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THE ASTRONOMER Electronic Circular No 2621      2010 Feb 06 14.55UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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U SCORPII
Further to the note on TA E-Circular 2619 regarding the outburst of U 
Sco, Bradley Schaefer of Louisiana State University has contacted us to 
appeal for IMMEDIATE time series photometric observations in its 
decline. He comments:
Just about now, the recurrent nova U Sco (now at V=13 in its
eruption just one week old) will start showing eclipses, and there is 
high science from getting detailed time series of these eclipses. So I 
am appealing that all hands take long time series of U Sco around these 
eclipses when U Sco is up in the pre-dawn sky. (Even northern hemisphere 
observers can get a few hours, and we need this to patch together the 
whole eclipse light curve).
In quiescence, U Sco has a deep *total* eclipse.  This provides a unique 
and wonderful opportunity to do important front-line science between now 
and early March, even with smallish-telescopes.  Here are three big-time 
science items perfect to get:
(1) Eclipse mapping of the light distribution across the photosphere. As 
the companion star blocks out the optical light from the photosphere 
around the white dwarf, the variations will define the size and 
structure of the photosphere. Such has never been done before.
(2) The centre of the eclipse timings can be used with the centre of 
eclipse timings in the tails of the last two eruptions (1987 and 1999) 
to determine the change in the orbital period across the 1999 eruption. 
This change will by Kepler's Law translate directly into a mass ejected. 
This method is the only reliable way to get the ejected mass. Knowing 
the ejected mass will tell us whether the white dwarf is net gaining in 
mass over each eruption cycle, and this will directly answer the old 
important Type Ia supernova
progenitor problem.
(3) The eclipse depth varies from early in the eruption when the 
expanding shell is optically thick until late in the tail when it is 
optically thin.  A measure of the eclipse depth will directly give the 
optical depth from the centre of the binary all the way out. This will 
then (for knowing the opacity of the shell from physics) translate into 
the total mass ejected.  This is a totally new way of measuring the mass 
ejected, and I want to try this on the U Sco eruption.
So we've got big stakes to play for, and amateurs with CCDs are 
perfectly set up.  First, you have a good distribution in longitude, 
with this allowing for coverage of multiple eclipses. Second, you have 
multiple observers with distribution in longitude, as this is `required 
to patch together light curves to get a whole eclipse.  The problem is 
that U Sco is close enough to the Sun, that any one observer can only 
cover 1-3 hours between U Sco rise and dawn, so multiple observers are 
required to cover the ~6 hour duration elcipses.  Third, you don't have 
large scopes, and you are set up perfectly for time series photometry. 
U Sco is now at V=13, and by the end of March it will be V=14.  This is 
just perfect for amateurs with CCDs on their scopes and useless for the 
big scopes.

To be continued in E2622: editor

Guy M Hurst