(cba:news) high-amplitude dwarf novae

Joe Patterson jop at astro.columbia.edu
Tue Apr 21 01:48:08 EDT 2015

  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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