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RCB star declines

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CGCA
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Joined: 2010-10-06

It was great to see Mike Simonson's review of RCB lightcurves in the latest AAVSO newsletter. It is cool that so many stars, including  R CrB, itself, are in deep declines. It is still a bit of a mysetry why some of these stars seem to be in decline all the time and some very rarely go down. The only good study I know of that shows which stars are the most active is Jurcsik (1996, http://adsabs.harvard.edu/abs/1996AcA....46..325J). As Mike pointed out, one of the least active RCB stars is MV Sgr. Its only known declines are reported in Hoffleit (1959,  http://adsabs.harvard.edu/abs/1959AJ.....64..241H). MV Sgr is also unusual in that it is one of the rare hot (Temperature ~ 20,000 K) RCB stars. The apparent change in amplitude does look strange. If you look at Figure 8 of De Marco et al. (2002,  http://adsabs.harvard.edu/abs/2002AJ....123.3387D), it looks like MV Sgr has faded about a magnitude in the last 50 years. Keep observing it!

Geoff

RCB Star Declines
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WPX
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Joined: 2010-10-30

Yes, it is good to see the RCB stars getting some well deserved air time.  The declining amplitude of MV Sgr is interesting in that during the "early days" there was, I understand, no standard magnitudes for the comparison stars and different observers seem to have recorded different magnitudes.  Also, the reduced amplitude in recent years seems to coincide with a reduced number of observers and hence what may be scatter in the early light curve.

For example, compare the light curve with list of observers for the past 10,000 days vs 3,000 days.

Just a thought.....

Tks.

Peter Williams WPX

Heathcote NSW

I did my best to put all the
CGCA's picture
CGCA
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I did my best to put all the old observations from the plates on the same system as the more modern observations. I agree about the more recent observations. Some of the reduced scatter is that more and more observers are using CCDs.

Geoff

Evaluating lightcurves for
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Evaluating lightcurves for evidence of longterm fadings or brightenings is often far better done by examining data from just a single long term observer, if available, and NOT the unusual compilation.

I followed MV Sgr for many years and if you plot my data alone, virtually no change is evident in MV's mean magnitude. What long term fluctuation is suggested amounts to no more than 0.2-0.3 magnitudes and certainly not anything upwards of a full magnitude. 

J.Bortle   (BRJ)

Then again, some stars
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Then again, some stars classified as RCB may turn out not to be so. Another inactive star is UV Cas (according to Mike S!) and another, in my experience, is V482 Cyg.

There are three criteria for
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There are three criteria for determining that a star is an RCB star, the lightcurve, the spectrum, and the infrared emission from the dust around the star. It is often difficult to determine whether a star is an RCB just from the lightcurve because there is such a wide variation in activity. There are 5 HdC stars that have no activity at all, and no IR emission, but have identical spectra to RCB stars. UV Cas has IR emission and is definitely an RCB star from its spectrum, but hasn't had a decline since JD 2435000. V482 Cyg is also definitely an RCB star in that it has IR emission and is an RCB star from its spectrum. Also, it has had several declines in the last 50 years. 

We don't understand what turns the dust formation on and off in RCB stars, which is why we desperately need longterm lightcurves for these stars.

Geoff

R Cor Boring
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Yes, I have complained in the past about some RCBs being less than spectacular targets, if you like to see things change. UV Cas is probably the one I've complained about the most in the past. She's a flat liner. If her light curve were an EKG we'd say she was dead.

V482 Cyg at least shows signs of life in her past, so it's just a matter of time and patience.

SV Sge has been biding her time, testing our patience also. As I said in my article, I think this is the year she rewards us with a dramatic fade, based on past performance and an unscientific hunch.

"UV Cas"
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BRJ
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"UV Cas has IR emission and is definitely an RCB star from its spectrum, but hasn't had a decline since JD 2435000." 

and

"UV Cas is probably the one I've complained about the most in the past. She's a flat liner. If her light curve were an EKG we'd say she was dead."

 

In point of fact, this star does show occasional if subtle activity. It definitely experienced minor declines in/around both 1974 and 2005.

The problem has been that too many observers become accustomed to seeing this star as steady in brightness over very long periods, then when one of the minor dips does occur they don't take notice and instead keep right on reporting their usual steady "maximum" magnitude.

One often sees this slip-up made with R CRB itself. After a long period at maximum, at the onset of a new decline one nearly always sees a handful of observers who keep right on reporting the star at maximum...until the amount of decline in brightness has become so great that it can no longer be ignored. 

Every effort must be made to avoid any pre-assumption of what a variable's magnitude is going to be each night and observations should absolutely never be rushed. Taking one's time to make sure each and every estimate is totally unbiased and independent is critical when observing stars that exhibit only rare episodes of activity. We all "think" we follow that guideline, but I"m afraid that it isn't necessarily so.

J.Bortle   (BRJ)

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