Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 04/22/2022 - 17:09
Hi, just downloaded a chart for another CV and this reminded me of the 'period' shown on the chart. This is always the rotation period of the CV binary rather than the outburst 'period' - yes, I know these periods are not exact (and I am sure that most if not all observers know this too, that's half the fun). Surely observers are not really interested in the rotation time, but do like to have some idea of when a star might be in outburst. Can we not therefore replace the rotation period with the outburst 'period' instead?
Hello Mike,
Thanks for the suggestion. Let me clear up a couple of things before going forward. First, the "period" that you see is actually not the rotation period, but the orbital period as all CVs are binaries. Second, the information comes from VSX. It tracks the orbital period because that is the only static (unchanging) period for the system. You can use this along with the ephemeris information to calculate the specific time of any orbital event, such as an eclipse. Since the outburst period is at best an educated guess, the time of the outburst changes every cycle and we need to know that time to predict when the next one might be. This is at best, unreliable, and even if we did keep this information in VSX it would be almost impossible to keep it up to date. I do think predicting the next outburst would be useful and if we could do it reliably, that would be great. However, as it stands now many of the CV alerts and campaigns come from researchers who have a prediction for the next outburst and would like people to keep an eye on it. Hope this helps put things in perspective.
Thanks,
Bert Pablo
Staff Astronomer, AAVSO
All that Bert said above: yes.
Also for information if you want to dig some more... a paper that some of us put out investigating these very phenom:
Publications of the Astronomical Society of the Pacific. Vol. 119, No. 862 (December 2007), pp. 1361-1366
SS Cygni Outburst Predictors and Long Term Quasi-periodic Behavior
~john
<BKL>
Sorry - I was in…
Hi Bert,
Sorry - I was in fact using the term 'rotation' to mean 'orbital period' not the rotation period of individual stars, so yes, I can see how that could have been misinterpreted.
However, only a minority of CVs are eclipsers, and this fact could be included, something like (I am using SS Cyg as an example, with rough numbers off the top of my head - I know it's not an eclipser, but it is for this instance!):
SS Cyg
Magn. 7.8 - 12.4
Period ~50d: + ecl
At the basis of all this is that CHARTS are for OBSERVERS*, and nought else. I should think that the majority of those of us who observe CVs know that their outburst periods are approximate.
The orbital parameters and other values wouldn't change and would remain in VSX for everyone to use to the ends you mention. When a star might go into outburst is, by your own admission, unpredictable and of course that's why they need constant watching - by observers with charts...
Mr Pablo to serve ;-)
*caps for emphasis only. No shouting of course intended.
It’s useful to know P_orb for dwarf novae as knowing if it’s above or below the period gap is helpful for identifying if it is UGSU and hence going to show superoutbursts/superhumps
Of course that's relevant too, but only if you know what the period gap is! My point relates purely to charts and the information shown thereon for the benefit of a chart's user (i.e., observer) - and the type/subtype will come from VSX so the observer doesn't need to analyse the orbital period in order to determine a subtype in any case, because VSX is constantly being updated. So I maintain that the outburst 'period' (however nebulous that may sometimes be) should appear on a chart.