WWhat are Cataclysmic Variables?
|Most simply stated, a Cataclysmic
Variable is a two star system in which the stars orbit closely
enough together that they exchange stellar material. CVs are composed
of a white dwarf and a red dwarf. A white dwarf is the burnt out core
of a star nearing the end of its life. It is a small, extremely dense,
bright star. On the other hand a red dwarf is a smaller version of the
sun which is much larger, less massive, and burns less brightly than
the white dwarf. The clock-like orbiting and exchange of material
between stars cause dramatic changes in brightness which have attracted
astronomers attention for years. Four decades ago Robert Craft
established the current model of Cataclysmic Variables (CVs) and since
then much work has gone into classifying various types of CVs.
How is material exchanged between
the two stars?
Cataclysmic Variables vary?
|Solitary stars are spherical pulled
together in the smallest configuration by gravity. In a binary star
system the gravity exerted by each star can affect the other. If the
binary stars are separated by a great distance gravity does not have an
affect on the shape of the planets, but for close binaries, gravity can
change the shape of the stars. In CVs the red dwarf orbits the white
dwarf closely enough that the gravitational pull of the white dwarf is
strong enough to elongate the red dwarf. Gravity pulls the lighter
outer layers of the red dwarf towards the white dwarf and eventually
the gravitational attraction of the white dwarf on this material
exceeds the pull from the red dwarf. This material then falls towards
the white dwarf and creates an accretion disk around the white dwarf.
This effect is known as a tidal force and has consequences on earth
such as the tides in the oceans.
|The main source of brightness
variability comes from the orbital period. The red dwarf orbits the
white dwarf every one to twelve hours. The white dwarf
outshines the red dwarf and thus every time the red dwarf passes
between the earth and the white dwarf the brightness of the CV drops.
There is also a bright spot associated with the point where material
flowing from the red dwarf contacts the disk surrounding the white
dwarf. CVs have the greatest variability in their outbursts and
What are outbursts?
What are Novae Eruptions?
|Outbursts are semi regular events
when the brightness of many CVs increases many magnitudes in the span
of a day and remains bright for the span of about a week. Outbursts are
believed to occur from instabililities in the accretion disk due to
transfer rates of material from the red dwarf being faster than the
accretion disc can transport smoothly. Through a complex process this
material is eventually accreted onto the white dwarf increasing the
brightness of the system and draining the disc of material. The
brightness of the CV drops again until the accretion disc is rebuilt
and becomes unstable again.
|The most cataclysmic event in a
CV's life is a novae eruption. Novae eruptions cause the CV's
brightness to increase 8-15 magnitudes compared to the 3-5 magnitudes
associated with outbursts. These occur when the pressure and
temperature on white dwarf's surface explodes in a nuclear chain
reaction like a nuclear bomb. It is just like a hydrogen bomb with
thirty times the mass of the earth.
|I worked with Dr. Alan Whiting from
CTIO and Dr. Linda Schmidtobreik from ESO to find new CVs by taking
spectrum of candidate stars and identify period information through
high resolution spectrum and optical photometry. The project was
basically divided into three stages: Choosing candidates, taking the
data, and reducing and analyzing the data.
|The first part of my project
consisted of choosing which stars we would look at for our
observing time on the telescopes. We had time on the 1.5m and .9m
telescopes at CTIO. The 1.5m telescope is used for taking spectra of
stars while the .9m is used for optical imaging. I was responsible for
choosing candidate CVs. These are stars that star surveys or people
have noticed may be changing in magnitude and thus may be CVs. The
easiest way to identify it as a CV is to take a spectra of a candidate
see if it has the characteristics of CVs. I used the Catalog and Atlas of Cataclysmic
Variables to find all of the candidates that would be
visible on our observing nights. I then narrowed this list down based
on the brightness of the candidates and by examining the previous
research on the candidates. Linda worked on picking objects
for the high resolution spectra and optical imaging. These objects were
known CVs but did not have any period information.
Resolution (Click to see some examples)
|The REU program observed for 12
nights, 6 nights on each telescope, from Feb. 5th to Feb. 14th. I
observed for two nights on each telescope. We obtained spectra of 11
different CVs. The table below lists the CVs that we took spectra of.
We observed 9 CVs at low resolution which allows us to observe a wider
range of wavelengths and high resolution to focus on the Hydrogen alpha
spectral line and see slight changes in it.
Reducing and Analyzing Data
|Once the data was taken I used
standard Iraf procedures to reduce the data. The low resolution
candidates were flux calibrated using standard stars HR3544 and HR1544
depending on which night they were taken. The low resolution relative
flux calibrated spectra can be seen here.
Click here for some high
resolution images of RR Pic and KQ Mon.
High resolution data
|There is no single feature in a CV
spectra that determines its classification. Instead the whole spectrum
needs to be examined and compared to other known CVs of various types.
In general CVs are hot objects so the peak of their spectrum should be
in the blue wavelengths and they should have some emission lines
especially in the Hydrogen Balmer series and Helium I series. After
looking at these characteristics it is necessary to look at other CV
spectrum and see how they compare.
|The high resolution data is
determine periods. For this data I fit a gaussian to the Hydrogen alpha
and recorded the center position of the peak. Then I plotted all the
peak positions versus time to see how the peak changes. The peak
changes position due to ............. This allows us to determine the
period of rotation. Then we analyzed the data using the pdm package in
iraf to determine the significant periods.
In the tables below I list the
classification that we determined for each CV and the periods
calculated for KQ Mon and RR Pic.
||Not CV, B-type
||CV, Dwarf Nova
||CV, Dwarf Nova
||CV, Dwarf Nova
||CV, Dwarf Nova