R Coronae Borealis stars (RCBs) are a small group of hydrogen poor, carbon rich supergiants that decline in brightness unpredictably and rapidly by up to 9 magnitudes, and remain at or near minimum light for several weeks or months, even years in some cases. It is generally accepted that the declines are the result of the formation of a cloud of carbon soot that obscures the stellar photosphere, and that this condensation takes place in matter that has been ejected from the stellar surface toward the observer.
Some RCBs exhibit more or less regular variations that may be interpreted as pulsations. The amplitudes of these changes are small, on the order of a few tenths of a magnitude, and have periods of approximately 30 days to 150 days. This pulsation appears to have no relationship to the obscuring events, and has been seen to continue through fading episodes in several cases.
Possible evolutionary tracks
RCBs are intriguing because they challenge our models for stellar structure and evolution. At first, they were believed to be highly evolved post-AGB stars, but most scenarios fail to explain the hydrogen abundance or trace their evolution back to the AGB.
Two more recent ideas suggest that 1) these may be ‘born again’ planetary nebula, created when the last thermal pulse is delayed to the point that it occurs as the star reaches the white dwarf phase. If the pulse is intense enough it may re-ignite a helium burning shell and expand the star to giant dimensions, moving it to the AGB for a second time, or that 2) RCBs may be the result of the merger of helium and CO white dwarfs. The merger theory goes a long way to explaining the exotic chemical composition of these stars.
Whatever their origin, this is a small group of unique, unpredictable stars, just my cup of tea! I’ve come to know many of them well through a decade of observations and I have subdivided the class into my own categories based on their relative activity and interest as targets for the visual or CCD amateur astronomer or hobbyist. I call them simply the Good, the Boring and the Unknown. Let me introduce you to the ‘family’.
These are stars that provide a lot of entertainment value for the time invested in observing them. They are quirky and unpredictable, but most importantly, they are active. You don’t need to wait five years to see a fading event or some small blip in a light curve. These stars are crazy and not afraid to show it.
02 35 17.07 +56 08 44.7
Spec. type C4,5(R8)
Mag. range 10.6 - <13.2 V (16.5V aavso)
DY Per from 1992 to present
While this is certainly the light curve of an unpredictable star, the fading episodes to don’t follow the typical RCB pattern. Is DY Per a special case in a class of special cases? Instead of occasional fades every few years or so, lasting months at a time, these episodes are more or less regular and spend almost no time lingering at minimum or maximum light.
For the visual observer with a 10” or 12” telescope this star is visible during all but the very faintest excursions into 15th to 16th magnitude range, and as you can see, it will soon be back up if you just wait a couple weeks. DY Per is a solid performer and always a surprise from night to night.
05 49 03.73 +19 04 21.8
Spec. type G0-1Iep(C1,0 HD)
Mag. range 9.1 - 16.86 V (18.2V aavso)
SU Tau from 1987 to present
Discovered by Annie Jump Cannon, SU Tau is another entertaining star to follow. The AAVSO light curve for this one goes back to 1909. As you can see from the above light curve, SU Tau has been very active since 1994, with an extended period after an initial fade where it tried to climb back to maximum light in fitful starts but sputtered again and again. It is now recovering from the third fading episode in the last decade. As with DY Per, visual observers can follow all but the very faintest periods of this star’s unpredictable cycles.
15 02 01.48 +83 03 48.7
Spec. type C
Mag. range 10.8 - 17.5: V (18.5V aavso)
Z UMi from 1999 to present
Z UMi is another RCB that delivers a lot of action. Because it is circumpolar, the light curve is nearly continuous. As shown in this light curve, the fading episode prior to the current one was a record breaking affair, reaching an unprecedented minimum level and an extremely long, slow climb back to maximum light. After a typical interval at maximum light, Z UMi precipitously faded again and is currently recovering towards maximum. There is no guarantee it will make it to the top before sputtering or fading again, and that is what makes observing these stars on a regular basis so much fun.
15 48 34.41 +28 09 24.3
Spec. type C0,0(F8pep)
Mag. range 5.71 - 14.8 V (15.4V aavso)
Thirty years of R CrB
The prototype of the class, R CrB is a binocular star hovering around 6th magnitude most of the time. Then, unpredictably it fades rapidly, diving for cover in the inner sanctum at or near 14th magnitude. However, we live in interesting times, because the last fade of R CrB was one for the record books. In July of 2007, R CrB began to drop. By October it was 14th magnitude, but R CrB was only beginning to put on the show of the ages. In February 2009 observations of 15th magnitude began to come in, with no sign of a recovery. In fact, R CrB did not even get back to 14th magnitude until November of 2010. This fade was not only the deepest in recorded history, it was now the longest, and it isn’t over yet!
Observations in June put R CrB around 12.2 V as it slowly makes its way towards recovery. Will R CrB make a full recovery, or have a relapse and fade again before reaching maximum light. Only time and AAVSO data will tell. No wonder this is one of the most well observed stars in the AAVSO program. You can’t buy this kind of reality entertainment.
19 32 21.62 -00 11 30.9
Spec. type C
Mag. range 11.5 - <17.7 V
All the AAVSO ES Aql data, 1999 to present
Located on the celestial equator, this under-observed star is somewhat difficult for northern observers to monitor, including me, which is a shame because this is a great star to follow, if you can. That’s why I added it to my AAVSOnet queue to acquire CCD photometry of it on a regular basis. As luck would have it, just about the time I began obtaining CCD measures it went into the deepest death spiral in AAVSO recorded history, hitting 16.5V in February 2011. I watched it steadily recover to 13.7V in May only to witness another fast decline, again reaching 16.5V as I write this piece, on June 27, 2011. During this entire period I was the only one collecting data on this fascinating star.
