Forums / The Science / Spectroscopy / Newbee question: what is the spectrum telling us exactly?
Newbee question: what is the spectrum telling us exactly?
Hi!
Just a dumb question:
My understanding is that changes in the spectrum (e.g. shifting of lines to the red or blue end) are interpreted as hints to rotation of the eclipsing object.
I wonder: wouldn't we see some spectroscopic effects even if the eclipsing object was not rotating? The idea is that Epsilon Aurigae itself will be rotating, and unless the eclipsing object is in a retrograde orbit, it will first occult (on ingress) parts of Eps Aur that are moving towards Earth, so spectral lines should be thinning a bit, right?? After the "edge" of the eclipsing object has reached the center of Eps Aur, also parts of the super giant that are receeding relative to the observer are occulted, reversing the effect until 2nd contact. The same then repeats in reverse order after 3rd contact.
So....is this effect from occulting different regions of the the star observable (in theory/in real)? Is the rotation speed of Eps Aur known?
CS
Heinz
Hi!Hmm ... so if the Rossiter–McLaughlin effect is not observed with Eps Aur...is that telling us anything about the primary or the secondary?? Like an upper bound on the rotation speed of the primary?CUHeinz
Hi!Hmm ... so if the Rossiter–McLaughlin effect is not observed with Eps Aur...is that telling us anything about the primary or the secondary?? Like an upper bound on the rotation speed of the primary?CUHeinz
It might be there but it is quite possible that nobody has looked ;-) The rotation velocity, normally measured as v sin i as we just see the component in our direction, (If we see the star pole on, we would see no doppler effect due to rotation) can be estimated by comparing the width and shape of the spectral lines with a modelled line shape. I did a bit of a search and found that this has been done for eps Aur. (see table2 of this paper). http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1995ApJS...99..135A It does not appear to be a straightforward calculation though and macroturbulance can produce a similar effect. To measure the Rossiter–McLaughlin effect during eclipse, you would need to find lines which are uneffected by the eclipsing body. There are papers which mention lines which may meet this criterion (ones which need a high temperature than found in the eclipsing body) If the Rossiter–McLaughlin effect was detected it would confirm that something indeed passes in front of the star, the direction relative to the rotation of the primary and perhaps it might tell us something about the geometry. If it was not detected and calculations suggest we should have done then that would be much more interesting ;-) (Calculating the expected effect is way over my head unfortunately.) Robin
Hi Robin >(Calculating the expected effect is way over my head unfortunately.) Same for my head :-), but I guess it's safe to do an order of magnitude reality-check here. If I read that table in the referred to paper correctly, then v sin i ~= 45 km/s for eps Aur? If we assume the consensus model of the system, we are looking edge-on on this and I guess it's reasonable to assume sin i ~ 1.0. So I would think that to observe the effect we are talking about (very roughly) +-1 Å for wavelength resolution (e.g. for the 7699A KI line). Correct? Now, I'm not at all into spectroscopy so much but I think in terms of wavelength resolution this is not prohibitive and could even be within reach for modern amateur equipment? Given the fact that during eclipse half the light is blocked, one might think that enough of the surface of the primary is occulted to actually see the effect in the "shape" of the line if integrated over long enough a timespan...but again, it's more a fascination about spectroscopy and photometry that motivates these thoughts than knowledge. CS Heinz
Hello Heinz, The phenomenon you describe can indeed be observed in eclipsing binary systems, (I believe it has a name which escapes me) but I do not think I have seen it mentioned in connection with eps Aur. edit: It isknown asthe Rossiter–McLaughlin effect The effect is different in nature to what is observed in practise with eps Aur. What you would see with this phenomenon is a change in the shape of the stellar lines (Out of eclipse the line would be broadened by the star rotation. The shape would then be slightly modified during the eclipse, first on one edge during ingress and then on the other during egress.) In the case of eps Aur the changes in the spectrum during eclipse are additional absorption components from the secondary superimposed on the stellar spectrum which move from red to blue as the eclipse progresses, rather than a modification of the shape of the existing lines produced by the primary. You can see this most clearly in the 7699A KI line I have been following where a new component is seen to emerge quite separate from the line that is there outside eclipse. (In this case it is even possible that the line seen outside eclipse does not come from the primary at all but from interstellar material between us an eps Aur) Best Regards Robin