Pulsating or Eclipsing???

Unione Astrofili Italiani (Italy) (UAI)
Fri, 06/24/2022 - 10:41

Good morning!
I measured with CCD many short period variable stars recently discovered by surveys.
The doubt is always the same: am I observing pulsating stars or eclipsing binaries?
Collecting measurements in several bands (B, V, Ic) I notice that for many systems the B-V curve shows a different mean value compared to the V-Ic curve.
Does this mean, perhaps, that the star exhibits physical variations and, therefore, it is not an eclipsing but a pulsating, or am I making a mistake?
Thank you for any help!


American Association of Variable Star Observers (AAVSO)
Amplitudes, wavelengths and types

Well, there is no straightforward answer.
What you are referring to is different amplitudes depending on passbands and they may be different in both pulsating variables and eclipsing binaries.
In the case of eclipsing binaries, it depends on the tempreature of the components. A typical example is an EA/GS system where a hot star is paired with a cooler giant or supergiant. The eclipse will have a very different amplitude depending on the filter you use. The hot star is much brighter in bluer wavelengths so the amplitude will be much larger in B than in V or Rc.
In stars with two eclipses, you will see that the primary eclipse becomes deeper and the secondary becomes shallower in a given passband compared with others.
In the case of pulsating variables, the change in temperature as the star pulsates also causes amplitude changes. They are more noticeable, as you go to the red side of the spectrum, amplitudes become smaller.
Since you are talking about short period objects, EA/GS are out of the question, you might be in the EW domain, and since both components are similar, there won't be important differences in amplitude.
I guess that your question is mostly referring to separating RRC and EW or ELL stars.
Well, the shape of the light curve will be enough in most cases. RRC are slightly assymetric (most have rise durations around 40%) and maxima are sharper than the shallow maxima in EW stars (which also show sharper minima).
RRC stars also show some bumps at pre-maximum.
Also checking colours and spectral types will allow you to properly classify some ambiguous objects. EWs tend to be cooler (later than F) while pulsating variables are usually hotter.
Some pulsating variables also tend to have multiple periods, so their light curves will look more "scattery" (e.g. SPB, BCEP or GDOR stars) while the ELL or EWs will show "cleaner" light curves.
Amplitudes are also good indicators to classify an object.
Check out the VSX Variability Types document for more information on each type.