Cataclysmic Variables

Basic Information



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?
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.


   How do Cataclysmic Variables vary?
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 novae eruptions.


What are outbursts?

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.

What are Novae Eruptions?
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.

My Project

Overview

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.

Choosing Candidates

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 and 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.

Observing

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.


Low Resolution (Click to see some examples)
Cet Oct WY Cma
FQ Mon V591 Cen UY Pup
ZZ Lep Lib V888 Cen

High Resolution
RR Pic  
KQ Mon


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.

Determining classification

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.

High resolution data
The high resolution data is used to determine periods. For this data I fit a gaussian to the Hydrogen alpha peak 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. 


Results


In the tables below I list the classification that we determined for each CV and the periods
calculated for  KQ Mon and RR  Pic.

 


CV
Classification
Cet Not CV, B-type
FQ Mon CV, Dwarf Nova
ZZ Lep Not reduced
V591 Cen Too noisy
Oct CV, Dwarf Nova
UY Pup CV, Dwarf Nova
V888 Cen Old Nova
WY Cma Not CV
Lib CV, Dwarf Nova

CV
Period
Amplitude
Theta
RR PIC
3.773 Hours
5.0799 Angstroms
.37
KQ Mon
3.223 Hours
3.0999 Angstroms
.1