Open Season on FS Aur !!!!

Like duck hunting, Variable Stars have their "Seasons" also. A season being where the variable star in question is in the optimal position for "shooting with CCDs" ie observable for most of the night. FS Aur is a Cataclysmic Variable rising in the early evening, and in the current northern winter, its visable for a good 6-7 hours a night. Thus its "in season".

FS Aur is one of the strangest and complex objects in the extensive variable star family. (See also Steve's excellent article elsewhere in today's Carnival of Space.) It is thought to be a magnetic white dwarf binary CV with a third object acting on it in a circumbinary orbit. (Tovmassian et al., 2010)

Now its in season - astronomers want answers. When they want answers they often leverage the collective effort of the American Association of Variable Star Observers (AAVSO), to muscle up some more observations and data.

FS Aur has a number of photometric and spectroscopic cycles going on. On top of this it goes into outburst about every 12-15 days. The outburst period is so short (2-3 days) it has been regularly missed in the past - so this season Dr Neustroev has rallied a special effort. For the first time, members of the AAVSO will map the entire 15 day outburst cycle, several, hopefully a good number of times.

Dr Vitaly Neustroev is the University Researcher (Yliopistotutkija) at the University of Oulu, Finland and he is a key reseacher on FS Aur, working hard to better understand its erratic behaviour. Visit the observing campaign page here, and also follow on (Twitter).

I like following Cataclysmic Variables, after catching Simostronomy fever, and I have been a regular on FS Aur since Dr Neustroev's call to action.

During the past few weeks FS Aur has been dancing away with a little periodic wobble at about 15.9 with the odd subtle change in brightness, then on the 22nd it went really dull to 16.2 then literally within 24-36 hours it popped to Magnitude 14.3 on the 24th, and then grew to about 14.035 throughout the day.

In my image from the 24th, FS Aur can be seen in the Green annulus.

Now take a look at this image on the 22nd. FS Aur can hardly been seen. It is at magnitude 16.2 at its lowest point in its quiescent phase.

So what is going on with FS Aur - Dr Neustroev describes it this way:

“FS Aur is an extremely unusual cataclysmic variable renown for the presence of a variety of uncommon and largely incomprehensible periodic variabilities of brightness and radial velocity. The outlandish peculiarity of FS Aur is the existence of a very coherent photometric period of 205.5 min that exceeds the spectroscopic orbital period of 85.7 minutes by 2.4 times. Such a discrepancy in the photometric and spectroscopic periods is highly unusual for cataclysmic variables.

Based on the short orbital period, FS Aur has been classified as a SU UMa star. Nevertheless, long‐term monitoring of the system by several groups failed to detect any superoutburst in its light curve. The publicly available AAVSO light curve show instead a steady‐cyclic outburst pattern that is more similar to a SS Cyg–type dwarf nova light curve. However, this monitoring reveals an additional, discordant, and very long photometric period of ~900 days. We explain the latter by the result of eccentricity modulation of a close binary orbit induced by the presence of a third body on a circumbinary orbit (Tovmassian et al., 2010).”

So it is indeed a complex beast. This is a little time-series I ran when it was at quiescence.

When variables are in season you can get nice long lightcurves and do time-series analysis and this is very valuable data. Below is the light curve of FS Aur climbing to its peak during the 24th January. The light curve below spans almost continuous/contiguous 6 hours. It is very interesting as it shows quite clearly the erratic nature and massive forces involved in the outburst. (Each measurement is from a 120s image and with a precision of about 0.01-0.03mag.)

I have also submitted some color times-series in BVRI as this is also useful data, more so if I had got it at the peak of the outburst, alas due to weather I missed out. I'll have to wait another 13 days ;-).

Again, about 4hrs of continuous data can only only be gathered "in season".

So this is indeed a fascinating object and could indeed be a new class of object. Only time will tell. AAVSO members are playing an important role in gathering the data to refine the lightcurve. Once the observing campaign is complete we await with interest to see what the astronmers and astrophysicists can tell about what they have learned.

At AARTScope my mission is to "keep creating the sense of anticipation and discovery that keeps scientists asking questions" so if you are an AAVSO member and would like to join the campaign, please do.

Unlike ducks, no variable stars are harmed during "peak season".