Observations of the Spectroscopic Binary Y Cygni

This was a 2 week mini-project I had the opportunity to do while at Mt. Wilson in the summer of 2003 as part of the CUREA program. I had never done spectroscopy before so I took advantage of the expert help and equipment available.

I hope that one day I will fully understand the celestial mechanics of binary star systems, the basics of astrophysics, but for now I have been able to learn a few principles of what can be observed and what parameters can be determined.

Typical period - velocity plot of a binary star system on the left. 2 stars orbiting a common center of mass. In a spectroscopic binary system, the stars are too close to be resolved optically. Shifts in the spectral absorption lines can be measured and using the doppler relation (change in wavelength divided by original wavelength = v/c), the radial velocity can be calculated. Period and mass of the system can also be determined.

The challenge was finding a spectroscopic binary that could be observed and data reduced in 1 week! I discovered the astronomy database called 'SIMBAD' which was a huge help. It led me to the system Y Cygni, which has a published period of 2.9 days and was favorably positioned. We used the 16" Meade at f/10 and an SBIG ST8E camera with the SBIG spectrograph high resolution grating. A mercury vapor lamp is used to create a control spectrum. The observations were carried out over a period of 4 nights. The change in wavelength of the specific absorption line is plotted vs the number of days observed. Y Cygni is a spectral type B0IV subgiant (both stars). For myself, a complete newbie at spectroscopy, I was thrilled to be able to see a spectrum with any absorption lines at all! I was able to measure an actual shift in line position. The absorption lines were those of helium. We did not see a progression like that shown in the diagram above. Below are examples of the control spectrum and the actual observed stellar spectra. There is a software program that loads the spectrum and the control and is able to calibrate and calculate the line shift.

7724A

7384A

The emission spectrum above was the control mercury vapor spectrum with known emission line values as shown

The spectral line shift is quite obvious in the above 2 examples. These were 10 minute exposures. The estimated radial velocity was 105km/sec (actual was 51..oh well!) This was a lot of fun and hopefully I can do it again someday and get accurate results for all of the stellar parameters!

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