BW Vul, a pulsating star

BW Vul is a variable star whose brightness changes every 4h50min. Spectroscopic study such as the one I did last night shows that the atmospheric absorption lines shift over the same period. This is due to the star pulsating radially, same as other beta Cephei - and BW Vul is the largest amplitude beta Cep stars ever observed.

The following graphs shows two spectra taken at different times, around sodium doublet:

 When you look over time, one can see that the two sodium lines are fixed (as well as several other ones). The small ones are absorption lines from our own atmosphere (vapor lines), and are of course stable (ie: no radial velocity) over the 6h observing time. The two sodium are the signature of interstellar sodium material which resides somewhere between us and the star, they are also not changing velocities relative to us.

The large absorption line around 5876 Angstroems is helium in BW Vul atmosphere which first shifts toward the red (ie: toward the right of the graph) as the stellar atmosphere compresses itself... up to a point when it bounces back - and does it twice with line doubling phenomenon, due to shocks inside this atmosphere:

 The following graph shows the evolution of the spectrum (dispersion = horizontal axis) over time (vertical axis, time going up when you go up), one can clearly see the star pulsating over the observing time:

You may notice a small absorption line between the two sodium lines; this is nickel which also moves (as the atmosphere moves) but as the temperature of the BW Vul atmosphere changes due to the compression (density) changing, there are moment when the line is not visible; nickel atoms are always there, but temperature is too high for the transition (ie: absorption) to take place.

BW Vul spectra acquisition (PRISM v10)

Night picture taken during spectra acquisition (see previous blog post)