(cba:news) AM CVn campaign (only)

[Joe Patterson]

Hi CBAers,

The AM CVn campaign - the main effort this month - is going well.  But 
not much data at European longitudes; I hope the Euros will pick up the 
pace for a few weeks.  The star is ideally placed for March-April.

There are two objectives in this campaign.

(1) Timing the phase of the 1028 s orbital signal.  It's always 
invisible in the light curve - just 0.005 mag peak-to-trough - and 
dominated by the 1051 s ("positive") superhump, the 1011 s ("negative") 
superhump, and flickering.  However, because the periods are so short, 
one can average these signals over a long time series - and thus extract 
the phase of the orbital signal.  This is of great interest because the 
slow drift of orbital phase defines the rate of period change - one of 
the great unknowns in the theory of these pure helium stars.  I attach a 
paper we wrote for SAS in 2019; this year, we want to upgrade it into a 
regular journal paper (and we will!).

For this goal, VERY LONG RUNS are highly desirable.  Great for March-April.

(2) For goal (1), the two superhumps are noise - to be overcome! But one 
man's noise is another man's signal.  In theory, as positive superhumps 
drift to longer period, negative superhumps drift to shorter period. 
But this has never been tested.  Only two stars offer the test, because 
only  rise    winterest because The only star which reliably *offers* 
the test is AM CVn, because no other star reliably offers both types of 
superhump.

For this goal, what matters is the total length of the campaign.  We 
should definitely extend it through March-April-May.  But the runs need 
not be particularly long; 2-3 hours is fine after the next 2-3 weeks.

Info on other targets tomorrow...

joe
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