[BAA 00474] U Sco eclipses

[Rick]

BAA electronic bulletin No. 00474 -- http://www.britastro.org/
Bulletin transmitted on Mon Feb 8 20:26:52 GMT 2010
(c) 2010 British Astronomical Association

Guy Hurst has just forwarded the following message from Brad Schaefer (Louisiana State University) appealing for time-series observations of this object in the coming days and weeks.

Although exceedingly difficult from the UK our longitude does fill a gap in the coverage and so is to be strongly encouraged.

Roger Pickard,
Director BAA VSS

-------------------
First, I would like to recruit you and your students and the BAA Nova group observing U Sco. This is fast moving, and there is big-time science that even people with small telescopes can do. In particular, even far to the north as many of you are, U Sco is up long enough in the morning sky to get time series photometry, and England observers fill a gap in longitude.

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. And the CBA is perfect for getting exactly that. 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 that CBA is 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 center of the eclipse timings can be used with the center of eclipse timings in the tails of the last two eruptions (1987 and 1999) so as 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 (the other *old* ways are all horribly assumption ridden and they are accurate only to perhaps 1-2 orders of magnitude). And the 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 uber-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 center 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. This method is totally independent of the big-troubles of the old methods (like uncertain distances, filling factors, ionization
states...), and hopefully will be good. Then, as in item 2, we get the mass ejected and this plays into the uber-important Type Ia SN progenitor problem.

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 eclipses. 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 forthe big scopes. So here are the details of what to do. Use the list below to select which mornings will have U Sco observable for your location for times within 5 hours of the quoted eclipse center times. (We must have observations around 5 hours of so on either side of eclipse so as to know the uneclipsed level.) When U Sco rises high enough (push this a little if you can), start taking your normal time series and just keep at this until dawn makes you stop. Choose the shortest reasonable exposure time such that U Sco and one of the comparison stars (preferably COMP) are not saturated. For the comparison stars, see the attached files (in PDF and WORD formats*) for finder charts on all scales as well and the magnitudes for all the indicated stars. If you have a V-filter then use it (R-band is the only other option to use). If you need more signal or do not have a V or R filter, then simply run without any filter. After dawn, process your images (bias subtract, flat field...) in the usual way. Extract the magnitude of U Sco with aperture photometry in comparison with one or more of the stated comparison stars (preferably COMP). Send these magnitudes to myself (schaefer at lsu edu) or to the usual sites for AAVSO or CBA. It is all easy, and much of what you already do so well.

Here is a table with all the eclipse times from now to early March:

Code Select Expand

JD mid-eclipse UT mid UT date Where Visible
2455232.571      1:43   5-Feb South Africa
2455233.802      7:15   6-Feb Florida, Chile
2455235.032     12:47   7-Feb Calif, Hawaii
2455236.263     18:18   8-Feb NZ, Australia
2455237.493     23:50   9-Feb South Africa
2455238.724      5:22  11-Feb Chile
2455239.954     10:54  12-Feb Florida-Calif
2455241.185     16:26  13-Feb Hawaii, NZ, Australia
2455242.416     21:58  14-Feb Aus-SA
2455243.646      3:30  16-Feb South Africa
2455244.877      9:02  17-Feb Florida-Calif
2455246.107     14:34  18-Feb Hawaii, NZ
2455247.338     20:06  19-Feb Aus-SA
2455248.568      1:38  21-Feb South Africa
2455249.799      7:10  22-Feb Florida, Chile
2455251.029     12:42  23-Feb Calif, Hawaii
2455252.260     18:14  24-Feb NZ, Australia
2455253.491     23:46  25-Feb Aus-SA
2455254.721      5:18  27-Feb Chile
2455255.952     10:50  28-Feb Florida-Calif
2455257.182     16:22   1-Mar Hawaii, NZ, Australia
2455258.413     21:54   2-Mar Aus-SA
2455259.643      3:26   4-Mar South Africa
2455260.874      8:58   5-Mar Florida-Calif
2455262.104     14:30   6-Mar Hawaii, NZ
2455263.335     20:02   7-Mar Australia
2455264.565      1:34   9-Mar South Africa
2455265.796      7:06  10-Mar Florida, Chile
2455267.027     12:38  11-Mar Calif, Hawaii

The eclipses should be just starting as shallow events now, and after early March then U Sco will likely have a sudden drop in brightness making it hard to get eclipses.

This is easy, this is exciting, this is big-time science, and only the  amateurs with CCD can do this!

Brad (schaefer at lsu edu)

* The Word and or PDF files can be made available to interested observers if they care to email me at roger.pickard at sky com or alternatively a chart can be generated from the AAVSO web site http://www.aavso.org/ The preferred Comp is the star marked 15.9 to the NW of the variable. Enter U Sco where it asks for the chart and click Findcharts and then Go. Then choose the parameters to suit your telescope/CCD combination.