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Model-based Interpretation of Cepheid spectra

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k.gurney's picture
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Model-based Interpretation of Cepheid spectra

I gathered some high res spectra over the summer of V1334 Cyg - a  Cepheid variable with period 3.3 days. Looking carefully at the Ha minima, I spotted a blue shift which varied across observations. I thought I might be able to see some radial velocity component related to the pulsations of this star. Serendiptously, a paper appeared last year  which modelled V1334 and showed how, in this multiple (triple) system, radial velocities could be extracted for orbital and pulsation components...

Find attached an analysis of my data using this model.

Kevin

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Robin Leadbeater's picture
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RV photometry phase difference

Nice work detecting pulsations at this RV level with the LHIRES and separating them out from the orbital component (I was slightly worried when I saw your RV and light curves in phase as The RV curve in Cepheids is normally approximately antiphase but then I noticed your light curve  Y is plotted in increasing magnitude)

RV measurements can be tricky with the H alpha line because of its width.   Narrow metal lines are easier, particularly if you can measure a few of them but then in the absence of telluric lines, a stable reference is difficult to arrange. I used two Si II lines with lamp lines superimposed on the spectrum during the exposure to follow pulsations in Deneb for example. 

https://britastro.org/video/13862/14769

at 18:30 

which suggested a long term sigma precision of 0.5km/s  is possible

Cheers

Robin

k.gurney's picture
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Pushing the limits

Hi Robin

Thanks for reminding me about this talk - I may have been in the audience!

I'll have a look at the 'lamp in front of the scope' possibility... Is it readily available?

Kevin

Robin Leadbeater's picture
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residual offset

I wondered if the residual  +1km/s offset in the fitted curve in fig 3 could be due to the mean motion of the system but it is catalogued as being in the other direction at -1.8km/s (or is this already included in the correction in table 1?)

Cheers

Robin

k.gurney's picture
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Residual offset

The 'systemic velocity' is included in the orbital component in the model so, theoretically, I have already subtracted it. I was happy just to get something that was roughly sinusoidal and not too bady offset from zero!

Kevin

Robin Leadbeater's picture
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Impressively close

Yes in that case impressively close !

I have to confess the idea of using the superimposed lamp lines came from Christian Buil,the objective being to solve the issue of shifts due mechanical flexure in the LHIRES.

http://www.astrosurf.com/buil/isis/He_calibration/method.htm

about half way down (In French)

Those particular "Filly" lamps are not around any more but you would only need one reference line to correct the offset in the dispersion equation measured using the internal LHIRES lamp as you did using the water line so chances are even a neon lamp would do.  The intensity of the lamp line(s) has to be adjusted to match the exposure needed for the star (not a problem in this case with bright Deneb) eg by adding some filtering material in front or perhaps in a more sophisticated setup, by electronically  pulsing it on and off. 

Robin