This is another fun to observe, unpredictable star worth putting on your program if you can observe near the celestial equator in Aquila from your observing site. I have it on both my visual and CCD programs.
22 03 19.70 -16 37 35.3
Spec. type pec
Mag. range 10.6 - 15.9 V
U Aqr AAVSO data from 1976 to present
The AAVSO data for this star stretches all the way back to 1905. Not surprisingly, at minus 16 degrees declination in Aquarius, it is not a well-observed star by northern observers. The long-term nature of U Aquarii’s behavior seems to change each decade. From 1976 to 1986 it had 4 or five fadings, then from 1986 to 1998, it remained more or less at maximum except for a couple seasons where it averaged one magnitude fainter than maximum light. Then, in 1999, it faded to fainter levels than ever seen before and has had a decade of fitful recovery to maximum light, which is where it was when I added it to my robotic telescope queue.
Again, my beginner’s luck has struck gold. As soon as we picked it up coming out of conjunction I could see it was fading to a deep minimum again, and as of right now the Krajci 35cm telescope holds the record for the faintest recorded magnitude of U Aqr in the AAVSO database, 17.2V on June 26.
Obviously, a CCD target when this faint, when it creeps back into the 14th magnitude range its liable to provide an interesting subject for visual observers who can pierce the haze from mid-northern latitudes. This chapter of U Aqr is far from over and no one knows how the story will develop from here.
These are RCBs that for whatever reason just aren’t very exciting to follow. They exhibit little, if any, activity and it has been years or decades since they showed any real sign of life at all.
19 08 11.78 +17 37 41.2
Spec. type C0-3,2-3(R2)
Mag. Range 11.5 - 16.2 p
SV Sge since 1966
It wasn’t too long ago this star was a lot of fun to monitor. It seemed to be fading and recovering fairly often, bit the last time it did anything was 2004. I’ve grown tired of observing it at 10.5V for years on end, but I know better than to drop it, because that is the day it will go into a tailspin and fade to record depths.
This is probably normal behavior for this star as the light curve above shows. I’m just impatient. It looks long overdue for a fade, but for now it’s a yawner. When it finally does cough up a dust cloud I’ll take it off the Bad list and put it back in the Good category.
18 44 31.97 -20 57 12.9
Spec. type B2p(HDCe)
Mag. range 12 - 16.05 B
MV Sgr since 1987
With an amplitude of 1.5 magnitudes, this light curve looks a lot more like a semi-regular variable than an RCB. The spectrum is of a hydrogen deficient carbon star so it has the requisite color and composition, it just hasn’t done anything since we’ve been monitoring it. Twenty-five years isn’t long in the history of an RCB, so maybe we just need to keep an eye out for activity. So far, it hasn’t been much fun.
19 59 42.57 +33 59 27.9
Spec. type --
Mag. range 11.8 - <15.5 p
V482 Cyg 1967 to present
The last fade was in 1996, three years before I started observing variables regularly. So in the time I’ve accumulated nearly 70,000 observations, V482 Cyg has kept me waiting, based on the promise of a fade from 11th magnitude to 13.5 in the last century. If I wasn’t already observing dozens of stars in Cygnus I probably would have dropped it a long time ago. All I can say is I hope the next fading event is worth waiting for, or my relationship with this bad star will suffer.
23 02 14.67 +59 36 36.6
Spec. type F0Ib-G5Ib
Mag. range 11.8 - 16.5 p
UV Cas since 1959
I remember a couple years ago some people got all excited because this star had a half magnitude dip in its light curve, confirming what I already thought about this star. It is one of the most R Cor Boring stars in the sky. Apparently, a “deep fade” of two magnitudes 40 years ago is enough to keep some observers monitoring this RCB. Not me, I dropped it long ago.
This category is made up of stars for which there are little or no data in the AAVSO database. These stars are ripe for the picking. No one else is watching, you never know what you might find.
18 45 14.84 -09 25 36.1
Spec. type --
Mag. range 13.4 - 16.8 p
FH Sct from 2001 to present
I knew next to nothing about this RCB when I added it to my CCD program in 2009. I soon discovered it resides in the open cluster M26 in Scutum. There was no sequence for it when I began taking data, but now there is BVRI data available from the AAVSOnet telescopes and a color magnitude sequence of comparison suitable stars has been created. In 2011 I caught the first fading episode recorded in BVRI in the AAVSO database. Normally around 12th magnitude at maximum, it faded abruptly to 15V and has been steadily recovering since. There is no excuse not to observe this star now. It simply suffers from neglect due to its southerly declination.
19 10 11.83 -20 29 42.1
Spec. type C(R)
Mag. range 11.5 - <14.5 V
The epitome of an ugly light curve, the data for this star is sparse and unreliable. At -20 degrees declination, this one stretches the definition of a northern RCB to the limit, but any reliable data starting right now can only improve the situation here. Amplitude, maximum and minimum magnitudes, and pretty much everything else are uncertain for this RCB. My own recent observations only range from about 12 – 11.8V, not enough to say anything for sure, except it is variable.
There is another “northern RCB” you may happen upon in the literature, but LT Dra is not variable, it is a constant star, so don’t waste your precious telescope time following this one.
There are not that many observers following these interesting stars regularly, so the chance to hit upon something new or unexpected is always there. That coupled with their irregular and unpredictable nature makes them fun and challenging at the same time. Add a few of these stars to your observing program and be prepared to be surprised.