(cba:news) high-amplitude dwarf novae
jop at astro.columbia.edu
Tue Apr 21 01:48:08 EDT 2015
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!
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