R Coronae Borealis At Faintest

R Coronae Borealis Field - J. Brimacombe


R CrB Chart - Credit: AAVSO

Monday, February 02, 2009

For those of you who like observing curiosities, it's time to take a look at R Coronae Borealis. As you may have guessed from the single letter designation, R is a variable star, but it's not just any old variable - it's the prototype of its class. What exactly is an R CorBor star, what does it do and why is taking the time to check it out now so important? Then step inside and find out…

R Coronae Borealis stars (RCB) type stars are one of the oldest known classes of variable star. In just a period of a few weeks, they can drop in brightness by factors of thousands and what they do is totally unpredictable. Within months, they recover again to their maximum brightness… But why? While astronomers don't fully understand the evolutionary origin and the physical mechanism behind what drives R CorBor types, they do know the stars pulsate - generating a sort of sooty dust cloud just above the surface. Like an old-fashioned oil lamp with its wick turned up too high, when R Cororonae Borealis stars burn their fuel, they smoke up their exterior - just like the lamp smokes its glass chimney and dims the light. What remains on the glass? That's right. Carbon. And the surfaces of RCB stars are unusually poor in hydrogen, and rich in carbon and nitrogen. Chances are very good that R CorBor stars are actually the remnants of more fully evolved stars.

Just a few days ago, M. Templeton of the American Association of Variable Star Observers (AAVSO) released Special Notice #145:

"R Coronae Borealis, the prototype of the R CrB class, is apparently at or near historic minimum; a number of observers have put this star below m(vis)=14.0 since early November 2008, and both visual and instrumental measures are now indicating R CrB is near or below V=14.5. R CrB began its current fading episode around JD 2454288 (2007 July 6 +/- 1 day), and faded from m(vis) ~ 6.0 to below m(vis) ~ 12.0 by JD 2454325 (2007 August 12). The star has continued to fade for the past 17 months. Current visual observations by a number of AAVSO visual observers estimate the star to be around m(vis) 14.3-14.5, and V-band CCD observations suggest the star may be at or near V=15.0. BAAVSS observer J. Toone also visually estimated the star is at m(vis) ~ 14.9 (via baavss-alert). Both visual estimates and instrumental photometry of R CrB are strongly encouraged at this time.

The duration of the current episode and its depth are similar to that observed during the previous extreme fading episode which began circa JD 2438200 (June 1963) and continued with only one brief interruption until circa JD 2439100 (December 1965). During the 1963-1965 event, a few AAVSO observers estimated that R CrB reached m(vis) around 14.9-15.0, although the average visual estimate remained around 14.2-14.3 at minimum. The current episode seems to have reached the same depth; there is no way to tell whether the fade will continue, although the light curve has been flat or trending weakly downward for several months. As J. Toone pointed out, the current magnitude is very close to if not fainter than the historic minimum for this star."

Of course, nearing magnitude 15 isn't within the territory of binoculars or small telescopes - but it is within the grasp of many of our amateur astronomer UT readers with larger equipment, clear skies and the willingness to seize the opportunity to record this historic astronomical event. (I dislike the term "amateur" - it only means you don't get paid for it, folks… Not that you're any less serious or talented!) One such astronomer is Dr. Joseph Brimacombe, who took up the gauntlet immediately. Although Joe hails from Australia where R Coronae Borealis isn't visible, today's astronomy world is far different than it used to be. Thanks to the magic of the Internet, he immediately set about the task of capturing the star on January 30, 2009 via a robotic telescope located in New Mexico and shared his results with us.

For those wishing to also participate in the quest for R Coronae Borealis, you'll find it located at the following (J2000) coordinates: RA: 15 48 34.40 , Dec: +28 09 24.0 and you may use this field chart provided by the AAVSO to further refine your observations. If R is too faint for your equipment now? Don't worry. It's a variable star and within a few months it will return to its easily spotted magnitude 6 self - and a very delightful red star in binoculars. As always, be kind to science and contribute! Please promptly submit all observations to the AAVSO using the name "R CRB" and take part in astronomy history!