post

UX UMa

After many false starts this year I finally got back out to the observatories and did some real observing!  And the target of the first nights of photometry for 2014 is the eclipsing nova-like (NL) star UX Ursa Majoris.  Discovered in 1933 by S. Beljawski, UX UMa was first thought to be a typical eclipsing binary star.  But once astronomers started observing the eclipses it was found the orbital period, as determined by the time between eclipses, was much shorter than most eclipsing systems.  UX UMa’s “period”, at just 0.1967 days (=4.7 hours), would normally indicate that it was a member of the W UMa subclass, which are stars orbiting so closely that they’ve almost merged, sharing a common “envelope” or outer atmosphere.  One observable consequence of such closely orbiting stars is that both stars are gravitationally distorted into ellipsoids so that the effective surface area of the stars as seen from our vantage point is always changing.  Thus there is no section of the light curve where the sum of the light we see from the system is constant for any period of time.   Visual observations of UX UMa showed it to have a fairly flat light curve except for the eclipses, so it obviously was not a W UMa type eclipsing binary.

By the middle of the last century photometric and spectroscopic observations had revealed that UX UMa was, in fact, an eclipsing nova-like system.  For a really good description of this star and a quick synopsis of the nova-like class check out this page from the American Association of Variable Star Observers (AAVSO).

In the past month or so I’ve been able to get three nights of data on UX UMa; May 18/19, May 25/26, and June 15/16.  The light curves for each are shown, below.  The plots show the brightness (increasing in the vertical axis) against time (increasing along the horizontal axis) for UX UMa (the red dots) and a “check” star (the blue dots), each as compared to a common “comparison” star.   Both the comparison and check stars are thought to be of constant brightness, so the blue curve, if everything was perfect, should be a perfectly straight line.  There are many many reasons why that is not so, but the scatter in the blue curve is an indication of how well I’ve managed to measure the brightness of the variable star.  In fact the scatter in the points in the blue curve help determine the “error” in the measurements of the variable star (the red dots).  The small vertical spikes that you see on the red dots is an indication of the error in each measurement.

Note that you can click on each of the plots to see them in full resolution.

2014_05_1819

 

 

2014_06_1516

 

2014_06_2223

2014_06_2829

There is a lot going on in the light curve of UX UMa!  The most obvious feature in the light curve is, of course, the eclipse, where the total light from the system is diminished by more than a full magnitude.  The eclipses are not symmetrical – a reflection of the fact that three different significant sources of light, each with it’s own characteristics (the accretion disk, the white dwarf, and the “hot spot” caused by the stream of material being sucked off of the red dwarf star impacting the outer edge of the accretion disk) are eclipsed.  Even outside of eclipse there is some interesting flickering going on that hints at being periodic.   And there is, perhaps counter intuitively, a gradual brightening towards when the red dwarf star passes behind the accretion disk.  The binary actually appears brighter due to the fact that the side of the red dwarf facing the white dwarf is being heated (something called the reflection effect, though it’s not a reflection) by radiation from the white dwarf and accretion disk,  thus making it’s surface much brighter on that side.  Got it?  That’s a fine run-on sentence!

Leave a Comment

*