seen in 1911, 1937, 1971, 1973, and 1975. Such
events are thought to be the result of increased mass transfer from the giant or by instabilities in the
accretion disk itself (Kenyon et al. 1991). It is interesting to note that virtually none of the outbursts
have exhibited the same behavior. The most distinctive episode in CI Cyg's observable light curve occurred
in the 1970s when a larger outburst followed two smaller outbursts. The activity observed during this era
provided astronomers the first opportunity to look at CI Cyg in a different "light." The early phases of the
1975 outburst were observed in the optical and infrared wavelengths, while the later stages of decline were
also covered in the ultraviolet (Kenyon et al. 1991). The importance of multiwavelength campaigns is that it
gives astronomers an opportunity to investigate a system in a wide range of wavelengths. This is extremely
helpful to the theorists trying to piece the physics of the system together. Since the 1970s, CI Cyg has
been fairly quiet in terms of outbursts, so it will be interesting to see the behavior of the next potential
bout of nova-like activity! Observe CI Cygni
 |
| The constellation of Cygnus is one of the most recognizable
in the sky. Click on the above image for an enlarged view with the approximate location of CI Cyg indicated.
Image: Akira Fujii, DMI. |
Observers
perusing through the constellation of Cygnus may wish to stop and take a look at this interesting symbiotic
star. Located just over a degree northwest of the 3rd magnitude star Eta Cyg, observers will find that this
variable may change in brightness from about 9th to 12th magnitude at its extremes. Identification of the CI
Cyg field can be made with the 'a' scale chi Cyg
chart, while the 'e' scale chart of CI Cyg
should be used to make brightness estimates of the variable throughout its range of variability. CI Cyg,
as well as other symbiotic stars, should be observed once a week, with the decimal portion of the Julian
Date recorded to one decimal place. For those interested in a more challenging project, observers may choose
to look for possible small-magnitude, short-period variability sometimes associated with the symbiotic
variables. If you are interested in looking for such features, then be sure to observe every clear night and
report the decimal portion of the Julian Date to 4 decimal places. Observations may then be submitted to the AAVSO where they will contribute
to the long-term light curve of CI Cyg for present and future generations of astronomers to study and ponder
over!
- AAVSO 'a' scale chart for 1946+32 Chi
Cyg (for the identification of CI Cyg) and the 'e' scale chart for 1946+35 CI Cyg
- AAVSO constellation
finder chart for Cygnus
- AAVSO VSOTM for August 2000: CH Cygni
- AAVSO VSOTM for October 2000: Z Andromedae
- Greenstein,
N.K. "Light Curves for Three Variables with Peculiar Spectra." Bulletin of the Harvard College
Observatory, 906, 1937, 3-6.
- Hoffleit, D. "CI Cygni and a Similar New Variable Star." The
Irish Astronomical Journal, 8, 1968, 149-152.
- Kenyon, S.J., N.A. Oliversen, J. Mikolajewska,
and M. Mikolajewski, R.E. Stencel, M.R. Garcia, and C.M. Anderson. "On the Nature of the
Symbiotic Binary CI Cygni." The Astrophysical Journal, 101, 1991, 637-654.
- Kenyon, S.J.
The Collected History of the Symbiotic Stars. Urbana: University of Illinois, Department of
Astronomy, 1983.
- Mattei, J.A. "The Symbiotic Star
CI Cygni." Journal of the AAVSO, 10, 1981, 92-95.
- Mikolajewska, J. "On the Nature of CI
Cygni." Acta Astromica, 35, 1985, 65-94.
- Mikolajewska, J. "Symbiotic Variable Stars." In
Percy, J.R., J.A. Mattei, and C. Sterken eds., Variable Star Research: An International
Perspective. New York: Cambridge UP, 1992, 267-278.
- Shapley, H., Harvard College
Observatory Bulletin, 778, 1922.
This month's Variable Star of the
Month was prepared by Kerri Malatesta, AAVSO Technical Assistant.
|
