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(cba:news) high-amplitude dwarf novae (Joe Patterson) [2015-04-21T05:49:03Z]


 Hi CBAers,

Amid all the dwarf novae - and *new* dwarf novae - popping off up there, which are the ones of special interest? No one knows for sure (that would require advance knowledge of the stars, not available on this planet) - but a good bet is the high-outburst-amplitude guys. Why?

Well, dwarf novae in full outburst are pretty much a standard candle, depending weakly on orbital period, but basically Mv = +4.5 for the most common dwarf novae, the short-period guys. That's at maximum light. What about minimum light? You have the secondary star (usually negligible for the short-period guys, where the secondaries are all M6 or cooler)... and the white dwarf... and the accretion disk. The last is hard to calculate, but let's take the extreme case of imagining that the disk contributes ZERO light. So the limiting case is that at minimum, the light is purely the WHITE DWARF. That's a useful assumption, because cool white dwarfs with masses typical of those in CVs have an absolute magnitude = +12. So that means the maximum amplitude (between max and min) should be about 7.5 magnitudes. Even WZ Sge, deservedly considered the most extreme of that class, has an amplitude of just 7.3 mag.

So the high-amplitude guys (anything over 6.5-7 mag) are particularly interesting, because they represent the most intrinsically faint of the dwarf novae - and probably the oldest, because plenty of evidence shows that stars get fainter as they get older. We really want to study the oldest stars, because we have no idea how CVs end their lives. Theory suggests that CVs should be 5-10x more abundant in the sky than they actually are - and therefore theory is probably wrong as regards the very late stages of evolution.

So when the outburst amplitude even exceeds that of WZ Sge, the star is likely to be important. And the brightest dwarf novae are the nearest, since the DN is roughly a standard candle. So "bright and high amplitude" is a strong reason to observe that particular star.
This new guy, with an amplitude of 8.5 mag, is definitely in that category.

But you guys are getting the light curves, not me. It's also true that the light curves themselves often tell you what's worth pursuing. If it looks interesting, then it is!

joe p
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