27 December 2015 at 2:16 pm #573513Andy WilsonKeymaster
This may be a little stretch for the variable star forum as Kappa Cassiopeia is a star that varies spectroscopically rather than showing any significant variation in brightness. It is one of my favourite stars, a hot luminous supergiant (spectral type B1Ia) that is losing mass by a strong stellar wind. Here are a couple of high resolution spectra of the hydrogen alpha emission line taken 2 months apart, in October and then December. Note the H-Alpha line is not from the star itself but the strong stellar wind.
In the first graph I’ve applied a heliocentric correction to the spectra, while leaving in the telluric lines from the Earth’s atmosphere. You can see that the telluric lines are shifted. This is due to the heliocentric correction, mainly correcting for the speed of the Earth in its orbit and a smaller correction for the Earth’s rotation. This is one of my first attempts at this, but I am confident that it has worked since the interstellar absorption band at 6613 Angstroms line up exactly.
In the second graph I’ve removed the telluric lines using Christian Buil’s ISIS software package. The removal of the telluric lines is a bit imprecise but I think it has worked well. I’ve also shown this as a velocity plot centred on H-Alpha, and corrected for the radial velocity of kap Cas. This shows hydrogen gas that is moving away from the star and towards the Earth as a negative velocity, while hydrogen gas that is moving away from both the star and the Earth shows a positive velocity. I’ve left in the interstellar absorption band at 6613 Angstroms to give confidence to this comparison since it lines up almost exactly between the 2 spectra. Note the spectra are not flux calibrated so the relative strength of the H-Alpha emission should be treated with caution.
The spectra clearly show a change in the H-Alpha line profile with a shift in the peak emission of around 70 km/s away from the Earth. There is also a change in the emission toward the Earth with an “emission foot” from 120-400 km/s being replaced by what appears to be a slight P Cygni style absorption feature.
I plan to continue monitoring this star, and I don’t think any of this behaviour is unusual for kap Cas. The main reason why I like the star!
Attachments:27 December 2015 at 4:55 pm #577188
Excellent report Andy, must checkout Kappa Cass next obs run!. Don`t think I would be able to detect any shift with my Alpy 600 but worth a shot 😉
Steve27 December 2015 at 5:23 pm #577189Andy WilsonKeymaster
Thanks Steve, it was nice to actually have clear skies so I could do some astronomy on the 23rd December. A rare occurrence recently! There is also gamma Cas nearby which is another bright star with interesting changes in H-Alpha.
Andy27 December 2015 at 6:35 pm #577191
Know exactly what you mean Andy! Only clear skies I`ve had lately were last night at 3am but I was doing my Gas emergency thing with the floods in York, maybe another night 😉
Steve27 December 2015 at 6:36 pm #577190
Know exactly what you mean Andy!
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